Properties Of Amines

PHYSICAL PROPERTIES

1. Among aliphatic amines, the lower members are gases while higher members are liquids. Among arylamines, the lower members are liquids while higher members are solids. Methyl amine and ethylamine have ammonical smell but higher amines have fishy smell. Most of aromatic amines are colourless but become coloured due to oxidation in air.

2. Boiling points:

All amines except tertiary amines are capable of forming intermolecular hydrogen bonds due to the presence of polar N – H bonds.

Due to this, amines have higher boiling points than non – polar compounds of comparable molecular mass. Among isomeric amines, 1° amines have highest boiling point due to more extensive H – bonding while 3° amines have the least boiling point due to their inability to form hydrogen bonds.

3. Solubility:

Aliphatic amines of lower molecular mass are soluble in water. With increase in molecular mass, solubility in water decreases, while that in ether increases.

CHEMICAL PROPERTIES

1. Basic Nature:

Aliphatic bases

As increasing strength in nitrogenous bases is related to the readiness with which they are prepared to take up protons, and therefore, to the availability of the unshared electron pair on nitrogen, we might expect to see an increase in basic strength on going : NH₃ → RNH₂ → R₂NH→ R₃N, due to the increasing inductive effect of successive alkyl groups making the nitrogen atom more negative. An actual series of amines was found to have related pKa values as follows, however:

It will be seen that the introduction of an alkyl group into ammonia increases the basic strength markedly as expected. The introduction of a second alkyl group further increases the basic strength, but the net effect of introducing the second alkyl group is very much less marked than the first. The introduction of a third alkyl group to yield a tertiary amine, however, actually decreases the basic strength in both the series quoted. This is due to the fact that the basic strength of an amine in water is determined not only by electron - availability on the nitrogen atom, but also by the extent to which the cation, formed by uptake of a proton, can undergo solvation, and so become stabilised. The more hydrogen atoms attached to nitrogen in the cation, the greater the possibilities of powerful solvation via hydrogen bonding between these and water:

Thus on going along the series, NH₃ → RNH₂ → R₂NH → R₃N, the inductive effect will tend to increase the basicity, but progressively less stabilisation of the cation by hydration will occur which will tend to decrease the basicity. The net replacing effect of introducing successive alkyl groups thus becomes progressively smaller, and an actual changeover takes place on going from a secondary to a tertiary amine. If this is the real explanation, no such changeover should be observed if measurements of basicity are made in a solvent in which hydrogen - bonding cannot take place; it has, indeed, been found that in chlorobenzene or in gas phase the order of basicity of the butylamines is

BuNH₂ < Bu₂NH < Bu₃N

Aromatic bases

In aniline the nitrogen atom is again bonded to an sp² hybridised carbon atom but, more significantly, the unshared electron pair on nitrogen can interact with the delocalised π orbitals of the nucleus:

The aniline molecule is thus stabilised with respect to the anilinium cation, and it is therefore ‘energetically unprofitable’ for aniline to take up a proton; it thus functions as a base with the utmost reluctance (pKa = 4.62, compared with cyclohexylamine, pKa = 10.68). The base weakening effect is naturally more pronounced when further phenyl groups are introduced on the nitrogen atom ; thus diphenylamine, Ph₂NH, is an extremely weak base (pKa = 0.8), while triphenylamine, Ph₃N, is by ordinary standards not basic at all.

Introduction of alkyl, e.g. Me groups, on to the nitrogen atom of aniline results in small
increase in pKa:

C₆H₅NH₂ C₆H₅NHMe C₆H₅NHMe₂ MeC₆H₄NH₂

4.62 4.84 5.15 o-4.38

m-4.67

p-5.10

Unlike on such introduction in aliphatic amines this small increase is progressive: suggesting that cation stabilisation through hydrogen - bonded solvation, responsible for the irregular behaviour of aliphatic amines, here has less influence on the overall effect. The major determinant of basic strength in alkyl-substituted anilines remain mesomeric stabilisation of the aniline molecule with respect to the cation; born out by the irregular effect of introducing Me groups into the o-, m- and p-positions in aniline.

A group with a more powerful (electron - withdrawing) inductive effect, e.g. NO₂ is found to have rather more influence. Electron withdrawal is intensified when the nitro group is in the o- or p-position, for the interaction of the unshared pair of the amino nitrogen with the delocalised π orbital system of the benzene nucleus is then enhanced. The neutral molecule is thus stabilised even further with respect to the cation, resulting in further weakening as a base. Thus the nitro - anilines are found to have related pKa values:

The extra base - weakening effect, when the substituent is in the o-position, is due in part to the short distance over which its inductive effect is operating, and also to direct interaction, both steric and by hydrogen bonding, with the NH₂ group. o-Nitroaniline is such a weak base that its salts are largely hydrolysed in aqueous solution, while 2, 4 - dinitroaniline is insoluble in aqueous acids, and 2, 4, 6 - trinitroaniline resembles an amide; it is indeed called picramide and readily undergoes hydrolysis to picric acid (2, 4, 6 - trinitrophenol).

With substituents such as OH and OMe that have unshared electron pairs, an electron - donating, i.e. base-strengthening, mesomeric effect can be exerted from the o- and p-, but not from the m-positon, with the result that the p-substituted aniline is a stronger base than the corresponding m-compound. The m-compound is a weaker base than aniline itself, due to the electron - withdrawing inductive effect exerted by the oxygen atom in each case. As so often, the effect of the o−substituent remains somewhat anomalous, due to the interaction with the NH2 group by both steric and polar effects. The substituted anilines are found to have related pKa values as follows:

Illustration 16. The sequence not correct for basic strength of compounds are

(A) Dimethylamine > methylamine > trimethylamine in aqueous solution

(B) diethylamine > triethylamine > ethylamine in aqueous solution

(C) methyl amine > pyridine > aniline

(D) aniline > pyrole > pyridine

Solution. Pyridine is more basic than aniline.

