General Organic Chemistry for IIT

GENERAL ORGANIC CHEMISTRY 1.

Heterolytic fission of covalent bond produces charged species.

2.

Homolytic fission of covalent bond produces free radicals having a odd i.e. unpaired electron.

3.

The ionic intermediate carrying a positive change at the central carbon atom is called carbonium ion in which carbon atom has 6 electrons (electron deficient). The central atom of carbon in carbonium ion is sp2 hybridized (planar). The intermediate in which central carbon atom is negatively charged is called carbanion, which has unshared pair of electron at the central carbon atom. A carbanion is isoelectronic with an amine and is sp3 hybridized (pyramidal).

4.

The stability of carbonium ion follows the order, tertiary > secondary > primary > methyl carbonium ion.

5.

The stability of carbanion follows the reverse order, i.e. methyl > primary > secondary > tertiary carbanion ion.

6.

Decreasing order of these carbanions is : Triphenyl methyl > Diphenyl methyl > Benzyl carbanion.

7.

Free radicals are odd electron species and may be regarded as having planar configuration analogous to carbonium ion or a pyramidal structure similar to carbanion, which is capable of rapid interconversion.

8.

Less is the homolytic bond dissociation energy greater will be the stability of free radicals. The stability of follows the order Tertiary > Secondary > Primary > Methyl free radical. Free radicals are paramagnetic species.

9.

Carbenes are electron deficient neutral carbon intermediates having one non bonded electron pair in sextet (the central carbon atom has six electrons). Carbenes exist in singlet and triplet states. The latter being more stable than the singlet state. Singlet carbine is stabilized by the presence of halogen atom or atom carrying non−bonding electron pair directly linked with carbenic carbon.

10.

The process of electron shift along a chain of atoms due to the presence of polar covalent bond in it is called inductive effect. When the hetero atom is such that it attracts the electron pair towards itself, it is said to exert - I effect or electron with drawing inductive effect. When the hetero atom or group of atoms pushes the electrons away from itself, it exerts + I effect or electron releasing inductive effect. (a) inductive effect involves the permanent displacement of an electron pair in a molecule (b) presence of an attacking reagent is not essential (c) Presence of multiple bond is also not essential (d) Polarity of bond is, however, essential (e) The displaced electron pair does not leave its molecular orbital. A distortion in the shape of the molecular orbital, however, takes place. (f) in it, there is a partial charge separation and ions are formed (g) Inductive effect of an atom or group of atoms diminishes rapidly with distance and is almost negligible beyond two carbon atoms.

11.

Electron releasing groups such as alkyl groups decrease the acidity, while electron withdrawing groups such as Cl, Br, OH, CN, etc increase the acidity.

12.

The decreasing order of electron withdrawing inductive effect (-I effect) of certain atoms / groups is NO2 > CN > F > COOH > CI > Br > I > OCH3 > C6H6. The decreasing order of the electron releasing inductive effect of some atoms/ groups is (CH3)2 C > (CH3)2CH > C2H5 > CH3.

13.

The phenomenon due to which compound is said to be hybrid of various cannonical forms is called resonance. The latter includes mesomeric and inductive effect both.

14.

The important points of resonance theory are: a) When a molecule is represented by two or more structures that differ from one another in the arrangement of electrons and not the atomic nuclei, the molecule is expected to involve what is known as resonance. The molecule is the resonance hybrid of all such structures, but none of these structures presents the actual molecules. Each of these structures however contributes to the resonance hybrid. b) Each contributing structure has the same number of unpaired electrons. c) The greater the stability of the contributing structure, the greater will be its contributes to the hybrid d) Resonance becomes more important when the contributing structures have almost same stability of the same energy content. e) The resonance hybrid is more stable than any of the contributing structures. This increase in stability of the hybrid is due to resonance energy or delocalization energy. f)

Contributing structures which involve distinct charges are less stable that those which do not involve any charges

g) The greater the number of contributing structures for a hybrid molecule, the greater will be its stability. h) The greater the number of bonds in a contributing structure, the grater will be the stability of that structure. 15.

