GOC Allen
October 4, 2022 | Author: Anonymous | Category: N/A
Short Description
Download GOC Allen...
Description
Read more at www.puucho.com
JEE – M
IN
REACTION MECHANISM
Reaction : Breaking of old bond and formation of new bond is known as chemical reaction A B +
X Y
X +
A
B
New bonds
Old bonds
A sequential account of each step, describing details of electron movement, energetics during bond cleavage and bond formation, and the rates of transformation transformation of reactants into products (kinetic (kinetics) s) is referred to as reaction mechanism. Species on which reagent is attacking is known as substrate substrate or reactant. Species which attack on substrate, is known known as reagent.
Type of cleavage of bond : (A)
Hete Hetero rolyt lytical ical cl clea eava vage ge/fi /fiss ssio ion. n.
(B)
Homo Homoly lyti tica cal l cle cleav avag age/ e/fi fiss ssio ion. n.
C
C
.
×
Z
×
C
+ ×
Z
+
| Ionic cleavage V or | Heterolytic fission ZW
+
C
× Z
Reaction intermediate
(A)
Cleavage in which unequal distribution of electrons electrons takes place during the bond cleavage is known as heterolytical cleavage. Due to unequal distribution of electrons, ions are formed. That’s why it is also known as ionic cleavage or heterolytical cleavage.
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
Un Ionic cleavage cleavage or Homolytic fission
+ ×
C
Z
C × Z
C
(B)
×
+
Z
Cleavage in which equal distribution of e–s takes place during the chemical reaction is known as homolytical cleavage. C
×
C
Z
+
×
Z
Due to equally distribution of electrons, without charge unpaired electrons species is formed, which is known as free radical and cleavage is known known as unionic cleavage/homolytical fission.
By both cleavage [ionic/non ionic] three type of species are formed [One carrying positive charge, other carrying negative charge and third one is neutral with unpaired electrons] is known as reaction intermidiate.
If reagent compound is organic, then reaction intermediate are ( i ) Carbocation ( ii ) Carban Carbanion ion (iii) Carbo Carbon n free free redica redical l
63
Chemistry
Carbocation :
Carbon on which positive charge is present is called carbocation. carbocation.
It is electrons defficient species.
It is incomplete octet species species also because it has six electron in outer most sh shell. ell.
It's hybridisation state is sp2.
It's Geometry is Trigonal planner.
Bond angle is of 120°.
Due to electron deficiency it act as a electrophile and always attack on electron richer site.
Carbocation are of three types :
(i) 1 or primary, º
(i)
(ii) 2 or secondary, º
(iii)3 ortertiary º
1º or primary : When positive charge present present on 1 or primary carbon. º
Example :
CH3
ethyl carbocation
n–propyl ca carbocation
CH3 CH2 CH2 CH2
(ii)
CH2 CH2
CH3CH2
CH2 CH CH CH2
n–butyl carbocation Allyl carbocation Benzyl carbocation 2° or secon seconda dary ry carbo carboca catio tion n : When positive charge present on 2° or secondary carbon.
Example :
CH3
CH
CH3
CH3
Sec. butyl carbocation
CH
CH2
CH3
iso-propyl carbocation
CH
CH 3
CH
CH2 CH 2 CH3
Active secondary pentyl carbocation (iii)
diphen phenyl methyl carbocation ion
3° or tertiary carbocation : when positive charge present on 3° or tertiary carbon. CH3 CH3
Example :
C
CH2
Tertiary butyl carbocation
Tertiary pentyl carbocation
Triphenyl methyl carbocation
Carbanions :
64
CH3
Carbon on which the negative charge is present is called carbanion. carbanion.
5 6 P . C O G 3 0 \ T R A Y P R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
It has eight electron in outermost shell shell so it is complete octet species.
It is an electron richer species because it has extra electron.
It has non bonding electron pair.
It's hybridisation state is sp3.
It's Geometry is pyramidal.
Bond angle is of 107°.
Due to presence of non bonding electron it it act as a nucleophile also.
On the basis of presence of negative charge on carbon atom it is of three types (i) 1 o or r primary (ii) 2 o or r secondary (iii)3 ortertiary º
( i )
º
IN
º
1° or Primary carbanion : When negative charge present on primary carbon like, CHCH 2 3
CH2
ethyl carbanion
3
2
2
n-propyl carbanion
be b enzyl carbanion
Other example of primary carbanion are same as that of carbocation only negative charge will be present on carbon instead of positive charge. (ii)
2° or secondary carbanion : When negative charge present on secondary carbon. carbon. Other example
of secondary carbanion are same as that of carbocation only negative charge will be present on carbon instead of positive charge.
CH
CH3–CH–CH3
(iii)
5 6 P . C O G 3 0 \ T R A Y P R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
3° or tertiary carbanion : When negative charge present on tertiary carbon. CH3
CH
CH3 ,
C
CH3
Free Radical :
Carbon on which a unpaired electron is present and also electrically neutral then it is known as carbon free radical.
It is always formed by the homolytical fission of bond or it is form when reaction occurs in the presence of U.V.
light or reaction occurs at high temperature or reaction occurs in presence of non polar solvent.
It has seven electron or odd electron in outermost shell.
It is incomplete octet species so it is also electron electron deficient species.
It's hybridisation state is sp2.
Geometry is trigonal planner.
Bond angle is 120°.
65
Chemistry On the basis of presence of unpaired electron on carbon. It is also of three types :
(i) 1° or primary free radical ( i )
(ii) 2° or secondary free radical
(iii) 3°ortertiary free radical
1° or primary free radical : When an unpaired electron present on primary carbon. carbon.
Examples : .
.
CH3CH2 CH2 ,
CH3 CH2 ,
.
CH3
CH
CH2
CH3
( i)
2° or secondary free radical : When unpaired electron present on secondary carbon. .
.
CH 3CH CH 3 , CH 3CH CH 2CH 3
(iii)
3° or tertiary free radical : When unpaired electron present on tertiary carbon.
Examples :
CH3
.
C
CH3
.
,
C
CH3
CARBENES (CH2:) :
Carbenes are neutral carbon species in which the carbon atom is bonded bonded to two monovalent atoms or groups and carries two electrons.
It is Bivalent radical.
It is neutral.
6 e– in outermost shell.
4 e– are bonded and two are nonbonded e–.
Carbenes are of two types :
(A)
Singl ingle et carben rbene e : Carbene in which both non-bonded e– lies in same orbital with opposite spin.
