Functional Isomerism

Isomerism of Class 11

Functional Isomerism

When any two compounds have the same molecular formula but possess different functional groups, they are called functional isomers and the phenomenon is termed functional isomerism. In other words substances with the same molecular formula but belonging to different classes of compounds exhibit functional isomerism. Thus,

1. Diethyl ether and butyl alcohol both have the molecular formula C4H6O, but contain different functional groups.

C2H5–O–C2H5                        C4H9–OH

 diethyl ether                      butyl alcohol

The functional group in diethyl ether is (–O–), while in butyl alcohol it is (–OH).

2. Acetone and propionaldehyde both with the molecular formula C3H6O are functional isomers.

CH3–CO–CH3                  CH3–CH2–CHO

acetone                  propionaldehyde

    In acetone the functional group is (–CO–), while in propionaldehyde it is (–CHO).

3. Cyanides are isomeric with isocyanides:

4. Carboxylic acids are isomeric with esters.

5. Nitroalkanes are isomeric with alkyl nitrites:

6. Sometimes a double bond containing compound may be isomeric with a triple bond containing compound. This also is called as functional isomerism. Thus, butyne is isomeric with butadiene (molecular formula C4H6).

7.  Unsaturated alcohols containing three or more carbon atoms isomeric to aldehydes as well as ketones:

8.  Aromatic alcohols may be isomeric with phenols:

9. Primary,secondary and tertiary amines of same molecular formula are also the functional isomers.

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