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Introduction of Alkenes


Alkenes are unsaturated hydrocarbons containing a double bond. Alkenes are also known as olefins. The doubly bonded carbon atoms are sp2 hybridized. The double bond in alkenes consists of a sigma (σ ) bond and a pi (π ) bond. Hydrogen atoms and the carbon atoms in ethylene are in one plane, i.e. the ethylene molecule is planer. The bond angle is 120° . IUPAC names of alkenes are derived from the IUPAC names of corresponding alkanes by changing the "ane" ending of alkenes to the "ene" ending in alkenes. IUPAC and common names of some alkanes are given in Table 16.3.

Table 16.3 IUPAC and Common Names of Some Alkenes

Molecular formula

Structural formula

IUPAC name

Common name

C2H4

CH2=CH2

ethene

ethylene

C5H6

CH3 CH=CH2

propene

propylene

C4H8

CH3.CH2CH=CH2

1-butene

butylene

C4H8

CH3 CH=CH CH3

2-butene

butylene

According to the IUPAC system. Alkenes are named after the names of the corresponding alkanes according to the following rules.
  1. Select the longest chain of carbon atoms containing the double bond. Then select the suffix ane from the name of corresponding alkane and add a new suffix ene.
  2. Count the carbon atoms of the longest chain, so as to assign the lowest number to the carbon atom where the double bond begins.
  3. Write the above numbers before the name of the alkene. Sometimes this number is mentioned after the name of the alkene.
  4. In case a branched chain is present, state its position and nature even before the name of the alkene with respect to the position of the double bond.
For example, the names of the some alkenes are given in the table.

IUPAC Names of Some Alkenes

Structural Formula

IUPAC name

CH2 = CH2

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

CH3-C(CH3)2-CH=CH2


CH3-C(CH3)2-CH(CH3)-CH=CH2

 

Ethene (old name Ethylene)
Propene (old name Propylene)
1 - butene
2 - butene

 
 


3,3, dimethyl 1-butene


3-methyl 4; 4 dimethyl 1- pentene
or 3,4,4 - Trimethyl 1-pentene





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