Chemistry - Why is formic acid a stronger acid than acetic acid?
Solution 1:
We are discussing the following equilibrium
We can make the acid a stronger acid by pushing the equilibrium to the right. To push the equilibrium to the right we can
destabilize the starting acid pictured on the left side of the equation, and \ or
stabilize the carboxylate anion pictured on the right side of the equation.
Comparing acetic acid ($\ce{R~ =~ CH3}$) to formic acid ($\ce{R~ =~ H}$), the methyl group is electron releasing compared to hydrogen. Therefore the methyl group will stabilize the dipolar resonance form of the starting acid where there is a partial positive charge on the carbonyl carbon. This should stabilize the starting acid. Further, this electron releasing ability of the methyl group will tend to destabilize the resultant carboxylate anion which already has a full unit of negative charge.
Therefore, because the methyl group 1) stabilizes the starting acid and 2) destabilizes the carboxylate anion product, the methyl group will push the equilibrium to the left, compared to the case where the methyl group is replaced by a hydrogen. Consequently, acetic acid is a weaker acid than formic acid.
Solution 2:
The strength of carboxylic acid can be discussed by positive inductive effect. Here, in ethanoic acid there is an electron releasing inductive effect from the alkyl group. The larger the positive inductive effect, the lesser will be the strength of an acid. Similarly in formic acid there is no alkyl group and hence no inductive effect takes place. So formic acid is a stronger acid than ethanoic acid.
Solution 3:
Yes the reasoning is correct . The more the charge is increased on a molecule the more it gets destabilised.