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Basicity of Amines and Amides – Explain and compare

comparing basicity of amines and amides - how to explain and how to determine relative basicity

Explaining and comparing the basicity of amines and amides are often tested in H2 A Level Chemistry exams. In this post, let’s look at :

What makes a compound basic?

There are many definitions of bases. If we use the Bronsted Lowry theory, then a base is a proton acceptor. If a compound accepts a proton (H+) , it will be acting as a base.

Let’s use ammonia as an example.

NH3 + H2O NH4+ + OH

In the equation above, ammonia (NH3) accepts a proton (H+) from water to form NH4+ . Since it accepts a proton, it is a base.

Why are amines basic?

The lone pair electron on nitrogen in an amine is able to form a dative covalent bond with a proton. Since it accepts a proton, amines can act as bases.

Primary amine accepting a proton:

basicity of amines: Primary amine acts as a base by accepting a proton.

Secondary amine accepting a proton:

secondary amine acts as a base by accepting a proton.

Tertiary amine accepting a proton:

tertiary amine acts as a base by accepting a proton.

Note: Every time an amine accepts a proton, the number of H bonded to nitrogen increases by 1, and the charge also increases by 1.

Factors affecting basicity of amines

The basicity of amines is determined by its availability of the lone pair electron on the nitrogen atom to accept a proton. The higher the charge density, the greater the availability of this lone pair of electrons, and hence the more basic the amine is.

Since alkyl group (or R groups) are electron donating, tertiary amines are generally more basic than secondary amine, and secondary amines are more basic and primary amines.

Comparing basicity of aliphatic amines and phenylamines

Aliphatic amines are more basic than phenylamines.

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.

Phenylamines are less basic than amines due to the ability of the lone pair electron on N to delocalise into the benzene ring
Phenylamines are less basic than amines due to the ability of the lone pair electron on N to delocalise into the benzene ring

Why are amides neutral?

We have talked about amines for most of this post. In this post, let’s talk about why amides are neutral.

structure of amides
Structure of amides

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, amides are neutral.

Resonance structure of amides, showing the delocalisation of the lone pair of electron on nitrogen into the C=O group. Hence, amides are not basic, but instead neutral.
Resonance structures of amides (Note: not required for H2 A Level exams)

p/s: From past A Level exams, “why amides are neutral” is a common question tested in H2 A Level Chemistry.

Explanation Questions for Organic Chemistry

In organic chemistry, explanation questions are very common. Asking students to explain the different basicity of amines and amides (i.e. nitrogen compounds) is one of them. There are also other types of explanation questions that are tested.

This entire post is on basicity of organic compounds. If you are also interested in acidity of organic compounds, check out this post here.

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