Q76E - Organic Chemistry (Mcmurry)

Question

Ketones react with sodium acetylide (the sodium salt of acetylene,$\mathit{N}{\mathit{a}}^{\mathbf{+}\mathbf{-}}\mathbf{:}\mathbf{C}\mathbf{\equiv }\mathbf{CH}$) to give alcohols. For example, the reaction of sodium acetylide with 2-butanone yields 3-methyl-1-pentyn-3-ol

(a) Is the product chiral?

(b) Assuming that the reaction takes place with equal likelihood from both Re and Si faces of the carbonyl group, is the product optically active? Explain.

 

Step-by-Step Solution

Answer

(a) The product obtained is chiral.

(b) The product is not optically active because it is two enantiomers formed in 50:50 ratio.

Step 1: Chirality

Chirality is defined as the property of the ions or molecules that do not have superimposable mirror images of each other.

A chiral (having four different groups attached to carbon atom) molecule and its non-superimposable mirror images are termed as special types of stereoisomers known as enantiomers. For example, 3-methyl-1-pent-3-ol and its mirror images are non-superimposable.

Step 2: Racemic Mixture

Enantiomers are compounds having the same molecular formula but different molecular structures. When these enantiomers are present in equal quantities in a mixture then the mixture is called a racemic mixture.

The chemical properties are almost the same for both compounds, therefore a mixture of both compounds is also known as a racemic.

Step 3: Identify (a) and (b)

(a) The product obtained is chiral, it has two enantiomers formed in a 50:50 ratio it is an optically inactive racemic mixture.

 

(b) The product is not optically active because it is an optically inactive racemic mixture as the reaction takes place with equal likelihood from both Re and Si faces of the carbonyl group as shown below.