H
empty empt y unhybridized p–orbital p–orbital
103°
–
lone pair of e
C 2
sp hy hybridi bridized carbon
H
Orbital structure of singlet carbene Characteristic of singlet carbene :
(B)
Both non-bonded e– lies in one orbital with opposite spin.
It is electron deficient so acts as electrophilic reagent.
Geometrical shape – bent or V-shape or angular shape.
It is diamagnetic in nature.
Triplet c ca arbene ene : Carbene in which both non-bonded e– lies in two different orbitals with same spin.
C sp carbon
two unhybridized p–orbital p–orbital
Orbital structure of triplet carbene 66
Hybridization – sp2
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
IN
Characteristic of triplet carbene :
Both non-bonded e– lies in two different orbitals with same spin.
Geometrical structure – linear
It is also electron deficient so acts as electrophilic reagent. reagent.
Triplet carbene is more stable than singlet carbene
Hybridization – sp
It is paramagnetic in nature.
A T T A C K I N G
REAGENTS
The species which attack on a substrate molecule or intermediate intermediate and form a product is called as attacking reagent. These are of two types : (A)
Ele Electr ctroph ophili ilic c reage reagent nt or ele electr ctroph ophile iles: s:
Electrophilic
(electro
+
philic)
(electron +
loving)
The reagent which attacks on the negative of the molecule molecule or loves electrons are called electrophiles. electrophiles. Electrophiles may be positively charged or electron deficient molecule (molecule with sextet or septet). (i)
Po Posit sitiv ively ely charge rged d el elect ectro roph phile iles s:
H , S O3 H , NO , NO 2 , X , R , C 6 H 5 N2 , C H
, OH CH3
2
C, CH2
CH
O
(ii)
Ne Neutr utral el elect ectro rophi philes :- Which possess a electron dificiency.
(a)
All Lewis acids as : BF3, AlCl3, SO3, ZnCl2, BeCl2, FeCl3, SnCl2, CO2, SnCl4.
5 6 P . C O G 3 0 \ T R A Y P R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
(b)
The neutral eutral atom that that accep accept electr electrons from the substra trates tes : * * *g – X, – Cl, CH – C * = O, R C OCl, R – M N > C I 3
The star (*) indicates the atom that accept electrons. (c) (B)
Free radicals, carbenes & nitrene acts as electrophiles.
Nucle Nucleoph ophil ilic ic reag reagen ent t or nucle nucleoph ophil iles es :
Which attack on the positive site of the substrate or loves nucleus. Nucleophilic
(Nucleo + philes)
(Nucleus + loving) Nucleophiles may be negatively negatively charged ions or possess a lone pair of electron or dona donate te an unshared electron pairs. ( i )
Negative vely ly charged nucleophil iles es.
, CN, X, R , R – COO, NH H, OH, OR NH2, CH2 – COR, SH, HSO4, N NO O3, RS, CO3, CH2 = CH
67
Chemistry ( i i )
All Lewis base which contains lone pairs :
R , H 2 S H, R S H 2 O , R O H , R O R , N H 3 , R N H 2 , R 3 N , R S
* * * * * – Li, LiAlH R – Mg – X, R 4, CH3 – CN, NaBH4
(iii)
The star (*) indicates the atom which donates electrons to the substrate. A Am mbident nucleophile : Nucleophiles which have two sites of electron rich centre or in which two or more
atoms bear an unshared pair of electrons. Example : K—O—N O, N H — OH 2
CH2
C
CH2
C O
O
Resonating structures are also abmbident nucleophile.
S.No .
El Electrophile ectrophile
Nucleophil Nucleophilee
1
Acce p ts th e e le ctr on p a ir
S up p lie s t he e le ct r o n p a ir
2
Ele ctr o n de ficie n t
Ele ctr on r ich
3
Attac ttacks ks the poi points nts of high high el electr ectron on dens densiity
Att ttacks acks the poi point nt of low el elect ectron ron densi density
4
Le wi wis a ci cid
Le wi wis ba ssee
5
Possess an emp ty orbi Possess orbital tal to re ceiv ceivee the electron pair
Possess an electron pair which is loosely held and can be supp lied ea si silly
6
U sua lllly p o ossitive lly y cha r g e d sp eeccie s
U su a lllly ne g gaa ttiive lly y ch a r g e d sp ec ecie s
7
Forms an extra bon bond d with the nuc nuclleophil eophile
Inc ncreases reases its coval covalency ency by one un uniit
ELECTRONIC
EFFECTS :
There are four effect which affect the chemical reaction are – (1) Inductive effect (2) Mesomeric effect (3) Hyper conjugation 1.
(4) Electromeric effect
INDUCTIVE EFFECT :
Polarity developed in Carbon–chain due to the shifting of bond electron by the group or atom present on carbon-chain is known as inductive effect,.
Discovered by scientist Ingold.
It is effective upto 3 or 4 carbons. After 3 or 4 carbons it becomes neutralized. So M Ma agnitude of I effect
1
distance
Inductive effect is measure with respect respect to hydrogen atom that means inductive effect of hydrogen is always zero.
68
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A A T D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
IN
It is a permanent effect.
Some atoms or groups have a greater tendency to attract the shared electron of the covalent bond. Such atoms or groups acquire partial negative charge by receiving electron density from the covalent bonds of the chain. Therefore, these are classified as the groups exerting negative inductive (– I) effect.
If shifting of electron takes place from from carbon chain to group or away from the carbon c chain hain or towards the group , then it is known as (–) I effects. C – C 3
C C 2
Z
(I) 1
C1 ( ) > C2 () > C3 ()
Due to –I effect positive charge develops over carbon chain or carbon chain becomes electron deficient such type of carbon chain will be mo more reactive towards nucleophile or less reactive will be towards electrophile. Group which shows (–)I are The decreasing order of negative inductive effect of some important atoms and groups is given below
Order of – I effect
OR2 > –NR3 >– >–N NH3 > –NO2 > –SO3H > –CN > –C – COR > –X> –OH > –OR > –NH2 > –C6H5 > –H
Some groups are electron donor and therefore acquire partial positive charge by by increasing electron density in the covalent bonds of a chain. chain. Such groups exert po positive inductive (+ I) effect.
