JEE Main 2020 · 2 September · morning · Single correct · +4 / −1
The increasing order of the following compounds towards HCN addition is :
- (a) (iii) < (iv) < (ii) < (i)
- (b) (i) < (iii) < (iv) < (ii)
- (c) (iii) < (i) < (iv) < (ii)
- (d) (iii) < (iv) < (i) < (ii)
Answer and solution
Answer (c) (iii) < (i) < (iv) < (ii)
Solution
Chemistry ma’am · 3 steps
HCN attacks the carbonyl carbon, so rank each ring by how much it pulls electrons away from CHO.
- What decides rate. CN⁻ is a nucleophile, and it attacks the carbonyl carbon. The more electron-poor that carbon is, the faster HCN adds. Electron-withdrawing groups speed it up, and electron-donating groups slow it down.
- Position matters. Resonance (± R) works only from the ortho and para positions. From meta, a group acts only through its inductive effect. So -OCH₃ donates by +R at ortho, but at meta it shows only a weak -I pull. -NO₂ withdraws by -R and -I at ortho, but only by -I at meta.
Compound Group and position Effect on the C=O carbon Reactivity (iii) -OCH₃, ortho +R pushes electrons in, carbon less positive lowest (i) -OCH₃, meta no +R, only weak -I second (iv) -NO₂, meta strong -I only third (ii) -NO₂, ortho -R and -I together, carbon most positive highest - So. The increasing order is (iii) < (i) < (iv) < (ii), which is option (c).
The idea to keep. For nucleophilic addition, rank by how electron-poor the carbonyl carbon is. Check the position first: resonance acts only from ortho and para, and from meta only the inductive effect counts.
Answer from the NTA final answer key. Solution by PrepWiser.