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Monday, 27 February 2017

Amines - II

Physical properties of amines:
(i) Solubility: Lower aliphatic amine is soluble in water because they can form hydrogen bonding with water. Solubility decreases with increases in molar mass of amines due to increase in size of hydrophobic group

(ii) Boiling points: Among the isomeric amines primary and secondary amines have high boiling point because they can form hydrogen bonding. Tertiary amine cannot form hydrogen bonding due to the absence of hydrogen atom available for hydrogen bond formation. Hence order of boiling of isomeric amines is Primary>Secondary> Tertiary

Chemical properties of amines:
(a) Basic character of amines:
Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base. Basic character of amines can be better understood in terms of their Kb and pKb values
Greater Kb value or smaller pKb indicates base is strong.

(b) Comparison of basic strength of aliphatic amines and ammonia: Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom.

(c) Comparison of basic strength of primary, secondary and tertiary amines

(i) The order of basicity of amines in the gaseous phase follows the expected order on the basis of +I effect: tertiary amine > secondary amine > primary amine > NH3

(ii) In aqueous solution it is observed that tertiary amines are less basic than either primary or secondary amines. This can be explained on basis of following factors:

(a) Solvation effect: Greater is the stability of the substituted ammonium cation formed, stronger is the corresponding amine as a base. Tertiary ammonium ion is less hydrated than secondary ammonium ion which is less hydrated than primary amine. Thus tertiary amines have fewer tendencies to form ammonium ion and consequently are least basic.

On the basis of solvation effect order of basicity of aliphatic amines should be primary amine>secondary amine>tertiary amine.

(b) Steric factor: As the crowding of alkyl group increases from primary to tertiary amine hinderance to hydrogen bonding increases which eventually decreases the basic strength. Thus there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state.
When the alkyl group is small like CH3 there is no steric hindrance to hydrogen bonding. In this case order of basicity in aqueous medium is
When alkyl group is ethyl group order of basicity in aqueous medium is
(c) Comparison of basic strength of aryl amines and alkylamines:
Generally aryl amines are considerably less basic than alkyl amines .Taking an example of aniline and ethylamine it is observed that ethyl amine is more basic than aniline. In aniline –NHgroup is directly attached to benzene ring. Hence unshared pair of electron on nitrogen is less available for protonation because of resonance. Below mentioned are resonating structures of aniline.

In the above resonating structures there is a positive charge on nitrogen atom making the lone pair less available for protonation. Hence aniline is less basic than ethyl amine which has no resonating structures. Less basicity of aniline can also be explained by comparing the relative stability of aniline and anilinium ion obtained by accepting a proton. Greater the number of resonating structures, greater is the stability of that species.
Aniline is resonance hybrid of five resonating structures whereas anilinium ion has only two resonating structures.

Thus aniline has less tendency to accept a proton to form anilinium ion.
(d) Effect of substituent on basic character of amines:
Electron donating or electron releasing group/groups (EDG) increases basic strength while electron withdrawing (EWG) decreases basic strength.

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