Cis/trans and E/Z alkene isomers

A concrete case

Two compounds called but-2-ene

But-2-ene, ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CHCH}}_3$CH3​CH=CHCH3​, has four carbons in a row with a double bond in the middle. Each carbon of the double bond carries one ${\mathrm{CH}}_3$CH3​ and one H. The two ${\mathrm{CH}}_3$CH3​ groups can sit on the same side of the double bond or on opposite sides.

Those are two different compounds, with the same formula, ${\mathrm{C}}_4{\mathrm{H}}_8$C4​H8​, and the same atoms joined in the same order. The one with both ${\mathrm{CH}}_3$CH3​ groups on the same side boils at about $3.7 °\mathrm{C}$3.7 ∘C and melts at $-139.3 °\mathrm{C}$−139.3 ∘C. The other boils at about $1 °\mathrm{C}$1 ∘C and melts at $-105.8 °\mathrm{C}$−105.8 ∘C, more than $33$33 degrees higher.

The first is cis-but-2-ene and the second trans-but-2-ene. In current names they are (2Z)-but-2-ene and (2E)-but-2-ene. This lesson explains why the two do not turn into each other, and how to name every such pair.

The rule

Cis and trans

When each carbon of a double bond carries one group and one hydrogen, name the arrangement by the two groups. They are cis if they are on the same side of the double bond and trans if they are on opposite sides. The prefix goes in front of the name, with no number, as in cis-but-2-ene.

The E/Z rule

On each carbon of the double bond, rank its two groups against each other with the CIP rules. Compare the bonded atoms by atomic number, and move outward on a tie. Then look at the two higher-ranked groups, one on each carbon.

If they are on the same side, the alkene is Z, from the German zusammen, together. If they are on opposite sides, it is E, from entgegen, opposite. The letter goes in parentheses before the name, with the number of the double bond, as in (2E)-3-methylpent-2-ene.

For an alkene with one group and one hydrogen on each carbon, the group outranks the hydrogen, so cis is Z and trans is E. IUPAC’s current names use E and Z, and keep cis and trans for that simple case.

Why it works

Why the groups cannot swap sides

The two carbons of a double bond are joined by a sigma bond and a pi bond. The pi bond is made by two p orbitals, one on each carbon, standing side by side. Twisting one end of the bond turns the p orbitals out of line and breaks the pi bond, which takes far more energy than molecules have at room temperature. The groups on each carbon therefore stay where they are, and the two arrangements do not turn into each other.

A single bond has no such lock. The two ends of butane spin freely, so butane has no cis and trans forms.

Cis and trans need a reference, and they fail once a double bond carries three or four groups. In 3-methylpent-2-ene, carbon 3 carries a methyl and an ethyl group, so “on the same side as the methyl on carbon 2” could mean either of two things. E and Z replace the reference with a ranking, so every alkene gets one answer.

The method

Assigning E or Z in four steps

1. Check that each carbon of the double bond carries two different groups. If either carries two identical groups, there are no E and Z forms, and you can stop.

2. On the first carbon, rank its two groups by the atomic number of the bonded atom, and break a tie one bond further out.

3. Do the same on the second carbon, without comparing its groups with those on the first.

4. Find the higher-ranked group on each carbon. Same side is Z, and opposite sides is E. Write the letter with the number of the double bond.

Where it breaks

Where it breaks

Some alkenes have no isomers of this kind. In 2-methylbut-2-ene, ${\mathrm{CH}}_3\mathrm{CH}=\mathrm{C}{\left({\mathrm{CH}}_3\right)}_2$CH3​CH=C(CH3​)2​, carbon 2 carries two ${\mathrm{CH}}_3$CH3​ groups, and swapping them changes nothing.

Cis and trans do not always match Z and E. In 2-bromobut-2-ene the two ${\mathrm{CH}}_3$CH3​ groups can be on opposite sides while the alkene is Z, because bromine, not ${\mathrm{CH}}_3$CH3​, is the higher-ranked group on its carbon. Courses still say cis and trans for the simple case, and the names do not conflict there.

Z is not “the main chain bends back”. The letter follows the ranking, which can point to a branch rather than to the chain.

A double bond in a small ring has only one arrangement. In cyclohexene the ring holds both chain ends on the same side, so no E form exists, and Chalk does not label a double bond in a ring of fewer than eight atoms.

Check your answer

Check your answer

Rank each carbon on its own. A slip that ranks a group on one carbon against a group on the other carbon is the most common error. Write the two rankings out as separate lines before you look at the sides.

Swap and reread. Exchanging the two groups on either carbon must change E to Z or Z to E.

Test the simple case. If each carbon carries one group and one hydrogen, your E or Z should match trans or cis. If it does not, recheck the sides.

Chalk reads E and Z from a drawing. Draw the alkene and choose Analyze → Stereochemistry, and Chalk shows the ranking on each carbon and the letter beside the double bond. A name entered under Analyze → Target is checked letter and all. On the drawing of (2E)-3-methylpent-2-ene, the name (Z)-3-methylpent-2-ene reads ✗ with “The name says Z, but the drawing shows E.”