[IShouldBeRevising_]

Why are some alkenes e/z isomers?

[charco]

(Original post by IShouldBeRevising_)
Why are some alkenes e/z isomers?

When there are two ways of arranging the groups at the double bond, this gives rise to E/Z isomers.

[clownfish]

When answering a question on why and when do E/Z isomers occur make sure you write both the following points:

1) as the double bond is non-rotational
2) when the carbons on the C=C have two different groups attached

The marking schemes are typically looking for these two points.

[countrybumpkin]

there must be two different groups attached to each carbon in the double bond.

H2-C=C-H2 doesn't show e/z isomerism because the groups attached to each carbon are the same ( H and H)

but for example, HCH3-C=C-HCH3 does (each carbon in the double bond has an "H" and a "CH3" group attached, which are obviously different from eachother)
The z isomer is where both CH3 groups are "pointing up" and the e isomer is when one's up and one's down

[AGrumpyMole]

(Original post by IShouldBeRevising_)
Why are some alkenes e/z isomers?

An E (Trans) isomer has atoms or molecules on opposite sides of a double bonded carbon chain.
A Z (Cis) isomer has atoms or molecules on the same side of a carbon chain.

Example:

Z isomer of pent-2-ene would have all of the hydrogen atoms on one side of the carbon chain.
The E isomer of pent-2-ene would have one of the hydrogen atoms near the double bond on one side of the carbon chain and another on the other side.

So there is (cis) pent-2-ene and there is (Trans) pent-2-ene.

Another example:

(Z) 1,2-dichloroethane - with both chlorine atoms on the same of the carbon chain.
(E) 1,2-chloroethane - with one chlorine atom on one side of the carbon chain and the other chlorine atom on the other side of the carbon chain.

[lildevil]

e/z isomers are geometric isomers which occur in alkenes which have two different groups on each of the two carbons making up the double bond. They arise as there is restricted rotation about the carbon-carbon double bond, and the carbons on either side of the carbon-carbon double bond have two different groups bonded to them. This means it is possible to get two different structures for the molecules known as the cis (now called Z) and trans (now called E) structures. But-2-ene is a classic example of this.

[AbominableSnowman]

The E/Z comes from German. E; "entegen" which means opposite, and Z; "Zusammen" which means together.

E/Z isomers are basically stereo-isomers, they have the same structural formula, but the arrangement of the atoms, and more importantly, the functional groups on the Carbon skeleton is different. Obviously, in an E isomer the functional group atoms (methyl group, halogen, etc) will be on opposite sides of the double bond, and in a Z isomer, the functional group atoms are on the same side of the double bond.