Question

Question: Given that syn addition of H₂ occurs from both sides of a trigonal planar double bond, draw all stereoisomers formed when each alkene is treated with H₂ .

a.

b.

c.

Short answer

Answer

a.

b.

c.

Step-by-step solution

Stereospecific reactions

If by taking a stereochemically different product (cis or trans), a stereospecific product is obtained, then such a reaction is called a stereospecific reaction.

The addition of molecular hydrogen to an alkene is an example of a stereospecific reaction.

Stereochemistry of addition of molecular  H2 to an alkene

When a cis stereomer is subjected to molecular hydrogenation in the presence of a catalyst, a single product is formed, stereochemically meso.

When a cis stereo is subjected to molecular hydrogenation in the presence of a catalyst, two different products are obtained in equal amounts. They are enantiomers of each other, and therefore the overall product is a racemic mixture.

Hydrogenation of an alkene

a. The product obtained is a racemic mixture of two enantiomers (mirror image).

Hydrogenation reaction of alkene resulting product as a racemic mixture

b. The products obtained are diastereomers (non-mirror image).

Hydrogenation reaction of alkene resulting meso product

c. The products obtained are diastereomers (non-mirror image).

Hydrogenation reaction of alkene resulting in meso product