Alkene or Olefin in Organic Chemistry

Alkenes names as olefins are the unsaturated hydrocarbon containing at least one double or olefinic chemical bond having different types of geometric structure and structural isomerism. Alkene or olefin structure is naming by different methods like common olefins molecules, substituted name, and IUPAC nomenclature of organic compounds in chemistry. Ethylene, propylene, butylene, etc define the common examples of alkene or olefin.

The common molecular formula of alkene CnH2n, where n = 1, 2, 3, etc.

IUPAC nomenclature alkene or olefin ethylene, propylene, butylene compounds and organic structure

Why Alkenes are Called Olefins?

The double bond of the alkene is known as ‘olefinic bond or ‘ethylenic bond’. But the organic naming of olefin molecules define from ethylene which is known ‘olefiant gas'( oil-forming gas molecules). Since alkenes form an oily liquid when treated with chlorine or bromine. Thus alkenes are called olefins.

The original name given to this homologous series was olefine. But later decided to reserve the suffix – ine for basic substances only. Since the naming of olefins compounds gained and use widely for alkenes. For examples, some common alkenes molecules are

CH2=CH2 ethylene
CH3-CH=CH2 propylene
CH3-CH2-CH=CH2 butylene

Common naming of Alkene or Olefin

Common naming of alkenes or olefins compounds from the corresponding alkanes molecules and named according to the following rule

  1. The total number of carbon atom counted in the olefin and the name of the corresponding alkane.
  2. changing the name of the corresponding alkane, the suffix -ane of the latter into – ylene.
  3. The define position of the olefin bond indicated by numbers 1, 2, 3, 4…., or Greek letters α, β, ⋎, ઠ, …., These letters are known as locants.
  4. The locants of the double bond carbon atom placed before the name of the olefin.
  5. A hyphen has written between the locants and the name.

The locants are used only to name alkenes containing more than three carbon atoms. Therefore the alkenes with low molar mass only have the common names. Examples of such naming of alkene molecules are

CH2=CH2 ethylene
CH3CH2CH2CH=CH2 1 – pentylene
or α – pentylene
CH3CH=CHCH2CH3 2 -pentylene
or β – pentylene

Substituted naming Ethylene, Propylene, Butylene

In this method, ethylene considers as the parent substance. Therefore the higher member is the derivatives of ethylene.

When we naming of mono-substituted organic compounds no difficulty arises. But the disubstituted derivative of ethylene, naming as an asymmetrical and symmetrical hydrocarbon.

  1. When the groups attached to the same carbon atom of the olefins nomenclature as the asymmetrical organic compounds.
  2. But when the groups attached to the different carbon atom of the alkene or olefin nomenclature as the symmetrical in organic chemistry.
CH3CH=CH2 methyl ethylene
CH3CH2CH=CH2 ethyl ethylene
CH3 (H3C)C = CH2 as-dimethyl ethylene
CH3CH = CHCH3 sym-dimethyl ethylene

IUPAC Nomenclature Alkenes or Olefins

  1. According to the IUPAC nomenclature of organic compounds like alkenes or olefins, the suffix name of the olefins = ene. Thus the series becomes the alkene series.
  2. Find the longest carbon chain structure containing the olefinic bond name as the parent alkene in the IUPAC nomenclature.
  3. 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.
  4. Thus the name of which is obtained by changing the suffix – ane of the corresponding alkanes into – alkenes.

Take an example of IUPAC name alkene containing a double bond and a side chain in the structure


Isomerism of Ethylene, Propylene, Butylene

An organic compound that has the same molecular weight but differs in its structure said to the structural isomerism.

Except for ethylene, propylene all the alkenes show structural isomerism. For example, butylene C4H8 has three structural isomers.

Structural isomers of pentene molecules

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. Therefore isomeric pentenes show five structural isomers nomenclature of these compounds in organic chemistry.

CH3CH2CH2CH=CH2 pent-1-ene
CH3CH2CH=CHCH3 pent-2-ene
CH3CH2C(CH3)=CH2 2-methylbut-1-ene
CH3CH(CH3)CH=CH2 3-methylbut-1-ene
CH3C(CH3)=CHCH3 2-methylbut-2-ene

Geometric isomerism of Ethylene and Propylene

Isomers that have the same chemical structure but different configuration name to be stereoisomerism. There are two types of stereoisomers

  1. Optical isomerism
  2. Geometrical isomerism

In optical isomerism, all or at least some of the isomers are optically active.

But in geometric isomerism name as cis-trans isomerism, there are exist different configurations for a given chemical structure of ethylene and propylene

These isomers may or may not be optically active because the optical activity is not define criterion for geometric isomers of alkene or olefin.