Or if shifting of electron takes place from group to carbon chain or away from the group or towards the carbon chain then it known as (+) I effect C – C 3
C 2
C Z 1
(II) 5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
C1 ( ) > C2 () > C3 ()
Due to +I effect electron density increase increase on carbon chain or negative charge comes on carbon chain chain mo ore reactive towards electrophile or will be less reactive towards such type of carbon chain will be m nucleophile.
Group which shows +I effect are.
O || – O, – O – C, all alkyl groups
In alkyl groups megnitude of +I effect size of alkyl alkyl group – CH3 <
– CH2CH3 <
–CH2CH2CH3
<
–CH2CH2CH2CH3
In isomeric alkyl group megnitude of +I effect number of branches. CH3 –C –CH2CH2CH2CH3 < –CH2–CH–CH3 < –C–CH3 CH3
CH3
69
Chemistry
APPLICATION OF I-EFFECT I-EFF ECT
(1)
Stab Stabil ilit ity y of carb carboc ocat atio ion n:
(a)
If number of + I groups increases then stability of carbocation increases.
(b)
If number of –I groups decreases then stability of carbocation increases.
(c)
Therefore En er ergy ch ar arge
1 stability
S ta b biility o f C ar arbo ca ca ti tio n No. o f I g grro u up ps
Stabillit Stabi ityo yo f carbocati carbocation on
1 No.o f I gro up upss
CH3
Question : Stability order :
CH3 2°
CH3 3°
Reas Reason on :
Ans. An s. (2)
CH3 CH CH CH 3 2
CH3 C
CH3
1°
More More no. no. of of +I +I gro group up. .
more stable carbocation. St Stab abil ilit ity y of of c car arba bani nion on : (a)
If number of – I groups increases then stability of carbanion increases.
(b)
If number of +I groups decreases then stability of carbanion increases.
(c)
Therefore St a b biility o f C ar arba ni nio n No.o f I gr o u up ps Stabillityo f Carbanion Stabi
1 No.o f I gro up s
CH3
Example: (1) CH
3
CH CH CH3 2°
(2)
C CH3 3°
3
(3) CH CH 3
2
(4) CH3
1°
More No. of +I group. Less stable carbanion. So stability order
1 < 2 < 3 < 4
Example : (1) CH2 CH CH2
CH3
F
Minimum distance of –F. Maximum –I of –F. Minimum negative charge. Maximum stable. So stability order 1 > 2 > 3 70
(2) CH2 CH2 CH F
CH3 (3) CH2 CH2 CH2 CH2 F
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A A T D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
IN
Aci Ac idi dic c and bas asic ic st str rengt ength h:
A Ac cidic strength : +
H dona nar r Acid e – acceptor
( i )
If No. of – I groups increases then acidic strength increases.
( i i )
If No. of + I groups increases then acidic strength decreases.
acidi acidicc strength No. of - I groups
1 N o . o f + I g rro oups
Example :
( i )
CH C H3 C O
OH
Cl CH2 C O
–H – H
CH3
–H – H
C
O
Cl
O +I of –CH3
( i)
OH
CH2 C
O
O –II of –Cl, so – so anion is more sta tab ble
so anion is less stable stable and corresponding corresponding acid is more acidic. CH3 CH2 CH COOH > CH3 CH CH2 COOH > CH2 CH2 CH2
F
F
COOH
F
minimum distance of F from –COOH maximum –I of F. So maximum acidic.
Basic strength : –
OH dona onar r 5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A A T D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
Base
e – donar +
H acceptor
( i )
If No. of + I groups increases then basic strength increases.
( i i )
If No. of – I groups increases then basic strength decreases.
Basic strength strength No. of + I groups
1 No. of – Igroups Igroups
Example :
CH3 CH3
C
OH
>
CH3
CH3
CH
>
OH
CH3
CH2 OH
CH3
Maximum +I. Maximum tendency to donate l.p. Maximum basic.
71
Chemistry Question : Explain— Basicity orde order in aq aqueo ueous solution and in liquid phase.
Et2 NH > Et3 N > Et NH2
Due to steric hindrance in 3° amine, it is less basic, than 2° amine.
Ans. An s.
Steric hindrance of three –C2H5 group protect the lone pair of nitrogen from the attack of H. H H H C HH C H H C C .. H H N H
H
C
H
H
C
H
H
In solution or in aqueous phase basic order is R2N > R–NH2 > R3N > H–NH2 But in gaseous phase basic order is
R3N > R2N > R–NH2 > H–NH2
Some other basic order of different amine if alkyl group would be change Alkyl groups (R–)
Relative base strength
( i )
CH3 –
R2NH > RNH2 > R3N > NH3
( i i )
C2H5 –
R2NH > RNH2 > NH3 > R3N
(iii) (iv)
(CH3)2CH – (CH3)3C –
RNH2 > NH3 > R2NH > R3N NH3 > RNH2 > R2NH > R3N
Question : Ethyl amine is more basic then aniline, why ? Ans. An s.
Due to the + I effect of ethyl group.
Question : Cl–NH2 is less basic then methyl amine, why ? Ans. An s.
Due to – I effect of –Cl –Cl group and p–d conjugation.
Question : Correct basic order of the following different amine in chlorobenzene medium is -
(A) CH3 – CH2 – NH2
(C) N
(B)
(D)
CH3 Ans. An s.
N | H
C > D > B > A
Question : Which is most basic among the following :
(A) CH3NH2 Ans. An s.
(B) CH3CH2NH2
(C) NH3
(D) (CH3)2CHNH2
(D)
Question : Which is most acidic compound – OH
OH
(A) Ans. An s. 72
(B)
NO2
(B)
OH
OH
(C) NO2
(D) NO2
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A A T D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M 2.
MESO MESOME MERI RIC C
EFFE EFFECT CT OR RESO RESONA NANC NCE E
IN
EFFE EFFECT CT
Polarity developed in conjugate system by the complete transfer of non–bonding electron or –bond electron due to the group or atom attach with conjugate system is known known as mesomeric effect. If transfer of pi-bond electron takes place from conjugate system to group then it is known as negative mesomeric (–M) effect. CH2
O
C CH C C C
O O CH2
C CH C C C O
CH2 = C
O
C C C C
O
O
CH2 CH
C CH C CH O
For –M effect, group should have either be positive charge or should have vacant orbital.
Due to –M effect positive charge comes over conjugate system system or due to –M effect electron density decrease in conjugate system, such type of conjugate system will be more reactive towards nucleophile or will be less
reactive towards electrophile. Group which shows –M effect are –NO2 , –CN, –SO3H , – CHO, –COR, –COOH, –COOR , –COX, –CONH2 etc. If transfer of non bonding electron takes place from group to conjugate system then it is known as po positive m me esomeric (+M) effect. ..
