Comparing Acidity of Organic Compounds

In this video we want to compare acidity of some organic compounds that we learn in A Level Chemistry.

Let's consider the dissociation of a weak acid HA to form H+ and conjugate base A-.

comparing acidity of organic compounds dissociation of weak acid

If the conjugate base is more stable, the system will favour the formation of it by shifting the position of equilibrium towards the right.

This releases more H+ hence it makes weak acid HA more acidic.

Therefore when we want to compare acidity in organic chemistry, we focus on the stability of conjugate base - the more stable the conjugate base, the stronger the acid.

comparing acidity of organic compounds stronger acid has more stable conjugate base

Comparing Organic Compounds

Here is the acidity trend of functional groups.

comparing acidity of organic compounds acidity trend of functional groups

For each of the functional groups, we can look at the conjugate base and deduce its stability to explain the acidity of each compound.

Carboxylic Acid

Acids will dissociate to give conjugate base carboxylate and H+.

comparing acidity of organic compounds dissociation of carboxylic acid

The carboxylate is resonance stabilised to a very significant extent, as the negative charge on oxygen is delocalised extensively between 2 electronegative oxygens.

comparing acidity of organic compounds stability of carboxylate

This makes the carboxylate very stable, hence carboxylic acid is the most acidic functional group.

Phenol

Phenols will dissociate to give conjugate base phenoxide and H+.

comparing acidity of organic compounds dissociation of phenol

The phenoxide is also resonance stabilised as the negative charge on oxygen can be delocalised into the pi electron system of benzene.

comparing acidity of organic compounds stability of phenoxide

This makes phenoxide more stable hence phenol is more acidic than water.

Water

Even though water is not an organic compound, putting water into the comparison is useful.

Water will dissociate to give hydroxide and H+.

comparing acidity of organic compounds dissociation of water

Hydrogen attached to oxygen atom is neither electron donating nor electron withdrawing.

Thus it does not have any destabilising or stabilising effect on the negative charge on oxygen.

comparing acidity of organic compounds stability of hydroxide

This makes water neutral and a useful benchmark to compare acidity of other organic compounds.

Alcohol

Finally, alcohols will dissociate to give alkoxides and H+.

comparing acidity of organic compounds dissociation of alcohol

The oxygen atom is attached to electron donating alkyl group which intensifies the negative charge on oxygen.

comparing acidity of organic compounds stability of alkoxide

This destabilises the alkoxide conjugate base and hence alcohol is less acidic than water.

For the detailed step-by-step discussion on how to compare acidity of organic compounds, check out this video!

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

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