# How can I draw the molecular formula for an unsaturated alkyl chloride C_5H_9Cl with optical activity and E, Z – isomerism?

Jun 11, 2015

First, draw the possible ($E$) and ($Z$) pentene isomers. Then substitute every possible $\text{H}$ atom with $\text{Cl}$ until you have generated all possible chiral molecules.

#### Explanation:

Step 1. Draw the isomeric pentenes.

There is only one pentene that can have ($E$)and ($Z$) isomers. That is pent-2-ene.

You can have ($E$)-pent-2-ene

or ($Z$)-pent-2-ene.

Step 2. Identify the prochiral carbon atoms.

A $\text{C}$ atom is prochiral if the replacement of one of its groups by a different group would create a new chiral centre.

It must be bonded to exactly three different groups before any change is made.

In pent-2-ene, C-4 is bonded to a methyl group, a propenyl group, and two hydrogens.

This fits the criterion of being bonded to exactly three different groups, even though one group is represented twice.

$\text{C-4}$ is a prochiral carbon.

If we replace one $\text{H}$ atom, we get an ($E$) isomer. If we replace the other $\text{H}$ atom, we get a ($Z$) isomer.

So there are four possibilities:

($2 E , 4 R$)-4-chloropent-2-ene and ($2 E , 4 S$)-4-chloropent-2-ene

and ($2 Z , 4 R$)-4-chloropent-2-ene and ($2 Z , 4 S$)-4-chloropent-2-ene