Reaction Mechanism
March 1, 2017 | Author: Varsha Dange | Category: N/A
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REACTION MECHANISM
Contents Topic
Page No.
Theory
01 - 10
Exercise - 1
11 - 19
Exercise - 2
20 - 28
Exercise - 3
29 - 33
Exercise - 4
34 - 35
Answer Key
36 - 40
Syllabus Organic Reagents, Alkyl halides : Nucleophilic substitution reactions, elimination reactions, Alcohols : dehydration, reaction with sodium, phosphorus halides, ZnCl2/concentrated HCl, Ethers : Preparation by Williamson’s Synthesis; Nucleophilic substitution reactions.
Name : ____________________________ Contact No. __________________
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REACTION MECHANISM (Alkyl halides, Alcohols & Ethers) KEY CONCEPTS Types of solvents
•
Polar protic solvent
Polar aprotic solvent
Capable of H-bonding
Incapable of H-bonding
even thought polar protic solvents are not the best SN2 and E2, these reactions are often run in these solvents because nucleophiles and bases are often easily prepared in these solvents.
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REACTION MECHANISM # 1
1.
Solvents H2 O
Non Polar
Polar
Protic
Aprotic
2.
CH3OH
× × × × × ×
– – – – – –
3.
CH3CH2OH
4.
H–COOH
5.
CH3–COOH
6.
NH3
7.
×
×
8.
×
×
DMSO–Dimethyl
9.
×
×
DMF –
Dimethyl formamide
10.
×
×
DMA –
Dimethyl acetamide
11.
×
12.
×
13.
C–C–C–C–C–C
×
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sulphoxide
REACTION MECHANISM # 2
Organic Reagents Organic reagents can be classified in two categories : (1) Electrophile : Electron pair acceptor are electrophile. (2) Nucleophile : Electron pair donor are nucelophile.
(1)
Electrophiles It can be classified into two categories : (A) Charged electrophiles (E) (B) Neutral electrophiles (E) (A) Charged electrophiles : Positively charged species in which central atom has incomplete octet is charged electrophile
H
, X R ,
,
= O,
Note : Generally All cations are charged electrophiles except cations of A, A group elements,
(B)
Al+++ and . Neutral electrophiles : It can be classified into three categories : (B) Neutral covalent compound in which central atom has incomplete octet is neutral electrophile,
••
AlX 3, C H , 2
••
C X 2 BF3, BCl3, BBr3, BI3
(B) Neutral covalent compound in which central atom has complete or expended octet and central atom has unfilled -d-shell is neutral electrophile SnCl4, SiCl4, SbCl5 Note : (i) Cl2, Br2 and I2 also behave as neutral electrophiles. (ii) Electrophiles are Lewis acids.
(2)
Nucleophiles Nucleophiles can be classified into three categories : (A) Charged nucleophiles : Negatively charged species are charged nucleophiles. , O , R – (B)
, C
,
H, R–
Neutral nucleophiles : It can be classified into two categories : Neutral covalent compound, in which central atom has complete octet, has at least one lone pair of electrons and all atoms present on central atom should not be electronegative, is neutral nucleophile. , R–
, R2
H– –H, R– –H, H– –H, H3, (C)
3,
R–
R H2,
–H,
,
R3 ,
–NH2
(Oxygen nucleophiles)
R– –R R –
R2 H,
(Nitrogen nucleophile)
– R R3
(Sulphur nucleophiles) (Phosphorus nucleophiles)
Ambident nucleophile : Species having two nucleophilic centres, one is neutral (complete octet and has at least one lone pair of electrons) and other is charged (negative charge) behaves as ambident nucelophile.
–
Note : (A) Organometallic compounds are nucleophiles. (B) Nucleophiles are Lewis bases. ETOOS ACADEMY Pvt. Ltd F-106, Road No.2 Indraprastha Industrial Area, End of Evergreen Motor, BSNL Lane, Jhalawar Road, Kota, Rajasthan (324005) Tel. : +91-744-242-5022, 92-14-233303
REACTION MECHANISM # 3
Organic compounds which behave as electrophile as well as nucleophile : Organic compound in which carbon is bonded with electronegative atom (O, N, S) by multiple bond/ bonds behaves as electrophile as well as nucleophile.
O O || || R CH, R CR, O || R C OR , R–
O || R C OH ,
O || R C NH2 ,
O || R C Cl
R–CN,
Note : During the course of chemical reaction electrophile reacts with nucleophile.
NUCLEOPHILIC SUBSTITUTION REACTIONS (SN) AT SATURATED CARBON NUCLEOPHILIC SUBSTITUTION REACTIONS (SN) : Replacement (displacement) of an atom or group by an other atom or group in molecule is known as substitution reaction. If substitution reaction is brought about by a nucleophile then it is known as nucleophilic substitution reaction. Generally substitution takes place at sp3 carbon. R–g +
R – Nu +
UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTIONS (SN1) : Nucleophilic substitution which involves two step process (a) First step : - Slow step involves ionisation to form carbocation R–g R+ + g – (b) Second step : - Fast attack of nucleophile on carbocation to result into product . R+ + Nu –
R–Nu
SN1 Reaction of Alkyl halide Mechanism :
Ionisation of alkyl halide Slow step (rds)
Characteristics of SN1 reactions : 1. It is unimolecular, two step process. 2. Carbocation intermediate is formed so rearrangement is possible in SN1 reaction. 3. It is first order reaction 4. Kinetics of the reaction
Rate [Alkyl halide]
Rate of SN1 reaction is independent of concentration and reactivity of nucleophile.
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REACTION MECHANISM # 4
5. Energetics of the SN1
Figure : Free energy diagram for the SN1 reaction.
SN1 Reaction of Alcohols (A)
Reaction with hydrogen halides A common method is to treat the alcohol with a hydrohalic acid, usually HI or HBr. These acids are used to convert alcohols to the corresponding alkyl halides. (i) In acidic solution, an alcohol is in equilibrium with its protonated form. Protonation converts the hydroxy group from a poor leaving group to a good leaving group (H2O). If the alcohol is protonated all the usual substitution and elimination reactions are feasible, depending on the structure (1°, 2°, 3°) of the alcohol. (ii) Halides are anions of strong acids, so they are weak bases. Solutions of HBr and HI contain nucleophilic
and
ions.
(iii) Concentrated hydrobromic acid rapidly converts t-Butyl alcohol to t-Butyl bromide. The strong acid protonates the hydroxyl group, converting it to a good leaving group. The hindered tertiary carbon atom cannot undergo SN2 displacement, but it can ionise to a tertiary carbocation. Attack by bromide ion gives the alkyl bromide. The mechanism is similar to other SN1 mechanism. (iv) 1-Butanol reacts with sodium bromide in concentrated sulfuric acid to give 1-Bromobutane by an SN2 displacement.
