organic compounds with functional groups containing...
TRANSCRIPT
1
Organic Compounds with Functional GroupsContaining Nitrogen (AMINES)
SYLLABUS
1. Structure of amino groups (primary, secondary and tertiary)
2. Nomenclature
3. Important methods of preparation
4. Physical properties – Basic character of amines
5. Chemical reactions
6. Separation of primary, secondary and tertiary amines
7. Some commercially important compounds
1 General Introduction
Amines may be regarded as amino derivatives of hydrocarbons. They may also be regarded as derivatives ofammonia in which one or more H-atoms have been replaced by alkyl or aryl groups. Depending upon whetherone, two or all the three H-atoms of ammonia have been replaced by alkyl or aryl groups, amines are classified asprimary (1°), secondary (2°) and tertiary (3°) respectively.
H|
R N HPrimary amine(1 Amine)
− −
°
H|
R N RSecondary amine(2 Amine)
− −
°
R|
R N RTertiary amine
(3 Amine)
− −
°
(where R may be alkyl or aryl group)
Thus, the characteristic or the functional group for primary, secondary and tertiary amines are – NH2 (amino) >NH (imino) and > N – (tertiary nitrogen atom) respectively.
Amines are further divided into two categories :
1. Aliphatic amines: Amines in which the nitrogen atom is directly linked to one, two or three (same or different)alkyl groups are called aliphatic amines. These may be primary, secondary or tertiary. For example,
2 5 2C H NHEthylamine (1 )° C H2 5
C H2 5
NH
Diethylamine (2°)
2 5
2 5 2 5
C H|
C H N C HTriethylamine (3°)
− −
2. Aromatic amines: These are of two types :
(a) Arylamines or nucleus substituted amines: Amines in which the nitrogen atom is directly linked to one,two or three (same or different) aromatic rings or aryl groups are called arylamines. Like aliphatic amines,arylamines can also be primary, secondary or tertiary. For example,
NH2
Aniline (1°)
NH
Diphenylamine (2°)
6 5 3(C H ) NTriphenylamine(3°)
2
(b) Aralkylamines or side chain substituted amines : Amines in which the nitrogen atom is attached to theside chain/s of one, two or three (same or different) aromatic rings are called aralkylamines. These can alsobe primary, secondary and tertiary. For example,
CH NH22
Benzylamine (1°)
6 5 2 2(C H CH ) NHDibenzylamine(2°) 6 5 2 3(C H CH ) N
Tribenzylamine(3°)
Simple and mixed amines: Secondary and tertiary amines are further classified as simple and mixed aminesrespectively according as all the alkyl or aryl groups attached to the nitrogen atom are same or at least two aredifferent.
Simple amines :
3 2(CH ) NHDimethylamine( )Secondary
3 2 3(CH CH ) NTriethylamine
( )Tertiary
6 5 2(C H ) NHDiphenylamine( )Secondary
6 5 3(C H ) NTriphenylamine
( )Tertiary
Mixed amines :
CH3
CH3CH2
NH
Ethylmethylamine(Secondary)
CH3
CH2CH3
Ethylmethyl –n – propylamine
(Tertiary)
CH CH CH–N3 2 2
Quaternary ammonium compounds : Apart from three types of amines, there is another class of compoundscalled quaternary ammonium compounds . These compounds may be regarded as derivatives of ammoniumsalts in which all the four H-atoms of the ammonium ion have been replaced by alkyl or aryl groups. For example,
3 4[(CH ) N] ITetramethylammoniumiodide
+ − 3 2 4[(CH CH ) N] OH
Tetraethylammoniumhydroxide+ −
6 5 3 3C H N(CH ) BrTrimethylphenylammoniumbromide
+−
2 Electronic Structure of the Amino Group
The ground state of N-atom has five electrons in the valence shell and thus needs three more electrons to completeits octet. These three electrons are acquired by mutual sharing of electrons with monovalent atoms/groups. In 1°amines, N-atom shares its three electrons, one each with the carbon atom of the alkyl (or aryl) group and two H-atoms; in 2° amines with carbon atoms of two alkyl (or aryl) groups and one hydrogen while in 3° amines withcarbon atoms of three alkyl (or aryl) groups. In all these, the remaining two electrons are present as a lone pair ofelectrons. Thus, the electronic structures of 1°, 2° and 3° amines are shown below
or or or
In terms of orbitals, the ground state electronic configuration of N is
3
1 s2 2 s2 2 1zp 2 1
yp 2 1zp . The 2 s-orbital and three 2 p-orbitals hybridize to form four sp3-hybridized orbitals; three
of these orbitals contain one electron each while the fourth contains a lone pair of electrons. In 1° amines, one ofthe three half filled sp3-orbitals overlaps with sp3-hybridized orbital of the carbon atom of the alkyl group (or sp2-hybridized orbital of the carbon atom of the alkyl group) and the remaining two overlap with s-orbitals of hydrogenatoms thereby forming one C – N and two N – H σ-bonds.
In 2° amines, two of the three sp3-hybridized orbitals of N overlap with sp3-hybridized orbitals of the carbon oftwo alkyl groups (or sp2-hybridized orbitals of carbon atoms of the aryl groups) and one with s-orbital of the H-atom thereby forming two C–N and one N–H, o-bonds.
In 3° amines, all the three sp3-hybridized orbitals of N overlap with sp3-hybridized orbitals of carbon atoms ofthree alkyl groups (or sp2-hybridized orbitals of the carbon atoms of the aryl groups). In all these three amines, thefourth sp3-orbital contains the lone pair of electrons as shown in Fig. 15.2.1.
Since lone-pair-lone pair repulsions are much greater than bond pair-bond pair repulsions, therefore, the bondangle between any two adjacent H-atoms or alkyl groups decreases from the tetrahedral angle of 109 28'⋅ to 107°in 1° and 2° amines. However, in case of 3° amines, due to steric hindrance between the three bulky alkyl groups,the bond angle increases from 107° in ammonia to 108° in trimethylamine. Thus, all the three amines (1°, 2° or3°) like NH3 have pyramidal shape.
N
sp3-ORBITAL
LONE PAIR
1° Amine
RH H
3 Nomenclature
1. Aliphatic amines: In the common system, aliphatic amines are named by the following two methods.
(i) According to one method, aliphatic amines are called alkylamines. The common names of aliphatic primaryamines are then obtained by adding the suffix amine to the name of the alkyl group attached to the nitrogenatom. In case of mixed secondary and tertiary amines, the names of the alkyl groups are arranged in alphabeticalorder and the suffix amine is then added. However for simple secondary and tertiary amines respectively,the prefixes di and tri are used before the name of the alkyl group.
Similarly, the names of aralkylamines are derived by adding the suffix amine to the name of the aralkylgroup (or groups arranged in alphabetical order).
(ii) According to the second system, aliphatic amines are called aminoalkanes. In this system, the primaryamines are named by adding the prefix amino to the name of the parent alkane corresponding to the longestpossible straight chain. The position of the amino group and that of the substituents, if any are indicated bythe arabic numerals with the carbon atom bearing the amino group getting the lowest possible number.
Secondary and tertiary amines are named respectively as N-alkylaminoalkanes and N-alkyl N-alkylaminoalkanes (or N, N-dialkylaminoalkanes in case the two substituent alkyl groups are the same).The largest alkyl group forms a part of the aminoalkane while the smaller alkyl groups are considered assubstituents. The prefixes N-and N, N-simply mean that the alkyl groups are attached to the nitrogen atomrather than to a carbon atom.
In case of aralkaylamines, the position of the aryl group is indicated by a suitable number.
In the IUPAC system, aliphatic amines are called alkanamines. These names are obtained by removingthe final 'e' from the name of the corresponding alkane and adding the suffix amine.
It may be noted here that regardless of the system of nomenclature, the complete name of an amine isalways written as one word.
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The common and IUPAC names of some aliphatic amines are given below :
AMINE COMMON NAME IUPAC NAME
Primary amines :
3 2CH NH Methylamine or Aminomethane Methanamine
3 2 2CH CH NH Ethylamine or Aminoethane Ethanamine
3 2 1
3 2 2 2C H C H C H NH n-Propylamine or 1-Aminopropane 1-Propanamine or
Propan-1-amine
3 2 1
3 2 3
2
CH C H C H|
NH
− − Isopropylamine or 2-Aminopropane Propan-2-amine
Secondary amines :
CH3 – NH – CH3 Dimethylamine or N-Methylmethananmine
N-MethylaminomethaneCH3CH2 – NH – CH3 Ethylmethylamine or N-Methylethananmine
N-Methylaminoethane
Tertiary amines :
3
3 3
CH|
CH N CH− − Trimethylamine or N, N Dimethylmethanamine
N, N-Dimethylaminomethane
2 3
3 2 3
CH CH|
CH CH N CH− −Deithylmethylamine or N-Ethyl-N-methylethanamine
N-Ethyl-N-methylaminoethane
Aralkylamines :
CH2NH2 Benzylamine or Phenylaminomethane Phenylmethanamine
CH2CH N H2 2β
2 1β-Phenylethylamine or 2-Phenylethanamine
2-Phenylaminoethane
2. Aromatic amines. In the common system, aromatic amines are called arylamines. The name of an individualamine is then obtained by adding the suffix amine to the name of the aryl group.
More often they are named as derivatives of the simplest aromatic amine called aniline.
In the IUPAC system, the simplest aromatic amine is called benzenamine and other amines are named asderivatives of this amine.
It may be noted here again that the complete name of an amine is always written as one word .
