Question

State reasons for the following:
(1) Monochloroethanoic acid has a higher pK value than dichloroethanoic
acid
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Answer 1

Answer:

monochloroethanoic acid has a higher pKa value than dichloroethanoic acid. ... Thus it implies that dichloroacetic acid is stronger than monochloroacetic acid. Acidity of carboxylic acid depends on how easily the H+ is lost and how stable is the conjugate base.

Answer 2

Monochloro ethanoic acid has a higher pKa value than dichloro ethanoic acid because of the decrease in the electron density on the carbon of dichloro ethanoic acid caused by the increased negative inductive effect due to two chlorine groups as compared to one.

• The greater is the value of pKa of an acid the lesser is the acidic character.
• Chlorine group produce a negative inductive effect.
• The greater is the negative inductive effect, more acidic is the compound.
• Since, there is a greater negative inductive effect on dichloro ethanoic acid due to the presence of two chlorine, dichloro ethanoic acid is more acidic and hence have a lesser pka value compared to monochloro ethanoic acid.