+2 UNIT 15 PAGE- 2
Effect of substituents on the basic character of Aromatic amines
(i) Electron releasing substituents like –OCH3, -CH3 increases the basic strength , while electron withdrawing groups like –X (halogen) , -NO2, -CN decreases the basic strength.
pKb values of some aromatic amines are given below :
Activating group | Deactivating group | ||
Amine | pKb | Amine | pKb |
p-C6H4(CH3)NH2 | 8.92 | p-C6H4(Cl)NH2 | 10.02 |
p-C6H4(OCH3)NH2 | 8.66 | p-C6H4(Br)NH2 | 10.14 |
p-C6H4(NH2)NH2 | 7.85 | p-C6H4(CN)NH2 | 12.26 |
C6H5NH2 | 9.38 | p-C6H4(NO2)NH2 | 13.00 |
(ii) The base weakening effect of electron withdrawing group and base strengthening effect of electron releasing group is more marked at p-position than at m-position.
(iii) Every o-substituted aniline (electron releasing or electron withdrawing ) is less basic than aniline. This is due to o-effect which is probably due to steric and electronic factors.
It may be noted that the base weakening effect of electron withdrawing group is very large when present at o-position than from m- or p-position(note aniline and o-nitroaniline).
Similarly,
Effect of Substitution on the Nitrogen atom
When hydrogen atoms of the amino group of arylamines are replaced by electron donating alkyl groups, the basic character of the resulting arylamine increases. For example, N-Methylaniline is a stronger base than aniline, N,N-dimethylaniline is even stronger than N-methylaniline. Thus,
On the other hand, when hydrogen atoms of amino group are replaced by electron withdrawing groups (such as phenyl group) , the basic character of the resulting aryl amine decreases. For example,
(d) Comparison of basic character of aralkylamines
The lone pair on the N-atom of aniline is delocalised over the benzene ring. However, in case of aralkylamines, the lone pair of electrons on N-atom in aralkyl amines is more readily available for protonation than that on the N-atom of aniline. Thus, the aralkylamines are more basic than arylamines. For example, benzyl amine is a stronger base than aniline.
Chemical Reactions of Amines
The reactions of amines are mainly due to participation of unshared pair of electrons of nitrogen which makes them react as a nucleophile or a base. A nucleophile is a species that attacks an electron-deficient carbon and a base is a species that attacks electron-deficient hydrogen i.e., a proton. In aromatic amines, the same unshared pair of electrons facilitates electrophilic substitution in the phenyl ring. The number of hydrogen atoms on the amine nitrogen also affect the course of some reactions. Some reactions of amine are discussed below :
1. Salt formation : Amines , being basic reacts with acids to form salts.
These salts are usually soluble in water which on treatment with aqueous hydroxide regenerate the amines.
2. Reaction of amines with metal ions
Like ammonia , amines combine with metal ions such as Ag+ or Cu2+ ions to form complex compounds. The lone pair of electrons in NH3 is used to form a co-ordinate bond of amine with metal ions. For example, silver chloride dissolves in methylamine to form a soluble silver amine complex.
Similarly, cupric chloride reacts with methylamine to give deep blue coloured complex.
In these complexes , there is a co-ordinate bond between metal ion and amine molecules as shown below.
3. Alkylation
Primary and secondary amines react with alkyl halides to form tertiary amines. The tertiary amines finally react to form quarternary ammonium salts.
For example,
The process of converting an amine (1°, 2° or 3°) into its quarternary ammonium salt on treatment with excess of alkyl halide is called exhaustive alkylation.
If the alkyl halide used is methyl iodide, the process is called exhaustive methylation.
Aromatic amines also undergo similar reactions. For example , when aniline is treated with methyl iodide under pressure, quarternary salts are formed.
The tramethylammonium salt or quarternary ammonium halide on reaction with moist silver oxide forms tetramethylammonium hydroxide and silver halide gets precipitated. This on heating (400 K) decomposes to give tertiary amine and alcohol.
If one of the alkyl groups is other than methyl, the quarternary ammonium hydroxide , gives tertiary amine, an alkene and water on heating. For example,
The pyrolysis of quaternary ammonium hydroxide to give alkenes is called Hofmann’s elimination reaction. This can be used for elucidation of structure of amines.
5. Acylation
Aliphatic and aromatic primary amines undergo acylation through nucleophilic substitution of acid derivatives such as acid chlorides or anhydrides to form amides.