The common and IUPAC names of some arylamines are given below :
5
NH2
Anilineor Benzenamine
NHCH3
N-Methylanilineor N-Methylbenzenamine
CH3 – N – CH3
N, N-Dimethylanilineor N, N-Dimethylbenzenamine
NH2
CH3
1
2
o-Toluidineor 2-Methylaniline
or 2-Methylbenzenamine
NH2
CH3
1
3m-Toluidine
or 3-Methylanilineor 3-Methylbenzenamine
NH2
CH3
1
3
p-Toluidineor 4-Methylaniline
or 4-Methylbenzenamine
2
4
NH2
CH3
1
3
2, 4-Dimethylanilineor 2, 4-Dimethylbenzenamine
2
4
CH3
NH2
Br1
3
2, 4, 6-Tribromoanilineor 2, 4, 6-Tribromobenzenamine
2
4
Br
Br
5
6
NH
N-Phenylanilineor N-Phenylbenzenamine
For sake of further illustration, the common and IUPAC names of some complex aliphatic and aromatic aminesare given below :
3
23 2 3
2
CH|35 4 1
CH CH CH CH CH|NH
− − − −
3
3 2 3
1 3
CH| 2 43
CH N CH CH CH|
CH2-(N, N-Dimethylamino)butane2-(N,N-Dimethyl)butanamine
− − − −
Common: 2–Methyl–3–aminopentanes
IUPAC : 2–Methyl–3–pentanamine
3 2 3
13 3
2 3 4 5CH NH CH CH CH CH
| |CH CH
: 2 (N Methylamino) 4 methylpentane:N,4 Dimethylpentan 2 amine
− − − − −
− − − −− − −
CommonIUPAC
CH3 – N – CH CH2 3
Common : N–Ethyl–N–methylaniline
IUPAC : N–Ethyl–N–methylbenzenamine
CH3 – CH – NH2
2 1
Common : 1-Phenylaminoethane
IUPAC : 1-Phenylethananmine
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In case of polyfunctional compounds, the amino group is considered to be a substituent. For example,
2 2 2
2 1H N CH CH OH2 Aminoethanol
− −−
4 3 2 12 2 2 2NH – C H C H C H COOH
4 Aminobutanoicacid−
−
Since amines are better known by their common names, therefore, in this unit, we shall be using common namesmore frequently though here and there some IUPAC names have also been given.
4 Isomerism in Amines
Amines show the following four types of isomerism:
1. Chain isomerism: Aliphatic amines containing four or more carbon atoms show chain isomerism. For example,
3 2 2 2 2CH CH CH CH NHButan–1–amine− − − −
3
3 2 2
CH|
CH CH CH NH2 Methylpropan 1 amine
− − −− − −
3
3 2
3
CH|
CH C NH|
CH2 Methylpropan 2 amine
− −
− − −
2. Metamerism. Aliphatic amines having the same molecular formula but different alkyl groups on either sideof the nitrogen atom show metamerism. For example,
3 2 2 3CH CH NH CH CHDiethylamine
− − and 3 2 2 3CH –NH CH CH CHMethyl propylamine
−−n
or 3 3 2CH NH CH(CH )Isopropylmethylamine
− −
are metamers.
3. Position isomerism: Aliphatic amines containing three or more carbon atoms show position isomerism dueto difference in position of the amino group. For example,
(i) 3 2 2 2CH CH CH NHPropan 1 amine
−− −
and 3 3
2
CH CH CH|
NH
− −
Propan-2-amine
(ii) 3 2 2 2 2CH CH CH CH NHButan 1 amine
−− −
and 3 2 3
2
CH CH CH CH|NH
Butan 2 amine
− −
− −
Similarly aromatic amines also show position isomerism. For example,
CH3
NH2
o-Toluidine
CH3
NH2
m-Toluidine
H C3 NH2
p-Toluidine
4. Functional isomerism: Primary, secondary and tertiary amines having the same molecular formula showfunctional isomerism among themselves. For example,
3 2 2 2CH CH CH NHPropylamine(1 )
−− °n
3 2 3CH CH NH CHEthylmethylamine (2 )
− −°
3 3(CH ) NTrimethylamine (3 )°
are functional isomers.
7
5 Methods of Preparation
Amines are generally prepared by the following methods:
1. From alkyl halides and ammonia or amines (Hofmann's ammonolysis Method). Amines are produced when analcoholic solution of ammonia ( or an amine) and an alkyl halide is heated in a sealed tube at 373 K. Thisreaction is called ammonolysis and usually gives a mixture of primary, secondary and tertiary amines along withsome quaternary ammonium salts as illustrated below :
(i) `+
–3RNH X
1°Aminesalt
+2 4RNH + NH X
1 Amine−
°
(ii) + –
2 2R NH X2°Aminesalt
+2 4R NH + NH X
2 Amine−
°
(iii)
R N H X 3
+ –
3° Amine salt
R N + 3
3° Amine
NH3 + -4NH X
(iv)
This reaction is a typical example of a nucleophilic substitution reaction in which ammonia (or the amine)acts as a nucleophile due to the presence of a lone pair of electrons on the nitrogen atom. The ammonia moleculefirst attacks an alkyl halide molecule to form a primary amine salt which then further reacts with ammonia to givethe corresponding free primary amine and the ammonium halide (reaction i).
The primary amine thus formed being almost as nucleophilic and basic as ammonia attacks another moleculeof alkyl halide to form a secondary amine (reaction ii) which, in turn, reacts with another molecule of alkyl halideto form a tertiary amine (reaction iii) and finally quaternary ammonium salt (reaction iv).
The actual composition of the reaction mixture depends upon the ratio of the alkyl halide and ammonia used.IF excess of alcoholic ammonia is used, primary amine is the main product. However, the mixtures obtained arevery complex and difficult to separate. Therefore, this method is used in the manufacture of amines and not as alaboratory preparation.
Limitation: This method cannot be used for the preparation of arylamines since arylhalides are much lessreactive than alkyl halides towards nucleophilic substitution reactions.
2. Reduction of nitro compounds: Both aliphatic and aromatic primary amines can be easily prepared by thereduction of the corresponding nitro compounds. This reduction can be achieved in a number of ways as discussedbelow :
(i) Catalytically with hydrogen in the presence of Raney Ni, Pt or Pd as catalyst at room temperature.
R – NO2 or Ar – NO2 + 3H2 R – NH2 or Ar – NH2 + 2 H2O
8
For example,
3 2 2 2CH CH NO 3HNitroethane
− + 3 2 2 2CH CH NH 2H O
Ethylamine− +
6 5 2 2C H NO 3HNitrobenzene
− + 6 5 2 2C H NH 2H O
Aniline− +
(ii) With an active metal such as, Fe, Sn, Zn etc. and conc. hydrochloric acid. For example,
3 2 2CH CH NO 6[H]Nitroethane
− + Sn/HCl→ 3 2 2 2CH CH NH 2H OEthylamine
− +
+ 6 [H] Fe/HCl
orSn/HCl→
A mixture of SnCl2 and conc. HCl has also been used for the reduction of aromatic nitro compounds.
The reduction of nitro compounds to primary amines is one of the most convenient methods for thepreparation of aromatic primary amines since they cannot be prepared from the corresponding aryl halideson treatment with ammonia. The required nitro compounds can be easily prepared by the nitration of arenes.
(iii) With lithium aluminium hydride (LiAlH4)
3. By the reduction of nitrilies (or cyanides) and isonitriles (isocyanides) : Aliphatic and aralkyl primary aminescan be easily prepared by the reduction of the corresponding nitriles either catalytically with H2 and Raney nickelor chemically with lithium aluminium hydride or sodium and alcohol (reduction with sodium and alcohol is calledMendius reduction).
R – C NAlkyl nitrile
≡or
Ar – C N Aryl nitrile
≡ 2 2 2 2RCH NH or ArCH NH
1Amine°
For example,
3CH C NAcetonitrile
≡ 3 2 2CH CH NH
Ethylamine−
9
Reduction of isocyanides gives secondary amines i.e., N-methylamines.
R – N = C + 2H or 4 [H]2
Alkyl isocyanide 3R – NH – CHN–Methylalkylamine (2 )°
e.g., 3 2 3CH CH NH – CH Ethylmethylamine
−
6 5 3C H NH – CHMethylphenylamineorN–Methylaniline
−
Limitation : This method cab be used only for the preparation of 2° amines in which one of the groups is alwaysmethyl.
Synthetic importance : Since alkyl cyanides can be easily prepared by the action of alcoholic NaCN or KCN onalkyl halides, this gives us an excellent method for converting alkyl halides into primary amines having onecarbon atom more than the parent alkyl halides. Thus,
R XAlkylhalide
− R CN
Alkylcyanide−
2 2RCH NH1 Amine
−°
4. Reduction of amides : Primary, secondary and tertiary amines can be prepared by the reduction of thecorresponding amides with lithium aluminium hydride (LiAlH4).
2R CONH1 Amide−°
2 2R CH NH
1 Amine−°
e.g., 3 2CH CONHAcetamide
3 2 2CH CH NHEthylamine
6 5 2C H CONHBenzamide
6 5 2 2C H CH NHBenzylamine
Secondary and tertiary amines can be prepared by the reduction of secondary and tertiary amides respectively.
3 3CH CONHCHN Methylacetamide(2 )− ° 3 2 3CH CH NHCH
Ethylmethylamine(2 )°
CHCO – N3
CH3
CH3
N, N–Dimethylacetamide (3°)
5. Reduction of oximes : Primary amines can be prepared by the reduction of oximes of aldehydes and ketones witheither lithium aluminium hydride or with sodium and alcohol.
