Question

The strongest acid among the following aromatic compound is:
A: o-Nitrophenol
B: p-Chlorophenol
C: p-Nitrophenol
D: m-Nitrophenol

Answer 1

Hint : Acids are the species which donate hydrogen ions while dissolved in water. Strong acids dissociate completely when dissolved. Aromatic compounds come under unsaturated chemical compounds.

Complete step by step solution :
Ortho, meta and para are prefixes used in organic chemistry to indicate the position of non-hydrogen substituents on aromatic rings. If the substituent is attached at an adjacent position to the functional group then it is called ortho position. There are two ortho positions in one compound. Position adjacent to ortho is called meta position. There are two meta positions in one compound and the position opposite to the functional group is called para. There is only one para position in a compound. o is used for ortho position, m is used for meta position and p is used for para position.
In all the given options phenol word is also present this means the given compound contains a phenol group. -$OH$ is called phenol group. Phenols are acidic in nature as when it releases its hydrogen phenoxide ion is formed and this ion is highly stable. It's stability is because when it releases ${H^ + }$ ion it gains negative charge, this charge goes in resonance. This means negative charge is not localised. It keeps on moving. It goes on to ortho and para position. This means ortho and para phenols will be highly acidic. Among given options option A, B and C has this phenol.
Now another factor which is responsible for acidic nature is electron withdrawing group (EWG). As the name suggests these groups withdraw electrons from the group thus making them lose ${H^ + }$ ions (because they will become electron deficient and they will not leave that ion). Nitro group $(N{O_2})$ is more electronegative than chloro group $(Cl)$.Therefore answer will be either option A or C. As both are acidic but due to less steric hindrance p-Nitrophenol is correct. So the answer to this question is option C i.e. p-Nitrophenol.

Note : To find acidic compounds first identify at which position group is present. Then check if some electron withdrawing group or donating group is present. Electronic withdrawing groups will make compound acidic and electron donating will make the group basic.