Question

Select the best reaction sequence to accomplish the following transformation

• A. $\frac{HNO_3}{H_2SO_4}$ $\longrightarrow$ $\frac{Cl}{AlCl_3}$ $\longrightarrow$ $\frac{Excess}{Zn(Hg), HCl}$ $\longrightarrow$
• B. $\frac{HNO_3}{H_2SO_4}$ $\longrightarrow$ $\frac{Cl}{AlCl_3}$ $\longrightarrow$ $\frac{Excess}{Zn(Hg), HCl}$ $\longrightarrow$
• C. $\frac{HNO_3}{H_2SO_4}$ $\longrightarrow$ $\frac{Zn(Hg)}{ClH}$ $\longrightarrow$ $\frac{Cl}{AlCl_3}$ $\longrightarrow$ $\frac{Excess}{Zn(Hg), HCl}$ $\longrightarrow$
• D. $\frac{Cl}{AlCl_3}$ $\longrightarrow$ $\frac{HNO_3}{H_2SO_4}$ $\longrightarrow$ $\frac{Excess}{Zn(Hg), HCl}$ $\longrightarrow$

Answer 1

Answer: (D)

D

Answer 2

The transformation requires converting benzene into 3-isobutyl-aniline. The key aspects are introducing an amino group and an isobutyl group, and ensuring they are in a meta-relationship on the benzene ring.

Let's analyze the directing effects of functional groups:

• Alkyl groups (-R) and Amino groups (-NH2) are ortho/para-directing and activating.
• Nitro groups (-NO2) and Acyl groups (-COR) are meta-directing and deactivating.

To achieve a meta-relationship between the final amino and isobutyl groups, at least one of the groups introduced early in the synthesis must be a meta-director when the second group is added.

Option D is the most chemically sound and practical sequence because the Friedel-Crafts reactions in options A, B, and C are problematic due to deactivation or complexation of the aromatic ring.

Option D follows the sequence:

1. Friedel-Crafts Acylation: Benzene is acylated with isobutyryl chloride ($\text{Cl-CO-CH(CH}_3)_2$) using $\text{AlCl}_3$ to form isobutyrophenone. This introduces the carbon chain.
2. Nitration: The isobutyrophenone then undergoes nitration with $\text{HNO}_3/\text{H}_2\text{SO}_4$. The acyl group ($-\text{COR}$) is a meta-director, so the nitro group ($-\text{NO}_2$) adds to the meta position, establishing the desired meta-relationship.
3. Reduction: The resulting 3-nitro-isobutyrophenone is treated with excess $\text{Zn(Hg), HCl}$ (Clemmensen reduction conditions). This reagent simultaneously reduces the ketone to an alkane (converting the isobutyryl group to an isobutyl group) and the nitro group to an amino group, yielding 3-isobutyl-aniline.