Question

Give plausible explanation for the following:

A Why are alcohols more acidic than amines of comparable molecular masses?

B Why do tertiary amines have lower boiling points than primary amines?

C Why are aromatic amines weaker bases than aliphatic amines?

Answer 1

A.If we want to compare the acidity of alcohol and amines, first we will do the deprotonation of the given species and then we check the relative stability of the formed anion. Anion which is more stable is also more acidic.

Deprotonation of amine and alcohol is shown below:

On deprotonation of amines gives amide ion.

$R-N{H}_2\to R-N{H}^{-}+H^{+}$

Similarly, alcohol loses a proton to give alkoxide ion.

$R-OH\to R-O^{-}+H^{+}$

In an amide ion, $N-$ atom has the negative charge on it, while an alkoxide ion, $O-$ atom has the negative charge on it. Since we know, $O$ is more electronegative than $N$, so it can accommodate the negative charge more easily than $N$.

Hence, due to this, the alkoxide ion is more stable than the amide ion. Therefore, alcohols are more acidic than amines of comparable molecular masses.


B.There are no $H-$ atoms present in a molecule of tertiary amine while in case of primary amines two hydrogen atoms are present. Primary amines undergo extensive intermolecular $H-$ bonding because of the presence of $H-$ atoms but tertiary amine does not have $H$ atom so it is not able to form $H$ bonding.



Hence, if we want to boil both amines, we need high amount of energy in case of primary amine to break all hydrogen bonds and detach the molecules of primary amines.

Therefore, tertiary amines have lower boiling points than primary amines.

C.Since we know, the alkyl groups are electron donating groups and we know that the stronger the electron donating group, the higher the $+l$ effect and stronger is the base.

Therefore, in aliphatic amines the $-N{H}_2$ group is attached to an electron donating alkyl group and thus show $+l$ effect and due to this, the electron density on nitrogen increases.

So, it can easily donate their lone pair of electrons to acid or electron deficient species.

But in case of aromatic amines nitrogen atom is directely attached to benzene ring or present in the ring.
So due to resonance, it cannot easily donate their lone pair of electrons to acid or electron deficient species. Hence, due to this aromatic amine weaker bases than aliphatic amines.