RCH NOH 4[H]Aldoxime
= + 2 2 2RCH NH H O
1 Amine− +
°
10
For example,
3CH CH = NOH Acetaldoxime 3 2 2CH CH – NH
Ethylamine
6. By Hofmann degardation of primary amides: (Hofmann bromamide reaction). When a primary amide is treatedwith an aqueous solution of potassium hydroxide and bromine (or potassium hypobromite, KOBr), it gives aprimary amine which has one carbon atom less than the original amide:
2 2R CONH Br 4KOH1 Amide− + + →
°2 2 3 2R NH K CO 2KBr 2H O
1 Amine− + + +
°
For example,
3 2 2CH CH CONH Br 4KOHPropionamide
+ + → 3 2 2 2 3 2CH CH NH K CO 2KBr 2H OEthylamine
− + + +
2+ Br 4KOH+ → + K2CO3 + 2KBr + 2H2O
This reaction is widely used for stepping down or descent of homologous series.
7. Gabriel phthalimide reaction : This is a very convenient method for the preparation of pure aliphatic andaralkyl primary amines. Potassium phthalimide (obtained by the action of ethanolic solution of KOH on phthalimide)on treatment with a suitable alkyl or aralkyl halide gives N-substituted phthalimides. These upon subsequenthydrolysis with dil. HCl under pressure or with alkali give primary amines.
H/H+
2O
or OH/H–
2O COOH
COOH+ C H2 5NH2
Phthalic acid
Ethylamine
Phthalic acid can again be converted into phthalimide and used over and over again.
Similarly benzylamine can be prepared by using benzyl chloride and glycine (NH2CH2COOH) by using chloroaceticacid in place of ethyl iodide in the above reaction.
Hydrazinolysis (Cleavage by hydrazine) of N-alkylphthalimides is a more convenient and effecient method ascompared to acidic/basic hydrolysis for obtaining 1° amines using Gabriel synthesis. For example,
2R NH1 Amine
− +°
11
8. By reductive amination of aldehydes and ketones: Aldehydes and ketones react with ammonia in presence of areducing agent such as H2/Raney Ni or NaBH3CN (sodium cyanoborohydride) to form primary amines. Thereaction, in fact, occurs through the intermediate formation of an imine. For example,
3R CH = O + NHAldehyde−
H O2
∆→−
R – C H NHAnimine
= 2 2R CH NH
1 Amine− −
°
3
R '|
R C O NHKetone− = + H O2
∆→−
R '|
R C NHAnimine
− =
2
R '|
R CH NH1 Amine− −°
e.g., 3 3CH CH O NHAcetaldehyde
− = + H O2
∆→−
3CH – C H NHAcetaldimine
= 3 2 2CH – C H NH
Ethylamine−
3
3 3
CH|
CH C O NHAcetone− = + H O2
∆→−
3
3
CH|
CH C NHDimethylketimine
− =
3
3 2
CH|
CH CH NHIsopropylamine
− −
This reaction leading to the conversion of an aldehyde or a ketone to the corresponding amine on treatment withammonia in presence of a reducing agent, i.e. H2/Ni or NaBH3CN is called reductive amination.
If instead of NH3, a primary amine is used in the above reaction, secondary amine is obtained. For example,
3
3 2 6 5
CH|
CH C O H NC HAcetone Aniline
− = + H O2
∆→−
3
3 6 5
CH|
CH C NC HIsopropylideneaniline
− =
3
3 6 5
CH|
CH –CH–NHC HN-Isopropylaniline
(2 )° Amine
CONCEPTUAL QUESTIONS
Q.1. tert-Butylamine cannot be prepared by the action of NH3 on tert-butyl bromide. Explain why ?
Ans. tert-Butyl bromide being a 3° alkyl halide on treatment with a base (i.e. NH3) prefers to undergo eliminationrather than substitution. Therefore, the product is isobutylene rather than tert-butylamine.
3
3
3
CH|
CH C Br|
CHButylbromidetert
− −
−
3
3 2
3
CH|
CH C NH|
CHButylaminetert
− −
−
2
3
3
CH H|
CH C Br|
CHButylbromidetert
−
− −
−
3
3 2
CH|
CH C CHIsobutylene
− =
Q.2. Can we prepare aniline by Gabriel-phthalimide reaction?
Ans. No. The preparation of aniline by Gabriel-phthalimide reaction requires the treatment of pot. phthalimde withC6H5Cl or C6H5Br. Since aryl halides do not undergo nucleophilic substitution under ordinary laboratory conditions,therefore, C6H5Cl or C6H5Br does not react with pot. phthalimide to give N-phenylphthalimide and hence anilinecannot be prepared by this method.
12
Q.3. Suggest chemical reactions for the following conversions :
(i) Cyclohexanol → Cyclohexylamine
(ii) n- Hexanenitrite → 1 - Aminopentane
Ans. (i)
(ii) 3 2 2 2 2CH CH CH CH CH C NHexanenitrilen
≡−
3 2 4NH3CH (CH ) COOH
Hexanoicacid→
3 2 4 4CH (CH ) COONH
H O2
∆→− 3 2 4 2CH (CH ) CONH
Hexanamide3 2 2 2 2 2CH CH CH CH CH NH1 Aminopentane−
6 Physical Properties :
Some important physical properties of amines are discussed below:
1. Solubility: All the three classes of aliphatic amines (1°, 2° and 3°) form H-bonds with water. As a result, loweraliphatic amines are soluble in water.
Methylamine and ethylamine are gases but they are highly soluble in water. That is why they are sold in themarket as their 34% aqueous solutions. However, as the size of the alkyl group increases (with increase in molecularmass), the solubility decreases due to a corresponding increase in the hydrocarbon part of the molecule. Theborderline solubility is reached with amines of about six carbon atoms in the molecule. However, amines are quitesoluble in organic solvents such as benzene, ether, alcohol etc.
Aromatic amines, on the other hand, are insoluble in water. This is due to the larger hydrocarbon part whichtends to retard the formation of H-bonds. Thus, aniline is almost insoluble in water. However, it is quite solublein benzene, ether, alcohol etc.
2. Boiling points: Amines have higher boiling points than hydrocarbons of comparable molecular masses. This isdue to the reason that amines being polar, form intermolecular H-bonds (except tertiary amines which do not havehydrogen atoms linked to the nitrogen atom) as shown below and thus exist as associated molecules.
(where R is any alkyl or aryl group)
13
Further, since the electronegativity of nitrogen is lower than that of oxygen, therefore, amines form weaker H-bonds as compared to alcohols and carboxylic acids. As a result, amines are not associated to the extent ofalcohols and carboxylic acids and hence the boiling points of amines are lower than those of alcohols and carboxylicacids of comparable molecular masses. For example, ethylamine (mol. mass 45) boils at 292 K while ethyl alcohol(mol. mass 46) and formic acid (mol. mass 46) boil at 351 K and 374 K respectively.
Compound Mol. mass b.p. (K)
C2H5NH2 45 292
C3H8 44 231
C2H5OH 46 351
HCOOH 46 374
3. Colour and odour: Pure amines are almost colourless but develop colour on keeping in air for long time. This isdue to the reason that amines, especially aromatic amines, are readily oxidised in air to form coloured oxidationproducts.
Lower aliphatic amines are gases and smell very much like ammonia but the higher amines are liquids withfishy odours. Lower aromatic amines are liquids having characteristic unpleasant odours but the higher ones arelow melting solids which are almost odourless.
CONCEPTUAL QUESTIONS
Q.1. Although trimethylamine and n-propylamine have the same molecular weight, the former boils at a lower temperature(276 K) than the latter (322 K). Explain.
Ans. n-Propylamine has two H-atoms on the N-atom and hence undergoes intermolecular H-bonding thereby raisingthe boiling point.
Trimethylamine, (CH3)3 N being a 3° amine does not have a H-atom on the N-atom. As a result, it does notundergo H-bonding and hence its boiling point is low.
4. Basic Character of Amines: All the three classes of amines (1°, 2° and 3°) because of the presence of a lone pairof electrons on the nitrogen atom behave as bases. Their basic character is shown by the following reactions.
(i) Reaction with water: Due to the presence of a lone air of electrons on the N-atom, amines like NH3 arestronger bases than H2O. Therefore, they react with water to form alkyl or arylammonium hydroxides whichionize to furnish hydroxyl ions:
2 2 3RNH H O R N H O H1 Amine Alkylamm.hydroxide
+−+
°��⇀↽��
3R N H OH+
−+��⇀↽��
(cf. NH3 + H2O 4NH OH��⇀↽�� 4NH OH )+ −+��⇀↽��
Due to the formation of hydroxide ions, the aqueous solutions of amines are basic in nature.
The presence of the hydroxide ions in the aqueous solutions of amines is detected by the precipitation ofmetal hydroxides such as aluminium hydroxide (gelatious white ppt.) or ferric hydroxide (brown ppt.) whenAlCl3 or FeCl3 is added to their aqueous solutions.
3 3AlCl 3OH Al(OH) 3ClGelatinouswhiteppt.
− −+ → +
3 3FeCl 3OH Fe(OH) 3ClBrownppt.
− −+ → +
14
(ii) Reaction with acids: Being basic, all types of amines react with mineral acids such as HCl, NHO3,H2SO4 etc. to form soluble salts.