The mesomeric effect is the effect to electron redistribution that can take place in unsaturated and especially in conjugated systems via their π orbital. The effect caused by resonance in a molecule is called resonance effect (R - effect) or mesomeric effect (M - effect). It is permanent effect.

16.

When groups such as - C = O, - NO2 - C≡ N, - COOR etc are adjacent to multiple bond they withdraw π electron from the multiple bond through R or M - effect. These groups withdraw electrons from the adjacent carbon - carbon multiple bonds and cause - R or - M effect.

17.

Groups such - OH, - NH2, NHR, Cl-, I- etc which can release or donate electrons through resonance are said to cause + R or + M effect. Mesomeric effect is not common in substituted aromatic systems.

18.

Electromeric Effect is a temporary effect operating only at the demand of nearby reagent and takes place in compounds containing multiple bonds such as C= O, C = C, C≡N etc, or atoms with a lone pair of electrons adjacent to the covalent bond. (a) Electromeric effect involves a temporary displacement of a pair of π electrons (b) The presence of an attacking reagent is essential (c) The presence of a multiple bond (double or triple bond) is essential (d) The electron pair which is completely transferred leaves its molecular orbital and takes up a new position (e) There is a complete charge separating and ions are formed (e) When the inductive as well as electromeric effect occur together in a molecules they can assist or oppose each other. When they oppose each other, the E - Effect generally dominates over I-Effect.

19.

Hyperconjugation arises from the delocalization of σ electrons of an alkyl group into an adjacent π bond.

20.

Conjugated dienes are more stable than corresponding diene which is not conjugated. It can be explained on the basis of delocalization of electrons. In addition to conjugation, the alkyl groups attached to doubly bonded carbon atoms tend to increases the stability of alkenes. The greater the number of alkyl group attached to C = C group, the greater would be the contributing structures and hence grater would be the stability of such alkene. For example 2−methyl propene [CH3)2 C = CH2] and but -2- ene, CH3CH = CHCH3 are more stable than propene CH3CH=CH2

22.

Electrophiles are electron seeking or electron loving species. Examples are NO 2+, Cl+, H3O+, RN2+, Ag+, CH3CH2+ and electron deficient atoms such as S in SO3 or SOCl2 etc. Lewis acids are also electrophiles, Neutral substance such as BF3, FeCl2, ZnCl2, Br2, H2O2, O3 etc containing an electron deficient atom are also electrophilie.

23.

Nucleophiles are nucleus loving and electron rich species. Lewis base are all nucleophiles. Examples are OCH3-, OH-, CN-, H-, CH3CO-, HSO4-, NH3, NH2-, LiAlH4, RMgX etc.

24.

An electron deficient electrophiles will attack at centers of high electron density in benzene ring. A nucleophilie will attack at the electron deficient centre.

25.

Compounds having the same molecular formula, but different physical and chemical properties are called isomers and the phenomenon is known as isomerism. There are seven isomers of molecular formula C4H10O. Out of these four are alcohols and three are ethers. When two or more compounds have the same molecular formula, but different carbon chains, they are called chain isomers and the phenomenon is called chain isomerism. For example, n - butane and isobutene, n-pentane and Isopentane etc are chain isomers. Position isomers differ mainly in the position of a substituents or a functional differ mainly in the nature of the functional group. For example, ethyl alcohol and dimethyl ether are functional isomers.

26.

Metamers differ in the nature of alkyl groups attached to the functional group. For example C2H5OC2H5, CH3O. CH2CH2CH3 and CH3.OCH(CH3)2 are metamers. It should be noted that metamerism is not exhibited by alkenes. Tautomerism is exhibited by a compound which is a mixture of two labile forms in dynamic equilibrium. Examples are aceto acetic ester (iketo - enol tautomerism) etc. Keto - enol Tautomerism in aldehydes and ketones is possible only when they contain at least one α - hydrogen atom.

27.

Geometrical isomerism is exhibited by alkenes or compounds containing double bond. Examples are maleic acid and fumaric acid, but - 2 - ene, 2- Dichloroethylene etc.

28.

In cis isomer similar groups are present on the same side and in trans isomer, they occupy opposite position.

29.