CH2 ==CH – CH = CH – CH = CH – NH2
CH2 = CH – CH = CH – CH — CH = NH2 5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A A T D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
CH2 = CH – CH – CH = CH – CH = NH2
CH2 – CH = CH – CH = CH – CH = NH2
For +M effect, group should have either either be lone pair of electron or should have negativ negative e charge. Due to + M effect negative charge comes over conjugate system or electron density increase on conjugate system such type of conjugate system will be more reactive towards electrophile electrophile or will be less reactive towards nucleophile. Group which shows + M effect are are -
Cl, – NHCOR , –O–COR etc. – O , –NH, –NR2 , –NHR, –NH2 , – OH, –OR, –SH – SR, – F, – Cl,
In mesomeric effect polarity or charge migrate migrate from one end to another end. During c charge harge transfer, energy releases from the conjugate system which increase the stability of conjugate conjugate system also.
Due to charge transfer compound form more then one structure. without change in atomic orientation these structures are known as resonating structures.
73
Chemistry
These structures are helpful in explanation of chemical reactivity or the chemical reaction of the compound thats why we can say resonance phenomenon is the result of mesomeric effect or delocalisation. delocalisation. ( i )
Resonating structure are not the real structures of conjugated compounds.
( i i )
The real structure of conjugated compound is a hybrid of all resonating structures. This phenomenon is known as resonance, mesomerism or delocalisation.
(iii)
Thus resonance is nothing but hybridisation of resonating structures and resonance phenomenon will take place in conjugated compounds.
(iv) (a)
Conditions of Resonating Structures : Resonance structures should fulfil following conditions : All resonating structures must have the same arrangement of atomic nuclei. Resonance differs from
tautomerism in this very important aspect. O | R – C = O – H
O || R – C – O – H :
:
Positions of atomic nuclei in (I) and (II) are same. 3
3
O OH || | CH3 – C – CH3 CH3 – C = CH2 1
1
(I)
(II)
Position of hydrogen nuclei in (I) and (II) are different, hence (I) and (II) are not resonating structures, they are tautomer. (b)
The resonating structures must have the same numbers of paired and unpaired electrons. However, they differ in the way of distribution of electrons.
••
•
••
• O = =N N—•O • •• •
Total number of paiire pa red d elec ectr tron ons = 16 16 unparied electron =1
• •
••
•
••
O —N=O •• ••
Total num number of paiire pa red d electr tron ons s = 16 16 unparied electron =1
The energy of the different resonating structures must be the same or nearly the same.
All atoms that are part of the delocalisation system must be in in a plane or be nearly planar. (v)
All atoms of the resonating structure should follow the octet rule. For Example: All atoms follow octet rule.
NH3
NH3 Nitrogen does not follow it s octet rule hence (II) is not resonating structure (I)
I
1.
CONDITIONS FOR RESONANCE : If there are two bonds at alternate position then e– of one bond are transferred towards another bond.
(According to I–effect). 74
II
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A A T D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
Chemistry
Hybrid resonating structure :
CH2 1
CH CH CH2 2
3
4
CH2
CH CH CH2
1
2
I
3
4
CH
2
1
II
CH CH CH2 2
3
4
I
In the structure I C —C — C2 has double bond while in II and III C —C — C2 has single bond, so C —C — C2 shows bond 1 1 1 length in between single and double bond similarly in structure I C —C — C has single bond. While in structure 2 3 II and III C — —C C3 has double bond, so C — —C C3 shows bond length in between single and double bond. 2 2 So hybrid resonating structure from all its resonating structures is : CH2
CH
CH
CH2
Hybrid structure
Example : ..
( i )
CH2
CH OH
CH2 CH OH
I
( i)
C 1H
RESONANCE
CH2
CH
OH
2 C 2H C 3H
ENERGY
(Hybrid structure) –
H2 CH CH2 C 1 2 3
(Resonating structure)
II (Resonating structure)
2
–
CH
2
CH
CH2
(Hybrid structure)
:
The difference in the experimental and calculated energies (heat of hydrogenation) by which the compound is stable, is known as the resonance or delocalization energy. Higher the value of resonance energy, greater is the resonance stabilization.
RESONANCE ENERGY (R.E.) OF BENZENE : 5 6 P . C O G 3 0 \ T R
The Resonance energy of benzene is calculated from from the heat of hydrogenation as given below :
+
H2
+ 3H2
+
28.6 28. 6 Kcal cal..
+ 3 ×28.6 Kcal. (=85.8)
but experimental value is 49.8 Kcal. so, Resonance energy = Calculated value – Experimental value value = 85.8 – 49.8 = 36 Kcal.
76
P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
IN
APPLICATION OF R OR M-EF M-EFFECT FECT Stability of carbocation :
(a)
Stability of carbocation is increased by resonance.
(b)
Aro Aromat matic ic compou compound nd are more more stabl stable e than non non aroma aromatic tic co compo mpound und. .
Example : Compare stability order of :-
( i )
CH CH2
CH2
>
CH3
stable by resonance CH2 <
( i)
<
CH2
CH CH
CH2
–I of Alkenyl group
2°
(iii)
>
CH2
+I of Alkyl group
1°
CH2
CH3
3°
CH2
CH2
CH2
Re sonance increases,stabi increases,stabili lityincreases tyincreases
,
(iv)
(I) stable by resonance
,
(II) mor ore e reson esona ance
(III) localized ve charge
stability order II > I > III
Stability of carbanion :
(a)
Stability of carbanion is increased by resonance.
Example : Compare stability order of :
( i )
CH2
CH CH2
,
stable by resonance 5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
CH2
CH
,
CH3
CH2
– ve char charge ge on on more more EN EN atom atom
stability order I > II > III
( i)
,
,
(I)
(II)
stable by resonance
mo more re re reso sona nanc nce e
stability order II > I. (iii)
NO2 CH — 2
H —N O2 CH —C 2 2 no resonance
stable by resonance
C H — — N O 2 CH 3 stable by resoncance, but +I of CH3
stability order I > III > II
Stability of free radicals :
(a)
Stability of free radicals is increased by resonance.
Example : Compare stability order of : .
.
( i )
CH2
CH
CH2
less resonance
CH2
CH
no resonance
.