2 3RNH HCl RNH Cl1 Amine Alkylammoniumchloride
+−+ →
°
22 2 4 2 2 2 42R NH H SO [R NH ] SO
2 Amine Dialkylammoniumsulphate
+ −+ →°
3 3 3 3R N HNO R NH NO3 Amine Trialkylammoniumnitrate
+ −+ →°
For example
3 2 2 3 2 3CH CH NH HCl [CH CH NH ]ClEthylamine Ethylammoniumchloride
+ −+ →
3 2 3 2 2 2(CH ) NH HCl [(CH ) CH N H ]ClDimethylamine Dimethylammoniumchloride
+ −+ →
6 5 2 6 5 3C H NH HCl [C H N H ]ClAniline Aniliniumchloride
+ −+ →
(cf. 3 4NH HCl NH Cl+ −+ → )
These amine salts are ionic compounds. They are non-volatile solids and generally decompose before theirmelting points are reached. They are soluble in water but are insoluble in non-polar solvents such as benzene,chloroform, ether etc. In contrast, higher amines are generally insoluble in water but are soluble in organicsolvents. This difference in solubility behaviour of amines and their salts is often used to detect amines andseparate (or purify) them from non-basic compounds.
Further, amines react with chloroplatinic acid (H2PtCl6) to form insoluble salts called chloroplatinates.
22 2 6 3 2 62RNH H PtCl ( R N H ) PtCl
Chloroplatinate
+−+ →
Chloroplatinates of amines are used for the determination of equivalent and molecular masses of amines.
Explanation of the basic character of amines
(a) Aliphatic amines. All the three classes of aliphatic amines contain a lone pair of electrons on the nitrogenatom. Therefore, they have a strong tendency to donate this lone pair of electrons to electron acceptors. As aresult, all the amines behave as bases.
Aliphatic amines are stronger bases than ammonia. All the three classs of aliphatic amines are strongerbases than ammonia. This is due to the reason that alkyl groups are electron-donating groups. As a result, theelectron density on the nitrogen atom increases and thus they can donate the lone pair of electrons more easilythan ammonia.
15
On the basis of the electron-donating inductive effect of the alkyl groups alone, the expected order ofbasicity in aqueous solution should be
3° Amine > 2° Amine > 1° Amine > NH3
However, the actual order of basicity in aqueous solution is
2° Amine > 1° Amine > 3° Amine > NH3.
The reason why the actual order is different from the expected order can be explained as follows :
The basicity of an amine in aqueous solution does not entirely depend upon the electron density on the N-atom but also depends upon the stability of the conjugate acid formed by accepting a proton from the solution.The stability of the conjugate acid, in turn, depends upon the extent of H-bonding. Obviously, greater the numberof H-atoms on N-atom, more stable is the conjugate acid. Thus, the conjugate acid of a 1° amine is the most stablesince it has three H-atoms which can form H-bonds with H2O; the conjugate acid of the 2° amine is less stablesince it has two H-atoms while that of the 3° amines is the least stable since it has only one H-atom which can formH-bonds with H2O as shown below :
Thus, on the basis of the stability of the conjugate acids alone, the basic strength of amines in aqueoussolution should follow the order :
1° Amine > 2° Amine > 3° Amine, i.e.,
RNH2 > R2NH > R3N
In actual practice, these two opposing factors balance each other in case of 2° amines. This makes 2° aminesto be the strongest. 3° Amines are weaker bases than 2° amines since their conjugate acids are less stable than 2°amines since their conjugate acids are less stable than those of 2° amines while 1° amines are less basic than 2°amines since the electron density on the N-atom is less and hence the lone pair of electrons is less easily availablefor protonation.
ADITIONAL INFORMATION
In gas phase or in non-aqueous solvents such as chlorobenzence etc., the solvation effects, i.e., the stabilizationof the conjugate acid due to H-bonding are absent and hence in these media the basicity of amines depends onlyon the + I-effect of the alkyl groups. Thus, the basicity of 1°, 2° and 3° amines in the gas phase or in non-aqueoussolvents follows the order: 3° Amine > 2° Amine > 1° Amine > NH3
The basic strength of an amine is determined by its basicity constant, Kb.
2 2 3RNH H O R N H OH+
−+ +��⇀↽��
16
According to the law of mass action, the equilibrium constant K may be expressed as
3
2 2
[RNH ][OH ]K
[RNH ][H O]
+ −=
Since water is taken in large excess, its concentration, [H2O] remains constant, therefore, the above equationmay be rewritten as
3
2
[RNH ][OH ][RNH ]
+ −
= K [H2O] = Kb
Evidently, greater the value of Kb, stronger is the base.
pKb values. Alternatively, the basicity of an amine can also be expressed in terms of its pKb value which isthe negative logarithm of the basicity constant, Kb i.e., pKb = – log Kb
Evidently, smaller the value of pKb stronger is the base.
The Kb and pKb values of some amines are given below :
AMINE Kb pKb AMINE Kb
NH3 1.8 × 10–5 4.75 ( CH3CH2)3N 5.6 × 10–4 3.25
CH3NH2 4.5 × 10–4 3.38 C6H5NH2 4.2 × 10–10 9.38
(CH3)2NH 5.4 × 10–4 3.27 C6H5NHCH3 5.0 × 10 –10 9.30
(CH3)3N 0.6 × 10–4 4.22 11.5 × 10–10 8.92
CH3CH2NH2 5.1 × 10–4 3.29 C6H5 – CH2NH2 2.0 × 10–5 4.70
(CH3CH2)2NH 10.0 × 10–4 3.00
(b) Aromatic amines are weaker bases than ammonia and aliphatic amines. Aromatic amines are far lessbasic than ammonia and aliphatic amines.
For example, aniline (Kb = 4.2 × 10 –10) is a weaker base, than ammonia (Kb = 1.8 × 10–5) as well asethylamine (Kb = 5.1 × 10 –4). The weaker basic character of aniline may be explained as follows :
(i ) Due to resonance in aniline. Aniline may be regarded as a resonance hybrid of the following structures :
NH2
I
NH2 NH2
II III
I
NH2
IV V
NH2
I
I
As a result of resonance, the lone pair of electrons on the nitrogen atom gets delocalized over the benzenering and thus is less easily available for protonation. Therefore; aromatic amines are weaker bases thanammonia.
(ii) Lower stability of anilinium ion than aniline. If we compare the relative stabilities of aniline and the aniliniumion, which aniline forms by accepting a proton, we find that whereas aniline is a resonance hybrid of fivestructure. i.e. I–V, anilinium ion is a resonance hybrid of only two structures, i.e. VI and VII. Structuresananlogous to III, IV and V are not possible in this case due to localization of the lone pair of electrons onnitrogen because of the formation of an additional nitrogen-hydrogen bond.
17
NH3
VI VII
NH3
In other words, aniline is more stable than anilinium ion. Hence, aniline has very little tendency tocombine with a proton to form anilinium ion. On the other hand, in case of ammonia and aliphatic amines,delocalization of the lone pair of electrons on the nitrogen atom by resonance is not possible. Furthermore,the electron density on the nitrogen atom is increased by electron-donating inductive effect of the alkylgroups. As result, ammonia and aliphatic amines are much stronger bases than aniline and other aromaticamines.
Effect of substituents on the basicity of aromatic amines. The basic character of an aromatic amine dependsupon the nature of the substituents present in the benzene ring.
(i ) In general, electron-donating groups such as – CH3, – OCH3, – NH2 etc. increase the basicity while electron-withdrawing substituents such as NO2, – CN, – X (halogen) etc. decrease the basicity of amines.
Electron-donating group (EDG) releases electrons, stabilizes the conjugate acid (cation) and thus increasesthe basic strength.
NH3NH2
+ H+
EDG EDG
(where EDG = CH3, – OCH3, –NH2 etc.)
Electron-withdrawing group (EWG) withdraws electrons, destabilizes the conjugate acid (cation) and thusdecreases the basic strength.
NH3NH2
+ H+
EWG EWG
(where EWG = – NO2, – CN, – X (halogen) etc.)
(ii) The base-strengthening effect of the electron-donating groups and base-weakening effect of the electron-withdrawing groups is more marked at p-positions than at m-positions. Thus,
NH2
CH3 -Toluidine
(K = 12 × 10 )
p
b–10
>
NH2
CH3
-Toluidine
(K = 5.0 × 10 )
m
b–10
and
NH2
NO2 -Nitroaniline
(K = 1.0 × 10 )
p
b–13
<
NH2
NO2
-Nitroaniline
(K = 2.9 × 10 )
m
b–13
18
(iii) o-Substituted anilines are usually weaker bases than anilines regardless of the nature of the substituentwhether electron-donating or electron-withdrawing. This is called ortho-effect and is probably due to acombination of steric and electronic factors. Thus, the basicity of o-, m- and p-toluidines relative to anilinefollows the sequence.
