The molecular formula of alkenes or olefins
Alkenes or olefins are the unsaturated hydrocarbons containing one double bond and the general molecular formula
where n = 1, 2, 3, 4 etc.
Thus they contain two hydrogen atoms less than the alkanes. Examples of some simple alkenes are
Why alkenes are called olefins?
The double bond of the alkene is called ‘olefinic bond or ‘ethylenic bond’. Thus alkenes are called olefins.
But the name olefin arose from the fact that ethylene was called ‘olefiant gas'( oil-forming gas). Since alkenes form oily liquids when treated with chlorine or bromine.
Thus the original name given to this homologous series was olefine. But it was later decided to reserve the suffix – ine for basic substances only. Since the name olefins gained and use widely.
Common naming of alkenes
One method of nomenclature is to name the olefins from the corresponding alkanes. This naming is given according to the following rule
- The total number of carbon atom counted in the olefin and the name of the corresponding alkane determined.
- changing the name of the corresponding alkane, the suffix -ane of the latter into – ylene.
- The position of the double bond indicated by numbers 1, 2, 3, 4…., or Greek letters α, β, ⋎, ઠ, …., These letters are known as locants.
- The locants of the double bond carbon atom are then placed before the name of the olefin.
- A hyphen is written in between the locants and the name.
The locants are used only to name alkenes containing more than three carbon atoms. But the alkenes of low molecular weights only have common names.
|CH3CH2CH2CH=CH2||1 – pentylene|
or α – pentylene
or β – pentylene
Substituted or derived naming of alkenes
In this method, ethylene considers as the parent substance. Thus the higher member is the derivatives of ethylene.
Thus the naming of mono-substituted derivative no difficulty arises. But the disubstituted derivative of ethylene, the alkyl groups are of attached the same or different carbon atoms.
- When the groups attached to the same carbon atom of the olefins named as the asymmetrical hydrocarbon.
- But when the groups attached to the different carbon atom of the olefins named as the symmetrical hydrocarbon.
|CH3 ㄧ(H3C)C = CH2||as-dimethyl ethylene|
|CH3ㄧCH = CHㄧCH3||sym-dimethyl ethylene|
IUPAC rules for naming alkenes
- According to the IUPAC system of nomenclature, the class suffix of the olefins – ene. Thus the series becomes the alkene series.
- The longest carbon chain containing the double bond chosen as the parent alkene.
- The position of the double bond and side chains indicated by numbers, the lowest number possible being given to the double bond, and placed before the suffix.
- Thus the name of which is obtained by changing the suffix – ane of the corresponding alkanes into – alkenes.
Take an example of alkene containing a double bond and a side chain
Naming alkenes practice with answers
Structural isomers of alkenes
Organic compounds that have the same molecular weight but differ in their structure said to the structural isomerism.
Except for ethylene, propylene all the alkenes show structural isomerism.
The molecular formula butylene C4H8. Thus isomeric butene has three structural isomers.
Structural isomers of pentene
The molecular formula of the pentene
Now take each one in turn and introduce one double bond, starting at the least substituted end and shifting the double bond inwards. By this isomeric pentenes shows five structural isomers.
Geometric isomerism in alkenes
Isomers that have the same structure but different configuration said to be stereoisomerism. There are two types of stereoisomers
- Optical isomerism
- Geometrical isomerism
In optical isomerism, all or at least some of the isomers are optically active.
But in geometric isomerism or cis-trans isomerism, there are exist different configurations for a given structure of alkenes or olefins.
These isomers may or may not be optically active because of optical activity not a criterion for geometric isomers of alkenes or olefins.