CH3 (CH2 )2 CH2 OH 1 bu tan ol
NaBr, H SO
2 4
CH3 (CH2 )2 CH2Br 1 bromobutane (90%)
Protonation converts the hydroxy group to a good leaving group, but ionization to a primary carbocation is unfavourable. The protonated unbranched primary alcohol is well suited for the SN2 displacement. (v) Secondary alcohols also react with HBr to form alkyl bromides usually by the SN1 mechanism. e.g.
HBr
(vi) HCl (Hydrochloric acid) reacts with alcohols in much the same way that as the hydrobromic acid. (vii) Chloride ion is a weaker nucleophlile than bromide ion because it is smaller and less polarizable. Lewis acid, such as ZnCl2, is sometimes necessary to promote the reaction of HCl with primary and secondary alcohols.
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REACTION MECHANISM # 5
Mechanism :
Reactivity of
RDS R R O H2 –H2O HX : HI > HBr > HCl
Reactivity of
ROH :
R –OH
R – X
allyl benzyl > 3° > 2° > 1°
SN1 REACTIONS OF ETHERS (A)
Reaction with HX Ethers are unreactive towards most bases, but they can react under acidic conditions. A protonated ether can undergo substitution or elimination with the expulsion of an alcohol. Ethers react with conc. HBr and HI because these reagents are sufficiently acidic to protonate the ether, while bromide iodide are good nucleophiles for the substitution. If R or R’ is 3º then mechanism will be SN1 otherwise SN2 . Mechanism : R – O – R`
– ROH
X R` – X
SN2 Reaction of Alkyl halide :
Mechanism :
+
Characteristic of SN2 1. It is bimolecular, one step concerted process 2. It is second order reaction because in the rds both species are involved 3. Kinetics of the reaction rate [alkyl halide] [nucleophile] rate = k[alkyl halide] [nucleophile] If the concentration of alkyl halide in the reaction mixture is doubled, the rate of the nucleophilic substitution reaction is double. If the concentration of nucleophile is doubled the rate of reaction is also double. If the concentration of both are doubled then the rate of the reaction quadriples. 4. Energetics of the reaction :
Figure : A free energy diagrams for SN2 reaction 5. No intermediates are formed in the SN2 reaction, the reaction proceeds through the formation of an unstable arrangment of atoms or group called transition state. ETOOS ACADEMY Pvt. Ltd F-106, Road No.2 Indraprastha Industrial Area, End of Evergreen Motor, BSNL Lane, Jhalawar Road, Kota, Rajasthan (324005) Tel. : +91-744-242-5022, 92-14-233303
REACTION MECHANISM # 6
6. The stereochemistry of SN2 reactions As we seen earlier, in an SN2 mechanism the nucleophile attacks from the back side, that is from the side directly opposite to the leaving group. This mode of attack causes an inversion of configuration at the carbon atom that is the target of nucleophilic attack. This inversion is also known as Walden inversion.
Inversion
SN2 Reaction of Alcohol (A)
Reaction with HX : The protonated unbranched primary alcohol is well suited for the SN2 reaction. Mechanism : X R O H2 R – X + H2O
R –OH
CH3 CH2 CH2 CH2 CH2Cl
e.g CH3 CH2 CH2 CH2CH2OH
n Pentyl chloride
n Pentyl alcohol
(B)
Reaction with phosphorus trihalides Several phosphorus halides are useful for converting alcohols to alkyl halides. PBr3, PCl3, & PCl5 work well and are commercially available. Phosphorus halides produce good yields of most primary and secondary alkyl halides, but none works well with tertiary alcohols. The two phosphorus halides used most often are PBr3 and the P4/I2 combination. 3R – OH + PX3 Mechanism :
3R – X + H3PO3
Step : 1
Step : 2
RCH2X+ HOPX2
Remarks The mechanism for the reaction involves attack of the alcohol group on the phosphorus atom, displacing a halide ion and forming a protonated alkyl dihalophosphite In second step a halide ion acts as nucleophile to displace HOPX2, a good leaving group due to the electronegative atoms bonded to the phosphorus.
(C)
Reaction with thionyl chloride in presence of pyridine Thionyl chloride (SOCl2) is often the best reagent for converting an alcohol to an alkyl chloride. The by products (gaseous SO2 and HCl) leave the reaction mixture and ensure that there can be no reverse reaction.
O || R – OH + Cl S Cl
Heat
Pyridine
R – Cl + SO2 + HCl
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REACTION MECHANISM # 7
Mechanism :
.. .. R – O .. – S – H
.. O:
+ HCl
Cl Chlorosulphite ester
R – Cl + SO2
In the first step, the nonbonding electrons of the hydroxy oxygen atom attack the electrophilic sulphur atom of thionyl chloride. A chloride ion is expelled a proton and gives test of chloro sulphite ester. Second step is an SN2 mechanism SOCl 2 Py
e.g.
Reaction with thionyl chloride ROH + SOCl2 RCl + SO2 + HCl In this mechanism an internal nucleophile attacks from the same side of leaving group , means retension of configuration . It is an SNi mechanism , where i means internal Mechanism :
.. .. –H R – O .. – S
.. O:
+ HCl
Cl Chlorosulphite ester
R – Cl + SO2
e.g.
SN2 Reaction of Ether (A)
Reaction with HX A protonated ether can undergo substitution reaction. Ether react with conc. HBr and HI because these reagents are sufficiently acidic to protonate the ether. If R or R’ is 3º then mechanism will be SN1 otherwise SN2.
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REACTION MECHANISM # 8
Mechanism :
+
X R
+
HX
X – R + X – R
alkyl halide
SN Reaction of Epoxide Epoxides are much more reactive than ether because of angle strain in three membered ring therefore epoxide readily undergo nucleophilic substitution reaction. In basic medium mechanism is SN2. Nucleophile atacks on less hindered carbon. Mechanism : Nu Nu | | H R — CH — CH2 R — CH — CH2 | | O OH
e.g.
In acidic medium mechanism is SN1 type. Nucleophilic attacks on more substituted carbon. Mechanism :
H
Nu | R — CH — CH2 | OH
ELIMINATION REACTIONS : E1 Reaction : Proton and leaving group depart in two different step. (a) First step : - Slow step involves ionisation to form carbocation (b) Second step : Abstraction of proton Mechanism : Step 1 : Formation of the carbocation (RDS)
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REACTION MECHANISM # 9
Step 2 : Base (
) abstracts a proton (fast) + B – H
E2 Reaction : Dehydrohalogenation is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene. (i) Hot alcoholic solution of KOH, EtO¯/EtOH (ii) NaNH2
(iii) t-BuO¯ K in t-BuOH This is one step reaction, hydrogen and leaving group depart simultaneously Mechanism :
+ BH
(iii) E1 cB Reaction (Unimolecular conjugate base reaction) In the E1 cB, H leaves first and then the X. This is a two step process, the intermediate is a carbanion. Mechanism : Step - 1: Consists of the removal of a proton,
, by a base generating a carbanion
H | | CC X | | Step - 2: Carbanion looses a leaving group to form alkene
–C C – | |
Condition : For the E1 cB, substrate must be containing acidic hydrogens and poor leaving groups.
