Question

Which of the following correctly ranks the aryl halides in increasing order of reactivity towards sodium methoxide in methanol?

• A.

  c1c(Br)cc(N(=O)=O)cc1 < c1c(F)c(N(=O)=O)cc(N(=O)=O)c1 < c1cc(Br)cc(N(=O)=O)c1N(=O)=O < c1c(Br)c(N(=O)=O)cc(N(=O)=O)c1
  

• B.

  c1cc(Br)cc(N(=O)=O)c1 < c1cc(Br)cc(N(=O)=O)c1N(=O)=O < c1c(F)c(N(=O)=O)cc(N(=O)=O)c1 < c1c(Br)c(N(=O)=O)cc(N(=O)=O)c1
  

• C.

  c1c(F)c(N(=O)=O)cc(N(=O)=O)c1 < c1c(Br)c(N(=O)=O)cc(N(=O)=O)c1 < c1cc(Br)cc(N(=O)=O)c1N(=O)=O < c1cc(Br)cc(N(=O)=O)c1
  

• D.

  c1cc(Br)cc(N(=O)=O)c1 < c1cc(Br)cc(N(=O)=O)c1N(=O)=O < c1c(Br)c(N(=O)=O)cc(N(=O)=O)c1 < c1c(F)c(N(=O)=O)cc(N(=O)=O)c1
  

Answer 1

Answer: (D)

c1cc(Br)cc(N(=O)=O)c1 < c1cc(Br)cc(N(=O)=O)c1N(=O)=O < c1c(Br)c(N(=O)=O)cc(N(=O)=O)c1 < c1c(F)c(N(=O)=O)cc(N(=O)=O)c1

Answer 2

The reaction of aryl halides with sodium methoxide in methanol is a Nucleophilic Aromatic Substitution (SNAr) reaction, specifically an addition-elimination mechanism. The reactivity of aryl halides in SNAr reactions is primarily governed by:

1. Electron-withdrawing groups (EWGs): Strong EWGs, particularly at the ortho and para positions relative to the leaving group, stabilize the negative charge in the intermediate Meisenheimer complex. Nitro groups (-NO₂) are strong EWGs. Meta-positioned EWGs only exert an inductive effect, which is less effective.

2. Nature of the leaving group: For SNAr reactions, the leaving group ability generally follows the order F > Cl > Br > I. The strong inductive effect of fluorine effectively stabilizes the negative charge, facilitating the attack.

Based on these factors, the correct order is:

Compound 1 (One meta -NO₂) < Compound 3 (Two meta -NO₂) < Compound 2 (Two ortho/para -NO₂) < Compound 4 (Two ortho/para -NO₂ with F)