How do you name and draw stereoisomers for 2-pentene?

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Answer 1 · 2015-12-24T14:41:42

Start by drawing a chain of five carbon atoms.

Pentane

Draw a double bond between carbons 2 and 3.

Pentene

Note that there are two ways to insert the double bond.

One has the $"H"$ atoms on opposite sides of the double bond; the other has the $"H"$ atoms on the same side of the double bond.

Cis/trans

cis-Pent-2-ene has the $"H"$ -atoms on the same side of the double bond.

trans-Pent-2-ene has the $"H"$ -atoms on opposite sides of the double bond.

You can also use the Cahn-Ingold-Prelog system to name the isomers.

On each end of the double bond, the alkyl groups (ethyl and methyl) have higher priority than the $"H"$ atoms.

The first isomer has the higher-priority groups on opposite sides. This is ( $E$ )-pent-2-ene.

The second isomer has the higher-priority groups on the same side. This is ( $Z$ )-pent-2-ene.