Table-2:Homologous series of Alkenes
Alkane No. of Carbons Structure Formula
Ethene 2 CH2=CH2 C2H4
Propene 3 CH3-CH=CH2 C3H6
Butene 4 CH3-CH2-CH=CH2 C4H8
Pentene 5 CH3-CH2-CH2-CH=CH2 C5H10

Alkenes have general molecular formula CnH2n,where'n' is 2,3,4,....

Table-3: Homologous series of Alkynes
Alkane No. of Carbons Structure Formula
Ethyne 2 HC = _ CH C2H2
Propene 3 CH3- C = _ CH C3H4
Butene 4 CH3-H2C-C = _CH C4H6
Pentene 5 CH3-CH2-CH2-C = _CH C5H8

Alkynes have general molecular formula CnH2n-2,where'n' is 2,3,4,....

Nomenclature of organic compounds

We have millions of organic compounds. As number of organic compound is very big it is difficult to remember their names individually. To overcome this problem they have to be properly named. For this, the International Union of Pure and Applied Chemistry (IUPAC) had been formed, and one of its responsibilities is to name the organic and inorganic compounds in a systematic order. The basic idea behind the systematic nomenclature is that there should be only one name for the given structure throughout the world and also there should be only one structure for the given name.

The IUPAC name of an organic compound consists of three parts : 1) Word root 2) Prefix 3) Suffix.

1) Word Root : The number of carbon atoms present in the molecules or principle chain is called word root.

C1 -Meth; C2 -eth; C3 -prop; C4 -but; C5 -pent; C6 -hex;
C7 -hept; C8 -oct; C9 -non; C10--dec; and so on.

2) Prefix :The substituents in the molecule are shown as the prefix.Prefix again has several parts known asprimary prefix secondary prefix,numerical prefix and number prefix.

Primary prefix :It is useful only for cyclic compounds (Cyclo). If the compounds are not cyclic, this part of the name is absent.

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