NH2
CH3 -Toluidine
(K = 12.0 × 10 )
p
b–10
>
NH2
CH3
-Toluidine
(K = 5 × 10 )
m
b–10
>
NH2
Aniline
(K = 4.2 × 10 )b–10
>
NH2
-Toluidine
(K = 2.6 × 10 )
o
b–10
CH3
Similarly, the basicity of o-, m- and p-nitroanilines relative to aniline follow the sequence :
NH2
Aniline
(K = 4.2 × 10 )b–10
>
NH2
-Nitroaniline
(K = 2.9 × 10 )
m
b–13
NO2>
NH2
NO2 -Nitroaniline
(K = 1.0 × 0 )
p
b–13
>
NH2
-Nitroaniline
(K = 6.0 × 10 )
o
b–14
NO2
Effect of substituents on the nitrogen atom. When the hydrogen atoms of the amino group in primaryarylamines are replaced by electrondonating alkyl groups, the basicity of the resultant arylamine increases. Forexample, N-methylaniline is a stronger base than aniline and N, N-dimethyl-aniline is even stronger than N-methylaniline. Thus,
CH3
CH3N, N – Dimethylaniline
( K = 8.92)p b
C H –N6 5 > 6 5 3
b
H|
C H N CHN Methylaniline( K 9.30)
− −−
=p
> 6 5 2
b
C H NHAniline
( K 9.38)
−
=p
On the other hand, replacement of hydrogen atoms(s) of the amino group by electron-withdrawing groups(such as phenyl groups) decreases the basic character. For example, aniline is a stronger base than diphenylaminewhich, in turn, is a much stronger base than triphenylamine. Thus,
6 5 2
b
C H NHAniline
( K 9.38)=p
> 6 5 2
b
(C H ) NHDiphenylamine
( K 13.2)=p
> 6 5 3(C H ) NTriphenylamine
(c) Basic character of aralkylamines . The lone pair of electrons on the N-atom of aniline is delocalized overthe benzene ring. However, in aralkylamines, the lone pair of electrons on the N-atom is not conjugated withthe benzene ring and hence is not delocalized. In other words, the lone pair of electrons on the N-atom inaralkylamines is more easily available for protonation than that on the N-atom of aniline. Thus, aralkylaminesare stronger bases than arylamines. For example, benzylamine is a stronger base than aniline.
6 5 2 2
b
C H CH NHBenzylamine( K =4.70)p
> 6 5 2
b
C H NHAniline
( K =9.38)p
19
Further due to electron-withdrawing inductive effect (or –I- effect) of the aryl group, the one pair of electronson the N-atom of aralkylamines is less easily available for protonation than that on the N-atom of aralkylamines isless easily available for protonation than that on the N-atom of alkylamines and ammonia. Therefore, aralkylaminesare weaker bases than alkylamines and ammonia. For example,
3 2
b
CH NHMethylamine( K =3.28)p
> 6 5 2 2
b
C H CH NHBenzylamine( K =4.70)p
> 3
b
NHAmmonia( K =4.75)p
7 Chemical Reactions
Some important chemical reactions of amines are discussed below:
1. Reactions with electrophilies
Due to the presence of a lone pair of electrons on the nitrogen atom, amines like ammonia are good nucleophilesand hence react with a variety of electrophiles (electron-deficient compounds) such as metal ions, alkyl halides,acid chlorides, acid anhydrides, chloroform etc. as discussed below:
1. Reaction with metal ions: Like ammonia, amines also form soluble co-ordination compounds with transitionmetal ions such as Ag+, Cu2+ ions etc. Thus, silver chloride dissolves in methylamine solution due to the formationof a soluble complex salt as shown below :
3 2AgCl 2CH NHSilverchloride Methylamine( )
+ →
Insoluble
3 2 2 3[CH NH Ag NH CH ] Cl+ −→ ← or 3 2 2[Ag(CH NH ) ] Cl+ −
Bis (methylamine) silver (I) chloride (Soluble)
3 3 2( .AgCl 2NH [Ag(NH ) ] Cl+ −+ →cf
Similarly like ammonia, amines such as methylamine, ethylamine etc. react with Cu2+ ions to form a deepblue solution due to the formation of the following soluble co-ordination compound.
2 3 2CuCl 4CH NHMethylamine
+ →
22 3
3 2 2 3
2 3
NH CH
H CH N Cu NH CH
NH CHTetrakis(methylamine)copper(II)chloride
( )Soluble
+ ↓ → ← ↑
4 3 2 2CuSO 4CH CH NHEthylamine
+ →2 2
3 2 2 4 4[Cu(CH CH NH ) ] ]SOTetrakis(ethylamine)copper(II)sulphate
( )
+ −
Soluble
22 3 3 4 2( .CuCl 4NH [Cu(NH ) ] (Cl ) )+ −+ →cf
20
2. Alkylation – Reaction with alkyl halides. The process of introducing an alkyl group into any molecule is calledalkylation.
(a ) With aliphatic amines. All the three classes of amines undergo alkylation when treated with alkyl halides.Thus a primary amine can be converted into secondary and tertiary amines and finally into quaternaryammonium salts. For example, when treated with excess of ethyl bromide, ethylamine undergoes alkylationto produce a mixture of diethylamine, triethylamine and tetraethyl-ammonium bromide.
3 2 2 3 2CH CH NH CH CH BrEthylamine Ethylbromide
+ → 3 2 2(CH CH ) NH HBrDiethylamine
+
3 2 2 3 2(CH CH ) NH CH CH BrDiethylamine Ethylbromide
+ → 3 2 3(CH CH ) N HBrTriethylamine
+
3 2 3 3 2(CH CH ) N CH CH BrTriethylamine Ethylbromide
+ → 3 2 4(CH CH ) N BrTetraethylammoniumbromide
+ −
Quaternary ammonium halides on treatment with moist silver oxide or methanolic KOH are converted intotheir corresponding hydroxides
4[R N]X AgOH+
− + → 4[R N]OH AgX+
− + ↓
These quaternary ammonium hydroxides are white deliquescent crystalline solids which are as strongly basicas NaOH or KOH.
(b ) With aromatic amines. Aniline when heated with excess of methyl iodide under pressure gives first N-methylaniline, theh N, N-dimethylaniline and finally trimethylanilinium iodide.
NH2
Aniline
+ CH3I Heat
– HI
NHCH3
N–Methylaniline
CH I, Heat3
– HI
N(CH )3 2
N, N–Dimethyl- aniline
CH I, Heat3
N(CH )3 3H–
Trimethyl-anilinium
iodide
+
This process of converting an amine (1°, 2° or 3°) into its quaternary ammonium salt on treatment withexcess of an alkyl halide is called exhaustic alkylation. However, if the alkyl halide used is methyl iodide, theprocess is commonly known as exhaustive methylation.
When the quarternary ammonium halide obtained through exhaustive methylation of any amine is treatedwith moist silver oxide, silver halide gets precipitated and the corresponding quarternary ammonium hydroxide isproduced. This when heated to 400 K or above undergoes an elimination reaction to produce an alkene,trimethylamine and water. For example,
3
3 2 3
3
CH|
CH CH N CH I|
CHEthyltrimethylammoniumiodide
+
−
− − −
moist Ag O2
Agl
3
3 2 3
3
CH|
CH CH N CH OH|
CHEthyltrimethylammoniumhydroxide
+
−
− − −
400 K
21
3
2 2 3 2
3
CH|
CH CH CH N H OEthene |
CHTrimethylamine
= + − +
This pyrolysis of quarternary ammonium hydroxides to produce alkenes is called Hofmann eliminationreaction and is used for determination of the structure of amines.
If all the alkyl groups in quaternary ammonium hydroxide are methyl groups i.e., tetramethylammoniumhydroxide, then pyrolysis does not produce an alkene but instead gives trimethylamine and methanol.
3 4 3 3 3400K(CH ) N OH (CH ) N CH OH+ − → +
ADITIONAL INFORMATION
In Hofmann elimination reaction, it is the less sterically hindered β-hydrogen that is removed and hence inthis reaction, less substituted alkenes are the major products. For example,
3 2 3
3 3
CH CH CH CHOH|
N(CH )Butyltrimethylammonium
hydroxidesec
β α β−
+
− − −
−
3 3 2(CH ) N, H O
∆→
− − 2 2 3 3 3CH CHCH CH CH CH CH CH1 Butene(95%) 2 Butene(5%)
( )
= + − = −− −
cisandtrans
In contrast during saytzeff elimination reaction, i.e., base-catalysed dehydrohalogenation of alkyl halidesand acid- catalysed dehydration of alcohols, it is always the more highly substituted alkene that predominates. Forexample,
CH – CH – CH CH | Cl
3 2 3
2 – Chlorobutane
CH – CH – CHCH3 2 3 | OH 2 –Butanol
∆
– KCl, – H O2
Conc. HSO2 4
433 – 433 K ( – HO)2
CH – CH = CH – CH + CH CH CH = CH3 3 3 2 2
2 – Butene (80%) ( )cis and trans
1 – Butene (20%)
3. Acylation– Reaction with acid chlorides and acid anhydrides. The process of introducing an acyl group
O||
(R C )− − into any molecule is called acylation.
(a ) With aliphatic amines. Primary and secondary amines (but not tertiary amines because they do not contain aH-atom on the N-atom) undergo acylation when treated with acid chlorides or acid anhydrides to form N-substituted amides. Thus,
Acidchloride
O||
R ' C Cl− − + 21°AmineRNH →
N–Substitutedamide
O||
R ' C NHR− − + HCl
22
22 Amine
O||
R ' C Cl R NH°
− − + → 2N,N–Disubstitutedamide
O||
R ' C NR− − + HCl
If acetyl chloride or acetic anhydride is used as an acylating agent, an acetyl(CH3– CO –) group is introducedinto the molecule and the process is called acetylation. For example, acetylation of ethyamine givesN-ethylacetamide and that of diethylamine gives N, N-diethylacetamide.
