Positional Isomers of Alkenes

James Richard Fromm


Another hydrocarbon family is the family of compounds containing double bonds between carbon atoms.  These compounds are termed the ALKENES or OLEFINS.  They are named just as their parent compounds, the alkanes, are named, except the ending -ene replaces -ane.   The ending -ene signifies a double bonding of carbon atoms.  The first five members of the alkene family are ethene, propene, butene, pentene, and hexene.   Many organic compounds contain double bonds.  Such compounds are termed UNSATURATED, since they can combine with other elements or compounds without breaking the carbon chain, by adding on at the double bond.  The alkenes constitute a homologous series with the general formula CnH2n.  Physical and chemical properties of alkenes vary with increasing molecular weight, in the same way as do the properties of the alkanes.

The location of the double bond is unambiguous in propene (also known as propylene), but two butenes are possible.  These two isomers are POSITIONAL ISOMERS.  They have been called n-butene, CH2=CHCH2CH3, and isobutene, H3C-CH=CHCH3, but are more commonly and correctly called 1-butene and 2-butene.  Two butadienes, which each contain two double bonds, are known also: 1,2-butadiene, H2C=C=CHCH3, and 1,3-butadiene, H2C=CHCH=CH2.

CH2=CHCH2CH3 H3C-CH=CHCH3
1-butene 2-butene

 

H2C=C=CHCH3 H2C=CHCH=CH2
1,2-butadiene 1,3-butadiene

Alkenes are considerably more reactive than are alkanes, and the addition of reactants across the double bond is common. Hydrogen will add across the double bond, as will halogens and hydrogen halides.  An oxidation of the double bond, with a strong oxidizing agent such as potassium permanganate, often leads to cleavage of the molecule at the double bond with formation of carboxylic acids, which will be taken up in later sections, on both fragments.

Compounds with double bonds, which exist in isomeric form, are named by placing the position number of the carbon atom on which the double bond begins, before the name of the parent compound.

Alkenyl radicals:

CH2=CH- CH3-CH=CH- CH2=CH-CH2- CH3-CH2-CH=CH- CH3-CH=CH-CH2- CH2=CH-CH2-CH2-
ethenyl 1-propenyl 2-propenyl 1-butenyl 2-butenyl 3-butenyl

Carbon atoms in alkenes are always numbered so that the lowest possible position number is assigned to the first carbon atom to which the double bond is attached.  The parent compound is named from the longest continuous chain containing a double bond. For example:

CH2=CHCH(CH3CH2CH2)CH2CH2CH3

is 3-propyl-1-hexene.

It is required that the parent chain contain the double bond.  Thus, a six carbon parent chain rather than a seven carbon chain containing only single bonds.

If more than one double bond occurs in a molecule, the endings -adiene, -atriene, and so on are used. The compound:

CH3-CH=CH-CH=CH-CH2-CH3

is 2,4-heptadiene.

Cycloalkenes, like cycloalkanes, are possible and their reactivities are very similar to those of the straight-chain alkenes. Cycloalkenes are named in the same manner as cycloalkanes.  Only the ending of the name is changed, to -ene.  The position is designated by a numeric prefix only when necessary, which it would not be in a compound such as cyclohexene.

cyclohexene.jpg (1884 bytes)                    1,3-cyclohexadiene.jpg (1989 bytes)                    1,4-cyclohexadiene.jpg (1796 bytes)                    benzene.jpg (1993 bytes)

                     cyclohexene                        1,3-cyclohexadiene                   1,4-cyclohexadiene              1,3,5-cyclohexatriene, Benzene


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Copyright 1997 James R. Fromm