cisdubbele

Cisdubbele binding, commonly referred to as a cis double bond, is a form of geometrical isomerism observed in alkenes. It describes the relative orientation of substituents attached to the two carbon atoms of a carbon–carbon double bond. When the substituents of interest are on the same side of the double bond, the molecule is the cis isomer; when they are on opposite sides, the molecule is the trans isomer.

For alkenes with two different substituents on each double-bond carbon, the preferred systematic descriptors are E and Z, according to the Cahn–Ingold–Prelog priority rules. The term cis is still used in many contexts, especially for simple cases, and cis roughly corresponds to Z when the higher-priority groups lie on the same side. In cases where the priority of substituents differs, E and Z should be used to avoid ambiguity.

Examples and implications: cis-2-butene and trans-2-butene illustrate how orientation affects properties. The cis isomer is typically less thermodynamically stable due to steric interactions between substituents on the same side, and it often exhibits a measurable dipole moment, influencing physical properties such as boiling point and solubility. In fatty acids, cis double bonds introduce kinks that prevent close packing, lowering melting points and increasing fluidity; trans double bonds permit tighter packing and generally raise melting points, making trans fats more similar to saturated fats.

The concept of cis remains common in everyday chemistry, but the IUPAC E/Z system is the formal standard for naming geometrical isomers.