IUPAC Names of Hydrocarbons

The naming of various hydrocarbons according to the IUPAC rule is discussed in the lesson above.

Alkanes

• Longest chain rule -

If the given organic compound is in branched chain structure, the longest possible chain of carbon atoms is selected as word root and the remaining branches are regarded as prefixes,

• Substituent rule -

If the given alkane (parent chain) contains only one branch or substituent in the parent chain, the numbering of carbon atoms is started from the nearest terminal carbon of the substituent.

• Lowest sum rule -

If more than one similar or different substitutes are present in the main chain, the numbering of carbon atoms is done in such a way that sum of the substituents gives the lowest number.

• Alphabetical order -

(a) If two or more than two branch chains are present in the parent chain, the prefix names of these groups are arranged in alphabetical order. The same prefix if present in the different position, the positional number of each group (branch chain) is separated by commas and suitable prefixes like di, tri, tetra etc. (which are not regarded in alphabetical order).

(b) If two different substituents are located at the equivalent positions, the numbering of the chain is done in such a way that the substituent which comes first in alphabetical order gets the lower position.

• Complex alkyl substituent rule -

If the alkyl substituent is further branched, it is named as substituted alkyl group, in which the carbon atoms of the alkyl group are separately numbered in such a way that the carbon atom directly attached to the parent chain is given the number 1.

Alkene

Alkene or olefin (Greek word: olefiant= oil forming).

Hydrocarbons containing carbon-carbon double bond (C=C) is known as an alkene. Its general formula is C_nH_2n.

Word root -alk

Primary suffix -ene

IUPAC name -> alk + ene= alkene

Some examples of alkenes are given below-

Molecular formula	Common name	IUPAC name
CH2=CH2	Ethylene	Ethene
CH3-CH=CH2	Propylene	Propene
CH3-CH2-CH=CH2	α- butylene	but- 1- ene or 1-butene
CH3-CH=CH=CH3	β- butylene	but- 2- ene or 2- butene
CH3-CH2-CH=CH-CH2-CH3	γ- hexylene	hex- 3- ene or 3- hexene
	iso- butylene	2- methylpropene

Alkyne

Hydrocarbons containing carbon-carbon triple bond (-C≡C-) are known as alkyne. Its general formula is (C_nH_2n-2).

Word root:Alk

Primary suffix:yne

IUPAC name ->alk + yne= alkyne

The IUPAC and common names of some important members of alkyne family are given below-

Molecular formula	Common name	IUPAC name
HC ≡ CH	Acetylene	Ethyne
CH3- C ≡ CH	Methyl acetylene	Propyne
CH3- C ≡ C - CH3	Dimethyl acetylene (Crotonylene)	But- 2- yne or 2- butyne
CH3- CH2- C ≡ C - CH2- CH3	Diethyl acetylene	hex- 3- yne or 3- hexyne

Points to remember while naming the compounds containing single, double, and triple bonds-

1) Hydrocarbons containing carbon-carbon single bond in a chain is denoted by normal (n). Example-

CH₃-CH₂-CH₂-CH₂-CH₃ (Pentane) [Common name: n-pentane]

2) If carbon-carbon branch is present in hydrocarbons, it is denoted by iso. 

Common name - iso-pentane

IUPAC name - 2- methyl butane

3) If central carbon is attached to three carbon atoms, it is denoted by tert. (tertiary).

4) If central carbon is attached to four carbon atoms, it is denoted by neo. For example-

5) If double and triple bond both are present in the equivalent position from the terminal carbon, the numbering is done from the terminal carbon to the double bond.

CH = C - CH₂- CH₂ - CH₂- CH = CH₂ (1) {Numbering is done form this side}

So, Hept- 1- ene- 6- yne.

6) If more than one double bonds or triple bonds are present di, tri, tetra etc. prefixes are added to 'ene' or 'yne'. Like-

CH₂ = CH - CH₂ - CH = CH₂(pent-1, 4- diene)