Q5-5-70E - Organic Chemistry (Mcmurry)

Question

(S)-1-Chloro-2-methylbutane undergoes light-induced reaction with ${\mathrm{Cl}}_2$to yield a mixture of products, among which are 1,4-dichloro-2-methylbutane and 1,2-dichloro-2-methylbutane.

1. Write the reaction, showing the correct stereochemistry of the reactant.
2. One of the two products is optically active, but the other is optically inactive. Which is which?

Step-by-Step Solution

Answer

Step 1: Stereochemistry and optical activity

First, let us take an example, if you see in the figure, B is the mirror image of molecule A containing one stereocenter, and if you try to superimpose A onto B, you will not be able to do so. So by definition A is a chiral molecule. A and B are optical stereoisomers. 

Optical isomers areof two types, those molecules which are mirror images of each other and non- superimposable  are enantiomers while those stereoisomers which are non mirror images and non superimposable are called diastereomers. The existence of these optical stereomermolecules, is determined by a property, chirality.

A molecule is said to be chiral, if it rotates the plane of polarized light (optical activity) and possesses non-superimposable mirror image. We can also say that a molecule shows optical activity if it contains atleast a chiral centre (stereocentre).

Step 2: Stereochemistry of (S)-1-Chloro-2-methylbutane and reaction

As we can see from the reaction that both the products have chiral centres, so both the products 1,4-dichloro-2-methylbutane and 1,2-dichloro-2-methylbutane are optically active.