2021 P1 Q21 - Comparing Basicity of Amines

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Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 21.

2021 p1 q21 question

We want to compare and explain the relative base strength of amines.

To explain basicity of nitrogen compounds in organic chemistry, we focus on the availability of lone pair on nitrogen.

The more available the lone pair on nitrogen is for donation, the stronger the base.

There are 2 effects that will affect amine basicity.

1. Electronic effect

Electronic effect is based on electron donating and electron withdrawing groups.

Donating and withdrawing groups affect the electron density of the functional group which then affect its reactivity or stability.

When amine is attached to more electron donating alkyl groups, nitrogen becomes more electron rich.

This makes its lone pair more available for donation.

Hence its basicity will get stronger.

2021 p1 q21 electronic effect

From the electronic effect we will expect tertiary amines to be most basic and ammonia to be least basic.

2021 p1 q21 electronic effect trend

2. Steric effect

Steric effect is based on steric hinderance of the groups attached to the functional group.

These groups act as physical barriers that block the entry of attacking groups hence lower its reactivity.

The more bulky alkyl groups attached to nitrogen, the greater the steric hinderance.

It becomes harder for the lone pair of nitrogen to be donated hence basicity will decrease.

2021 p1 q21 steric effect

From the steric effect we will expect ammonia to be most basic and tertiary amines to be least basic.

2021 p1 q21 steric effect trend

Notice the electronic and steric effects oppose each other.

The electronic effect is dominant for gaseous amines in A Level syllabus hence we will deduce the basicity trend based on electron donating effect of alkyl groups.

2021 p1 q21 compare electronic vs steric

Therefore we will expect tertiary amine to be most basic and primary amine to be least basic as predicted by electron donating effect of alkyl groups.

2021 p1 q21 answer

The answer to this question will be option A.

Topic: Nitrogen Compounds and Proteins, Organic Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2021 A Level H2 Chemistry Paper 1.

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