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REACTION MECHANISM # 10
PART - I : OBJECTIVE QUESTIONS * Marked Questions are having more than one correct option.
Section (A) Organic reagent A-1.
Which of the following molecules can behave both as a nucleophile and an electrophile ? (A) CH3NH2 (B) CH3Cl (C) CH3CN (D) CH3OH
A-2.
Which one of the following statements is not correct for electrophile : (A) Electron deficient species are electrophile (B) Electrophiles are Lewis acids (C) All + ive charged species are electrophile (D) AlCl3 and SO3 are electrophiles
A-3.
Which of the following is an electrophilic reagent ? (A) H2O (B) OH– (C) NO2+
(D) none
A-4.
Which of the following reaction intermediates are electrophilic in character ? (a) Carbocation (b) Carbanion (c) Free radicals (d) Carbenes (A) Only b (B) a and c (C) a and d (D) a, b, c and d
A-6.
Which one of the following has maximum nucleophilicity ? (A)
A-7.
(B)
(B)
(C) Et3N
A-10.
(D) CH3COO– < C6H5O– < OH– < CH3O–
The correct nucleophilicity order is/are : (A) (CH3)3 O > CH3
(B) CH3S > CH3SH
(C) CH3CH2CH2O > (CH3)3CO
(D) (CH3CH2)3N > (CH3CH2)3P
Which among the following species is an ambident nucleophile ?
(A) CH3 – C H2 A-11.
(D)
Correct arrangement of the following nucleophiles in the order of their nucleophilic strength is (A) C6H5O– < CH3O– < CH3COO– < OH– (B) CH3COO– < C6H5O– < CH3O– < OH– (C) C6H5O– < CH3COO– < CH3O– < OH–
A-9.*
(D)
Which one of the following has maximum nucleophilicity ? (A) CH3 S
A-8.
(C)
(B) CH2 = CH2
(C) C N
••
(D) NH3
Decreasing order of nucleophilicity of the following nucleophile is
–
(1) CH3O-
(2) CN
(3)
(4) CH3 CO–2
(A) 4 > 3 > 2 > 1
(B) 2 > 1 > 4 > 3
(C) 2 > 1 > 3 > 4
(D) 1 > 2 > 3 > 4
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REACTION MECHANISM # 11
Section (B) SN1 Reactions B-1.
Following is the list of four halides. Select correct sequence of decreasing order of reactivity for SN1 reaction using the answer codes given below :
(1) H5C6
CH
Br
(2) C6H5–CH2–Br
(3) H5C6
CH
CH3
I
(4) C6H5–CH2–Cl
CH3
Answers codes : (A) 3, 1, 2, 4 B-2.
(B) 1,3, 2, 4
(C) 2, 4, 3, 1
(D) 4, 2, 3, 1
Which one of the following compounds will be most reactive for SN1 reactions:
I (A)
Cl
O
(B)
Cl
Br O
O
(C)
(D)
O B-3.
Arrange the following compounds in order of decreasing rate of hydrolysis for SN1 reaction: (I)
CH2–Br
(II) H3C
CH2–Br
H3C (III) CH3–CH2
CH2–Br
(A) II > III > IV > I B-4.
CH
(IV)
H3C
(B) IV > III > II > I
(C) III > IV > II > I
CH2–Br (D) I > II > III > I
Among the bromides I–III given below, the order of reactivity in SN1 reaction is:
(I)
(II)
(III)
(A) III > I > II
(B) III > II > I
(C) II > III > I
(D) II > I > III
B-5.
Which of the following compounds is most rapidly hydrolysed by SN1 mechanism. (A) C6H5Cl (B) Cl–CH2–CH = CH2 (C) (C6H5)3CCl (D) C6H5CH2Cl
B-6.
Which among the following will give SN1 reaction
C 6 H5
CH3 (1) H3C
CH
Br
C6H5
(2) H3C
C
Br
(3) CH3–CH2–I
CH3
Select the correct answer from the codes given below : Codes : (A) 1, 2 and 3 (B) 1, 2 and 4 (C) only 3
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(4) H5C6
C
Cl
CH3
(D) 2 and 4
REACTION MECHANISM # 12
B-7.
What will be the major product of the following reaction ?
H CH3 | | CH3 — C — C — CH — CH3 | | | H CH3 Br
B-8.
CH3OH, 30C
H CH3 C H3 | | | (A) CH3 — C — C — C — CH3 | | | OCH3 H H
H | (B) CH3 — C — | H
H | (C) CH3 — C — | H
H CH3 | | (D) CH3 — C — C — | | H CH3
CH3 CH3 | | C — C — CH3 | | OCH3 H
CH3 OCH3 | | C — C — CH3 | | CH3 H H | C — CH2 | | H OCH3
Which one of the following compounds will give (d) and () form in SN1 reaction (as major product)
(A)
(B)
B-9.
(C)
(D)
(C)
(D)
Product, Identify the major product :
(A)
B-10.
(B)
Aq.
(B) AgNO3 Which statement is correct about the above reaction ?
(A) Product (B) is
(B) Product (B) is
(I) by SN1 mechanism.
(II) by SN2 mechanism.
(C) Product (B) is (I) by SN2 mechanism. (D) Product (B) is (II) by SN1 mechanism.
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REACTION MECHANISM # 13
Section (C) : SN2 Reactions C-1.
The given compound CH3–O–CH2–Br gives which one of the following reactions: (A) Only SN1 (B) Only SN2 (C) SN1 as well as SN2 (D) E1
C-2.*
In the given pair in which pair the first compound is more reactive than second to SN2 reaction.
(A)
Cl
CH2Cl
Cl
(B)
Cl
Cl (C)
C-3.
Cl
(D)
Cl
The decreasing order of rate of SN2 reaction is :
CH3 C CH2 Cl || O
CH3–Cl (I)
(II)
(A) IV > III > II > I C-4.
Cl
(B) II > III > I > IV
CH3–CH2–Cl (III) (C) II > I > IV > III
(IV) (D) none
In the following reaction the most probable product will be :
(A)
(B)
(C)
(D)
(C)
(D)
PBr
3 (X) , X is :
C-5.
(A)
(B)
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REACTION MECHANISM # 14
SOCl 2 Product,
C-6.
Identify the product
(A)
C-7.