During the reaction, the acid generated can form salt of the amine which will lose nucleophilic character and the reaction will not proceed to completion. So a base is added to facilitate the reaction.
Unlike in the case of alkylation reaction of amines, the amide formed here does not react further with organic halide because the amide is non-basic and poor nucleophile due to its nitrogen unshared pair of electrons being in conjugation with carbonyl group.
Amongst the acid derivatives, acid chlorides are stronger acylating agents than anhydrides and esters react slowly. Carboxylic acids as such form only salts with amines and not the amides.
Tertiary amines fail to react with acid derivatives because they cannot lose a proton to stabilise the product. So in acylation reaction of an amine, in addition to its nucleophilic character, presence of an H atom on nitrogen is also necessary.
Aniline reacts in a similar manner. However, acylation of aromatic amines is usually carried out in the presence of a base such as aqueous NaOH or pyridine.
1. Benzoylation
Aliphatic and aromatic amines react with benzoyl chloride in the presence of a base such as pyridine or aqueous NaOH to form benzoyl derivatives in which C6H5CO – group is introduced. This reaction is called benzoylation.
Benzoylation of compounds containing an active hydrogen such as alcohols, phenols or amines with benzoyl chloride in the presence of dilute aqueous NaOH solution is called Schotten Baumann reaction.
2. Schiff’s base formation
(reaction with aldehydes)
Both aliphatic and aromatic primary amines react with aldehydes to form Schiff’s bases or anils.
Benzaldehyde reacts with aniline to give a typical Schiff’s base known as benzal aniline.
Schiff’s base on reduction gives secondary amine.
Therefore , this method can be used to prepare secondary amine.
8. Oxidation
Amines undergo oxidation by powerful oxidising agents such as caro’s acid (H2SO5) , potassiumpermanganate (KMnO4), hydrogen peroxide (H2O2) etc. The products of oxidation depend upon the nature of the amine and oxidising agent. For example,
(a) Oxidation with potassium permanganate
(i) Primary aliphatic amines on oxidation with potassium permanganate followed by hydrolysis give aldehydes and ketones.
(ii) Secondary aliphatic amines on oxidation with potassium permanganate give tetra alkyl hydrazines.
(b) Oxidation with Caro’s acid
(i) Primary amines react in the following ways :
(ii) Secondary amines with caros’s acid give corresponding N-hydroxyamine.
(iii) Tertiary amines are not oxidised by KMnO4 , but are oxidised by Caro’s acid , ozone or H2O2 to corresponding N-oxides.
Aniline can be readily oxidised in the presence of K2Cr2O7 and H2SO4, give p-Benzoquinone.
9. Carbylamine (or isocyanide test)
Aliphatic or aromatic primary amines when warmed with chloroform and an alcoholic solution of KOH, form isocyanide or carbylamine which has very unpleasant smell.
Secondary and tertiary amines do not give this test. Therefore , this test can be used to distinguish between primary amines from secondary and tertiary amines.
10. Reaction with nitrous acid
Nitrous acid (HNO2) is an unstable acid, therefore , it must be freshly prepared by treatment with sodium nitrite with cold dilute HCl or H2SO4.
Different classes of amines react differently with nitrous acid. This reaction serves as an important reaction to distinguish between primary, secondary and tertiary amines.
(i) Primary amine
Primary aliphatic amines react rapidly with nitrous acid to form alcohol and evolve nitrogen.
Primary aromatic amines react with nitrous acid in cold (below 273 K) to form diazonium salts . The process of formation of diazonium salts by the reaction of aromatic amino compound and nitrous acid is diazotisation. For example,
However, if temperature is more than 278 K, aromatic amines form phenol with evolution of N2 gas.
(ii) Secondary amines
Secondary aliphatic and aromatic amines react with nitrous acid slowly in the cold to form yellow oily nitroso amines.
The yellow oily nitrosoamine gives a green solution when warmed with phenol and con. Sulphuric acid. On dilution with water, the colour changes to red , but it changes to greenish blue to violet on the addition of sodium hydroxide. The overall reaction is called Libermann’s nitroso reaction. This test is used for secondary amines.
(iv)Tertiary amines
Tertiary amines dissolve in cold nitrous acid to form salts which decompose on warming to nitrosoamine and alcohol.
Aromatic tertiary amines react with nitrous acid to give coloured substituted nitroso compound.
11. Reaction with Grignard reagent
Primary and secondary amines react with Grignard reagents to form alkanes.
