Question

Which of the following is the strongest acid?
A. CH${}_3$COOH
B. CH${}_2$ClCOOH
C. CHCl${}_2$COOH
D. CCl${}_3$COOH

Answer 1

Hint: The acidity of a compound, or molecule depends upon the halogen group attached to the acid. In this question, the attached halogen is chlorine, chlorine shows –I effect. The presence of the attached group increases the acidity.

Complete step by step answer:
1. Firstly, let us discuss the acidity of carboxylic acids, they are more acidic than alcohols, or phenols. The carboxylic acids are acidic in nature as the hydrogen group belongs to the –COOH group.
2. Now, if we see the options, the first option is acetic acid, there is no group attached to it. So, it will be a less acidic compound.
3. Talking about the second option, there is one chloro group attached to the acetic acid.
4. Similarly, in the third, and fourth option there are 2, and 3 chloro groups attached respectively.
5. As we know, the greater the number of electron withdrawing groups (halogens), stronger will be the acid.
6. Thus, the order of acidity is: CCl${}_3$COOH > CHCl${}_2$COOH > CH${}_2$ClCOOH > CH${}_3$COOH.
7. So, the strongest acid is CCl${}_3$COOH.

Therefore, the correct option is D.

Note: Don’t get confused between the +I effect, and –I effect. The presence of +I effect leads to the decrease in acidity. It is represented by the chemical species having the tendency to release, or donate electrons like alkyl groups, but the groups having –I effect have the tendency to accept electrons like halogens.