(B)
(C)
(D)
Consider the following reaction. SOCl 2 ether In the above reaction which phenomenon will take place :
(A) Inversion
(B) Retention
(C) Racemisation
(D) Isomerisation
C-8.*
Which of the following conditions favour SN2 mechanism ? (A) Strong nucleophile (B) High conc. of nucleophile (C) 3º alkyl halide (D) Polar protic solvent
C-9.*
OH Y CH 3
H O18
2 X, Identify X & Y : H
H
(A) X =
(B) Y =
(C) X =
(D) Y =
vkSj Y gSa %
Section (D) : E1 Reaction D-1.
D-2.
Which one of the following compounds undergoes E1 reaction most readily ?
(A)
(B) CH3 – CH2 – CH2 – Br
(C) CH3 – CH2 – CH2 – I
(D)
Which of the following will be most reactive for E1 reaction ?
(A)
(B)
(C)
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(D)
REACTION MECHANISM # 15
D-3.
CH3OH [X]
In the given reaction,
[X] as the major product among the elimination products is : CH3 | C = CH2
(A)
(B)
(C)
(D)
H SO 4 2
D-4.
In the above reaction the major product is shown, which is formed through the intermediate (carbocation) given below :
Which bond will migrate to form the above product ? (A) p (B) q (C) r
D-5.
(D) s
Major product is :
(A)
(B)
(C)
(D)
Section (E) : E2 Reaction E-1.
Which of the following cannot undergo E2 reaction ? (A)
E-2.
(B)
(C)
(D) none of these
Arrange the following in decreasing order of stability of their transition state during elimination by strong base
(A) II > I > III
(B) II > III > I
(C) I > III > II
(D) I > II > III
E-3.
2-Bromopentane is heated with potassium ethoxide in ethanol. The major product obtained is (A) 2-Ethoxypentane (B) pent-1-ene (C) cis-pent-2-ene (D) trans-pent-2-ene
E-4.
In which of the following reaction, regioselectivity can be observed.
CH3
CH3
alc . KOH / (A) CH3–C–CH2–Cl
H
Cl CH3
CH3 alc . KOH / (C) CH3–C–CH2–CH3
Cl
alc . KOH / (B) CH3–C–CH3
alc . KOH / (D) CH3–C–CH2–Cl
CH3 ETOOS ACADEMY Pvt. Ltd F-106, Road No.2 Indraprastha Industrial Area, End of Evergreen Motor,
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REACTION MECHANISM # 16
E-5.
The most probable product in the following reaction is :
Trans
(A)
E-6.
(B)
(C)
(D)
Which one of the following hexachlorocyclohexane is least reactive and which one is most reactive for E2 reactions with a strong base for dehydrohalogenation.
(A) I least & II most
(B) II least & I most
(C) III least & I most
(D) III least & II most
(C)
(D) All of these
Section (F) : E1cB Reaction F-1.
E1 cB reaction is given by which of the following : (A) CF3 – CHCl2
(B)
PART - II : MISLLANEOUS QUESTIONS Comprehensions Type Questions : Comprehension # 1 Read the following passage carefully and answer the questions. One of the most interesting and useful aspects of stereochemistry is the study of what happens to optically acitive molecules when they react. The product isolated from the reaction of a chiral starting material can tell us a great deal about the reaction mechanism . We observe SN2 --------- Inversion of configuration SN1 --------- Racemisation SNi --------- Retention of configuration 1.
In the given reaction, mention reaction mechanism respectively
()
()
()
(V)
+
(A) SN2 , SN1 , SN2 , SNi
(B) SN2 , SN1 , SNi , SN2
(C) SN1 , SN2 , SNi , SN1
(D) SN2 , SNi , SN1 , SN2
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REACTION MECHANISM # 17
2.
The given reaction is an example of which type of Mechanism ?
C3H7 Ether
D
Cl + SO2 + HCl C6H5
(A) SN2 3.
(B) SN1
(C) SNi
(D) None
In which of the following reaction retention of configuration is observed ?
H Nal / acetone Br
(A) CH3
(B)
D CH3
C2H5 (C) CH3
(D) H
Br + HOH
Na OTs
C2H5
C3H7
Comprehension # 2 Alcohols undergo acid catalysed elimination reactions to produce alkenes. Because water is lost in the elimination , this reaction is called dehydration reaction. Secondary and tertiary alcohols always give E1 reaction in dehydration. Primary alcohols whose -carbon is branched also give E1 reaction. The reactivity of alcohol for elimination reaction is tertiary alcohol > Secondary alcohol > Primary alcohol.
4.
conc .H2SO 4 Alkenes
In the given reaction :
Total number of alkenes (Including stereo isomers) formed will be (A) Two (B) Three (C) Four 5.
(D) Five
Which of the following dehydration product (major) is incorrect ? (A)
(B)
conc .H2SO 4
conc .H SO
2 4
conc .H SO
2 4 (C) CH3 CH2 CH2CH2OH CH3–CH=CH–CH3
(D)
6.
conc .H2SO 4
Identify the product in the given reaction : Product
(A)
(B)
(C)
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(D)
REACTION MECHANISM # 18
Assertion / Reasoning Type Questions DIRECTIONS : Each question has 5 choices (A), (B), (C), (D) and (E) out of which ONLY ONE is correct. (A) Statement-1 is True, Statement-2 is True; Statement-2 is a correct explanation for Statement-1. (B) Statement-1 is True, Statement-2 is True; Statement-2 is NOT a correct explanation for Statement-1. (C) Statement-1 is True, Statement-2 is False. (D) Statement-1 is False, Statement-2 is True. (E) Statement-1 and Statement-2 both are False.
7.
Statement-1 : If the mixture of
and
reacts with the excess of NaSH in DMF,,
the molecuarity of the SN2 reaction will be two but not three. ..
Statement-2 : In the SN2 reaction two molecules e.g. R – X and N u take part in the formation of transition state.
8.
Statement-1 : Iodide (I–) is the strongest nucleophile (in protic solvent) among the halide anions Statement-2 : Iodide is the largest halide anion and is the most weakly solvated in a protic solvent
Match the column Type Questions 9.
Match the following :
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REACTION MECHANISM # 19
PART - I : MIXED OBJECTIVE Single Choice Questions 1.
Which one is the strongest nucleophilic site in the following species ?
(A) 1 2.
(B) 2
(D) 4
Decreasing order of relative nucleophilicity of the following nucleophiles in protic solvent is ,
3.
(C) 3
,
,
(A)
>
> H2O >
(C)
>
>
, >
> H2O >
H2O (B)
>
>
(D)
>
>
>
> H2O
>
> H2O
Which of the following is not expected to be intermediate of the following reaction ?
H2 O
(A)
4.