It may be noted that alkane is obtained from the alkyl part of Grignard reagent. Tertiary aliphatic amines do not react with Grignard reagent because they do not have hydrogen atom attached to the nitrogen atom.
12. Reaction with carbon disulphide
Primary amines react with carbon disulphide to form dithio-alkyl carbamic acids which decompose on heating with mercuric chloride (HgCl2) to give alkyl isothiocyanates. These have characteristic smell like mustard oil. For example,
This reaction is called Hofmann mustard oil reaction and is used as a test for primary amines.
However, aromatic amines react in slightly different manner. For example, when aniline is heated with ethanolic CS2 and solid KOH , it forms N-N’-diphenyl thiourea which on treatment with con. HCl gives phenyl isothiocyanate.
13. Reaction with carbonyl chloride (phosgene)
Primary and secondary aliphatic amines react with carbonyl chloride to form substituted urea.
Tertiary aliphatic amines form salts.
Aromatic primary amines react with COCl2 to form aryl isocyanate. For example,
13. Ring substitution in aromatic amines
Aromatic amines give the aromatic substitution reactions as given by benzene. Aniline is more reactive than benzene. The presence of amino group activates the aromatic ring and directs the incoming group preferably to ortho and para positions. This is clear from the following resonating structures in which electron density is more at ortho and para positions ( structures III to V)
Therefore , substitution mainly occurs at ortho and para positions. Due to strong activating effect of –NH2 aromatic amines undergo electrophilic substitution reactions readily. Therefore, it is difficult to stop the reaction to monosubstitution stage. However, in order to stop the reaction to monosubstitution stage, the activating effect of the amino group has to be reduced. This can be done by acetylation. Acetyl group is electron withdrawing group and therefore, the electron pair of N-atom is withdrawn towards the carbonyl group as shown by following resonating structures :
Therefore , the lone pair of electrons on nitrogen is less available and the activating power of –NH2 group is decreased. This method is called protection of the amino group by acetylation and can be used to control the rate of electrophilic substitution reaction. This also prevents the formation of di and tri-substituted products. The acetyl group is then removed by hydrolysis to get back the amine.
Some of these reactions are given below :
(i) Halogenation
Aniline reacts with bromine water readily to give a white precipitate of 2,4,6-tribromoaniline.
This reaction is used as a test for aniline.
However, if monosubstituted derivative is desired, aniline is first acetylated with acetyl chloride and then halogenation is carried out. After halogenation, the acetyl group is removed by hydrolysis and only monosubstituted derivative is obtained.
It may be noted that –NH2 group directs the attacking group at o- and p-positions and therefore both o- and p- derivatives are obtained.
(ii) Nitration
Nitration of aniline , unexpectedly gives 47% m-nitroaniline. It is due to the fact that aniline gets protonated to m-directing and deactivating +NH3 group.
For facilitating , nitration, aromatic amines are converted to amides which after performing nitration are hydrolysed back to amino groups. For controlling the bromination also, the amide derivatives are used.
(iii) Sulphonation
Sulphonation of aniline is carried out by heating aniline with sulphuric acid. The product formed is anilinium hydrogen sulphate which on heating gives sulphanilic acid.
The sulphanilic acid exists as a dipolar ion (structureII) which has acidic and basic groups in the same molecule. Such ions are called Zwitter ions or inner salts.
14. Coupling with diazonium salts
Aromatic amines react with diazonium salts to form azo compounds in acidic medium called azo dyes. The reaction is known as coupling or diazo reaction. For example , aniline couples with benzene diazonium chloride to form diazoaminobenzene which ultimately changes to p-aminoazobenzene on warming with a small amount of hydrochloric acid.
Test for Amines
1. All amines are basic in nature and hence are soluble in dil HCl. These amines can be regenerated from acid medium by adding excess alkali.
2. Primary amines (aliphatic or aromatic) can be detected easily by heating the amine with chloroform and alkali, when characteristic foul smell of isocyanide is produced. This test is called carbylamine test.
3. Aromatic amines give dye test ( reaction 14)
Uses of amines
The important uses of aliphatic and aromatic amines are given below :
1. Aliphatic amines of low molecular mass are used as solvents.
2. Amines are also used as intermediates in drug manufacture and as reagents in organic synthesis.
3. Aromatic amines are used for the manufacture of polymers, dyes and as intermediates for additives in rubber industry.
4. The quarternary ammonium salts derived from long chain aliphatic tertiary amines are widely used as detergents.
5. Aromatic amines are converted into arene diazonium salts which are used for the preparation of variety of aromatic compounds via substitution and coupling reactions.
