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 sp³ hybridized carbons occurs below 3000 cm⁻¹, while C - H stretching of sp² hybridized carbons occurs above 3000 cm⁻¹. Also, N - H stretching occurs above 3000 cm⁻¹.

Step2: Analyze first structure

The first structure has only sp³ hybridized C - H bonds. So, it has no major peak above 3000 cm⁻¹.

Step3: Analyze second structure

The second structure has aromatic (sp² hybridized) C - H bonds. Aromatic C - H stretching gives a peak above 3000 cm⁻¹.

Step4: Analyze third structure

The third structure has an amide group with N - H bonds. N - H stretching vibrations occur above 3000 cm⁻¹.

Answer:

The second and third structures.