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

1 Answer
Jun 11, 2015

Answer:

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

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

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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.

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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

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

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