Q55P Phenyl Grignard reagent adds to ... [FREE SOLUTION]

Chapter 13: Q55P (page 707)

Phenyl Grignard reagent adds to 2-methylpropanal to give the secondary alcohol shown. The proton NMR of 2-methylpropanal shows the two methyl groups as equivalent (one doublet at 未 1.1), yet the product alcohol, a racemic mixture, shows two different 3H doublets, one at 未 0.75 and one around 未 1.0
(a)Draw a Newmann projection of the product along the C1-C2 axis.
(b)Explain why the two methyl groups have different NMR chemical shifts. What is the term applied to protons such as these?

Short Answer

Expert verified

b) The two methyl groups are diastereotopic in nature and have different values of chemical shifts

Step by step solution

Step-1. Explanation of part (a):

A projection formula represents spatial arrangement of bonds on two adjacent atoms in a molecular entity. The bonds from the atom nearer to the observer are drawn in such a way that it meets the center of circle representing that atom. Those from the further atom are drawn as if projecting from behind the circle. In Newmann projection, we depict the 鈥渇ront鈥� atom as a dot and 鈥渂ack鈥� atom as a large circle. Bonds are not drawn as solid or dashed wedges in Newmann projection.

Step-2. Explanation of part (b):

In the starting aldehyde, the two methyl groups are equivalent and in the product of Grignard reagent, a new chiral center is created and two methyl groups are now no longer equivalent. From the Newmann projection, rotating the back carbon atom will not put two methyl groups in identical environments. In NMR spectrum, two separate signals with different chemical shifts are obtained. Thus, these two methyl groups are diastereotopic.