Question

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

Answer 1

First, draw the possible (`E`) and (`Z`) pentene isomers. Then substitute every possible `"H"` atom with `"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 `"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.

`"C-4"` is a prochiral carbon.

If we replace one `"H"` atom, we get an (`E`) isomer. If we replace the other `"H"` atom, we get a (`Z`) isomer.

So there are four possibilities:

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

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