Question

Which acid of the pair shown here would you expect to be stronger?

CH₂FCH₂CH₂CO₂H or CH₃CHFCH₂CO₂H

Answer 1

\[\begin{array}{cc} \phantom{...................}\ce{O}\\ \phantom{...................}||\\ \ce{\underset{4-Fluorobutanoic acid}{F <- CH <- CH2 <- CH2 <- C <- O <- H}}\ \end{array}\]

\[\begin{array}{cc} \ce{CH3}\phantom{..............}\ce{O}\phantom{........}\\ \phantom{.}|\phantom{.................}||\phantom{........}\\ \ce{\underset{3-Fluorobutanoic acid}{F <- CH <- CH2 <- C <- O <- H}}\ \end{array}\]

The inductive effect decreases with distance, so −I effect of F is stronger in 3-fluorobutanoic acid than in 4-fluorobutanoic acid. Hence, CH₃CHFCH₂COOH is stronger acid than FCH₂CH₂CH₂COOH.