Question

Arrange the following in order of increasing acidity.
$2-$Fluorobutanoic acid, $2-$iodobutanoic acid, $2-$bromobutanoic acid and butanoic acid

Answer 1

Hint : We know that we determine the nature and number of different types of substituents. Here, type of substituent refers to its electron donating nature or electron withdrawing nature. Identify the inductive effect present.

Complete Step By Step Answer:
Electron withdrawing groups show -I effect electron withdrawing groups show -I effect. Here, ‘I’ represents an inductive effect. When electron withdrawing groups are present, the acid strength increases and when electron donating groups are present, the acid strength decreases. With increase in the number of electron withdrawing groups, the acid strength increases. With increase in the strength of -I effect of electron withdrawing groups, the acid strength increases.
The acidity of the carboxylic acids are going to depend on the electron donating groups and electron withdrawing groups attached to them. At the same time the distance between the electron withdrawing groups from the carboxylic acid also decides the acidity of the carboxylic acids.
Electron donating groups are called +I groups, in the given carboxylic acids methyl groups act as electron donating groups and bromine atoms act as electron withdrawing groups (-I effect). We know that electron withdrawing groups increase the acidity of the carboxylic acids and electron donating groups decrease the acidity of the carboxylic acid molecules.
$C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}COOH < C{{H}_{3}}C{{H}_{2}}CHICOOH < C{{H}_{3}}C{{H}_{2}}CHBrCOOH < C{{H}_{3}}C{{H}_{2}}CHFCOOH$

Note :
Remember that a strong acid can easily donate a proton. The carbanion obtained by donation of a proton is stabilized due to inductive and resonance effects. Greater is the stabilization of the carbanion through inductive and resonance effects, greater is the acid strength of the molecule.