Alkenes are categorized as cis or trans (if there are two groups or identical groups) or by E and Z, which is the official designation of alkene stereochemistry. Stability of alkenes is determined by the degree of substitution of the alkene.
Alkenes have at least one double bond as part of the carbon structure. The double bond prevents free rotation of the carbons and affects the configuration. These configurations are differentiated in their names by the prefixes cis or trans. Cis stems from Latin and means "on this side," while trans stems from Latin and means "across." E and Z notation is the preferred IUPAC (International Union of Pure and Applied Chemistry) method for nomenclature. E/Z nomenclature is a method for naming organic compounds based on where the substituents are bonded. The Cahn-Ingold-Prelog system of nomenclature is used with the IUPAC system to identify enantiomeric structures by name alone. If the highest-priority substituents bond on opposite sides of the double bond, then the bond is described as E. If the highest-priority substituents are bonded on the same side of the double bond, the bond is described as Z. Z comes from the German zusammen, meaning "together." In chemistry, Z means "on the same side of the double bond." E comes from the German entgegen, meaning "opposite." In chemistry, E means on the opposite side of the double bond. While cis and trans describe the orientation of two groups in relation to a double bond, E and Z are used for describing the orientation of up to four groups around a double bond, including those that cannot be described using cis and trans. Cis is the geometric arrangement in which two substituents are on the same side of the central part of a molecule or ion. Trans is the geometric arrangement in which two substituents are on opposite sides of the central part of a molecule or ion. The more substituted an alkene, the more stable it is. Tetrasubstituted alkenes, meaning an alkene with four alkyl groups attached to it, are more stable than trisubstituted alkenes. Disubstituted alkenes are less stable than trisubstituted alkenes. Monosubstituted alkenes are the least stable. The stability of a molecule is a measure of its energy. The lower the energy of a molecule, the more stable it is. More-substituted alkenes are more stable than less-substituted alkenes because of the hyperconjugation, a stabilizing overlap between a molecular bonding orbital and a molecular bonding orbital, which lowers its energy. The more substitution around an alkene, the more stable that alkene becomes. The stabilization occurs via hyperconjugation from an adjacent bond, which cannot form from adjacent hydrogen atoms but can form from adjacent carbon atoms from a substituent.