CH2 CH CH CH CH2
more resonance
stability order III > I > II
77
Chemistry .
.
.
( i)
,
,
resonance
more resonance
localized
stability order II > I > III Equal resonating structures : Resonating structures in which there is same charge on same atom.
Example :
O H C O are equal R.S.
H C
O
O Unequal resonating structures : Resonating structures in which there is same charge on different atom
or different charge on same atom or different charge on different atom.
Example :
CH2
CH O CH2
..
CH O are unequal R.S.
CH2 OH CH2
OH
N No ote : Equal resonating structures structures are more stable than unequal resonating structures. structures.
A Aci cidi dic c and and Basi Basic c st stre reng ngt th : (a)
Acidi cidic c stre stren ngt gth h: A c id ic s t r e n g t h M I
1 M
1 I
Question : Carboxylic acids are more acidic than phenols, why ? R
Ans. An s.
C
OH
O
OH
–H – H
–H – H
R
C
O
R
C
O
O
O
O
2, equal R.S. more stable anion
O
5, unequal R.S. less stable anion
so corresponding acid is more acidic Question : Phenol is more acidic than alcohols why ? Ans. An s.
Ph
Ph
OH –H – H
R OH –H – H
O
R O
stable by resonance So, it is more acidic. (b)
no resonance.
Basi Basic cs str tren engt gth ho ord rder er :-
If there is more resonance of lone pair or negative charge then it will be more stable, means less basicity. Ba sic st re n gth M I
78
1 M
1 –I
5 6 P . C O G 3 0 \ T R P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
IN
Example : Give basic strength order : ..
( i )
..
..
NH2
CH2 NH2
NH CH3
l.P. is stabilized by
no r re esonance of of l. l.p.
l.p. is stabilized by resonance
resonance
so maximum basic
stabilized and +I of CH3
basic or order —
II > III > I
NH2
( i)
..
..
N H
stable by
localized
resonance
l.p. on more EN
basic order — III > II > I Question : Give basic strength order for : ..
..
NH
( i )
..
NH2
CH2 NH2
diphenyl amine
benzyl amine
more resonance
no resonance
3 > 2 > 1
Ans. An s.
Question : Aniline is less basic than alkyl amine, why ?
Due to delocalization of l.p. of nitrogen in aniline, aniline, aniline is less basic.
Ans. An s.
Question : Which is weakest base :
P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
7.
(ii) C6H —CH —NH—C H3 5 2
(iii) O2N—CH —NH 2 2
(iv) CH —NH—CHO 3
(iv) due to resonance of l.p.
Ans. An s.
5 6 P . C O G 3 0 \ T R
(i) C6H —CH —NH 5 2 2
Reactivity of benzene : Characteristic reaction of benzene is electrophilic substitution (ESR)
E
-H
H E
(product) E
In benzene ring due to more electron density first attack will be of electrophile. 8.
Rule Rules s fo for r stab stabil ilit ity y of r res eson onat atin ing g stru struct ctur ure e (R (R.S .S) ):
( i )
Non-polar R.S. is more stable than polar resonating structures.
( i i )
In polar R.S. complete octet is more stable than incomplete octet.
(iii)
For incomplete R.S. positive charge on more EN is less stable.
Example : Arrange the following for stability order.
( i ) CH CH Cl non-polar 2
2
CH
CH
Cl
complete octet
2
CH
CH
Cl
incomplete octet
stability order : 1 > 2 > 3
79
Chemistry
( i)
R C OH
R C O H
O
O
O
non-polar
complete octet
(iii)
R C OH
<
R C O
R
incomplete octet
C
incomplete octet
O
complete octet
(Acylium ion) (iv)
CH2 CH O
>
CH2 CH O
negative ch charge on on
negative ch charge on on
more EN
less EN
Example : Give stability order of :
CH2
CH2
CH2
Cl
CH3
OCH3
CH2
( i ) NO2 –M charge charge
–II – charge
So stability order
IV > III > II > I
M charge
CH2
+I charge
CH2
CH2
CH2
Cl
CH3
OCH3
( i) NO2
M –M charge
Stability order
–II – charge
+I charge
charge
I > II > III > IV
CH2
CH2
(iii)
CH2
CH2
NO2 NO2
–M and
M = 0
more –I only –I charge is minimum So stability order I > III > II > IV 80
NO2
–M and less–I
5 6 P . C O G 3 0 \ T R P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
IN
Question : Give acidic strength order for :
Ans. An s.
OH
OH
OH
OH
NO2
Cl
CH3
OCH3
–M
–I
+I
+M
So acidic order is
I > II > III > IV
Question : The mixture of ortho-nitrophenol and para-nitrophenol can be separated by steam distillation why and which can be distilled out ?
Ortho-nitrophenol is distilled out due to less B.P. of ortho-nitrophenol it is more volatile
Ans. An s.
Example : Give acidic strength order for
( i ) OH
OH
OH
OH
CH3 CH3 CH3
–H – H
–H – H
–H – H
O
–H – H
O
O
O
CH3 CH3 more +I and H-effect of of CH3 ,S ,So o anion anio n is minimum stable Acidic order 4 > 2 > 3 > 1
( i)
P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
OH NO2
NO2
NO2
maximum –M and –I so maximum acidic Acidic order 1 > 2 > 3 > 4 Example : Give basic strength order for : NH2 NH2 NH2
NH2
( i ) NO2
– –M M
Cl –I
CH3
OCH3
+I
+M
So basic strength order IV > III > II > I Question : What is the increasing order of resonance stabilization of benzene, naphthalene and anthracene.
Ans. An s.
E
OH
NO2 NO2
NO2
maximum stable anion anion so corresponding corresponding acid is maximum acidic acidic
OH
OH 5 6 P . C O G 3 0 \ T R
CH3 less +I and H-effect
H= 0 only +I
The values of resonance energy for benzene, naphthalene and anthracene are 36, 76 and 85 Kcal per mole respectively. It is clear from these values that the resonance stabilisation of naphthalene is more than that of benzene and less than that of anthracene. Thus, the increasing order of resonance stabilisation isasfollows.
81
JEE – M
IN
APPLICATION OF H-EFFECT
STABILITY OF CARBOCATION :
Stability of carbocation can be explained explained by M, I and H-effect.
If more hyperconjugation structures (more -H) then more stable cation.
Stability of carbocation No. of canonical structures No. of H.
