Question

Give plausible explanation for each of the following:

1. Why are amines less acidic than alcohols of comparable molecular masses?

2. Why are primary amines higher boiling than tertiary amines?

3. Why are aliphatic amines stronger bases than aromatic amines?

Answer 1

1. Loss of proton from amines give amide ion whereas loss of a proton from alcohol gives an alkoxide ion.

Since O is more electronegative than N, therefore, RO^- can accomodate the -ve charge more eaisly than RNH^- . Consequently, RO^- is more stable than RNH^- . Thus, alcohols are more acidic than amines.

2. Primary amines (RNH₂) have two hydrogen atoms on the N atoms and therefore, form inter molecular hydrogen bonding

Tertiary amines (R₃N) do not have hydrogen atoms on the N atom and therefore, these do not form hydrogen bonding in primary amines, they have higher boiling points than tertiary amines of comparable molecular mass. For example, boiling point of n- butylamine is 351 K while that of tertbutylamine is 319K.

3. Both arylamines and alkalamines are basic in nature due to the presence of lone pair on N-atom. But arylamines are less basic than alkyamines.

For example, aniline is less basic than ethylamine as shown by K_b 

values:

Ethylamine: K_b = 4.7 × 10^-4

Aniline: K_b = 4.2 × 10^-10

The less basic character of aniline can be explained on the basis of aromatic ring present in aniline.

Aniline can have the following resonating structures:

It is clear from the above resonating structures that three of these (II, III and IV) acquire some positive charge on N atom. As a result, the pair of electrons become less available for protonation. Hence, aniline is less basic than ethyl amine in which there is no such resonance.