Distinction between primary, secondary and tertiary amines
The following tests can be used to distinguish between primary, secondary and tertiary amines :
1. Carbyl amine test : The test is given by primary amines only (reaction 9)
2. Reaction with nitrous acid : The three types of amines react differently with nitrous acid (reaction 10)
3. Hinsberg’s test for amines : This test is used to distinguish primary, secondary and tertiary amines. In this test, the amine is heated with benzene sulphonyl chloride C6H5SO2Cl ( known as Hinsberg’s reagent) in the presence of excess of alkali. Different amines gives different observations.
(i) Primary amines give clear solution which on acidification yields an insoluble material.
(ii) Secondary amines give an insoluble substance which remain unaffected on addition of alkali.
(iii) Tertiary amines do not react and remain insoluble in alkali and can be dissolved in acids.
Dissolves in HCl to form R3N+HCl-.
Separation of Mixture of Primary, secondary and tertiary amines
The mixture of primary, secondary , tertiary and quarternary salt is distilled with potassiun hydroxide, when the amine distil over leaving behind the non-volatile quarternary salt. The mixture of amines obtained as distillate is separated by any one of the following methods.
1. Fractional distillation : The mixture of amines may be separated by fractional distillation because their boiling points are different. This method has been proved to most satisfactory and efficient and has been extensively used in industry.
2. Hofmann’s method : The mixture of primary, secondary and tertiary amines is treated with diethyl oxalate, when primary amine forms solid oxamide, secondary amine forms a liquid oxamic ester while tertiary amine remains unreacted.
The reaction mixture containing oxamide, oxamic ester and tertiary amine is distilled, when tertiary amine being most volatile, distils over and is collected. Now , the residual mixture in the distillation flask contains oxamide and oxamic ester. The solid oxamide is separated from oxamic ester by simple filtration and both are then separately treated with strong alkali regenerating corresponding amines. These are collected and purified by distillation.
3. Hinsberg’s method
The mixture of amines is reacted with benzene sulphonyl chloride and the product mixture obtained is treated with aq. HCl and filtered. The filtrate which contains tertiary amine as its hydrochloride is isolated as amine by addition of base. The acid insoluble residue is treated with aq. KOH solution and filtered. The filtrate on acidification gives sulphonamide of primary amine which is isolated after hydrolysis. The sulphonamide insoluble in alkali, is hydrolysed to get secondary amine.
Problems
06. How is classifications of amines different from that of alcohols ?
07. Why do quaternary ammonium salts having four different groups attached to nitrogen show optical activity ?
16. tert-Butylamine cannot be prepared by by the action of NH3 on tert-butyl bromide. Explain.
17. Suggest chemical reactions for the following conversions :
(i) Cyclohexanol to cyclohexyl amine
(ii) n-Hexanenitrile to 1-Aminopentane.
18. Sulphanilic acid is soluble in dil NaOH but not in dil HCl. Explain.
19. How will you convert the following ?
(i) Methylamine ® Ethylamine
(ii) Ethylamine ® methylamine
(iii) Aniline ® Benzoic acid
(iv) m-Nitroaniline ® m-chloroiodobenzene
(v) p-chloroaniline ® p-chlorobenzylamine
(vi) 4-Nitroaniline ® 1,2,3-tribromobenzene
(vii) p-toludine ® m-bromotoluene
(viii) aniline ® sulphanilamide
(ix) aniline ® 4-bromo-4’-aminoazobenzene
(x) propene ® n-butylamine
20. How will you convert aniline to :
(i) benzene (ii) benzonitrile
(iii) benzylamine (iv) phenol
(v) p-Nitroaniline
21. Draw the structures and give the names according to IUPAC system the five isomeric amines of formula C7H9N that contain a benzene ring.
22. How will you convert :
(a) aniline to chlorobenzene
(b) nitrobenzene to m-Bromochlorobenzene
23. An optically inactive compound (A) having molecular formula C4H11N on treatment with HNO2 gave an alcohol (B). B on heating at 440 K gave an alkene (C). C on treatment with HBr gave an optically active compound (D) having the molecular formula C4H9Br. Identify A, B, C and D and write down their structural formulae. Also write equations involved.