Modelo del limoneno Alkenes are hydrocarbons that contain carbon-carbon double bonds. The word olefin is frequently used as a synonym.

Alkenes abound in nature. Ethene is a compound that controls plant growth, seed germination and fruit ripening.

Alkenes are named by replacing the -ane ending of the corresponding alkane with -ene. The simplest alkenes are ethene and propene, also called ethylene and propylene at the industrial level.

Rule 1.- The longest chain that contains the double bond is chosen as the main chain. Numbering begins at the end that gives the double bond the lowest locant.
alkenes nomenclature 1
[1] 1-Butene
[2] trans-2-Hexene
Rule 2.- The name of the substituents precedes that of the main chain and is accompanied by a locator that indicates its position in the molecule. The molecule is numbered so that the double bond takes the lowest locant.
alkenes 2 nomenclature
[3] 2-Propylhept-1-ene
[4] trans-4-Methyl-2-pentene

2-buteno: compuesto insaturado Each ring or cycle of a molecule implies the loss of two hydrogens with respect to an alkane of formula CnH2n+2 . The degree of unsaturation is the number of cycles and double bonds present in a molecule.

The following models show the structure, distances, and bond angles of ethene. Each of the carbons in the molecule is $sp^2$ hybridized. Its geometry is flat, with bond angles close to 120º.


Alkenes have melting and boiling points close to the corresponding alkanes.


Dipole moment in alkenes. In carbon $sp^2$ the electrons in the s orbital are closer to the nucleus and are strongly attracted to it, so that a carbon $sp^2$ has a tendency to attract electrons towards itself, which generates dipole moments.


The heats released in the following hydrogenation reactions give us an idea about the different stability of alkenes.

stability alkenes 01
Increased stability

Alkenes can be prepared from haloalkanes and alkyl sulfonates by bimolecular elimination (E2). In the following example 2-bromo-2-methylbutane reacts with sodium methoxide to form a mixture of 2-methyl-2-butene and 2-methyl-1-butene.


The treatment of alcohols with mineral acids at high temperatures causes the loss of water, which occurs through E1 or E2 mechanisms.