See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Friedel-Crafts acylation is an electrophilic aromatic substitution (EAS) reaction. CH3COCl/AlCl3 generates the acetylium electrophile (CH3CO+), which attacks the aromatic ring of the indane substrate. Step 1 – Identify the substrate: The starting material is 5-tert-butyl-1,1-dimethylindane. The benzene ring of the indane is fused to a cyclopentane ring (with gem-dimethyl at C1). The tert-butyl group is at C5 of the benzene ring. Step 2 – Identify directing effects: The indane benzene ring has two substituents that direct EAS: (i) The C1 gem-dimethyl cyclopentane ring junction connects at C3a and C7a positions. The ring fusion makes C3a and C7a the junction carbons. The alkyl ring fusion activates the ring and directs ortho/para to the C-C bonds at the junction. (ii) The tert-butyl group at C5 is an ortho/para director (activating). Step 3 – Available positions: The benzene ring positions are C4, C5 (blocked by tBu), C6, and C7 (C3a and C7a are ring junction carbons, not available). So electrophilic attack can occur at C4, C6, or C7. Step 4 – Determine the most activated position: - The tert-butyl group at C5 directs ortho (to C4 and C6) and para (to C2, which is a ring junction — blocked). - The ring junction at C7a (bearing the gem-dimethyl cyclopentane) directs ortho/para: ortho to C7a is C7 and C1 (junction, blocked); para to C7a is C4. - The ring junction at C3a directs ortho/para: ortho to C3a is C4 and C2 (blocked junction); para to C3a is C6 (actually C7a, blocked) — so C4 is activated by C3a ortho. - C4 is activated by: ortho to tert-butyl (C5), ortho to C3a ring junction. Both directing effects converge on C4. - C6 is activated by ortho to tert-butyl and ortho to C7a ring junction, but C6 is between two bulky groups (tBu at C5 and ring junction at C7a), making it sterically hindered. - C4 receives cooperative activation from both the tert-butyl group (ortho) and the C3a ring fusion (ortho), and is less sterically hindered than C6. Step 5 – Steric consideration: C6 is flanked by the tert-butyl group (C5) and the C7a ring junction, making it more sterically crowded. C4 is flanked by tert-butyl (C5) on one side and only a hydrogen at the ring junction side (C3a), making it more accessible. Step 6 – Major product: Acylation occurs at C4, placing the acetyl group (COCH3) at C4, adjacent to the tert-butyl group at C5, on the same side as the five-membered ring junction (C3a). This corresponds to option (c). Why other options fail: - (a) places acetyl at C6, which is sterically hindered between tBu and the ring junction. - (b) places acetyl at C4 but drawn with a different orientation — examining carefully, (b) shows the acetyl between the tert-butyl and the ring junction on the cyclopentane side but attached differently (appears to show acylation adjacent to C7a rather than C3a). - (d) places acetyl at C7, which is only ortho to C7a ring junction but meta to tert-butyl — not the most activated position. Therefore, the correct answer is C.