Question

Arrange the following compounds in increasing order of their property as indicated:

CH₃CH₂CH(Br)COOH, CH₃CH(Br)CH₂COOH, (CH₃)₂CHCOOH, CH₃CH₂CH₂COOH (acid strength)

Answer 1

Carboxylic acid molecules with electron-withdrawing groups or electronegative atoms are more acidic. In addition, bonding the atom/group further away from the a-carbon reduces acidity. Thus, the acid strength is:

\[\begin{array}{cc}
\phantom{.....................................}\ce{Br}\phantom{.......................}\ce{Br}\\
\phantom{....................................}|\phantom{.........................}|\\
\ce{\underset{I}{(CH3)2CHCOOH} < \underset{II}{CH3(CH2)2COOH} < \underset{III}{CH3CHCH2COOH} < \underset{IV}{CH3CH2CHCOOH}}\
\end{array}\]

The lesser number of −CH₃ groups on the α-carbon in II results in increased acidity compared to I. The −CH₃ group increases electron density through the +I action on α-carbon which also affects the −COOH group, hindering the release of H⁺.