Comparing relative basicity of amines and amides is a common H2 A Level Chemistry question. We talked in detail on how to explain and compare the basicity of amines and amides in this post here.
In this post, let’s look at a sample question from the past A Level paper.
Question on relative basicity of amines and amides
Question: Describe and explain the relative basicities of ethanamide, CH3CONH2, ethylamine, CH3CH2NH2, and phenylamine, C6H5NH2, in aqueous medium.
Recall basicity is dependent on the availability of lone pair electrons on nitrogen. The more available this lone pair electron for dative covalent bonding with a proton (H+), the more basic the compound. Electron donating groups will enhance availability of this lone pair electrons by increasing its electron density, hence basicity is increased.
Ethylamine CH3CH2NH2 has an electron donating ethyl group attached to the amine group which will enhance availability of this lone pair of electrons on nitrogen.
For phenyl amine, this lone pair of electron is delocalised into the benzene ring. What’s the result? Decrease in availability of lone pair electron on nitrogen.
For ethanamide, we learnt that amides are neutral, as the lone pair is not available for forming a dative bond with a proton. Hence, ethanamide will be the least basic.
Comparing Relative basicity
With that, we can arrange the compounds in order of their relative basicity:
ethanamide(CH3CONH2 ) < and phenylamine (C6H5NH2) < ethylamine (CH3CH2NH2)
Explanation of relative basicity
And the explanation for this order of their relative basicity:
A base is a proton acceptor. The more available is the lone pair of electron on nitrogen for dative covalent bonding with nitrogen, the more basic the nitrogen compound.
In ethanamide, the lone pair of electrons on nitrogen can delocalise into the C=O group, making this lone pair of electrons on nitrogen not available for dative bonding with a proton. Hence, ethanamide is neutral, and the least basic of the three.
In phenylamine, the lone pair electron on nitrogen can delocalise into the benzene ring. This reduces the electron density of the lone pair electron on nitrogen, and hence its availability to form a dative bond with a proton.
In ethylamine, the ethyl group is electron donating and will increase electron density of the lone pair electron on nitrogen, making it more available for dative bonding with a proton.
Hence, ethylamine is more basic than phenylamine.
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