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Chemistry · General chemistry II · Worked example

Assign E or Z to 3-methylpent-2-ene

CH3CH=C⁢(CH3)CH2CH3 is drawn with the CH3 on carbon 2 and the CH3 on carbon 3 on the same side of the double bond. Is it E or Z?

List the groups on each carbon

Carbon 2 carries CH3 and H. Carbon 3 carries CH3 and CH2CH3. Each carbon has two different groups, so E and Z forms exist.

Rank the groups on carbon 2

The bonded atoms are carbon, atomic number 6, and hydrogen, 1. Carbon is higher, so CH3 outranks H.

Rank the groups on carbon 3

Both groups start with carbon, so look one bond out. The CH3 carbon carries (H, H, H) and the CH2 of the ethyl group carries (C, H, H). Carbon beats hydrogen at the first difference, so the ethyl group outranks the methyl group.

Compare the sides

The higher-ranked groups are the CH3 on carbon 2 and the ethyl group on carbon 3. The two methyl groups share a side, which puts the ethyl group on the other side from the carbon 2 methyl. Opposite sides make it E.

(2E)-3-methylpent-2-ene

Check the other arrangement

Swap the two groups on carbon 3 and the higher-ranked groups land on the same side. That compound is (2Z)-3-methylpent-2-ene, a different compound with the same connections.

Result

E. The compound is (2E)-3-methylpent-2-ene.

Your turn

1. One like the example

3,4-Dimethylhex-3-ene, CH3CH2C⁢(CH3)=C⁢(CH3)CH2CH3, is drawn with its two ethyl groups on the same side of the double bond. Is it E or Z?

Show the answer and explanation

Z. The compound is (3Z)-3,4-dimethylhex-3-ene.

Each carbon of the double bond carries a methyl and an ethyl group. On each, the ethyl carries (C, H, H) against the methyl’s (H, H, H), so ethyl ranks higher. The two higher-ranked groups share a side.

2. A variation

Give both isomers of but-2-ene, CH3CH=CHCH3, by cis or trans and by E or Z.

Show the answer and explanation

cis-But-2-ene is (2Z)-but-2-ene, and trans-but-2-ene is (2E)-but-2-ene.

Each carbon carries one CH3 and one H, and CH3 outranks H on both. With the CH3 groups on the same side, the higher-ranked groups share a side, so cis is Z. Opposite sides make trans E.

3. Somewhere new

Does 2-methylbut-2-ene, CH3CH=C⁢(CH3)2, have E and Z isomers?

Show the answer and explanation

No. It is one compound.

Carbon 2 carries two CH3 groups. Swapping them gives the same molecule, so there is no second arrangement to name, whatever the other carbon carries.

Keep exploring

Draw it, then choose Analyze → Stereochemistry: Chalk ranks each end of the double bond between carbons 2 and 3 and puts an E beside it. Draw the Z isomer on the same board, and Analyze → Compare reads “Cis/trans isomers, as drawn”. The two Draw buttons put each isomer in the Skeletal tool with its name as the target, and the Practice buttons import each practice structure without a name, for your own name to be checked.

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Sources and scope

Authored study material. Tool results depend on the stated inputs and model assumptions.

After reading

Study it, then try it

Practice the key terms as flashcards, then open the example in its tool, change a value and keep a useful result on your board.

Study the 5 key terms Draw (2E)-3-methylpent-2-ene Draw (2Z)-3-methylpent-2-ene Practice 1: import the structure and check your name Practice 2: import cis-but-2-ene and check your names Practice 3: import the structure and check your name Open worked example on a board Stereochemistry rules in Chemistry Reference

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