(B)
(C)
(D)
Which describes the best stereochemical aspects of the following reaction ? H Br Product
(A) Inversion of configuration occurs at the carbon undergoing substitution. (B) Retention of configuration occurs at the carbon undergoing substitution. (C) Racemization occurs at the carbon undergoing substitution. (D) The carbon undergoing substitution is not stereogenic. 5.
When the concentration of alkyl halide is tripled and the concentration of of SN2 reaction increases by : (A) 3 times (B) 2 times (C) 1.5 times
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ion is reduced to half, the rate (D) 6 times
REACTION MECHANISM # 20
SOCl
2 Product pyridine,
6.
Product of the above reaction is :
(A)
(B)
(C)
(D)
CH2OH
7.
Conc.H2 SO 4 [X] as major product
In the given reaction :
[X] will be :
(A)
CH2
CH3
(B)
(C)
CH3
(D)
8.
The product X and Y are respectively :
(A)
&
(B)
&
(C)
&
(D)
9.
&
Which of the following will undergo fastest elimination reaction with alcoholic KOH.
(A)
(B)
(C)
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(D)
REACTION MECHANISM # 21
10.
X and Y are respectively : (A)
and
(B)
(C)
and
(D)
and
and
More than one correct answer 11.
Which of these statements are correct about nucleophiles : (A) Nucleophiles have an unshared electron pair and can make use of this to react with an electron deficient species. (B) The nucleophilicity of an element (as electron donor) generally increases on going down a group in the periodic table. (C) A nucleophile is electron-deficient species (D) All good nucleophiles are good bases when we deal across the period.
12.
Which of the following is / are true for SN1 reactions ? (A) They occur through a single step concerted reaction. (B) They are favoured by polar solvents. (C) 3º alkyl halides generally react through this mechanism. (D) Concentration of nucleophile does not affect the rate of such reactions.
13.
SN2 reaction will be negligible in
Br (A)
14.
15.
(B)
(C)
Rate of SN2 depends on (A) Conc of Nucleophile (C) Nature of leaving group
(D)
(B) Conc of substrate (D) Nature of solvent
Which of the following reaction is correct ? Acetone (A) 1, 4-Dichlorohexane + NaI CH3 CH2 CH CH2 CH2 CH2 + NaCl
| Cl
1eq.
(B) 1, 2-Dibromoethane + NaS – CH2 – CH2 – SNa
| I
+ 2NaBr
Et O
2 (C) 4-Chloro-1-butanol + NaH C4H8ClONa
NH
CH3 I 3 C3H3Na C4H6 (D) Propyne + NaNH2
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REACTION MECHANISM # 22
PART - II : SUBJECTIVE QUESTIONS 1.
Arrange the given species in decreasing order of their nucleophilicity ? H2O
2.
Write nucleophilicity order in the following compound.
3.
Of the following pairs, which is the faster SN1 reactions? (a) (c)
(a) (c) 4.
and
(b)
and
(d)
rFkk
and CH3(CH2)5Cl and
rFkk CH3(CH2)5Cl
(b)
rFkk
(d)
rFkk
Give the decreasing order of SN1 reactions of the following compounds in H2O.
(a) (i)
(ii)
(iii)
(b) (i)
(ii)
(iii)
(c) (i)
(ii)
(iii)
5.
(a)
(b)
6.
What effect do you expect due to following changes in SN1 reaction of (CH3)3CBr with CH3OH ? (a) The Concentration of (CH3)3CBr is doubled and that of CH3OH is halved. (b) The concentration of both (CH3)3CBr and CH3OH are tripled.
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REACTION MECHANISM # 23
7.
Which SN1 reaction of each pair would you expect to take place more rapidly ? Explain. (a) (i) Me3CCl + H2O Me3COH + HCl (ii) Me3CBr + H2O Me3COH + HBr (b) (i) Me3CCl + H2O Me3COH + HCl (ii) Me3CCl + CH3OH Me3COCH3 + HCl
8.
Explain how AgNO3 increases the rate of solvolysis in SN1 reactions of alkyl halide
9.
Complete the following reactions and predict the nature of solution after product formation.
HO
2 P + Q
(a)
10.
Acetone 25C
(b)
R + S
Write the mechanism of the following reaction and mention the rate determining step.
(a)
(b)
HI A + B,
11.
(1eq )
Identify A and B. 12.
Propose a mechanism for the following reaction
O O CH 3 O / CH 3OH (a) CH 3 CH CH CH 2 CH CH CH CH OCH + Cl 3 2 3 | Cl
H (b) HO–CH2–CH2–CH2–CH2–OH
13.
O
+ HOH
How many structural isomers of monobromo cyclohexene are possible. Which will react most readily with NaCN?
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REACTION MECHANISM # 24
14.
Which of the following reactions (a) and (b) would give a better yield of the desired ether as product.
(a)
(b)
15.
Explain why thionyl chloride is considered the best reagent for converting alcohols to alkyl chlorides.
16.
Predict the product of the following reactions using one equivalent of HI.
(a)
17.
(b)
(c)
Give the products of the following reactions
(i) 18.
(d)
X
(ii)
Y
Few dialkyl ethers & cyclic ethers were allowed to react with excess of HBr with the following results. Identify the ether in each case.
HBr
(P) excess
+
CH3 – CH2 – CH2 – CH2 – Br
HBr
(Q) excess
HBr
(R) excess
HBr
(S) excess
19.
Observe the following reactions. O || CH3 – C – F O || CH3 – C – Br
H O 2 r1
H O r3
2
O || CH3 – C – Cl
2
O || CH3 – C – I
2
H O r2 H O r4
Identify the order of the rate of reaction r1, r2, r3 and r4, give reason.
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REACTION MECHANISM # 25
20.
Predict the products of the following reactions
(i)
+
O || (ii) Ph C Cl
+
21.
Explain the order of the rate of esterification of the following acid with MeOH : MeCH2COOH > Me2CHCOOH > Me3CCOOH > Et3CCOOH >> (i-Pr)2CHCOOH
22.
Write the correct reactivity order of nucleophilic substitution reaction for the following compounds.
23.
Write the correct reactivity order with NaOH of following compounds.
24.
When 1-Bromo-1-methylcyclohexane is heated in ethanol for an extended period of time, three products result: one ether and two alkenes. Predict the products of this reaction, and propose a mechanism for their formation. Also, mention the major elimination product.
25.
Predict the major product of E1 elimination of the following compounds.
(a)
(b)
(c)
(d)
26.
Why dehydration of alcohol takes place in acidic medium but not in basic medium.
27.
1º alcohols are poor starting material for synthesis of 1-Alkene. Explain
28.
What happens when ethanol is heated with conc. H2SO4 at 453 K. Explain the mechanism of this reaction.
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REACTION MECHANISM # 26
29.
Predict the major product of the sulphuric acid catalysed dehydration of the following alcohols : (a) (CH3)2C(OH)CH2CH3
(b) CH3CH2 CH2CH(OH)CH3
(c) (CH3)2C(OH)CH(CH3)2
(d) (CH3)3CCH2OH
H Product,
30.