Isomer which differ in the rotation of plane polarized light are called optical isomers. Optical isomerism is exhibited by compounds containing at least one chiral centre or asymmetric carbon atom. Number of possible optical isomers for a given compound may be calculated by 2 n, where 'n' is the number of dissimilar asymmetric carbon atoms. Asymmetry in the inner structure of an organic compound is the real cause of optical activity.

30.

A chiral centre or an asymmetric carbon atom is one which is attached to four different atoms or groups.

31.

Compounds that rotate the plane polarized light to the right are called dextro rotatory (+) compounds and those that rotate the plane polarized light to the left are called laevo rotaotry (-) compounds.

32.

Optical activity is measured by polar meter. Optical isomers of a compound are nonsuperimposable. A mixture of 50 % dextro rotatory and 50% laevo rotatory compounds is called racemic mixture, which is optically inactive because of external compensation. A meso form is inactive (e.g. mesotartaric acid) because of internal compensation.

33.

The optical isomers that are mirror images of each other are called enantiomers or enantiomorphism or enantiomorphs. For example d - and l-lactic acid. The optical isomers of a substance which are not mirror images of each other are called diastereomers. a) When the molecule is unsymmetrical (when it can not be divided into equal halves), then No. of d- and - I isomers (a) =2n No of meso forms (m) =0 Total no. of optical isomers = (a+m)=2n b) When the molecule is symmetrical and has even number of asymmetric carbon atom, then No. of d - and I - isomers (a) = 2(n-1) No of meso forms (m) = 2(n/2-1) Total no. of optical isomers = (a+m) c) When the molecules is symmetrical and has odd number of asymmetric carbon atoms, then No. of d- and - I - isomers (a) = 2(n-1) -2(n-1/2) No. of meso forms (m) = 2(n-1/2) Total no. of optical isomers = (a+m) = 2(n-1).

HYDROCARBONS 1.

Alkanes can be prepared by catalytic hydrogenation of alkenes and alkynes (unsaturated hydrocarbons) in presence of catalysts like Ni, Pd etc. If catalytic hydrogenation is carried out in presence of Raney Ni (Sabatier Sanderen's reaction), the reaction is possible , even at room temperature, because the energy of activation of the reaction is much decreased in presence of Raney nickel.

2.

Alkanes are also prepared by the reduction of aldehydes and ketones by zinc-amalgam and conc. HCl. The reduction of aldehydes and ketones to alkanes by hydrazine and NaOH solution is known as Wolff Kishner reduction.

3.

Carboxylic acids, alcohols, aldehydes, ketones and alkyl halides can also be reduced to corresponding alkanes using red phosphorus and HI at about 200° - 250°C

4.

Decarboxylation is also achieved by Kolbe's process, which consists in electrolysing sodium or potassium salts of fatty acids in concentrated solution. The anode reactions in Kolbe's synthesis are RCOO- → RCOO• → R• + CO2 R• + R• → R – R The cathode reaction in Kolbe's syntheses is 2H+ + 2e- → H2

5.

A molecule is expected to be reactive if it has (a) Polar covalent bond (b) Multiple bond (c) Lone pair of electrons (d) Electron deficient central atom. The alkanes or paraffins do not satisfy any of these conditions and in addition have strong C - C and C - H sigma bonds. These are inert under ordinary conditions. Thus alkanes are very unreactive and called paraffins.

6.

When alkanes are heated, the C - C bond rather than C - H bonds are broken. This is due to the fact that C - C bond have lower bonds energy than the C - H bond energy. The bonds with lower bond energy are broken more easily.

7.

Alkenes can be prepared by elimination reaction such as dehydrohalogenation of alkyl halides and these elimination reactions follow Saytzeff's rule, according to which hydrogen atom is preferentially removed in elimination from that carbon atom which has least number of hydrogen atom. The reactivity of halides towards dehydrohalogenation follows the order iodides > Bromides > Chlorides. However if the leaving group is a bad leaving group but very strongly electronegative like F, and/or if the base is very bulky, the alkene formed during dehydrohalogenation is the less substituted alkene rather than the more substituted alkene which being stable should be the major product as according to Saytcoff’s rule. The formation of less substituted alkene during elimination is known as Hoffmann’s elimination. The Alkenes are formed by the dehydration of alcohols. The common dehydrating agents are H2SO4, P2O5, ZnCl2 etc. Electrolysis of sodium or potassium salts of succinic acid in concentrated aqueous solution gives ethylene at the anode.