3Acetylchloride
O||
CH C Cl− − + 2 5 2EthylamineC H NH → 3 2 5
O||
CH C NHC H HClN EthylacetamideorN Ethylethanamide
− − +−
−
CH CO3
O + C H NH22 5
CH CO3
Acetic anhydrideEthylamine
3 2 5 3
O||
CH C NHC H CH COOHN Ethylacetamide Aceticacid
− − +−
3
O||
CH C Cl H N− − + −2 5C H
2 5Diethylamine
C H→
2 5
3 2 5N , N Diethylacetamide
O C H|| |
CH C N C H HCl−
− − − +
(b) With aromatic amines: Acylation of aromatic amines is usually carried out in presence of a catalyst. Forexample, acetylation with acetyl chloride is carried out in presence of pyridine as catalyst while with aceticanhydride it is carried out in presence of acetic acid or a few drops of conc. H2SO4 as catalyst. Benzoylationof aromatic amines is, however, carried out in presence of aqueous NaOH solution as shown below:
2 3Acetyl chloride
O||
NH CH C Cl+ − −
Aniline
Pyridine→ 3
O||
NH C CH HCl− − +
Acetanilide or N–Phenylethanamide
Aniline
32
3Acetic anhydride
CH CONH
CH CO+ O 2 4
3
Dropofconc.H SOorCHCOOH
→ 3NHCOCH
Acetanilide3CH COOH+
Aniline
2 6 5Benzoyl chloride
O||
NH C H C Cl+ − − Aq.NaOH→ 6 5Benzanilide
or N phenybenzamide
O||
NH C C H HCl
−
− − +
Benzoylation of compounds containing an active hydrogen atom such as alcohols, phenols and amines withbenzoyl chloride in presence of dilute aqueous NaOH solution is called Schotten Beumann reaction.
4. Reaction with aldehydes and ketones: Primary amines react with aldehydes and ketones in presence of a traceof an acid as catalyst to produce azomethines called Schiffs bases or anils.
H2
1 Amine AldehydeRNH O CHR
+
°+ = → 2
Schiff'sbaseorAnilRN CHR H O= +
23
e.g., 3 2 2 3Ethylamine Acetaldehyde
CH CH NH O CHCH+ = H+→
( )
3 2 3 2Ethylidineethylamine
CH CH N CHCH H O= +
ASchiff'sbase
3 2 2Ethylamine
CH CH NH O C+ =3CH
3CH
Acetone
H+→ 3 2CH CH N C=
3CH
3CH
Isopropylideneethylamine
Aniline
2NH O CH+ =
Benzaldehyde
H+→
Benzalaniline or Benzylideneaniline
N CH= 2H O+
Schiff's bases can be easily reduced to the corresponding secondary amines. Therefore, this method offers aconvenient route for their preparation. For example,
3 2 3 2Ethylideneethylamine
CH CH N CHCH H= + RaneyNi→ 3 2 2 3Diethylamine
CH CH NH CH CH−
Secondary amines also react with aldehydes and ketones containing α-hydrogen atoms/s to form firstcarbinolamines which being unstable readily lose a molecule of H2O to form stable enamines. For example,
( )
2Aldehydeor ketone
O||
R CH C R '− − −
R'=Horalkyl
+ 22 AmineR NH°
6 6C H ,PTS→← 2
2Carbinolamine
OH|
R CH C R '|NR
− − −
(unstable)
2H O−→←R CH C− =
R'
NR2Enamine
The equilibrium is shifted in the forward direction by removing H2O as an azeotrope with benzene. Thus,enamine formation is usually carried out by refluxing the benzene solution of an aldehyde or a ketone with asecondary amine in presence of a trace amount of an acid such as p- toluenesulphonic acid (PTS).
5. Reaction with chloroform–Carbylamine reaction or Isocyanide test. Both aliphatic and aromatic primaryamines when warmed with chloroform and an alcoholic solution of KOH produce isocyanides or carbylamineswhich have very unpleasent odours.
2 31°Amine ChloroformR NH CHCl 3KOH( )− + + alc. ∆→
AlkylcarbylamineAlkylisocyanideor
R N C→− = + 23KCl 3H O+
For example,
3 2 2 3Ethylamine ChloroformCH CH NH CHCl 3KOH( )− + + alc ∆→
Ethylcarbylamine
3 2 2Ethylisocyanideor
CH CH N C 3KCl 3H O→− = + +
NH2
Aniline
+ 3ChloroformCHCl 3KOH( )alc.+ ∆→
Phenyl isocyanide orPhenyl carbylamine
N C→=
+ 23KCl 3H O+
In contrast, secondary and tertiary amines (both aliphatic and aromatic) do not give this test. Therefore, this test isused to distinguish primary amines from secondary and tertiary amines.
24
II. Miscellaneous reactions: In addition to the two classes of reactions already discussed, amines give certain otherreactions as described below:
1. Reaction with nitrous acid: Primary, secondary and tertiary amines react differently with nitrous acid.Since nitrous acid is unstable, it is prepared in situ by the action of dilute hydrochloric acid on sodiumnitrite.
(a) Primary amines
(i) Aromatic primary amines react with nitrous acid at 273 – 278 K (0 – 5° C) to form arenediazonium salts.Thus,
2 2NaNO HCl HNO NaCl+ → +
NH2
Aniline
+ 2HNO HCl+ 273 278K−→
Benzenediazonium chloride
N NCl−≡
+ 22H O
This reaction of converting aromatic primary amines into diazonium salts by treatment with a cold(273–278 K) solution of nitrous acid is called diazotisation: If, however, the temperature rises above 278 K,the initially formed diazonium salts decompose to form phenols.
–6 5 2C H N NCl H O
+≡ + H
278K
+
> → 6 5 2
PhenolC H OH N HCl+ +
(ii) Aliphatic primary amines, on the other hand, react with cold nitrous acid to give alcohols with the quantitativeevolution of N2 gas:
273 278K2R NH HONO −− + → 2 2R OH N H O− + +
eg.,3 2 2
EthylamineCH CH NH HONO+
273 278K−→ 3 2 2 2Ethylalchol
CH CH OH N H O+ +
Since no other class of amines librerates N2 gas on treatment with HNO2,this reaction is used as a testfor aliphatic primary amines.
Aliphatic primary amines like aromatic primary amines also react with nitrous acid in the cold (273-278K) to form alkanediazonium salts. These, however, being unstable even in the cold (273-278 K) decomposeto form a mixture of alcohols, alkenes, alkyl halides along with the evolution of N2 gas:
HONO/HCl3 2 2 273–278K
EthylamineCH CH NH →
( )
3 2
Ethanediazoniumchloride
CH CH N NCl+
− − ≡
Unstable
Decompose(273 278K)−
→3 2 2 2 3 2 2
EtheneEthylalcohol Ethyl chlorideCH CH OH CH CH CH CH Cl N+ = + +
(b) Secondary amines: Both aliphatic and aromatic secondary amines react with nitrous acid to give N-nitrosoamines which being insoluble in dilute mineral acids separate out as yellow oily compounds:
3 2 2Diethylamine
(CH CH ) NH HONO+ →(Yellowoil)
3 2 2 2N-Nitrosodiethylamine
(CH CH ) N N O H O− = +
3CH|NH HONO+ →
N-Methylaniline
3
2
CH|N N O H O− = +
N-Nitroso-N-methylaniline( )Yellow oil
25
These N-nitrosoamines on warming with a crystal of phenol and a few drops of conc. H2SO4 form a green solutionwhich when made alkaline with aqueous NaOH, turns deep blue and then red on dilution. This reaction is calledLibermann's nitroso reaction and is used as a test for secondary amines.
Chemistry of the test: When a N-nitrosoamine is heated with an acid (H2SO4), the parent secondary amine isregenerated and nitrous acid is produced.
3
6 5 2N Nitroso N methylaniline
CH|
C H N N O H O− − −
− − = + H+→ 6 5 3 2
N MethylanilineC H NH CH HNO
−− − +
Nitrous acid thus produced reacts with phenol to form p- nitrosophenol (benzenoid form) which then changes tothe quinoid form. This then condenses with another molecule of phenol to give indophenol hydrogen sulphatewhich is blue. On dilution, it changes to free indophenol which is red. With excess of NaOH, sodium salt ofindophenol is produced which is blue.
OH
Phenol
2
HO N OH O− =
−→ OH
pBenzenoid form-Nitrosophenol
( )
O N=
→←p
Quinoid form-Nitrosophenol
( )
HON = O=
O = N= OHOH H+ 2 4
2
H SOH O−
→ N= OH][O =
2 4H SO→N=[HO =
+
Indophenol hydrogen sulphate ( )Deep blue
OH] HSO4 H O2
excess
N=O =
Indophenol ( )red
OHNaOH N=O =
Indophenol sodium salt ( )Deep blue
O Na– +
(c) Tertiary amines: Aliphatic tertiary amines on reaction with nitrous acid form soluble nitrite salts whilearomatic tertiary amines undergo electrophilic substitution reaction in the ring to form green-colouredp-nitrosoamines. For example,
3 2 3 2(CH CH ) N HNOTrethylamine
+ →+ –
3 2 3 2[(CH CH ) NH ]NOTriethylammoniumnitrite
( )Soluble
CH3
N – + HONOCH3
N, N–Dimethylaniline
CH3
N – N = O + H O2
CH3
p green–Nitroso–N, N–dimethylaniline ( )
Thus, it is evident from the above discussion that reaction with nitrous acid can be used for the distinction of 1°,2° and 3° amines and in some cases, it is also possible to determine whether the amine is aliphatic or aromatic.
