ALKENES are hydrocarbons that contain a carbon–carbon double bond. A carbon–carbon double bond is both an important structural unit and an important functional group in organic chemistry. The shape of an organic molecule is influenced by the presence of this bond, and the double bond is the site of most of the chemical reactions that alkenes undergo. Some representative alkenes include isobutylene (an industrial chemical), -pinene (a fragrant liquid obtained from pine trees), and farnesene (a naturally occurring alkene with three double bonds).

Bonding in Alkenes

In alkenes we have a C=C bond and one bond is a sigma bond and the other one is pie bond. In ethene (simplest alkene) , Each of the carbon atoms is sp2-hybridized, and the double bond possesses a component and a component. The component results when an sp2 orbital of one carbon, oriented so that its axis lies along the inter-nuclear axis, overlaps with a similarly disposed sp2 orbital of the other carbon. Each sp2 orbital contains one electron, and the resulting bond contains two of the four electrons of the double bond. The bond contributes the other two electrons and is formed by a “sideby-side” overlap of singly occupied p orbitals of the two carbons. 

The double bond in ethylene is stronger than the C-C single bond in ethane, but it is not twice as strong. The C=C bond energy is 605 kJ/mol (144.5 kcal/mol) in ethylene versus 368 kJ/mol (88 kcal/mol) for the C-C bond in ethane. Chemists do not agree on exactly how to apportion the total C=C bond energy between its and components, but all agree that the sigma bond is weaker than the pi -bond.

We give alkenes IUPAC names by replacing the -ane ending of the corresponding alkane with -ene. The two simplest alkenes are ethene and propene. Both are also well known by their common names ethylene and propylene.
Ethylene is an acceptable synonym for ethene in the IUPAC system. Propylene, isobutylene, and other common names ending in -ylene are not acceptable IUPAC names.
The longest continuous chain that includes the double bond forms the base name of the alkene, and the chain is numbered in the direction that gives the doubly bonded carbons their lower numbers. The locant (or numerical position) of only one of the doubly bonded carbons is specified in the name; it is understood that the other doubly bonded carbon must follow in sequence.

Carbon–carbon double bonds take precedence over alkyl groups and halogens in determining the main carbon chain and the direction in which it is numbered.
Hydroxyl groups, however, outrank the double bond. Compounds that contain both a double bond and a hydroxyl group use the combined suffix -en -ol to signify that both functional groups are present.

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