8.

The reactivity of hydrogen halides towards an alkene follows the order HI > HBr > HCl, and reactivity of alkenes towards hydrogen halides (HX) increases as the number of carbon atoms in the alkene increases. Butene > Propene > Ethene

9.

The detection of unsaturation in an organic compound can be carried out by adding alkaline KMnO4 solution, the pink colour of which is decolourised in the presence of unsaturation. 1% alkaline KMnO4 solution is known as Baeyer's regent.

10.

Reaction which takes place by addition usually proceed much more readily than those which require replacement of one atom by another as in displacement reactions.

11.

The outstanding chemical property of an olefin is its ability to undergo addition reactions and possible displacement reactions are of minor importance. In fact, they are generally called side reactions. Olefinic double bonds behave as nucleophilic substances in their addition reactions. They combine readily with electrophilic reagents such as strong acids (H+), halogens and oxidising agent. They fail to combine with other nucleophilic reagents such as base and Grignard reagents. The addition usually takes place stepwise in which an electrophilic agent initiates the reaction by sharing the π electrons to form a new bond. Hydrogenation of alkenes is the basis of an analytical method for the determination of double bonds. In petroleum industry the process is reversed by using heat (cracking) so as to produce olefins from saturated substances.

12.

The net effect of oxidation of an Olefin with KMnO4 involves (a) Cleavage of the molecule at the double bond with the appearance of two C = O groups (b) If there is any hydrogen attached to either of the initially doubly carbon atoms, it is oxidised to -OH (c) If only carbon - to - carbon bonds are present, these remain unaffected.

13.

The relative stabilities of alkenes are related to their heats of hydrogenations. The amount of heat evolved when one mole of an unsaturated compound is hydrogenated is called heat of hydrogenation. Almost every alkene has the heat of hydrogenation of about 125 kJ mol-1 for each double bond present in one mole of the alkene. For example, heat of hydrogenations of unsaturated alkenes, CH2 = CH2, RCH=CH2 RCH=CHR or R2C=CH2 and R2C=CHR are about 134, 125, 117 and 113 kJ mol-1 respectively. 1-butene, cis - 2- butene and trans -2- butene yield the same product, n-butane on hydrogenation, but they have different heats of hydrogenations. So these alkene are expected to have different energies and hence different stabilities. An alkene having lower heat of hydrogenation must have less energy and greater stability than its isomer. In general, lower the heat of hydrogenation of an alkene, greater is its stability. On this basis, the

stabilities of some alkene follow the order the order: 2-methyl but-2-ene > Trans - but - ene > 2methyl but-1-ene > cis - but-2-ene > propene > but-1- ene > ethene. The heats of hydrogenation of the above alkenes are 112.1 115.5, 119.2, 119.6, 125.9, 126.8 and 137.2 kJ mol -1 respectively. The greater the number of alkyl groups attached to the doubly bonded carbon atoms, the more stable is the alkene. In general, alkenes follows the following decreasing order of stability. R2C = CR′2 > R2C = CHR′ > R2C = CH2, RCH = CHR′ > RCH = CH2 > CH2 = CH2 This order can be explained in terms of Hyperconjugation. The greater the degree of substation in an alkene, the number of hyperconjugative forms and greater is the stability. 14.

The acidic nature of H-atom present at the end of the triple bonded carbon atom is due to the higher electronegativity of the sp- hybridised carbon. Acidic hydrogen is present in 1- alkynes, but not in 2- alkynes and hence 1-alkynes can be distinguished from 2- alkynes by the reaction of acidic hydrogen in 1- alkynes. Acetylene has unpleasant garlic odour due to the presence of minute amounts of PH3, H2S etc. in it. Acetylene is transported by dissolved in acetone because it becomes explosive above 2 atmosphere pressures. Electrolysis of an aqueous solution of sodium of potassium salt of maleic acid or fumaric acid produces acetylene at the anode. CaC 2 reacts with water to form acetylene.

