Question

Rank the following acids in decreasing order of their acid strength. In each case, explain why the previous compound should be a stronger acid than the one that follows it.

Short answer

Stronger electron-withdrawing groups stabilize the anion of the conjugate base more than weaker groups do, leading to stronger acids. Chlorine is more electronegative and a stronger withdrawing group than bromine, making chloropentanoic acid a stronger acid than bromopentanoic acid.

Step-by-step solution

     Step 1

Stronger electron-withdrawing groups stabilize the anion of the conjugate base more than weaker groups do, leading to stronger acids. Chlorine is more electronegative and a stronger withdrawing group than bromine, making chloropentanoic acid a stronger acid than bromopentanoic acid.

Step 2

Adding a chlorine atom to pentanoic acid increases its acidity, and the effect is larger if the chlorine atom is closer to the acidic group.

Step 3

In case of bromopentanoic acid, acidic effect is larger if the bromine atom is closer to the acidic group. Therefore 2-bromopentanoic acid is more acidic than 4-bromopentanoic acid.

Step 4

Multiple electron-withdrawing groups stabilize the conjugate base and increase the acidity more than a single group does. Therefore 2,2 dichloropentanoic acid is more acidic than 2-chloropentanoic acid.