Q8(d). Structure of C₉H₁₀O₂ from UV, IR and NMR

Reading of the question: a methyl ketone CH₃CO on an aromatic ring is usually near δ 2.5; the printed 2.0 is taken as this signal.

1. DBE = 9 - 10/2 + 1 = 5: a benzene ring (4) + one C=O (1).
2. IR 1680 cm⁻¹: conjugated (aryl) ketone C=O (lower than 1715 because of conjugation with the ring and the donor OMe).
3. UV 270 nm: π→π* of an aroyl chromophore with a para electron donor (benzoyl K-band shifted to longer λ).

Figure: 4-methoxyacetophenone

1. NMR 7.6 and 6.9 (2H each, d, J = 8 Hz): an AA'BB' para-disubstituted ring; 7.6 = H ortho to C=O (deshielded), 6.9 = H ortho to OMe (shielded by +R).
2. 3.9 (3H, s): OCH₃ on the ring. 2.0 (3H, s): CH₃-C=O (no neighbouring H, singlet).
3. Pieces: C₆H₄ + OCH₃ + COCH₃ = C₉H₁₀O₂. A methyl ester (CH₃C₆H₄CO₂CH₃) would show C=O near 1720 cm⁻¹ and an aryl CH₃ near 2.4, so it is excluded.

Conclusion. The compound is 4-methoxyacetophenone (1-(4-methoxyphenyl)ethan-1-one).

Application. 4-Methoxyacetophenone (acetanisole) is used in perfumery and is the Friedel–Crafts acylation product of anisole, the textbook example of para-selective acylation.