15.

Acetylene in aqueous solution are oxidised by permanganate to yield cleavage products. The reaction is similar to that shown by double bonds with KMnO4 except that when acetylenes are oxidised the linkage always formed is

ELECTROPHILIC AROMATIC SUBSTITUTION 1.

Cyclic conjugation is a necessary requirement for aromaticity but this alone is not sufficient. An additional requirement for aromaticity is that the number of π electrons in conjugated, planar, monocyclic species must be equal to 4n+2, where n is an integer. This is called Huckel's rule.

2.

Benzene, with six π electrons, satisfies Huckel's rule for n =1, cyclobutadine and planar cyclooctatetraene do not. Both are examples of system with 4nπ electrons and are antiaromic.

3.

Aromatic species include certain ions, eg cyclopentadiende anion and cycloheptatrienyl cation.

4.

Heterocyclic aromatic compounds are compounds that contain at least one atom other than carbon with in an aromatic ring.

5.

Huckel's rule can be extended to heterocyclic aromatic compounds unshared electron pairs (which are conjugated) of the heteroatom may be used as π electrons as necessary to satisfy the 4n+2 rule.

6.

On reaction with electrophiles i.e. electron deficient reagents, aromatic compounds undergo electrophilic aromatic substitution.

7.

The mechanism of electrophilic aromatic substitution involves two stages: attack of the electrophilie on the π electrons of the ring (generally slow, rate determining) followed by loss of a proton to restore aromaticity of the ring.

8.

Substituents on an aromatic ring can influence both the rate and regioselectivity of electrophilic aromatic substation. Substituents are classified as activating or deactivating according to whether they cause the ring to react more rapidly or less rapidly than benzene. With respect to regio selectivity substituents are either ortho, para directing or meta directing.

9.

An electron releasing substituent stabilises σ complex corresponding to ortho and para attack more than meta

11.

Conversely, an electron withdrawing substituent destabilises the σ complex corresponding to ortho and para attack more than meta. Thus metas substitution predominates.

12.

So generally activating groups are ortho, para directing and deactivating groups are meta directing. Halogens are very deactivating but ortho para orienting.

13.

When two or more substituents are present on a ring the regio selectivity of electrophilic aromatic substitution is generally controlled by the direction effect of the more powerful activating substituent.

14.

When both groups are ortho / para directing The directive power each group is generally in the following order O- > -NH2 > -NR2 > -OH > -OMe-NHAc > -Me > -X

15.

When both groups are deactivating and meta directing then it is difficult to introduce a third group and the directive power of each group is generally in the following order Me3N+ > -NO2 > -CN > -SO3H > -CHO > -COMe > -CO2H

16.

When the two groups direct differently, ortho para directing groups take precedence over meta directing groups.

17.

The active electrophilie in the nitration of benzene and its derivatives is nitronium ion

.

It is generated by reaction of nitric acid and sulphuric acid

Here nitric acid acts a base Very reactive arenes - those that bear strongly activating substituents undergo nitration in nitric acid alone. 18.

For sulphonation reagent used is fuming sulphuric acid or concentrated sulphuric acid. In either reaction the electrophilie appears to be SO3.

19.

Chlorination and bromination of arenes are carried out by treatment with the appropriate halogen in the presence of a Lewis catalyst. Very active arenes undergo halogenation in the absence of catalyst. Iodination should be performed in the presence of an oxidising agent. eq. nitric acid mercuric oxide etc.

20.

Hypohalous acids in presence of acid are also used as a reagent for halogenation.

21.

Carbocations usually generated form an alkyl halide and AlCl3 act as electrophilie in Feediel-Crafts alkylation. The arene must be at least as reactive as a halo benzene. Carbocation rearrangements can occur especially with primary alkyl halides.

22.

In Friedel - Crafts acylation acyl cation (acylium ions) generated by treating an acyl chloride or acid anhydride with AlCl3 attack aromatic ring to yield aromatic ketones.

23.

Vinylic halides and aryl halides do not form carbocation under conditions of the Friedel - Crafts reaction and so can not be used in place of an alkyl halide.

