Organic Chemistry MCQ - 1 (Advanced) - JEE MCQ

IV will be most acidic owing to -M effect of -NO₂. For comparison b/w halides, we use their +M effect(Just to check). F’s +M effect will be stronger than Cl’s as the F-C bond is shorter than the Cl-C bond.Also, electron density at F is greater than Cl. This will make electrons available to the acidic group thus decreasing its acidic strength. The last will be I. So the order is 
IV>III>II>I.

Acidic nature can be calculated by the stability of conjugated base. That acid which makes more stable conjugated base is more stable. The leaving of H+ ion will form conjugate base having negative charge. By the concepts of aromaticity we come to know that option A and C will form anti aromatic compound. Option C will form compound having resonance but it will be anti-aromatic. But option B will form aromatic compound which is more stable than others. So B is most acidic having most stable conjugated base.

All finally comes down to the 4n + 2 rule.

Hence B

pH of phenol, ethyl alcohol, formic acid and benzoic acid is 5-6, 7.33, 3.47 and 3.66 respectively

Every carbonyl group are electron deficient and stablised by resonance. In cyanide nitrogen exhibiting -3 oxidation state and of its ionic character it also under goes resonance. except (e) all elements in given option undergoes resonance and are EWG.

Resonance occurs in a conjugated system and in option b, d, e the lone pairs are joined in conjugation i.e. double bond then single bond and then lone pair. So, in these three cases the lone pair participates in resonance.

Aromatic compound formula is 4n + 2

where n = no. of π e's + lone pair e's whose orbital are parallel to adjacent C atom 's orbital

Non aromatic compound formula is 4n

In a,b ,d - n= 6 in each while in c option n= 4 as B has no lone pair of e's

An aromatic hydrocarbon always has a sigma as well as a delocalized pi bond found between the carbon atoms.

Substituents with lone pairs (e.g. −OCH₃, −NH₂) on the atoms adjacent to the ππ-system are electron donating groups (EDG) in resonance systems. And, substituents with ππ-bonds to electronegative atoms (e.g. −C=O, -NO₂) adjacent to the ππ-system are electron withdrawing groups (EWG) in a resonating structure

Therefore among the following groups electron-donating groups are:

(C) -OCOCH₃, and (E) −NHCOCH₃

Substituents with lone pairs (e.g. −OCH₃, −NH₂) on the atoms adjacent to the π-system are electron donating groups (EDG) in resonance systems. And, substituents with ππ-bonds to electronegative atoms (e.g. −C=O, −NO₂) adjacent to the π-system are electron withdrawing groups (EWG) in a resonating structure

Therefore among the following groups electron-donating groups are:

(C) −OCOCH_3, and (E) −NHCOCH₃