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Organic Compounds: Nomenclature-Alkanes

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Organic Compounds: Nomenclature-Alkanes - Lesson Summary

Hydrocarbons are organic compounds that contain only carbon and hydrogen. The Hydrocarbons which consists of Carbon – Carbon single bonds are said to be saturated and are called alkanes. Alkanes can be straight chain alkanes (or) branched chain alkanes based on their structure. There are two systems for naming organic compounds the common (or) trivial system and the IUPAC system.

The IUPAC name of an alkane basically contains two parts the root word and the suffix. A root word indicates the length of the continuous selected carbon chain.

A chain which consists of up to four carbon atoms is assigned special root words from their trivial names. Thus, the root word for one carbon atom is ‘meth’, two carbon atoms is ‘eth’, three carbon atoms is ‘prop’ and for four carbon atoms is ‘But’.

A chain that consists of more than four carbon atoms is assigned a root word derived from the names of Greek numerals.
             5           Pent              6           Hex              7           Hept              8           Oct              9           Non              10           Dec

A suffix indicates the nature of the carbon chain. As the carbon chain is saturated, Suffix “ane” is used. Molecule can be named by adding root word + primary suffix.
Ex: IUPAC name of an alkane having four carbon atoms is, but + ane, thus butane.
 
Rules to name the  branched chain alkanes:

(i) Select the longest carbon chain in the molecule.



(ii) Number the parent chain that has branches in such a way that branched Carbon atoms should get lowest possible number.
 
(iii) Name the branched chain alkane.

(iv) Name the identical alkyl groups at different positions of the carbon chain using prefixes like ‘di’, ‘tri’, ‘tetra’...etc.

Ex:


(v) Numbering different alkyl groups at equivalent positions: Lower number is assigned to the group that appears first in the alphabetical listing.

Ex:


(vi) If the substituent is on the parent chain is complex, then the carbon atom of the branch attached to the parent chain is given the position-1while numbering.

Ex:


The group that consists of one hydrogen atom less than an alkane is called Alkyl group. An alkyl group is obtained by removing one hydrogen atom from an alkane. The alkyl groups obtained from straight chain hydrocarbons are known as normal alkyl groups.

Ex:


Straight chain alkanes can either be acyclic or cyclic. Cyclic compound which consist of carbon-carbon single bond are named by prefixing cyclo to corresponding straight chain alkane.

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