24.

It is sometimes difficult to limit Friedel Crafts alkylation to mono alkylation. The first alkyl group that goes on makes the ring more reactive toward further substitution because alkyl group are activating substituents. Mono-acylation is possible because the first acyl group to go on is electron withdrawing and deactivates the ring toward further substitution.

25.

The order in which substituents are introduced into a benzene ring needs to be considered in order to prepare the desired isomer in a multi step synthesis.

26.

The problem of assigning position of substituents in disubstituted and higher substituted derivative of benzene is known as orientation. One of the methods is Korner's absolute method. The method is based on the principle that the introduction of a third substituent in the para isomer gives one

trisubstituted product, the ortho isomer gives two and the m-isomer gives three trisubstituted products. 26.

Benzylic hydrogen are reactive under free radical conditions because of resonance stabilisation of derived benzyl radical. For this reason benzylic carbon undergo relatively facile halogenation and oxidation, including side chain oxidation to the corresponding benzoic acid.

27.

Polycyclic aromatic hydrocarbon, examples of which include naphthalene, anthracene and phenanthrene are compounds with two or more fused rings with total of 4n + 2 π electrons.

28.

The α position of naphthalene is more reactive toward EAS than the β position because the σ complex obtained due to electrophilic attack to α carbon is more stabilized by resonance.

29.

Pyridine is less reactive than benzene towards EAS (reacting at the third position) due to the -I effect of the nitrogen atom.

30.

Pyrrole is more reactive than benzene towards EAS and reacts at the 2 position. This is a consequence of the fact that the non-bonding pair on nitrogen is part of the aromatic sextet.

ALKYL AND ARYL HALIDE Preparation 1.

From Alcohols (Replacement of OH by X)

2.

Halogenation of Hydrocarbons i.e.

3.

Addition of Hydrogen Halides to Alkenes

4.

Addition of Halogens to Alkenes and Alkynes

5.

Halide Exchange R-X + I-

RI + X-

Nucleophilic Substitution

The order of reactivity is RI>RBr>RCl>RF. RX + –OH → ROH + X–

Alcohol

RX + H2O → ROH

Alcohol

–

RX + OR' → R OR'

Ether

(Williamson synthesis)

RX + –C ≡ CR' → R – C ≡ CR' Alkyne RX + I– → RI

Alkyl iodide

RX + –CN → RCN

Nitrile

RX + R'COO– → R′ –

– OR Ester

RX + :NH3 → RNH2

Primary amine

RX + :NH2R' → RNHR'

Secondary amine

RX + :NH R'R" → RNR'R''

Tertiary amine

RX + SH- → RSH

Thiol (mercaptan)

RX + :SR' → RSR'

Thioether (sulfide)

RX + ArH + AlCl3 → Ar R

Alkyl benzene

(Friedel Craft reaction)

Nucleophilic Displacement by SN1 and SN2 Mechanisms SN2 Reaction 1.

Mechanism

Characteristic of SN2 reaction i)

Reaction is biomolecular

ii) R ∝ [Substrate] [Nucleophile] iii) Product formation takes place by (TS) iv) Reaction is favourable in the presence of polar aprotic solvent such as acetone, DME, DMSO which favours transition state. v) Reactioni s given mainly by primary and secondary alkyl halides in which β−carbon is either primary or secondary. vi) Reactivity in decreasing order is CH3X > p−alkyl halide > secondary alkyl halide 2.

Kinetics: The reaction between methyl bromide and hydroxide ion to yield methanol follows second order kinetics; that is, the rate depends upon the concentrations of both reactants : CH3Br +-OH → CH3OH + Brrate = K [CH3Br] [OH–]

3.

Stereochemistry: A reaction that yields a product whose configuration is opposite to that of the reactant is said to proceed with inversion of configuration.

inversion in configuration

4.

Reactivity: In SN2 reactions the order of reactivity of RX is CH3X>1o>2o>3o.

SN1 Reaction Mechanism and Kinetics The reaction between tert-butyl bromide and hydroxide ion to yield tert-butyl alcohol follows first order kinetics; i.e., the rate depends upon the concentration of only one reactant, tert-butyl bromide.