Write the major product of the reaction
Conc. H3PO4 A (Major) + B (Minor),
31.
Write structures of A and B 32.
Among the various isomers of C4H9Br which would undergo elimination most readily and why ?
33.
A halide with formula C6H13I is found to give two isomeric alkenes 2-methyl-2-pentene and 4-methyl-2pentene on dehydrohalogenation with alcoholic KOH. Suggest its structure.
34.
Bromocyclodecane on heating with ethanolic KOH, produces two alkenes Write the two products also mention the major one.
35.
A compound (X) [mol formula (C7H13Br)] on dehydrohalogenation gives only vinylcyclopentane, Write the strucuture of X.
36.
Explain why more alkylated alkene is formed predominatly if base is is obtained majorly when 37.
, while less alkylated alkene
base is used.
Give the product of elimination reaction with its stereochemistry
(a)
(b)
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?
REACTION MECHANISM # 27
38.
Which alkyl chloride would yield following pure alkene on reaction with alcoholic KOH ? CH3 | (i) CH3 C CH2
39.
(iii) CH3 CH 2 C CH 2 | CH3 The sum of molecular weights of the major products P and Q form at the followng reaction is (ii) CH3 – CH2 – CH2 – CH = CH2
KOH / P (organic product)
(i)
+ CH3I
(ii)
KOH / Q (organic product) + CH3CH2OH
40.
What are the essential conditons for any reaction to show E1cB mechanism ?
41.
Write down the increasing order of reactivity of following compounds via E1cB mechanism in basic medium
42.
43.
(a) C6H5 – CH2 – CH2 – Br
CF3 – CH2 – CF3
(b)
NO2 – CH2 – CH2 – Br
If ethanol containing EtOD is used as solvent, then deuterium exchange take place in E1cB mechanism. Why?
+
Write down the mechanism of the above reaction
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REACTION MECHANISM # 28
PART - I : JEE PROBLEMS (PREVIOUS YEARS) * Marked Questions are having more than one correct option. 1.
An SN2 reaction at an asymmetric carbon of a compound always gives (A) an enantiomer of the substrate (C) a mixture of diastereomers
[JEE-2001, 1/135]
(B) a product with opposite optical rotation (D) a single stereoisomer
2.
Explain why 7-bromo-1, 3, 5-cycloheptatriene exist as an ion while 5-Bromo-1, 3-cyclopentadiene does not form any ion even in the presence of Ag+ . Explain why ? [JEE 2004, 4/60]
3.
Compound (X)
is reacted with aqueous acetone it gives following
products.
(A) K, L
4.
[JEE-(scr.)2005, 3/144]
(B) K, M
(C) L only
Explain the following observations
(D) M only
[JEE(M)-05, 1/144]
Acidic solution
Neutral solution
5.
6.
Which is the best reagent to convert cyclohexanol into cyclohexene. (A) conc. HCl (B) conc. HBr (C) conc. H3PO4
H X, Identify X.
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[JEE-2005, 3/144] (D) HCl + ZnCl2
[JEE-2005, 2/144]
REACTION MECHANISM # 29
7.
Match the following (one term in column-I may match with more than one terms in column-II) [JEE-2006, 6/184]
8.
The major product of the following reaction is
(A)
9.
(B)
(C)
(D)
In the following carbocation; H/CH3 that is most likely to migrate to the positiviely charged carbon is [JEE-2009, 3/160]
(A) CH3 at C-4 10.
[JEE-2008, 3/163]
In the reaction (A) Br (C)
(B) H at C-4
(C) CH3 at C-2
(D) H at C-2
HBr OCH3 the products are :
OCH3 and H2 Br and CH3OH
[JEE-2010, 3/163]
(B)
Br and CH3Br
(D)
OH and CH3Br
11.
The total number of alkenes possible by dehydrobromination of 3-bromo-3-cyclopentylhexane using alcoholic KOH is : [JEE 2011 160/4]
12.
The compound that undergoes decarboxylation most readily under mild condition is -[JEE-2012, 3/136]
COOH
COOH CH2COOH
(A)
COOH O
(B)
CH2COOH COOH
(C)
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O (D)
REACTION MECHANISM # 30
PART - II : AIEEE PROBLEMS (PREVIOUS YEARS) 1.
Following reaction (CH3)3CBr + H2O (CH3)3COH + HBr is an example of : (1) Elimination reaction (3) Nucleophilic substitution
2.
3.
[AIEEE-2002] (2) Free radical substitution (4) Electrophilic substitution
SN1 reaction is feasible in :
[AIEEE-2002]
(1)
+ KOH (aq.)
(2)
(3)
+ KOH (aq.)
(4)
+ KOH (aq.)
+ KOH (aq.)
Maximum dehydration takes place that of :
(1)
(2)
[AIEEE-2002]
(3)
(4)
4.
Tertiary alkyl halides are practically inert to substitution by SN2 mechanism because of (1) steric hindrance (2) inductive effect (3) instability (4) insolubility
[AIEEE-2005]
5.
The decreasing order of nucleophilicity among the nucleophiles
[AIEEE-2005]
6.
7.
(a)
(b)
(c)
(d)
(1) (c), (b), (a), (d)
(2) (b), (c), (a), (d)
(3) (d), (c), (b), (a)
(4) (a), (b), (c), (d)
The decreasing order of nucleophilicity among the nucleophiles
[AIEEE-2005]
(a)
(b)
(c)
(d)
(1) (c), (b), (a), (d)
(2) (b), (c), (a), (d)
(3) (d), (c), (b), (a)
(4) (a), (b), (c), (d)
Fluorobenzene (C6H5F) can be synthesized in the laboratory (1) from aniline by diazotisation followed by heating the diazonium salt with HBF4 (2) by direct fluorination of benzene with F2 gas (3) by reacting bromobenzene with NaF solution (4) by heating phenol with HF and KF
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[AIEEE-2006]
REACTION MECHANISM # 31
8.
CH3Br + Nu– CH3 – Nu + Br– The decreasing order of the rate of the above reaction with nucleophiles (Nu¯) A to D is : [Nu¯ = (A) PhO¯, (1) D > C > B > A
9.
(B) AcO¯,
(C) HO¯, (D) CH3O¯]
(2) A > B > C > D
(3) B > D > C > A
[AIEEE-2006] (4) D > C > A > B
The structure of the major product formed in the following reaction is
[AIEEE-2006]
NaCN DMF
CH2Cl
(1)
CH2Cl
(2)
CH2CN
(3)
CH2CN
(4)
CN 10.