Example : Give stability order for :CH3
( i )
>
CH3 C CH3
>
CH3 CH CH
>
CH3 CH2
CH CH3
CH3
9 -H
6 -H
3 -H
Zero -H
Maximum stable
( i)
CH2
CH3
>
>
Stability of carbon free radicals radicals :
Stability of carbon free radical is explained by M and and H-effect.
More hyper conjugation structures (more -H) more stable free radical. Example : Give stability order for :CH3 .
( i )
.
CH3 C
.
CH3 C CH H
.
CH C H3
CH3 CH2
CH3
CH3
3°
2°
1°
9 -H
6 -H
3 -H
Zero -H
Maximum stable 5 6 P . C O G 3 0 \ T R P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
Stability order
Stability of alkenes : More hyperconjugation structures (more -H) more stable alkene.
( i )
C H — 3 CH
CH2
>
3 -H
CH 2
CH 2
Zero -H
more stable ( i i )
Stability order of alkenes will be CH3
C
C
CH3 CH3 H
C
C
CH3
CH3
CH3
CH3
CH3 H
CH3
C
C
H
C
C
CH3
CH3
H
CH3
H
H
H
H
C C
C
C
Question : which of the following alkene is maximum stable. (i) R2C CR2 (ii) R—CH C—R2 (iii) R—CH
An Ans.
E
I > II > III > IV
H
CH3
H
H
C
C
H CH3
H H
CH—R
(iv) R—CH
CH2
(i) due to more substituted alkene.
83
Chemistry
Heat of hydrogenation :
R—CH
CH2 + H2 R—CH —CH 2 3 + H (Heat of hydrogenation)
Heat evolved when any unsaturated hydrocarbon are hydrogenated is called heat of hydrogenation (H) If alkene is more reactive towards hydrogen then it will evolve evolve more H.
So,
Heat ofhydrog ofhydrogenati enation on
1
1
n um be r o f H
sta b ilit y o f a lke n e
Question : Which of the following has minimum heat of hydrogenation. (i) ethene (ii) Propene (iii)cis-2-butene Ans. An s. (iv)
(iv) trans-2-butene
maxim maximum um stab stable le alkene alkene means means minimum minimum reacti reactive. ve.
Question : If Heat of hydrogenation of 1-butene is 30 Kcal then heat of hydrogenation of 1,3-butadiene is ?
(i)30 Ans. An s. ( iii)
(ii)60
(iii)57
(iv) 25
1,3-butadiene 1,3-butadiene requires two moles of hydrogen so heat of hydrogenati hydrogenation on should be 60 Kcal but 1,3-butadiene is stabilized by resonance than propane so heat of hydrogenation of 1,3-butadiene will not be twice of 30. Actual H - 60 > H > 30 Kcal.
Question : Which of the following is maximum stable.
( i )
Co Conj njug ugat ated ed alk alkad adie iene ne (CH (CH2
( i i )
Isolated Isolated alkadie alkadiene ne (CH2
CH—CH —C —CH H 2
(iii)
Cumulated alkadiene alkadiene (CH (CH2
C
(iv)
All are equal.
Ans. An s. ( i )
CH—CH
CH2) CH2)
CH2)
Due to resona resonance nce co conjuga njugated alkadie alkadiene is maximum maximum stabl stable. e. Isol Isolated ated i is s more stabl table e than cumulat mulated ed alkadiene due to H-effect.
Reactivity of Benzene : H-effect of R groups increases electron density in benzene ring.
H C H
H
C H
H
H
due to CH3 group there is more e– density at ortho and para position so CH3 is ortho/para directing and activating group. If H-effect is more than e– density will be more. Example : Give electrophilic sustitution reaction order :
Maximum -H. -H. So maximum H-effect So maximum e– density So maximum reactive ESR order I > II > III > IV 84
5 6 P . C O G 3 0 \ T R P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M 4
IN
ELECTROMERIC EFFECT : (E (E E Ef ffec fect)
It is a temporary effect. The organic compounds compounds having a multiple bond (a double or tri triple ple bond) show this effect in the presence of an attacking reagent only. It is defined as the complete transfer of a shared pair of -electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented represented by E and the shifting of the electrons is shown by a curved arrow ( ( i )
). There are two distinct types of electromeric effect.
Positive Eelctromeric Effect (+ E effect) : In this effect the -electrons of the multiple bond are
transferred to that atom to which the reagent gets attached. For example :
+
C= C + H (attacking reagent)
( i)
C–C H
N Ne egative Electromeric Effect (-E effect) : In this effect the - electrons of the multiple bond are
transferred to that atom to which the attacking reagent does not get attached. For example.
C = O + CN (attacking reagent)
CH3
C
HCN
H CH3
C
C–O CN
HO CH3 C N CH3 C N
H ,
2
O
O
TYPE
OF
REACTIONS
Mainly reactions are of four types :
5 6 P . C O G 3 0 \ T R P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
1.
1.
Substitution reactions.
2.
Addition reactions.
3.
Elimination reactions.
4.
Rearrangement reactions.
SUBSTITUTI TUTION ON REACT EACTI IONS : Reactions in which one atom or a group of substrate is replaced by other
atom or group are called as substitution reactions. on the basis of attacking species, substitution reaction is also of three types : (a)
Free radical substitution reactions
(b)
Electrophilicsubstitutionreactions
(c)
Nucleophilic substitution reactions
(a)
Free radical substitution reactions :- These reactions are brought about by the attack of free radicals.
Example : ( i )
Ha Halo loge gena nati tion of al alka kane ne : h CH4 + Cl2 CH3Cl + HCl
M Me echanism : reaction occurs in three steps : I
Chain initiation step :
h Cl2 C l C l
85
Chemistry I
Chain propagation step :
C H 4 C l C H 3 H Cl
C H 3 C l2
C H 3 C l C l
(major product) III
Chai Chain n term termin inat atio ion n step step :
C l C l Cl2
C H 3 C H 3 CH3 – CH3 (minor) C H 3 C l CH3 – Cl
( i i )
Allylic or benzylic substitution by NBS or by Br2/h. CH3
Br 2
CH CH2 h
CH
CH2 + HBr
Br
Mechanis Mech anism m : I
CH2
Chain initiation step :
h Br2 B r + B r
I
Chain propagation step :
CH —CH 3
CH2 + B r C H —CH 2
CH2 + HBr (stable by resonance) .
C H —CH 2
CH2 + Br2 CH2 CH CH2+ Br Br
(Product) (b)
Electrohilic substitution reactions (ESR) :- These are brought about by the attack of a electrophiles.