Rate = K[RBr] SN1 reaction ⇒ follows first order kinetics. Nucleophilic Displacement By SN1 And SN2 Mechanisms S N1 Steps

Two :

(1) R:X

+

R +X

-

Carbonium ion

S N2 One : R:X + Nu- → RNu + Xor R:X + Nu → RNu+ X-

(2) R+ + NuRNu or R+ + :Nu → RNu+ Rate

=K [RX] (1st order)

=K[RX] [:Nu-] (2nd order)

TS of slow step Stereochemistry

Inversion and racemization

Inversion (backside attack)

Molecularity

Unimolecular

Bimolecular

3o> 2o> 1o> CH3 Stability of R+

CH3> 1o> 2o> 3o Steric hindrance in R group

RI> RBr> RCl> RF Rate increases in polar solvent

RI> RBr> RCl> RF with Nu- there is a large rate increase in polar aprotic solvents.

Reactivity structure of R Determining factor Nature of X Solvent effect on rate Effect of nucleophile

Rate depends on nucleophilicity I- > Br- > Cl- ; RS- > RO-

Catalysis

Lewis acid, eg. Ag+, AlCl3, ZnCl2

None

Competition reaction

Elimination, rearrangement

Elimination

Stereochemistry When (-)-2-bromo octane is converted into the alcohol under conditions where first-order kinetics are followed, partial racemization is observed. The optically active bromide ionizes to form bromide ion and the flat carbocation. The nucleophilic reagent then attaches itself to carbonium ion from either face of the flat ion. If the attack were purely random, we would expect equal amounts of two isomers; i.e. we would expect only the racemic modification. But the product is not completely racemized, for the inverted product exceeds its enantiomer. We can say in contrast to SN2 reaction, which proceeds with complete inversion; an SN1 reaction proceeds with racemization though may not be complete.

r.d.s ⇒ formation of carbonium ion. Reactivity of an alkyl halide depends chiefly upon how stable a carbonium ion it can form. In S N1 reactions the order of reactivity of alkyl halides is Allyl,benzyl>3o>2o>1o>CH3 X.

AMINES AND AMINO ACIDS 1.

Amines are alkyl derivatives of NH3 and are said to be of (3−n)0 where n = number of H atoms at N. RNH2 (1°), R2NH (2°), R3N(3°)

2.

–NH2, – NH – groups are called amino, imino groups respectively.

3.

4.

Gabriel−phthalimide synthesis is used to prepare 1° amine from R– X and NH3 (by blocking two active H of NH3) so as to prepare pure 1° amine.

5.

Amino, imino or nitrile groups are ring activating and hence o – p orienting in nature.

6.

Basic nature of amines (in aqueous medium) Me3N is less basic than 1° and 2° amines due to less degree of hydration of the protonated species i.e. the conjugate acid of 3° amine.

7.

Mixture of 1°, 2°
and
3°
amines
can
be
separately
−
Hindsbergs
method
using
C6H5SO2Cl
(Hinsberg’s
 reagent).






Primary
amines
give
N−alkyl
benzene
sulphonamide
when
treated
with
C6H4SO2Cl,
which
is
soluble
 in
aqueous
KOH
solution.









 



 Secondary
 amines:
 Secondary
 amines
 form
 N,
 N−dialkyl
 benzene
 sulphonamide
 with
 C6H5SO2Cl
 which
do
not
form
any
salt
with
KOH
and
is
insoluble
in
alkali
solution











Tertiary
amine:
Tertiary
amines
do
not
react
with
C6H5SO2Cl.


8.


Primary
amines
give
carbyl
amine
reaction..











This
is
used
to
distinguish
between
amine
from
2°
or
3°
amine.


9.


Primary
 aliphatic
 amine
 gives
 off
 nitrogen
 gas
 when
 treated
 with
 nitrous
 acid

 (NaNO2
+
HCl)



 



 Aromatic
primary
amines
form
diazonium
chloride.



