Compare Acidity of Organic Compounds - Worked Example

Let's take a look at this week's question.

We are required to sort the following compounds in increasing pH or decreasing acid strength:

chloroethanoic acid, ethanoic acid, ethanoyl chloride and ethyl ethanoate

In this list we have 2 carboxylic acids, 1 acid chloride and 1 ester.

Let's consider the acidity of each functional group.

1. Aqueous solution of Acid Chloride

Anhydrous acid chloride is neutral since there are no H+ to donate.

In aqueous medium however, acid chloride can react with water to form carboxylic acid and HCl.

Since HCl is a strong acid, it will dissociate fully to give a high concentration of H+(aq).

Hence an aqueous solution of ethanoyl chloride will be the most acidic.

2. Ester

Esters have no protons to donate hence they are neutral.

For this question we will expect ethyl ethanoate to have the highest pH.

Hence the answer to this question will be option B.

3. Comparing Acids

Let's compare the acidity of chloroethanoic acid and ethanoic acid.

For chloroethanoic acid, the electron withdrawing Cl group will disperse negative charge on O- of conjugate base.

The conjugate base will be more stable, hence chloroethanoic acid will be more acidic than ethanoic acid.

Topic: Carboxylic Acids and Derivatives, Organic Chemistry, A Level Chemistry, Singapore

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