01882 IUPAC Gold Book - E, Z 10.1351/goldbook.E01882 E01882 current 1 The approved stereodescriptors of stereoisomeric alkenes $\ce{R1R2C=CR3R4}$ ($\ce{R1} \neq \ce{R2} , \ce{R3} \neq \ce{R4}$); neither $\ce{R1}$ nor $\ce{R2}$ need be different from $\ce{R3}$ or $\ce{R4}$), cumulenes $\ce{R1R2C(=C=C)_{n}=CR3R4}$ and related systems e.g. $\ce{R1R2C=NOH}$, $\ce{HON=C\{[CH2]_{n}\}2C=NOH}$. The group of highest CIP priority attached to one of the terminal doubly bonded atoms of the alkene, oxime, etc. or cumulene (i.e. $\ce{R1}$ or $\ce{R2}$) is compared with the group of highest precedence attached to the other (i.e. $\ce{R3}$ or $\ce{R4}$). The stereoisomer is designated as $Z$ (zusammen = together) if the groups lie on the same side of a reference plane passing through the double bond and perpendicular to the plane containing the bonds linking the groups to the double-bonded atoms; the other stereoisomer is designated as $E$ (entgegen = opposite). The descriptors may be applied to structures with a fractional bond order between one and two; and to double bonds involving elements other than carbon. They are not used to describe ring substitution relationships. cis-trans isomers https://goldbook.iupac.org//terms/view/C01093 CIP priority https://goldbook.iupac.org//terms/view/C01082 alkenes https://goldbook.iupac.org//terms/view/A00224 bond order https://goldbook.iupac.org//terms/view/B00707 cumulenes https://goldbook.iupac.org//terms/view/C01440 stereodescriptors https://goldbook.iupac.org//terms/view/S05976 PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2206 (https://doi.org/10.1351/pac199668122193) https://goldbook.iupac.org/terms/view/E01882/html https://goldbook.iupac.org/terms/view/E01882/json https://goldbook.iupac.org/terms/view/E01882/plain Citation: 'E, Z' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.E01882 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-06-20T16:57:23+00:00