Hence (D) is not correct.

Illustration 17. An aqueous solution of ethylamine gives a red precipitate with ferric chloride. Explain

Solution. Ethyl amine acts as a base in water to produce appreciable OH− ions which then reacts with Fe³⁺ ions to give a red precipitate of ferric hydroxide

C₂H₅NH₂ + + H₂O C₂H₅NH₃⁺ + OH−

Fe³⁺ + 3OH− → Fe(OH)₃

Illustration 18. p-Methoxyaniline is stronger base than aniline but p – nitroaniline is a weaker base than aniline. Explain.

Solution: Methoxy group is electron repelling and thus intensifies the negative charge on N atom of NH2 group to increase the electron pair donating tendency whereas nitro group is electron attracting and thus intensifies positive charge on N atom of – NH2 group to decrease the electron donating nature.

Illustration 19. Which of the following is a better nucleophilie, aniline or anilinium ion and why?

Solution: Aniline. The presence of postive charge on anilinium ion C6H5NH3+ reduce the tendency to donate lone pair of electrons.

2. Reaction with acids to form salts:

The nitrogen is quadricovalent unielectrovalent in amine salts.

3. Alkylation:

Amines react with R – X to form amines of higher class. In this reaction, the amine acts as nucleophile bringing about nucleophilic substitution of alkyl halides.

These salts give a test for halide ion with AgNO3 solution.

4. Acylation and benzoylation of amines to form substituted amides

The above reactions are called Schotten – Baumann reaction.

Illustration 20. Which of the following will not produce amide when treated with ethanoyl chloride?

Solution. 3° amines do not have H-atom on nitrogen hence will not produce amide.

Hence (B) is correct.

Illustration 21. Tertiary amines do not undergo acylation. Why?

Solution: Tertiary amines, R₃N do not contain any replaceable hydrogen atom and thus can not be acylated.

5. Reaction with HNO₂ (NaNO₂ + HCl):

1° amines give diazotization reaction as follows :

1°aliphatic amines also react with HNO₂ to form diazonium salt but due to the absence of delocalization, it is unstable, decompose to yield mixture of alcohols, alkenes with the evolution of N₂ gas.

2° amine: Both aliphatic acid aromatic 2° amines react with HNO₂ to produce N - nitroso amines that are insoluble in the aqueous solution and separate out as a yellow oily layer.

The nitroso amines on warming with a little phenol and conc. H₂SO₄ produce red solution which changes to blue with NaOH (This is Liebermann nitroso test for 2° amine and phenol).

3° amine:Aliphatic 3° amines form nitrites while aromatic 3° amines undergo electrophilic substitution.

Illustration 22. Action of nitrous acid on ethylamine gives

(A) ethyl alcohol (B) acetamide

(C) Ethane (D) ethanoic acid

Solution:

Hence (A) is correct.

6. With CS₂: The below reaction is called Hofmann mustard oil reaction which is a test for 1° amines.

7. Ring substitution in aromatic amines:

Due to resonance effect –NH₂, -NHR, -NR₂ groups are ortho and para directing for electrophilic attack.

Following reactions clarify the point:

(i) Halogenation:

In order to introduce only one halogen atom, the activating effect of the –NH₂ group must be lowered using acetylation.

Illustration 23. While carrying out an electrophilic substitution reaction on aniline, Lewis acid is not used. Why?

Solution: Aniline possesses basic nature due to lone pair of electrons and thus it will form coordinate bond with Lewis acid.

(ii) Nitration: Direct nitration of aniline with nitric acid gives a complex mixture of mono – di and trinitro compounds and oxidation products. If however, NH₂ group is protected by acetylation, main product of nitration is p – nitro derivative.

Illustration 24. During nitration of benzene with a mixture of conc. Nitric acid and conc. Sulphuric acid, nitric acid acts as a base. Explain.

Solution: Both HNO₃ and H₂SO₄ are strong acids but H₂SO₄ is a stronger acid then HNO₃ therefore during nitration, H₂SO₄ acts as an acid releasing a proton. HNO₃, on the other hand, accepts this proton and thus acts as a base. The protonated nitric acid then loses a proton to form nitronium ion which then brings about the nitration.

(iii) Sulphonation:

Aniline reacts with conc. H₂SO₄ to form the salt anilinium hydrogen sulphate which on heating at 455 – 475 K gives sulphanilic acid (p – amino benzene sulphonic acid).

Sulphanilic acid exists as Zwitter ion i.e. a dipolar ion which exists in the form of internal salt structure. Such ion has positive as well as negative charge within same molecular structure.

Illustration 25. Which of the following reactions involve alkyl/aryl migration?

(A) Baeyer Villiger oxidation

(B) Hoffmann bromamide rearrangement

(C) Beckmann rearrangement

(D) All of these

Solution: (D) is correct.

Illustration 26. Which of the following reactions involve nitrene intermediate?

(A) Baeyer-Villiger oxidation

(B) Hoffmann elimination

(C) Hoffmann bromamide rearrangement

(D) Carbylamine reaction

Solution: (C) is correct.

Illustration 27. Why aniline acquire pink colour on long standing?

Solution: Because of aerial oxidation of aniline to give black mass known as aniline black.