Introduction to the Alkenes

James Richard Fromm

Alkenes are is the simplest of the unsaturated hydrocarbons, hydrocarbons which will react with hydrogen.  Alkenes contain one or more reactive double bonds between carbon atoms, and are easily indicated by the symbol =.    Since a double bond requires two carbon atoms, the simplest alkene is C2H4 or H2C=CH2, ethene.  The presence of a double bond is indicated by the change of ending from -ane to -ene.   An older form of the name, ethylene, is used in chemical industry.  The general formula is CnH2n.  All alkenes have at least one double bond.

Ethene (ethylene) C2H4
Propene C3H6
Butene C4H8
Pentene C5H10
Propene2.gif (480 bytes)

Alkenes and Cycloalkanes have the same general formula, CnH2n.   Therefore, the general formula does not identify the structure as an alkene nor an cycloalkane.  To further become problematic there are alkenes which contain more than one double bond.  Those with two double bonds have the formula, CnH2n-2.

Cycloalkenes have the general formula CnH2(n-m).   The letter m represents the number of double bonds.  Thus, cyclopropene has the formula C3H4 while that of cyclobutene is C4H6.   The properties of alkanes and alkenes are very similiar.

Organic compounds which have a more complex structure than simple alkanes are named as compounds which have been substituted.  By substituted is meant replacement of a hydrogen by some other atom or group of atoms, or the insertion of a different type of chemical bond in the molecule.  Each of the different types of carbon-carbon bonds is designated by a change in the ending of the name of the compound. For a carbon-carbon single bond, no ending change takes place because this is the normal structure of alkanes; the ending remains -ane.  Replacement of a single bond with a double bond, which requires loss of a molecule of hydrogen, is indicated by changing the ending to -ene.   Thus ethane, H3C-CH3, can be converted to ethene, H2C=CH2.   Replacement of a single bond with a triple bond is less common.  It is indicated by changing the ending to -yne. Thus ethane, H3C-CH3, can be converted to ethyne, HCCH, upon the loss of two molecules of hydrogen.

Substitution can occur in whichever direction gives a useful name.  Thus the compound CH3Cl can be named either as a methane which has had a chlorine substituted upon it (name: chloromethane) or as a chlorine which has had a methane substituted upon it (name: methyl chloride).  Both forms are correct, and both directions of substitution may be used in naming organic compounds.

If and only if there is more than one possible location for the substitution, its location is specified by the number of the carbon at which the substitution takes place.   This numbering is arbitrarily from one upwards.  For simple compounds it begins at one end of the longest carbon chain, but for more complex or ring structures the numbering is not necessarily obvious.  International standardizing groups establish an agreed appropriate numbering pattern for more complex structures.  This agreed numbering is given in reference works such as the Ring Index.

Simple Substitution

Simple substitution occurs when the substituent group replaces one hydrogen, or forms a single bond to the carbon chain (an equivalent statement).  Some of the simple substituent groups and their effect on the name are shown in the following Table. The examples are all for methyl but any alkyl (alkyl = substituent alkane) compound is named in the analogous way; R denotes any alkyl group.  Both directions of substitution can be used. The forms indicated by asterisks in the Table are less common forms.

Table: Simple Organic Substitution and Nomenclature
Compound Name Name
RCl Chloromethane Methyl Chloride
RBr Bromomethane Methyl Bromide
RI Iodomethane Methyl Iodide
ROH Hydroxymethane Methyl Alchohol (Methanol)
RNH2 Aminomethane Methyl Amine

When two or more different substituents appear on the same chain, they are named in alphabetical order.  When two or more of the same substituent appear in the same chain, the number of substituents is indicated by a numerical prefix (di, tri, tetra, ...). For example, butene has one double bond while butadiene has two.

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Copyright 1997 James R. Fromm