 



 The
 aryl
 diazonium
 ion
 is
 stabilized
 by
 the
 π−electron
 cloud
 of
 the
 aryl
 ring
 system
 whereas
 alkyl
 diazonium
 ion
 being
 unstable
 breaks
 down
 to
 give
 carbocationic
 species
 capable
 of
 rearrangement
 together
 with
 evolution
 of
 N2
 gas.
 Evolution
 of
 N2
 by
 the
 action
 of
 HNO2
 upon
 aliphatic
 1°−amine
 is
 used
 as
 a
 chemical
 test
 for
 the
 identification
 of
 1°
 amine.
 When
 ice
 cold
 solution
 of
 β−naphthol
 is
 added
in
above
solution
then
formation
of
orange
red
or
reddish
blue
dye
formation
takes
place.



 10.
 


2°
Amines
form
yellow
oily
liquid
or
white
precipitate.
 Libermann
nitrosoamine
reaction



 10.



 3°
amine
is
dissolved
in
HNO2
forming
[R3NH]+NO2–


11.


Amine
 acids
 (





 exist
 solely
 as
 Zwitter
 ion
 (α
 dipolar
 ion)
 due
 to
 internal


neutralisation.
 






 
 


(A)
Zwitter
ion
or
dipolar
ion
 
 A
is
cation
 in
acidic
reduced
(pH
7).





The
 hydrogen
 ion
 concentration
 of
 the
 solution
 in
 which
 a
 particular
 amino
 acid
 does
 not
 migrate
 under
the
influence
of
an
electric
field
is
called
the
isoelectric
point.
The
pH
of
solution
of
an
amino
 acid
at
which
its
iso−electric
point
occurs
is
equal
to
the
mean
of
the
two
pka
values
of
its
conjugate
 acid
i.e.



 





12.



 13.



 14.



 15.


The
reaction
of
amines
with
excess
of
alkyl
halide
to
form
a
quarternary
ammonium
salt
is
know
as
 exhaustive
alkylation
and
if
the
alkyl
group
is
methyl
then
it
is
known
as
exhaustive
methylation.
 The
 exhaustive
 methylation
 of
 amine
 is
 followed
 by
 treatment
 with
 moist
 silver
 oxide
 (Ag2O
 +
 H2O
 →
 2AgOH)
 and
 subsequent
 heating
 to
 form

 alkene
is
known
as
Hoffmann
elimination



 



 Hofmann
 elimination
 unlike
 Saytzeff
 elimination
 always
 favours
 the
 formation
 of
 less
 substituted
 alkene
which
is
due
to
some
carbanionic
nature
of
the
TS
formed.






 








ALDEHYDE AND KETONES Preparation
of
Aldehydes
 1.


From
1°
alcohols



 2.



 From
reduction
of
acid
chlorides











R
−
may
be
alkyl
or
aryl
group


3.


From
oxidation




 4.



 Reimer
Tiemann
Reaction



 5.



 Hydroboration
oxidation








Preparation
of
Ketones
 1.





From
oxidation
of
secondary
alcohol





2.



 


Unsaturated
2°
alcohol
can
be
oxidised
to
ketone
without
affecting
double
bond,


3.


Friedel
Crafts
acylation



 4.



 Reaction
with
acid
chlorides
with
organo
copper
compounds.








Reactions
of
Aldehydes
and
Ketones
 1.


Addition
of
Cyanide



 2.



 Addition
of
derivatives
of
ammonia.






 H2N
–
G


Product















 



 
 3.


Oxidation





a)
 Aldehydes











b)
 Methyl
Ketones








4.


Reduction















 5.



 Addition
of
Grignard
reagents











By
this
method
1°,
2°
and
3°
alcohol
can
be
prepared.


6.


Halogenation
of
Ketone











X2
=
Cl2,
Br2,
I2


7.


Addition
of
alcohols



 8.



 Cannizaro’s
reaction











In
 cross
 a
 cannizaro
 reaction
 of
 formaldehyde,
 it
 is
 always
 oxidised
 to
 acid
 and
 other
 is
 reduced
 to
 alcohol.


9.


Aldol
condensation











An
aldol
goes
easy
dehydration
to
α−β
unsaturated
aldehyde
or
ketone.


10.


Wittig
reaction