Example : (i) Nitration of benzene.
(ii) Reaction of Grignard reagent with active –H. (i)
Nitr Nitrat atio ion n of ben benze zene ne :- formation of nitrobenzene NO2
co conc nc HN HNO3 + conc H2SO4
(Nitrating mixture or nitrating agent)
Mechanis Mech anism m : (i)
Form Format atio ion n of of ele elect ctro roph phil ile e:
..
HO NO2 + H
H2SO4
H
86
H – O NO2 H2O + NO2
electrophile or attacking species.
5 6 P . C O G 3 0 \ T R P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ T A A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M (ii)
IN
Attack Attack of electr electroph ophile ile (E+) on benzene : H
NO2 NO
NO2 H
intermediate (iii)
Removal of o f H+ : NO2 H
NO2
–H – H
In nitration attacking species is nitronium ion (NO (NO2+).
In nitration H2SO4 is Bronsted acid while HNO3 is Bronsted base.
(c)
Nucleoph phili ilic substitutio ion n reaction ons s (NSR) : These are brought about by the attack of a nucleophile. In nucleophilic substitution substitution reaction a weaker nucleophile is replaced by stronger nucleophile.
OH R X R OH + X
Example :
weaker
stronger
nucleophile
nucleophile
NSR are of two types (i)
SN1 : In these reaction the rate of reaction depends only upon concentration of the substrate and not upon the attacking nucleophile. CH3
Example : CH3
CH3
C
OH CH3 Cl (hydrolysis)
CH3
OH + Cl
C CH3
M Me echan hanism : SN1 reaction occurs in two steps : (a) 5 6 P . C O G 3 0 \ T R P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ A T A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
Form Format atio ion n of carb carboc ocat atio ion n: CH3
CH3
Example :
CH3
C
Slow ste step p C + Cl (Rate determination step) CH3
Cl
CH3
(b)
CH3
Attack of of O OH H on C : CH3
CH3 Example : CH3
fast step C + OH CH3
C
OH
CH3
CH3
Rate of reaction concentration of 3° halides.
Order of reaction is 1, so reaction called as SN1
In SN1 reaction intermediate is carbocations so rearrangement can take place. place. CH2
Cl SN 1
–
OH
OH CH3
In SN1 reaction intermediate is carbocations carbocations so if substrates has chiral carbon atom (opticall (optically y active) then product will be racemic mixture, (opitically inactive). inactive).
87
Chemistry (ii)
SN2 : Bimolecular subsitution reaction.
+ Cl CH —Cl + OH CH —OH 3 3 M Me echanism : SN2 reaction occurs in one steps : H
Cl
HO + C
Example :
H
bre b rea ak
H H
C
form H
H
H Cl
H
C
OH H
H
Transition stage
Rate of reaction [OH] [1° halides].
Order of reaction is 2, so reaction called as SN2
AROMATIC AROMAT IC NUCLEOPHILIC NUCLEO PHILIC (i)
SUBSTITUTION SUBSTI TUTION
:
Decomposition of diazonium salts salts in polar medium and formation of various products in presence of various nucleophiles.
N
N
+ N2
H2O
Ph OH Cl Ph Cl
CN
(ii)
Ph
CN
Aryl halides activated by strong electron withdrawing withdrawing groups from o– or p– positions undergo substitu substitution tion reaction by nucleophile. Cl
OH
+ Cl
OH [NaOH] high temp.
NO2
NO2
Mechanis Mech anism m : Cl
+ O
N
OH
O
O
O
N
OH
fast
N
O
O
Stable by resonance by NO2 group
OH
OH
slow step
There is very much positive charge on carbon due to –M of NO2 and –I of Cl. Cl
Cl
+ Cl O
N
O
If there is no strong –M group (like NO2) on benzene ring then displacement is carried out only in presence of very strong bases such as NaNH2/liq. NH3.
li liq. q. NH 3 C6H5Cl + NH NH2 C6H5NH2 + Cl
88
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ T A A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
IN
NSR NS R in Acid cid de derr rri iva vat tiv ives es : Example : Hydrolysis of acid derrivatives : Nu
Nu
–Z – Z
R C Z R C Z R C Nu
O
O
Whe her re Z
—Cl, —O —OR, —N —NH2,
O
O C R O
here Z is a good leaving group then H and R as in carbonyl compounds, so carbonyl compound does not shows NSR. 2.
ADDIT ITI ION RE REACTIONS : It is also of three types :
(a)
Free radical addition reactions
(b)
Electrophilic addition reactions
(c)
Nucleophilic addition reactions
(a)
Fr Free ee rad radic ical al add addit itio ion n reac reacti tion ons s ::- Addition of HBr on alkene or alkyne in presence of peroxide.
CH —CH 3
CH 2
H Br ( )
CH3 ROOR
CH
CH2
H
Br
Mechanis Mech anism m : I
Chain initiation step :
2R O
R—O—O—R
R O + HBr ROH + B r
I
Chain propagation step : .
.
CH3 5 6 P . C O G 3 0 \ T R P A Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ T A A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
CH3 CH2 + Br
CH
.
CH
CH2 + CH3 CH CH2 Br
2° (more stable) .
CH3 CH
Br
1° (lessstable) .
CH3 CH2 + HBr
CH
CH2 + Br
Br
H
Br
(major) A An nti-Markovnikoff's Rule/An /Anti M.K .K. . Rule/Kharash eff ffe ect/perox roxide effect
In presence of peroxide negative negative part of a reagent attack on the sp2 –C which have more H.
Only HBr shows peroxide effect. Other H-X shows electrophilic addition reaction in presence of peroxide. Reason :
Reactivity order of H-X
HI > HBr HBr > H-C H-Cl l > HF HF
Bond energy order
H-F > H-Cl > HBr > HI
In case of HCl :
CH —CH 3
CH 2 +
C l CH3
.
CH
CH2 Cl
89
Chemistry .
CH3
CH
.
Cl CH3
CH2 + H
CH
CH2 + Cl
H
Cl
Cl
Strong bond
weak (endothermic step) bond
In case of Hl :
R O + Hl ROH + I
3 CH —CH
CH2 + I X (No reaction) More activation energy
Question : CH —CH 3
HCl CH3 ROOR
CH 2
CH
endothermic step.
