Acetophenone, p-methoxy INFRARED SPECTRUM 0.8 0.6 L04 D2 3000
The Correct Answer and Explanation is:
Correct Answer:
The key peaks to identify in the IR spectrum of p-methoxyacetophenone are:
- Aromatic C–H stretches: Around 3000–3100 cm⁻¹
- Alkyl C–H stretches (from CH₃ and CH₂): Around 2850–2960 cm⁻¹
- C=O stretch (ketone): ~1680–1725 cm⁻¹ (strong, sharp peak)
- Aromatic ring stretches (C=C): ~1450–1600 cm⁻¹
- C–O stretch (ether, from the methoxy group): ~1240–1275 cm⁻¹
- Aromatic out-of-plane C–H bending: ~700–900 cm⁻¹
Explanation
p-Methoxyacetophenone is a substituted aromatic ketone. The molecule has three significant functional groups: an aromatic ring, a methoxy (-OCH₃) group para to the ketone, and an acetyl group (-COCH₃) attached to the ring. Its IR spectrum provides insight into the presence and environment of these functional groups.
The carbonyl (C=O) stretch, typically strong and sharp, appears in the range of 1680–1725 cm⁻¹. In p-methoxyacetophenone, this appears around 1690–1705 cm⁻¹, slightly lower due to resonance from the aromatic ring and the electron-donating methoxy group.
The aromatic C=C stretching vibrations appear as medium intensity bands in the 1450–1600 cm⁻¹ region. These peaks indicate the presence of an aromatic ring.
The C–H stretches in the 3000–3100 cm⁻¹ region correspond to the aromatic hydrogen atoms. Additionally, the alkyl C–H stretches from the methyl and methylene groups show up around 2850–2960 cm⁻¹.
A distinguishing feature of p-methoxyacetophenone is the C–O stretch from the methoxy (-OCH₃) group, which appears around 1240–1275 cm⁻¹. This region helps confirm the presence of the ether linkage.
Finally, the out-of-plane C–H bending in the 700–900 cm⁻¹ region provides information on the substitution pattern on the aromatic ring. For para-substitution, you typically observe a strong single band near 825–860 cm⁻¹.
By analyzing these characteristic peaks, one can confidently identify the presence and structure of p-methoxyacetophenone using its IR spectrum.
