Analyzing the Setup
We are given a multi-step organic synthesis problem starting with compound P, which is an aromatic ester with a bulky tert-butyl group para to the ester-containing alkyl chain
The journey from P to the final product S involves a series of classic organic reactions: Grignard addition, intramolecular Friedel-Crafts alkylation, and finally, Friedel-Crafts acylation.
The Grignard Addition (P to Q)
Compound P contains an ester group, specifically −CO2Et
When we treat an ester with an excess of a Grignard reagent like methyl magnesium bromide (CH3MgBr), the nucleophilic methyl group attacks the electrophilic carbonyl carbon.
Because it's an ester, the first addition kicks out the ethoxy leaving group to form a ketone intermediate. However, since the Grignard reagent is in excess, a second molecule immediately attacks the highly reactive ketone. This double addition converts the ester into a tertiary alcohol, giving us compound Q. The chain is now −CH2−CH2−C(OH)(CH3)2.
Intramolecular Friedel-Crafts Alkylation (Q to R)
Next, compound Q is treated with concentrated sulfuric acid (H2SO4) at 0∘C
The strong acid protonates the hydroxyl group, turning it into a fantastic leaving group (water). As water departs, it leaves behind a highly stable tertiary carbocation: −CH2−CH2−C+(CH3)2.
This carbocation is an excellent electrophile, and it's tethered right next to an electron-rich benzene ring! The carbocation attacks the adjacent ortho position of the benzene ring in an intramolecular Friedel-Crafts alkylation. Because the chain has three carbons between the benzene ring and the carbocation center, the attack forms a stable five-membered ring fused to the benzene ring. This gives us compound R, an indane derivative. Notice that the gem-dimethyl group ends up right next to the benzene ring on the newly formed fused ring.
Friedel-Crafts Acylation and Steric Hindrance (R to S)
Finally, compound R undergoes Friedel-Crafts acylation with acetyl chloride (CH3COCl) and anhydrous aluminum chloride (AlCl3)
The challenge here is regioselectivity: where will the incoming acetyl group attach?
The benzene ring in R has two activating alkyl groups: the extremely bulky tert-butyl group and the fused indane ring. We must evaluate the steric hindrance at all available ortho positions:
1. The position between the gem-dimethyl group and the tert-butyl group is a steric nightmare. Attack here is practically impossible.
2. The position ortho to the tert-butyl group is also highly crowded due to the sheer bulk of the three methyl groups.
3. The position ortho to the −CH2− group of the fused ring is the least sterically hindered available site.
Therefore, the electrophile smartly chooses the path of least resistance and attacks the position ortho to the −CH2− group. This results in the final product S, where the acetyl group is at the bottom vertex of the ring, perfectly matching option (D).
Furthermore, the reaction sequence from Q to R is a Friedel-Crafts alkylation accompanied by dehydration, and R to S is a Friedel-Crafts acylation, which corresponds to option (B) for the first sub-question.
