Alkenes its Nomenclature and isomerism

ALKENES:

The members of this class of hydrocarbons contain two hydrogen atoms less than the corresponding alkanes. The two valency bonds left free on the adjacent carbon atoms unite to form a second union between them, establishing , π double bond in the molecule.

Since fill the valencies of the carbon atoms in alkenes are not fully satisfied, they can take up more H-atoms and are called Unsaturated hydrocarbons.

The hydrocarbons of this cla.ss are frequently called Olefines (Olefiant=oil forming) as the first member of the class, ethylene reacted with chlorine to form an oily product.

Cracking of petroleum:

Alkenes being much more reactive than alkanes seldom occur free in nature.They are, however, produced in Iarge amounts during the cracking of petroleum a:nd the decomposition of almost all organic substances. The cracking of cotton seed oil gives as much as 30 per cent yield of ethylene. The lower alkenes occur to a minor extent in coal gas.

Structure:

Bond hybridization:

Let us consider ethylene, in ethylene the carbon bonds are sp2 hybridize. They are attached to each otherby a sigma bond and a pi bond. The sigma bond results from the overlap of two sp2 hybrid orbitals( i.e. one from each carbon). The pi bond is formed from the overlap of the unhybridized p orbitals.

Remember:

1) The carbon-carbon double bond in alkenes is made up of one sigma bond and one pi bond

2) Alkenes are more reactive than alkanes. This is due to the availiability of the more exposed pi electrons.

NOMENCLATURE:

The COMMON NAMES:

The common names of the first four members (C₂ to C₅) are derived from those of the corresponding alkanes (having same No. of carbons) by changing the ending -ane to -ylene. Grcek letters are used to distinguish isomers having double bond at the first (α) or the second (β) carbon of the chain is illustrated above for Butylenes . Thus:

THE IUPAC NAMES:

The ipuac names of alkenes are derived from those of the corresponding alkanes by changing the terminal ·ane to ·ene, Thus to get the name of an individual member, first name the alkane with the same number of carbon atoms ana then replacing the ending ane by.ene, Thus:

When there are two or three double bonds in a molecule, the ending ane of the rcspective alkane is replaced by diene or triene to get the name of the hydrocarbon. Thus

AlkenyI radicals.

The monovalent radicals obtaincd from alkenes are called alkenyl radicals 'I'he common names vinyl radical and allyl radical are also the IUPAC names. All other alkenyl radicals arc named by replacing the 'e' of the parent alkene by '.yl.' The carbon atoms constituting the radical are numbered So that the carbon with free velency is always number 1. The IUPAC names of some aIkenyl radicals are given below:

ISOMERISM:

The presence of the double bond in alkenes increases the opportunity of isomerism. In addition to chain isomerism, they offer position isomerism due to the different positions of the double bond on the carbon chain.

The first two alkenes can have but one structural formula. The third member butene, however, can have three possible structural formulas

All the three butenes are actually known. The compounds (1) and (2) present a case of position isomerism as they differ only in the location of the double bond. The compounds (2) and (3) illustrate both position and chain isomerism.

Besides the structural isomerism, certain alkenes also show cis-trans isomerism. Thus 2-Butene is known to exist in two forms.