Nomenclature and Isomerism

The compound having same molecular formula but different physical and chemical are called as isomers and this phenomenon is called Isomerism.

Compounds which have same molecular formula, but their structural formulas are different, are called structural isomers.

First three alkanes – methane, ethane and propane have only one structure but higher alkanes can have more than one structure.

Four carbon atoms of C₄H₁₀ can be joined either in a continuous chain or with a branched chain in the following two ways:

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 -

o Primary (1°),

o Secondary (2°),

o Tertiary (3°) or

o 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.

Nomenclature of few organic compounds

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.

Example: Structure of 3-ethyl-2, 2–dimethylpentane

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: