Alkene Nomenclature -Hydrocarbons Notes

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What are alkenes?

The class of hydrocarbons which contains at least one carbon-carbon double bond are called alkenes.

Their functional group is C=C

They have at least one carbon-carbon double bond. Alkenes have hydrogen atoms less than the corresponding alkanes.

Alkene also forms a homologous series and each member of the series has one methylene group (-CH3) less than the next higher members.

Alkenes are unsaturated hydrocarbons having the general formula CnH2n.

when C=2 C2H4 ethene

when C=3 C3H6 propene

when C=4 C4H8 butene

when C=5 C5H10 pentene

On the basis of the number of double bonds per molecule,alkene may be monoene having one double bond, diene having two double bonds and triene having three double bonds.

Lower members of the alkene family on reaction with halogens produce oily products hence alkenes are also called olefins (meaning oil-producing).

Nomenclature:

  • Common System:

The common system names of alkenes are derived from the names of corresponding alkyl groups by adding the suffix “ene”.

To locate the position of double bonds Greek letters α, β, γ , δ etc are used.

The common names of the alkenes are derived from the common names of alkyl groups having the same number of carbon atoms.

The ending -ene is added to the name of the corresponding alkyl group.

Example:

Alkane —-> Alkyl group —-> Alkene

  • IUPAC System:
  • Select the longest continuous carbon chain containing a carbon-carbon double bond.
  • The ending -ane of an alkane is replaced by -ene .
  • The longest continuous chain is numbered from that side which gives the lowest possible number to the carbon-carbon double bond.
  • If the position of the double bond is the same from both sides, then the chain is numbered from that side where the smallest number is given to the substituent.
  • Rules for substituents are the same as those of alkanes.
  • Since a double bond appears between two carbons. Therefore, its position is indicated by the number of that carbon, bearing a lower number.

Examples:

CH2=CH2 Ethene

CH3-CH=CH2 Propene

CH3-CH2-CH=CH2 1-Butene

Name of alkenes are derived from the name of corresponding alkanes by replacing the Last three letters with new suffix “ene”.

If double bonds are more than one then the last two letters are dropped and suffix diene, triene etc is added.

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