Question

Which of the following is the correct order of acidic strength?
A: $C{l_3}CCOOH > C{l_2}CHCOOH > ClC{H_2}COOH > C{H_3}COOH$
B: $C{l_3}CCOOH > ClC{H_2}COOH > C{l_2}CHCOOH > C{H_3}COOH$
C: $C{H_3}COOH > C{l_3}CCOOH > C{l_2}CHCOOH > ClC{H_2}COOH$
D: $C{H_3}COOH > ClC{H_2}COOH > C{l_2}CHCOOH > C{l_3}CCOOH$

Answer 1

Hint: Acids are chemical compounds which donate ${H^ + }$ ions. More easily a compound will dissociate into ${H^ + }$ ions more strong acid that compound will be. So we need to find which compound will give hydrogen ions the easiest.


Complete step by step solution:
Compounds which will be deficient in electrons will withdraw electrons from hydrogen atom as a result carbon hydrogen bond will become weak and hydrogen atom will be separated as hydrogen ion. In this question the number of chlorine atoms is increasing in compounds (replacing hydrogen atoms). Chlorine acts as an electron withdrawing group. This means chlorine takes electrons from the atom with which it is attached. So, more the number of chlorine attached to carbon is more electron deficient than carbon will be, hence more electrons that carbon will take from hydrogen atom, which means more weaker carbon hydrogen bond will be in that atom. So compounds which contain more chlorine atoms will be most acidic. In option A compounds are arranged with decreasing number of chlorine atoms. So the correct order will be as shown in option A that is $C{l_3}CCOOH > C{l_2}CHCOOH > ClC{H_2}COOH > C{H_3}COOH$.
Additional information: Like electron withdrawing groups there exists a term called electron donating group. As the name suggests electron donating group is the group which donates electrons to the atom with which it is connected. Atoms which have an excess of electrons present in the form of lone pairs act as electron donating groups. Examples of electron donating groups are amine, alkyl groups, aryl groups etc. Electron withdrawing groups include halogens, aldehydes etc. Halogen acts as an electron withdrawing group because halogens have one electron in their outermost shell. So, they withdraw electrons from other compounds to complete their octet and become stable. Electron donating groups are also called activating groups as they activate the compound to which they are attached and electron withdrawing groups are also called deactivating groups as they deactivate the compound to which they are attached.



Note: Terms electron withdrawing group(EWG) and electron donating group(EDG) are key to this question. So, in such questions just identify which kind of group is attached to atoms. If EWG is a present option with maximum number of EWG will be the answer and if EDG is present then option with minimum number of EDG will be the answer.