Basicity of Amines | Tricks & Techniques | IIT Jee Mains, Advance | AIIMS , NEET | BITSAT and REE Video
'Basicity of Amines' is one of the most important topic for Engineering and Medical entrance examination. Here, we have tried to explain it up-to that level.
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.
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 with 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 stabilized. 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 the order of basicity of the butylamines is
BuNH2Bu2NH Bu3N (Greatest)
Tetralkylammonium salts, e.g. R4N+ I-, are known, on treatment with moist silver oxide, AgOH, to yield basic solution comparable in strength with the mineral alkalis. This is readily understandable for the base so obtained, R4N+ –OH, is bound to be completely ionised as there is no possibility, as with tertiary amines, etc.,
R3NH++ -OH → R3N: + H2O
of reverting to an unionised form.
The effect of introducing electron withdrawing groups, e.g. Cl, NO2, close to a basic center is to decrease the basicity, due to their electron withdrawing inductive effect. Thus the amine is found to be virtually non - basic, due to the three powerfully electron withdrawing CF3 groups. This can well be explained on the basis of more ‘s’ character on lone pair of N.
The change is also pronounced with C=O, for not only is the nitrogen atom, with its electron pair, bonded to an electron withdrawing group through an sp2 hybridised carbon atom but an electron withdrawing mesomeric effect can also operate:
Thus amides are found to be only very weakly basic in water [pKa for ethanamide(acetamide) is » 0.5], and if two C=O groups are present the resultant imides, far from being basic, are often sufficiently acidic to form alkali metal salts, e.g. benzene - 1, 2 - dicarboximide .
Aromatic Bases: (Aromatic Amines):
The exact reverse of the above is seen with aniline, which is a very weak base (pKa = 4.62) compared with ammonia (pKa = 9.25) or cyclohexylamine (pKa = 10.68). In aniline the nitrogen atom is again bonded to a sp2 hybridised carbon atom but, more significantly, the unshared electron pair on nitrogen can interact with the delocalised p orbitals of the nucleus. If aniline is protonated, any such interaction, with resultant stabilisation, in the anilinium cation is prohibited, as the electron pair on N is no longer available Anilinium cationThe 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 .
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