CH3
Cl
no effect simple EAR
Ans. An s. (b)
Electrop ophil hilic ad addit dition re react action :- Characteristic reaction of alkene alkene is EAR. Due to more e density in alkene first attack will be of electrophile. electrophile. There is unsaturation in alkene so there will be addition.
>C
E C >C
C<
Nu N u >C
E
intermediate e p hi hilic ad a dditio n re r e a ctio n
C<
Nu E
addition product
e – de n sity
I I
M M
Question : Give reactivity order towards EAR.
(a)
(i) CH2
CH2
(iii) C H — CH 3
(ii) CH —CH 3 CH—CH3
CH2
(iv) CH3 C CH CH3 CH3
reactivity order (b)
IV > III > II > I OH
(i)
(ii)
reactivity order
II > IV > I > III
(iii)
Cl
(iv)
Cl
Example : CH3 CH
Cl2
CH3 CH CH2 FeCl
CH2
3
Cl
Mechanis Mech anism m :
Cl2 + FeCl3 C C l + FeCl4 CH3 CH
C H2
Cl CH3 CH CH2 CH3 CH CH2 :Cl
Cl
• Complete octet • More stable • Cyclic halonium ion • Intermediate 90
CH3
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ T A A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M
IN
Cl +
CH3
C H2
CH
FeCl CH3 CH CH2 + FeCl3 4
+
Cl
Cl
In this reaction intermediate is cyclic halonium halonium ion. So there will be no rea rearrangement. rrangement. Br
Question : CH3 CH
Br
2 CH CH2 CH3 CH
CH3
CH
CH3
CH2 Br
Anti addition
M Ma arcovnikoff's Rule :- (Old rule) only for for unsymmetrical alkene (without rearrangement).
In addition of alkene negative negative part of a reagent attacks on the sp2 carbon which have less H.
In EAR attack of a nucleophile on the sp2 carbon which have less hydrogen.
N Ne ew Rule :- If there is carbocation as intermediate than nucleophile attack on more stable carbocation (with
rearrangement, only for unsymmetrical alkene). Question : Which of the following does not follow M.K. rule ?
(i) CH2
CH2
(ii) CH —CH 3
CH2
(iii) CH —CH 3
CH—CH3
(iv) CH —CH 3
CH— CH —CH 2 3
A An ns. (iii) it is symmetrical alkene. (c)
Nucl Nucleo eoph phil ilic ic a add ddit itio ion n re reac acti tion ons s:
NAR in alkynes
NAR in carbonyl compounds
Example : Addition of HCN. H/ R R–C O
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ T A A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
+
HCN –
wea eak k base OH H2O –
+
CN +H (Nucleophile)
H/ R
H/ R
R–C
H/ R
R–C–CN
R–C–CN +
O
3.
O
–
H +OH
OH
ELIMINATION REACTIONS : These reactions are brought about elimination of small molecule from the substrate Elimination
– –el eliminat atiion
. – –el eliminat atiion or 1, 1–elimination
–el –eliminat atiion
or 1, 2–elimination
-Elimination (1, 1-Elimination) : Removal of H and X from one C-atom
Example :
KOH CHCl3 :CCl2 (dichloro carbene)
Mechanis Mech anism m :
Cl Cl OH H C Cl C Cl + H2O (KOH) Cl Cl (acidic H)
91
Chemistry ..
Cl
–C – Cl
..
C ..Cl :CCl2 Cl
, Elimination (, elimination) : Removal of H and X from adjacent C-atoms
(a)
E1
unimolecular elim eliminat ination ion
E2 b biimolecular elimination
Unim Unimol olec ecul ular ar elim elimin inati ation (E1) :CH3 CH3 C
CH3 Cl + KOH CH3
CH3
C + KCl + H2O CH2
M Me echanism : occurs in two steps – Step-I Formation of Carbocation : CH3
CH3 CH3
C
slowstep
CH3 Rate Rate determinat determinatiion step step
Cl
C + Cl CH3
CH3
Step-II Abstraction of proton (H+) by base (KOH) : CH2 H
CH3
CH2
OH CH3 C –H – HO
C
2
CH3
CH3
Fast step
(Alkene)
Rate of reaction (3° halide)
Order of reaction = 1 (So reaction is called as E1)
In E1 reaction intermidiate carbocation is formed.
Special point : All the reaction in which intermidiate is carbocation. There will be rearrangement of carbocation to get more stable carbocation (if possible) Example : CH3
hydride shifting shifting (H)
CH CH2
CH3
CH CH3
H
1° (less stable)
2° (more stable) CH3
CH3
Example : CH3 C
CH2
methylide shifting
CH3
C
CH2
CH3
1°
3°
H
Example : CH3 C
92
CH2
CH3
C
CH3
CH3
1°
3°
CH3
CH3
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ T A A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
JEE – M 2.
ISOM SOMERI ERIZA ZAT TIO ION N REA REACTI CTIONS ONS : These reaction involves the interconversion of one isomer into the another
isomer. For example : ( i )
AlCl3 CH3 CH CH3 C H —C H — —C C H — —C C H 3 3 2 2 HCl
CH3
( i i )
C H — —C C H — —C CH 3 2
Al2 (S O 4 )3 C H 2 CH3 C CH2 + CH3 CH CH CH3
CH3
(iii)
CH —C —CH H —C 3 2
CH
Alc.KOH CH3
C
C
NaNH2
CH3 CH3 CH2
REACTION AT A GLANCE :
5 6 P . C O G 3 0 \ T R A P Y R O E H T 1 0 \ G N E \ C O G & M S I R E M O S I , E R U T A L C N E M O N \ Y R T S I M E H C \ P M S \ N I A M E E J \ A T O K \ 4 1 0 2 \ T A A D \ ) E ( 6 E D O N _ E \ 6 E D O N \ \
E
IN
S . N.
C la s s o f c o m p o u n d s
T y p e s o f re a c t io n s
(i)
Alk a n e
Fr e e r a d ic a l s u b st it u t io n
(ii)
Alk e n e , a lk yn e
Ele c t r o p h ilic a d d it io n
(iii)
Alk yl h a lid e
N u c le o p h ilic su b s t it u t io n
(iv)
Ald e h yd e , k e t o n e
N u c le o p h ilic a d d it io n
(v)
Ac id a nd nd t h e ir ir d e rriiva ti tive s
N u c le o op p hi h ilic su b s t it u t io n
(vi)
Ar o m a t ic c o m p o u n d s
Ele c t r o p h ilic ssu u b s t it u t io n
C
C Na
View more...
Comments