26
2. Reaction with Grignard reagents: Primary and secondary amines (but not tertiary amines since they do notcontain a N – H proton) react with Grignard reagents to form alkanes corresponding to the alkyl group of theGrignard reagent. Thus,
RNH– H + R' – MgX1° Amine Grignard reagent
NHR
XAlkaneR' – H + Mg
R N – H + R' – MgX22° Amine Grignard reagent
NR2
XAlkaneR' – H + Mg
e.g., CH CH NH H + CH MgBr3 2 3Ethylamine Methylmag.
bromide
NHCH CH2 3
BrMethaneCH + Mg4
(CH) N H + CHCH Mgl3 2 3 2
Dimethylamine Ethylmag. iodide
N(CH3 2)
ICH CH + Mg3 3
Ethane
3. Reaction with carbon disulphide (a) With aliphatic amines. When warmed with carbon disulphide, aliphaticprimary amines form dithioalkyl-carbamic acids which decompose on heating with mercuric chloride to givealkyl isothiocyanates having a characteristic smell like that of mustard oil. For example,
2RNH S C S
1 Amine Carbondisulphide+ = =
°
Warm→
S||
RNH C SHDithioalkyl
carbamicacid
− −−
2HgCl→ R N C S HgS 2HCl
Alkyl isothiocyanate− = = + +
e.g.
3 2 2CH CH NH S C SEthylamine
+ = = Warm→ 3 2
S||
CH CH NH C SHDithioethylcarbamicacid
− − 2HgCl→
3 2CH CH N C S HgS 2HClEthylisothiocyanate
− = = + +
This reaction is called Hofmann mustard oil reaction and is used as a test for primary amines.
(b) With aromatic amines: Aromatic primary amines, however, react in a slightly different manner. For example,when aniline is heated with ethanolic CS2, and solid KOH, it gives N, N'-diphenylthiourea which upontreatment with conc. HCl gives phenyl isothiocyanate.
6 5
6 5
C H NH HS C S 2KOH
C H NH H
− −+ = = +
− −Aniline
∆→ 6 5 6 5 2 2
S||
C H NH C NH C H K S 2H ON,N' Diphenylthiourea− − − − + +
−
6 5 6 5
S||
C H NH C NH C HN,N' Diphenylthiourea− − − −
− Cons. HCl
6 5 6 5 3C H N C S C H N H C lPhenylisothiocyanateAnilinehydrochloride
+−− = = +
N, N' -Diphenylthiourea (or thiocarbanilide or sym-diphenylthiourea) is used as an accelerator duringvulcanization of rubber.
27
4. Reaction with phosgene or carbonyl chloride: Primary and secondary aliphatic amines react with carbonylchloride to form substituted ureas.
2
O||
2 R NH Cl C Cl1 Amine Carbonylchloride
− + − − →°
O||
R NH C NH R 2HClDisubtitutedurea
− − − − +sym-
2 22R NH COCl2 Amine
+ →°
2 2R N CO NR 2HClTetrasubstitutedurea− − +
−sym
Tertiary aliphatic amines, however, form salts.
Aromatic 1° amines, i.e. aniline reacts with COCl2 to form aryl isocyanate. For example,
6 5 2 2C H NH COClAniline
+ HCl→
− 6 5C H NH COCl−HCl∆→
− 6 5C H N C OPhenylisocyanate
− = =
5. Oxidation: Oxidation of amines gives different products depending upon the nature of amine and the oxidisingagent. For example,
(i ) Primary aliphatic amines on oxidation with KMnO4 followed by hydrolysis give aldehydes or ketones.
2 2R CH NH1 Amine− −
° 4KMnO
→ R CH NHAldimine− =
+2H /H O
→ 3R CH O NHAldehyde
− = +
2 2R CH NH1 Amine
−°
4KMnO→
2R C NHKetimine
=+
2H /H O→
2 3R C O NHKetone
= +
(ii) Secondary aliphatic amines on oxidation with KMnO4 give tetra-alkylhydrazines.
22R NH2°Amine
4KMnO→
2 2R N – NR + H2OTetra–alkylhydrazine
With Caro's acid or H2O2, 2° aliphatic amines, however, give the corresponding N-hydroxy-amines.
2R NH2°Amine
2 5
2 2
H SOor H O
→ 2R N OHN–Hydroxydialkylamine
−
(iii) Tertiary aliphatic amines are not oxidised by KMnO4 but are oxidised to the corresponding amine N-oxidesby Caro's acid, ozone or H2O2.
3R N [O]3 Amine
+°
2 2 3
2 5
H O or Oor H SO
→ 3R N OAmineN–oxide
→
Aromatic amines, on the other hand, because of the presence of high electron-density on the benzene ring areeasily oxidised on exposure to air or oxidising agents leading to the formation of complex coloured products.For example,
Aniline + K2Cr2O7 + H2SO4 → A black dye called aniline black
Controlled oxidation of aniline with K2Cr2O7 + H2SO4, however, gives p-benzoquinone.
Aniline
NH2 2 2 7 2 4K Cr O H SO+→ O=O =
-Benzoquinonep
28
6. Electrophilic substitution reactions: In addition to the reactions of the amino group, aromatic amines alsoundergo typical electrophilic substitution reactions of the aromatic ring. In all these reactions, the NH2 groupstrongly activates the aromatic ring through delocalization of the lone pair of electrons of the N-atom over thearomatic ring (refer to structures I-V on page 9/41).
As a result, electron density increases more at o- and p-positions as compared to m-positions. Therefore, the– NH2 group directs the incoming group to o- and p-positions, i.e., NH2 is an o-, p-directing group.
Due to the strong activating effect of the NH2 group, aromatic amines undergo electrophilic substitutionreactions readily and it is difficult to stop the reaction at the monosubstitution stage. Usually the reaction proceedsto give 2, 4, 6-trisubstituted amines. However, sometimes a monosubstitution product is required. In order to doso, the activating effect of the amino group is reduced by aceylation. The acetyl group being electron-withdrawingattracts the lone pair of electrons of the N-atom towards itself. As a result, the activating effect of the amino groupis reduced. This method is known as protection of the amino group by acetylation and can be used to control therate of the substitution and to prevent the formation of di- and tri-substitution products.
3
O||
H N C CH|
− − − ←→ 3
O|
H N C CH|
− −=+
–
Some typical electrophilic substitution reactions of aromatic amines are discussed below :
(i) Halogenation: Due to the strong activating effect of the amino group, halogenation of amines occurs veryfast and the halogen enters the p-and both the o-positions even in the absence of a catalyst. For example,aniline on treatment with bromine-water gives 2,4, 6-tribromoaniline.
+ 3HBr
2,4, 6 – Tribromoaniline
If, however, a monohalogenated derivative is required, the amino group is first acetylated and then halogenationof the ring is carried out. After halogenation, the monohalogenated amine is obtained.
NH2 NHCOCH3
Aniline Acetanilide
3 23
(CH CO ) OCH COOH−
→
2BrCH COOH3
→
NHCOCH3
Brp
major– Bromoacetanilide
( )
+
NHCOCH3
Bro
minor– Bromoacetanilide
( )
Br
H /HO ( )(–CH COOH)
+2
3
Hydrolysis
NH2
Br
pmajor
– Bromoacetanilide( )
+
NH2Br
ominor
– Bromoacetanilide( )
29
(ii) Nitration: Nitric acid is not only a nitrating agent but also acts as a strong oxidizing agent. As a result, directnitration of aromatic amines is not a useful reaction since it often gives tarry oxidation products along withsome nitration products. However, under controlled conditions, nitration of aniline gives a mixture of p-nitroaniline and m-nitroaniline in the ratio, 1 : 2.
32 4
conc.HNOconc.H SO ,293K
→
NH2NH2
m-Nitroaniline(60%)
NO2 +
NH2NH2
p
o
-Nitroaniline(30%)
+ Some-isomer
NO2
The reason for formation of larger amount of m-nitroaniline over p-nitroaniline is that under strongly acidicconditions of nitration, most of the aniline is converted into anilinium ion and since
– 3N H
+ is a m-directing group, therefore, the major product is m-nitroaniline.
Therefore, to avoid these problems, the most convenient method to carry out the nitration of aniline is to firstprotect the amino group by acetylation. The acetyl group is finally removed by hydrolysis to give a mixture of o-and p-nitroanilines. Thus,
3 2
3
(CH CO) O
CHCOOH−→
Conc.HNO32 4Conc.H SO
288K+→
NHCOCH3
NO2
pmajor product
– Nitroacetanilide( )
+
NHCOCH3
NO2
ominor product
– Nitroacetanilide( )
2
3
H / H OCH COOH
+
−→
NO2
pmajor product
– Nitroacetanilide( )
NH2
+
NO2
ominor product
– Nitroacetaniline( )
NH2
30
(iii) Sulphonation: In spite of the presence of activating amino group, sulphonation of aromatic amines occursonly under drastic conditions. For example, when aniline is treated with conc. H2SO4 at 455 - 475 K, it gives p-aminobenzenesulphonic acid also called sulphanilic acid. This reaction is believed to occur through the aminesalt as depicted below :
+ H2SO4 →2
455 475KH O−
−→
Sulphanilic acid contains both an acidic (SO3H) as well as a basic (NH2) group. It is, therefore, quite understandablethat sulphanilic acid exists as an internal salt. During the formation of this salt, the NH2 group has accepted theproton donated by the SO3H group. Such types of internal salts are called dipolar ions or zwitterions. Due tozwitterion character, sulphanilic acid has high melting point and is practically insoluble in water and organicsolvents.
Sulphanilic acid is an important intermediate in the manufacture of dyes and drugs. Sulphailic acid and its derivativesare also used in the manufacture of well-known sulpha drugs such as sulphathiazole, sulphadiazine, sulphapyridineetc. which are widely used against bacterial infections.
