13.2.1 Nomenclature and Isomerism

You have already read about nomenclature of different classes of organic compounds in Unit 12. Nomenclature and isomerism in alkanes can further be understood with the help of a few more examples.

Common names are given in parenthesis. First three alkanes – methane, ethane and propane have only one structure but higher alkanes can have more than one structure.

Let us write structures for C4H10. Four carbon atoms of C4H10 can be joined either in a continuous chain or with a branched chain in the following two ways :

In how many ways, you can join five carbon atoms and twelve hydrogen atoms of C5H12? They can be arranged in three ways as shown in structures III–V

Structures I and II possess same molecular formula but differ in their boiling points and other properties. Similarly structures III, IV and V possess the same molecular formula but have different properties. Structures I and II are isomers of butane, whereas structures III, IV and V are isomers of pentane.

Since difference in properties is due to difference in their  structures, they are known as structural isomers. It is also clear that structures I and III have continuous chain of carbon atoms but structures II, IV and V have a branched chain.

Such structural isomers which differ in chain of carbon atoms are known as chain isomers. Thus, you have seen that C4H10 and C5H12 have two and three chain isomers respectively.

Based upon the number of carbon atoms attached to a carbon atom, the carbon atom is termed as primary (1°), secondary (2°), tertiary (3°) or quaternary (4°).

Carbon atom attached to no other carbon atom as in methane or to only one carbon atom as in ethane is called primary carbon atom. Terminal carbon atoms are always primary.

Carbon atom attached to two carbon atoms is known as secondary. Tertiary carbon is attached to three carbon atoms and neo or quaternary carbon is attached to four carbon atoms. Can you identify 1°, 2°, 3° and 4° carbon atoms in structures I to V ?

If you go on constructing structures for higher alkanes, you will be getting still larger number of isomers. C6H14 has got five isomers and C7H16 has nine. As many as 75 isomers are possible for C10H22. In structures II, IV and V, you observed that –CH3 group is attached to carbon atom numbered as 2.

You will come across groups like –CH3, –C2H5, –C3H7 etc. attached to carbon atoms in alkanes or other classes of compounds. These groups or substituents are known as alkyl groups as they are derived from alkanes by removal of one hydrogen atom. General formula for alkyl groups is CnH2n+1 (Unit 12).

Let us recall the general rules for nomenclature already discussed in Unit 12. Nomenclature of substituted alkanes can further be understood by considering the following problem:

If it is important to write the correct IUPAC name for a given structure, it is equally important to write the correct structure from the given IUPAC name. To do this, first of all, the longest chain of carbon atoms corresponding to the parent alkane is written.

Then after numbering it, the substituents are attached to the correct carbon atoms and finally valence of each carbon atom is satisfied by putting the correct number of hydrogen atoms.

This can be clarified by writing the structure of 3-ethyl-2, 2–dimethylpentane in the following steps :

i) Draw the chain of five carbon atoms: C – C – C – C – C

ii) Give number to carbon atoms: C1– C2– C3– C4– C5

iii) Attach ethyl group at carbon 3 and two methyl groups at carbon 2

iv) Satisfy the valence of each carbon atom by putting requisite number of hydrogen atoms :

Thus we arrive at the correct structure.

 

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