Question

which of the structures have a major peak above 3000 cm⁻¹? (select all that apply).

Explanation:

Step1: Recall IR - absorption rules

Infra - red (IR) spectroscopy: C - H stretching of sp² hybridized carbons (such as in aromatic rings) occurs above 3000 cm⁻¹, while C - H stretching of sp³ hybridized carbons occurs below 3000 cm⁻¹. Carbon - oxygen double - bond (C=O) stretching occurs around 1700 cm⁻¹.

Step2: Analyze each structure

The first structure is a cyclohexanone. It has only sp³ hybridized C - H bonds, so no major peak above 3000 cm⁻¹. The second structure is a benzoyl - benzene. It has aromatic C - H bonds (sp² hybridized C - H), which will show a major peak above 3000 cm⁻¹. The third structure is a cyclopentanone. It has only sp³ hybridized C - H bonds, so no major peak above 3000 cm⁻¹.

Answer:

The second structure (the benzoyl - benzene)