Q23P - Organic Chemistry (Mcmurry)

Question

How might you use IR spectroscopy to determine whether reaction between2-cyclohexenoneand lithium dimethylcopper gives the direct addition productor the conjugate addition product?

Step-by-Step Solution

Answer

Step-by-Step Solution Step 1: Infrared spectroscopy

Infrared spectroscopy is specifically used to detect/identify “functional groups” in organic molecules.

E.g., an organic compound with a carboxylic group can be distinguished from the organic compound with an alcohol group—very strong OH, C=O stretch is observed in carboxylic acid, and alcohol shows only OH stretch.

Step 2:Formation of direct addition product or the conjugate addition product

2-cyclohexenone is a cyclic alpha, beta-unsaturated ketone whose infrared absorption of the carbonyl group occurs at 1685${\text{cm}}^{\text{-1}}$ .

 

If the direct addition product 1-methyl-2-cyclohexen-1-ol (a) is formed, the carbonyl absorption vanishes as it is converted to hydroxyl group, the IR absorption of the hydroxyl group appears at 3300${\text{cm}}^{\text{-1}}$ .

 

The conjugate addition product 3-methyl cyclohexanone (b) is formed, shifting the infrared carbonyl absorption from 1685 to 1715${\text{cm}}^{\text{-1}}$ (the absorption of 6 membered saturated rings).

The formation of the direct addition product and the conjugate addition product is shown below.

Formation of 1-methyl-2-cyclohexen-1-ol and 3-methyl cyclohexanone