Name: cis, trans, and e/z alkene forms
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 1 s
Channel: JoVE

Alkenes differing in the position of the double bond exhibit constitutional isomerism. In addition, those with the same constitution show stereoisomerism when they differ in the spatial arrangement of atoms or groups across the double bond.
The stereoisomer is referred to as cis if the principal chain carbon atoms are on the same side of the double bond and as trans when the opposite is the case.
The cis and trans isomers of alkenes are not interconvertible by internal rotation about the double bond. Hypothetically, such a rotation would twist the p orbitals out of coplanarity, causing the bond to break. 
Alkenes are named according to the Cahn&#8211;Ingold&#8211;Prelog sequence rules to avoid ambiguity in the cis, trans nomenclature.
Each carbon atom across the double bond is envisioned separately, and the substituent with a higher atomic number connected to the carbon atoms is assigned a higher priority.
When high priority substituents are on the same side of the double bond, the alkene has a Z configuration; otherwise, it has an E configuration.
If the substituents on a carbon atom have identical first atoms, the earliest point of difference is decisive. Therefore, in this example, the subst

isomerism in alkenes

Isomerism in Alkenes

Alkenes like 1-butene and 2-butene exhibit constitutional isomerism, as they differ in the position of the double bond. Further, 2-butene exhibits stereoisomerism and exists as two distinct compounds differing in spatial arrangement.
An isomer is called cis-2-butene when the methyl groups are on the same side of the double bond, and the other stereoisomer, in which methyl groups are on the opposite side of the double bond, is called trans-2-butene. The cis and trans stereoisomers are not interconvertible at room temperature because of the restricted rotation of the double bond.
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			<td style="color: black; border: 1px solid black; text-align: center;"><img alt="Chemical structure diagram of cis-2-butene, showing double bond between carbon atoms." src="/files/ftp_upload/11767/11767_Figure-1_left.svg" style="text-align: center; height: 55px;" data-altupdatedat="1754904545000" data-alt="Isomer structure diagram, 2-butene molecular formula C4H8 for organic chemistry study." /></td>
			<td style="color: black; border: 1px solid black; text-align: center;"><img alt="Organic chemistry; hydrocarbon structure, C=C double bond, structural formula diagram." src="/files/ftp_upload/11767/11767_Figure-1_right.svg" style="text-align: center; height: 55px;" data-altupdatedat="1754904545000" data-alt="Ethene structural formula; diagram showing double bond and alkyl groups; organic chemistry." /></td>
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			<td style="color: black; border: 1px solid black; text-align: center;">cis-2-Butene</td>
			<td style="color: black; border: 1px solid black; text-align: center;">trans-2-Butene</td>
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For tri- or tetrasubstituted compounds, the E,Z nomenclature has been adopted over the cis and trans nomenclature. The E,Z nomenclature is based on the sequence rules. The carbon atoms across the double bond are envisioned separately, and the substituent with connecting atoms having higher atomic numbers is assigned the highest priority. If two substituents have the same atomic number, the first point of difference is decisive. When the high-priority substituents on both the carbon atoms of the double bond are on the same side, the alkene has a Z configuration, whereas when the high-priority substituents are on the opposite side of the double bond, the alkene is said to have an E configuration.
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			<td style="color: black; border: 1px solid black; text-align: center;"><img alt="Molecular structure diagram; E/Z isomerism around C=C, showing priority groups for stereochemistry." src="/files/ftp_upload/11767/11767_Figure-2_left.svg" style="text-align: center; height: 70px;" data-altupdatedat="1754904545000" data-alt="Cis-trans isomerism diagram; C=C bond with priority groups; stereochemistry concept." /></td>
			<td style="color: black; border: 1px solid black; text-align: center;"><img alt="Cis-trans isomerism diagram; C=C bond priority labeling in molecular stereochemistry study." src="/files/ftp_upload/11767/11767_Figure-2_right.svg" style="text-align: center; height: 70px;" data-altupdatedat="1754904545000" data-alt="cis-trans isomerism diagram, C=C bond priority configuration, stereochemistry analysis" /></td>
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			<td style="color: black; border: 1px solid black; text-align: center;">Z Configuration</td>
			<td style="color: black; border: 1px solid black; text-align: center;">E Configuration</td>
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Alkenes like 1-butene and 2-butene exhibit constitutional isomerism, as they differ in the position of the double bond. Further, 2-butene exhibits ...

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Alkene Structure and Reactivity

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cis, trans, and e/z alkene forms

Alkene isomers can differ in both bond position and spatial arrangement. 1-butene and 2-butene are constitutional isomers because the double bond is in a different place. 2-butene also shows stereoisomerism, which means its atoms are connected the same way but arranged differently in space.

In 2-butene, the two methyl groups can sit on the same side of the double bond or on opposite sides. When the methyl groups are on the same side, the compound is called cis-2-butene. When they are on opposite sides, it is called trans-2-butene. These cis and trans forms do not change into each other at room temperature because the double bond limits rotation.

For tri- or tetrasubstituted alkenes, chemists use the E/Z system instead of cis/trans. This system follows sequence rules. Each carbon of the double bond is considered separately, and the substituent whose attached atom has the higher atomic number gets higher priority. If two atoms have the same atomic number, the first point of difference decides the priority.

When the high-priority groups are on the same side of the double bond, the alkene has a Z configuration. When the high-priority groups are on opposite sides, the alkene has an E configuration. The E/Z labels help describe alkene geometry more clearly when cis/trans names are not enough.

Cis, Trans, and E/Z Alkene Forms

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Alkene isomers can differ in both bond position and spatial arrangement. 1-butene and 2-butene are constitutional isomers because the double bond is in a different place. 2-butene also shows stereoisomerism, which means its atoms are connected the same way but arranged differently in space. In 2-butene, the two methyl groups can sit on the same side of the double bond or on opposite sides. When the methyl groups are on the same side, the compound is called cis-2-butene. When they are on...

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key terms and concepts

Learn how alkene isomers differ by bond position and shape. Explore cis-2-butene, trans-2-butene, and the E/Z naming system for substituted alkenes.

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Cis, Trans, and E/Z Alkene Forms

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