CN
CN
Reaction of trans 2-phenyl-1-bromocyclopentane on reaction with alcoholic KOH produces : [AIEEE-2006] (1) 2–phenylcyclopentene (2) 1–phenylcyclopentene (3) 3-phenylcyclopentene (4) 4-phenylcyclopentene
11.
The alkene formed as a major product in the above elimination reaction is :
(1)
(2) CH2 = CH2
(3)
[AIEEE-2006]
(4)
12.
Which of the following is the correct order of decreasing SN2 reactivity ? (1) RCH2X > R3CX > R2CHX (2) RCH2X > R2CHX > R3CX (3) R3CX > R2CHX > RCH2X (4) R2CHX > R3CX > RCH2X
13.
The organic chloro compound, which shows complete stereochemical inversion during an SN2 reaction, is [AIEEE-2008, 3/105] (1) (CH3)3CCl (2) (CH3)2CHCl (3) CH3Cl (4) (C2H5)2CHCl
14.
Which of the following on heating with aqueous KOH, produces acetaldehyde ? [AIEEE-2009, 4/144] (1) CH3CH2Cl (2) CH2ClCH2Cl (3) CH3CHCl2 (4) CH3COCl
15.
From amongst the following alcohols the one that would react fastest with conc. HCl and anhydrous ZnCl2, is : [AIEEE-2010, 4/144] (1) 2-Butanol (2) 2-Methylpropan-2-ol (3) 2-Methylpropanol (4) 1-Butanol
16.
Consider the following bromides :
The correct, order of SN1 reactivity is (1) B > C > A (2) B > A > C
[AIEEE-2007, 3/120]
[AIEEE-2010, 4/144]
(3) C > B > A
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(4) A > B > C
REACTION MECHANISM # 32
17.
In the chemical reactions :
[AIEEE-2010, 4/144]
NH2
HBF4 NaNO 2 B A HCl, 278 K the compounds ‘A” and ‘B’ respectively are
18.
(1) nitrobenzene and fluorobenzene
(2) phenol and benzene
(3) benzene diazonium chloride and fluorobenzene
(4) nitrobenzene and chlorobenzene
The main product of the following reaction is :
[AIEEE-2010, 4/144]
conc. H2SO4 C6H5CH2CH(OH)CH(CH3)2
19.
(1)
(2)
(3)
(4)
An unknown alcohol is treated with the “Lucas reagent” to determine whether the alcohol is primary, secondary or tertiary. Which alcohol reacts fastest and by what mechanism : [JEE Mains 2013] (1) secondary alcohol by SN1 (2) tertiary alcohol by SN1 (3) secondary alcohol by SN2 (4) tertiary alcohol by SN2
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REACTION MECHANISM # 33
NCERT QUESTIONS 1.
Write the isomers of the compound having formula C4H9Br.
2.
Write the equations for the preparation of 1-iodobutane from (i) 1-butanol (ii) 1-chlorobutane (iii) but-1-ene.
3.
What are ambident nucleophiles? Explain with an example.
4.
Which compound in each of the following pairs will react faster in SN2 reaction with –OH? (i) CH3Br or CH3I (ii) (CH3)3CCl or CH3Cl
5.
Predict all the alkenes that would be formed by dehydrohalogenation of the following halides with sodium ethoxide in ethanol and identify the major alkene: (i) 1-Bromo-1-methylcyclohexane (ii) 2-Chloro-2-methylbutane (iii) 2,2,3-Trimethyl-3-bromopentane.
6.
Write the structure of the major organic product in each of the following reactions: (i) CH3CH2CH2Cl + NaI (ii) (CH3)3CBr + KOH
acetone heat
ethanol heat
(iii) CH3CH(Br)CH2CH3 + NaOH (iv) CH3CH2Br + KCN (v) C6H5ONa + C2H5Cl (vi) CH3CH2CH2OH + SOCl2 (vii) CH3CH2CH = CH2 + HBr (viii) CH3CH = C(CH3)2 + HBr
7.
water aq.ethanol peroxide
Write the mechanism of the following reaction :
EtOH H2 O nBuBr + KCN nBuCN
8.
Arrange the compounds of each set in order of reactivity towards SN2 displacement : (i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane (ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3-methylbutane (iii) 1-Bromobutane, 1-Bromo-2,2-dimethylpropane, 1-Bromo-2-methylbutane, 1-Bromo-3-methylbutane.
9.
Out of C6H5CH2Cl and C6H5CHClC6H5, which is more easily hydrolysed by aqueous KOH.
10.
p-Dichlorobenzene has higher m.p. and solubility than those of o- and m-isomers. Discuss.
11.
The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain.
12.
Primary alkyl halide C4H9Br (a) reacted with alcoholic KOH to give compound(b). Compound (b) is reacted with HBr to give (c) which is an isomer of (a). When (a) is reacted with sodium metal it gives compound (d), C8H18 which is different from the compound formed when n-butyl bromide is reacted with sodium. Give the structural formula of (a) and write the equations for all the reactions.
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REACTION MECHANISM # 34
13.
What happens when (i) n-butyl chloride is treated with alcoholic KOH, (ii) ethyl chloride is treated with aqueous KOH, (iii) methyl chloride is treated with KCN?
14.
Explain why propanol has higher boiling point than that of the hydrocarbon, butane?
15.
Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain this fact.
16.
Show how will you synthesise: (i) cyclohexylmethanol using an alkyl halide by an SN2 reaction. (ii) pentan-1-ol using a suitable alkyl halide?
17.
Give reason for the higher boiling point of ethanol in comparison to methoxymethane.
18.
Write the names of reagents and equations for the preparation of the following ethers by Williamson’s synthesis: (i) 1-Propoxypropane (ii) Ethoxybenzene (iii) 2-Methoxy-2-methylpropane (iv) 1-Methoxyethane
19.
Illustrate with examples the limitations of Williamson synthesis for the preparation of certain types of ethers.
20.
How is 1-propoxypropane synthesised from propan-1-ol? Write mechanism of this reaction.
21.
Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.
22.
Write the equation of the reaction of hydrogen iodide with : (i) 1-propoxypropane (ii) methoxybenzene and (iii) benzyl ethyl ether.
23.
Explain the fact that in aryl alkyl ethers (i) the alkoxy group activates the benzene ring towards electrophilic substitution and (ii) it directs the incoming substituents to ortho and para positions in benzene ring.
24.
Write the mechanism of the reaction of HI with methoxymethane.
25.
When 3-methylbutan-2-ol is treated with HBr, the following reaction takes place:
Give a mechanism for this reaction. (Hint : The secondary carbocation formed in step II rearranges to a more stable tertiary carbocation by a hydride ion shift from 3rd carbon atom.
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REACTION MECHANISM # 35
Exercise # 1 PART - I A-1. A-9.* B-5. C-2.* C-9.* E-2.
(C) (BC) (C) (BD) (AD) (A)
A-2. A-10. B-6. C-3. D-1. E-3.
