Question

Use the four compounds shown below to answer the following questions:

1. Why are the ortho-halo-substituted benzoic acids stronger acids than benzoic acid?
2. Why is o-fluorobenzoic acid the weakest of the ortho-halo-substituted benzoic acids?
3. Why do o-chlorobenzoic acid and o-bromobenzoic acid have similar pKa values?

Short answer

1. Ortho-halo-substituted benzoic acids are less reactive towards electrophilic substitution compared to benzoic acid. Decrease in reactivity will show increase in acid strength.
2. Fluorine is the weakest deactivating group compared to other halogens and it donates electrons. So, o-fluorobenzoic acid is the weakest.
3. Halogens are better at withdrawing electrons from the ring. But o-chlorobenzoic acid and o-bromobenzoic acid doesn’t withdraw electrons because 2p orbital of carbon is overlapping with 3p and 4p orbital of given halogens respectively. Hence, they have similar pKa values.

Step-by-step solution

Reason behind ortho-halo-substituted benzoic acids are stronger acids than benzoic acid (a)

Ortho-halo-substituted benzoic acid contain halogen groups, are electron withdrawing groups. They destabilize the benzene ring towards electrophilic substitution reaction and there by decreases reactivity.Benzoic acid is reactive towards electrophilic substitution reaction and hence, strong acid. Thus, decrease in reactivity will show increase in acid strength.

Reason behind o-fluorobenzoic acid the weakest of the ortho-halo-substituted benzoic acids (b)

Among halogens fluorine is the weakest deactivating group. Compared to other halogens it donates electrons and hence, o-fluorobenzoic acid is the weakest.

Reason behind o-chlorobenzoic acid and o-bromobenzoic acid have similar pKa values (c)

Among the halogen series Br is the most deactivating group compared to chlorine. Smaller halogens are better at donating electrons because 2p orbital of carbon overlaps better with 2p orbital of halogen other than 3p and 4p orbital of halogens.So, bromine and chlorine doesn’t withdraw electrons. Hence, both halogen containing benzoic acid is similar at their pKa value.