Fololow-up conceptual questions
1. In the following compounds :
1
I II III IVthe order of basicity is :(a) IV > I > III > II (b) III > I > IV > II(c) II >I > III > IV (d) I > III > II > IV
Ans. (i) In compound II (pyridine), the lone pair of electrons on N is present in a sp2 - ordbital while in compoundsI (piperidine) and III (morpholine), the lone pair of electrons on N is present in a sp3 -orbital. Since a sp2 -orbital has more s-character (33.33%) than a sp3 - orbital (25%), therefore, the lone pair of electrons on N ismore readily available for protonation in I and III than in II. In other words, compound (II) is less basic thanI and III. Among I and III, III contains an oxygen atom which has —I-effect. As a result, it will attract the lonepair of electrons on N towards itself. Consequently, the lone pair of electrons on N in III is less readilyavailable for protonation than in I. In other words, compound I is more basic than III.
(ii) Compound IV (pyrrole) is aromatic in character. Therefore, in accordance with Huckel's rule it has a cycliccloud of six π -electrons. Out of these, four are contributed by two double bonds while the remaining two arethe lone pair of electrons on the N-atom. In other words, the lone pair of electrons on N-atom is contributedtowards the aromatic sextet formation and hence is not at all available for protonation. Therefore, compoundIV is the least basic. In fact, it is such a weak base that it is weakly acidic in character and thus reactswith K metal when heated to form the corresponding potassium salt.
Thus, the basicity of the above four compounds decreases in the order : I > III > II > IV, i.e., option (d) iscorrect.
31
2. Sulphanilic acid is soluble in dil. NaOH but not in dil. HCl. Explain.
Ans. Sulphanilic acid exists as a zwitterion. In presence of dil. NaOH, the weakly acidic —3N H
+ transfers its H+ to
OH– to form a soluble p-NH2C6H4SO3–Na+. On the other hand, — SO3
– group is a very weak base and hencedoes not accept aH+ from dil. HCl to form p-NH3C6H4SO3H and hence it does not dissolve in dil. HCl.
8. Separation of Primary, Secondary and Tertiary Amines.
Primary (1°), secondary (2°) and tertiary (3°) amines can be separated from one another by the following methods:
1. Fractional distillation. A mixture of 1°, 2° and 3° amines can be separated into individual components by fractionaldistillation since their boiling points are fairly apart. For example.
CH3CH2NH2 (CH3CH2)2NH
1° Amine (290 K) 2° Amine (329 K)
(CH3CH2)3N
3° Amine (371 K)
2. Hinsberg's method— Reaction with benzenesulphonyl chloride. This is an excellent method for separating amixture of 1°, 2° and 3° amines. In this method, a mixture of 1°, 2° and 3° amines is treated with benzenesulphonylchloride (Hinsberg's reagent) and the resulting reaction mixture is basified with aq. KOH solution when the threeamines react differently as discussed below :
(i) A primary amine forms N-alkylbenzenesul-phonamide which because of the presence of an acidic hydrogenon the N-atom dissolves in KOH.
||
||S – N – R
O
O HN–Alkylbenzene–
sulphonamide
| 2
KOH( H O)−
→
(ii) A secondary amine forms N, N-dialkylbenzenesulphonamide which due to the absence of acidic hydrogenon N-atom does not dissolve in KOH.
HCl−
→
(iii) The tertiary amine under these conditions does not raect at all since it does not contain a replaceable hydrogenon the nitrogen atom.
The reaction mixture obtained after treatment with benzenesulphonyl chloride in presence of alkali is distilledwhen tertiary amine distills over. The remaining mixture is filtered and the filtrate on acidification gives thesulphonamide of 1° amine while the solid residue left on the filter paper is the sulphonamide of 2 ° amine.
32
The two sulphonamides thus isolated are hydrolysed separately to regenerate the corresponding 1° and 2°amines.
HCl
∆→
R
R
||
||S – OH +
O
OBenzene–
sulphonamide
NH
2° Amine
Recently benzenesulphonyl chloride is replaced by p - toluenesulphonyl chloride (tosyl chloride) since thesubstituted sulphonamides thus formed are stable solids which can be easily purified by crystallization.
3. Hofmann's method — Reaction with diethyl oxalate: In this method, a mixture of 1°, 2° and 3° amines istreated with diethyl oxalate when the three amines react differently as discussed below:
(i) The primary amine forms the corresponding substituted oxamide which is usually a crystalline solid.
→
(ii) The secondary amine forms a diethyl oxamic ester which is generally a liquid.
→ + C2H5OH
(iii) The tertiary amine under these coditions does not react at all since it does not contain a replaceable hydrogeatom, The reaction mixture containing the substituted oxamide, oxamic ester and unreacted tertiary amine isdistilled when the tertiary amine distils over. The residual mixture containing the substituted oxamide (solid)and the oxamic ester (liquid) is separated by simple filtration. Both the oxamide and the oxamic ester whenboiled separately with a strong base, regenerate the corresponding amines. Thus.
+ R2NH + C2H5OH
2° Amine
33
9 Tests for Amines
1. Solubility Test. All the three classes of amines (1°, 2° and 3°) being basic dissolve in mineral acids like HCl,H2SO4 to form the corresponding salts.
2. Hinsberg's Test: This is an excellent test for distinguishing primary, secondary and tertiary amines. In this test,the amine is shaken with benzenesulphonyl chloride (Hinsberg's reagent) in the presence of excess of aqueousKOH solution when
(i) A prinmary amine gives a clear solution which on acidification gives an insolubleN-alkylbenzene sulphonamide.
||
||S – Cl
O
OBenzenesulphonyl
chloride
+ –HCl
→ 2
KOH( H O)−
→
K+ HClKCl−
→
(ii) A secondary amine gives an insoluble N, N-dialkylbenzenesulphonamide which remains un affected onaddition of acid.
+ KOH→
O R|| |S N R HCl||O
− − − +
N,N–Dialkylbenzenesulphonamide(Insoluble in KOH)
(iii) A tertiary amine does not react at all: Therefore, it remains insoluble in the alkaline solution but dissolves onacidification to give a clear solution.
chloride
6 5 2BenzenesulphonylC H SO Cl + 3
3°AmineNR
KOH→
inKOHsolution)(3°Amineremainsinsoluble
Noreaction HCl→(SolubleinHCl)
3Trialkylammoniumchloride
R NHCl+ −
3. Carbylamine Test or Isocyanide Test: Both aliphatic and aromatic primary amines can be detected by heatingwith chloroform in presence of alcoholic KOH when isocyanides having extremely unpleasant smell are obtained.This test is called carbylamine test or isocyanide test.
RNH2 + CHC13 + 3KOH (alc) ∆→ R-N =ur
C + 3KCl + 3H2O
1° Amine Isocyanide
34
4. Azo dye test (only for 1° aromatic amines): Aromatic primary amines can be distinguished from aliphatic 1°amines by azo dye test. To carry out this test, dissolve the 1° amine in dil. HCl and cool it to 273-278 K and
then treat it with an ice-cold solution of HNO2 (NaNO2 + dil. HCl) at 273-278 K. The resulting solution is thenadded to cold alkaline solution of 2-naphthol (or 3- naphthol). Appearance of an orange or red dye confirms the
presence of an aromatic 1° amine.
2NH HONO HCl− + +
Aniline
273 278K−→
2N NCl 2H O+
−− ≡ +
Benzenediazonium chloride
N NCl+
−− ≡ +
Benxendediazonium chloride
2–Naphathol
OH
DilNaOHpH9 10−
→
1–Phenylazo –2–naphthol( )Orange dye
OH
N N− = −HCl+
10 Uses
1. Low molecular mass aliphatic amines such as diethylamine, triethylamine etc. are used :
(i) as reagents in organic synthesis.
(ii) as solvents in the laboratory and industry
(iii) as intermediates in the manufacture of drugs.
2. The quaternary ammonium. salts derived from long chain aliphatic tertiary amines are used as detergents. For
example, 3 2 15 3 3CH (CH ) N(CH ) Cl .
+−
n-Hexadecyltrimethylammoninm chloride
3. Aromatic amines such as aniline are widely used
(i) in the manufacture of dyes and drugs.
(ii) as additives (antioxidants) and vulcanization accelerators in rubber industry.
(iii) for the preparation of phenyl isocyanide needed for the manufacture of polyurethane plastics ;
(iv) for the preparation of arenediazonium salts which, in turn, are widely used for preparation of a variety ofaromatic compounds via substitution and coupling reactions.
11 Some Commercially Important Compounds
1. Hexamethylenediamine: It is manufactured by catalytic hydrogenation of adiponitrile prepared from adipicacid.
2 2
2 2Adipicacid
CH CH COOH|
CH CH COOH 3NH
Catalyst→
2 2
2 2Adiponitrile
CH CH CN|CH CH CN
−
−2H
Catalyst→
2 2 2 2
2 2 2 2Hexamethylenediamine
CH CH CH NH|
CH CH CH NH
−
−
Hexamethylenediamine is extensively used in the manufacture of nylon (For details refer to unit 16).
35
2. Aniline: It is manufactured by either of the follwoing two methods.
(i) From nitrobenzene: Aniline can be prepared commercially by vapour-phase reduction of introbenzene withhydrogen using cupric oxide or vanadium-platinum as catalyst at 680 K.
Nitrobenzene
2NO
CuOorV Pt2 680K
3H −+ →
Aniline
2NH
22H O+
(ii) From chlorobenzene: Aniline can be prepared commercially from chlorobenzene by the action of NH3 at475 K under high pressure (60 atm.) and in the presence of cuprous oxide (Cu2O), as catalyst.
Chlorobenzene
Cl
3 22NH Cu O+ +2475K60atm.
→
Aniline
2NH
2CuCl H O+ +2
Aniline is exensively used:
(i) in medicine, drugs and dyes
(ii) for making isocyanates required for polyurethane plastics
(iii) in the manufacture of anioxidants for and vulcanization acclerators for rubber.