(C) (C) (B) (C) (D) (D)
A-3. A-11. B-7. C-4. D-2. E-4.
(C) (B) (C) (B) (A) (C)
A-4. B-1. B-8. C-5. D-3. E-5.
(C) (A) (B) (A) (C) (C)
A-6. B-2. B-9. C-6. D-4. E-6.
(A) (A) (A) (A) (B) (B)
A-7. B-3. B-10. C-7. D-5. F-1.
(A) (A) (D) (B) (C) (D)
A-8. B-4. C-1. C-8.* E-1.
(D) (A) (C) (AB) (C)
(B)
6.
(C)
7.
(A)
(C)
6.
(A)
7.
(B)
PART - II 1. 8.
(B) (A)
2. 9.
(C) 3. (B) 4. (C) 5. (A) - p, r, s ; (B) - r, s ; (C) - r, s ; (D) - p, q
Exercise # 2 PART - I 1.
(D)
2.
(B)
3.
(A)
4.
(C)
5.
8.
(A)
9.
(A)
10.
(A)
11.
(ABD) 12.
14.
(ABCD) 15.
(BCD) 13.
(ABC)
(ABCD)
PART - II 1.
>
>
> H2O
2.
III > II > I > IV
b. (iii) > (i) > (ii)
3.
(a) (i), (b) i, (C)i (d) i
4.
a. (iii) > (ii) > (i)
c. (ii) > (i) > (iii) (allylic > 3° > 1°)
5.
(a)
6.
(a) Rate - doubled ;
7.
(a) ii, (b) i
8.
Ag+ ion has strong affinity for X– than a solvent molecule, the precipitation of AgX accelerates the dissociation of R–X bond.
9.
(a) P =
,
(b) R =
, S=
(b)
(b) Rate - tripled
Q=
pH is less than seven because acid HBr is formed.
pH is less than seven because acid HCl is formed
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REACTION MECHANISM # 36
10.
CH3 | (a) CH3 CH CH CH3 | OH
Slow rds
3 Methyl 2 butanol
H O
(b)
2 rds
+ CH3OH
11.
12.
(a)
3 O / CH 3OH N2 CH CH 3 CH CH CH 2 OCH 3 S CH3–CH–CH–CH2–O–CH3 CH3–CH–CH–CH2–O–CH3 SN 2 | | O Cl OH O
H
H (b) HO–(CH2)3–CH2OH ( CH 2 ) 3 CH 2 O H–O
H H
H 2O
O
H
H
O
13.
Three
will react most readily with NaCN.
14.
(a) Give better yield, because in the (b) nucleophile attacks on more hindered 2° carbon. So along with substitution, elimination also posible in the (b).
15.
ROH + SOCl2 R–Cl + HCl + SO2 gases Pure RCl is obtained, as the by products (HCl and SO2) are gases and get evaporated from the reaction mixture.
16.
17.
(a) I–H2C – CH2 – CH2 – CH2 – CH2–OH
(b)
(c)
(d)
(i) X = CH3CH(OH)CH2OH
(ii) Y = CH3CH(OH)CH2OMe
+ CH3I
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REACTION MECHANISM # 37
18.
P=
Q=
R=
S=
CH2OCH2
19.
r4 > r3 > r2 > r1
rate of SN2 Th is inversly proportional to basicity of leaving group.
20.
(i)
21.
As the size of the substituents on the C increases, the tetrahedrally bonded intermediate becomes more crowded. The greater the crowding, the larger is H+ of the TS and the slower is the reaction.
22.
II > I > IV > III
23.
25.
(a)
(b)
26.
In basic condition very poor leaving group –OH will eliminate but in acidic medium –OH will be converted
(ii)
II > III > I
24.
+
(c)
(d)
into – O H which is very good leaving group. 2
27.
1º carbocation R – CH2 would rearrange and 2-alkene would result. Even if 1-alkene is also formed but, it would tend to rearrange in acidic medium to 2-alkene.
28.
What happens when ethanol is heated with conc. H2SO4 at 453 K. Explain the mechanism of this reaction.
Ans.
.. + (a) CH3—CH2— O .. —H + H Ethanol
CH3—CH2— —H
Oxonium ion slow
(b)
CH2=CH2 + H2O
CH3
29.
(a) CH3 C CHCH3 CH3
(c)
CH3
CH3 C
CH3
(b) CH3 – CH2 – CH = CH – CH3
C
(d) CH3 C CHCH3
CH3
CH3
31.
H3 C
H O
30.
2
A:
+
H
– H
B:
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REACTION MECHANISM # 38
32.
Tert-butyl bromide would undergo elimination most readily as the alkene formed from it would be more stable.
33.
34.
+
E2
35.
The product is formed only by E2 elimination.
HBr
36.
t-
37.
(a)
38.
(i)
39.
196
is bulky base so Hoffmann product is formed as major product.
(b)
or
; (ii) CH3CH2CH2CH2CH2Cl ; (iii) CH3 CH2 CHCH2 Cl | CH3
O
P=
CH3
[M.F. C7H14O
M.W. 84+14+16 = 114]
[M.F. C6H10
M. W. 72 + 10 = 82]
CH3
Q=
CH3
M.W. (P + Q) = 114 + 82 = 196 40.
Reactant should have acidic -hydrogen and poor leaving group.
41.
(a) C6H5 – CH2 – CH2 – Br < CF3 – CH2 – CF3
42.
In E1cB mechanism carbanion is formed as intermediate and 1st step is reversible.
(b) NO2 – CH2 – CH2 – Br <
43.
2nd step
In E1cB 1st step is reversible step so deuterium exchange take place
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REACTION MECHANISM # 39
Exercise # 3 PART - I 1.
(D)
2.
7-bromo-1, 3, 5-cycloheptatriene on ionisation gives tropylium ion
which is aromatic & highly stable,
but ionisation of 5-bromo-1, 3-cyclopentadiene gives 1, 3-cyclopentadienyl cation
which is anti aromatic
& unstable. (non existent) 3.
(A)
4.
+ HBr (strong acid) +
(SN1 product)
No reaction. Because aryl halide have resonance stabilized C – X bond, and do not give SN reaction.
5.
(C)
6.
8.
(A)
9.
(D)
10.
7.
(A) - q ; (B) - q ; (C) - r, s ; (D) - p, s
(D)
11.
5 alkenes including stereoisomers
12.
(B)
PART - II 1.
(3)
2.
(1)
3.
(2)
4.
(1)
5.
(1)
6.
(1)
7.
(1)
8.
(4)
9.
(3)
10.
(3)
11.
(2)
12.
(2)
13.
(3)
14.
(3)
15.
(2)
16.
(1)
17.
(3)
18.
(1)
19.
(2)
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REACTION MECHANISM # 40
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