Question

In which of the following, functional group isomerism is not possible?
A: Alcohols
B: Aldehydes
C: Alkyl halides
D: cyanide

Answer 1

Hint : Isomerism is the name given to a phenomenon in which more than one compounds have the same chemical formula but different chemical structure. Compounds having the same molecular formula but different arrangement of atoms are called isomers.

Complete step by step solution :
We know in isomerism the chemical formula of compounds is the same but their arrangement is different. As the name suggests functional isomers are the isomers which have the same chemical formula but functional group present in the compound is different. Alcohols and ethers are functional isomers. In alcohol one atom of hydrogen is replaced by an alkyl group and in ether both hydrogen atoms are replaced by an alkyl group. Therefore alcohol shows functional isomerism. Coming to option B, aldehydes and ketones are functional isomers. In ketone oxygen atom is present on that carbon which has carbon atoms on both sides and aldehyde is connected to that carbon which has alkyl group on one side and hydrogen atom as remaining bond as carbon needs to complete it’s four bonds. Therefore aldehydes also show functional isomerism. Cyanide and isocyanide are isomers. Therefore cyanide is also not the answer. Remaining is option C i.e. alkyl halides, alkyl halides do not show functional isomerism. So, the answer to this question is option C i.e. alkyl halides.
Additional information: Types of isomers are structural and stereoisomers. Structural isomers are divided into chain, positional, functional, metamerism, tautomerism and ring chain isomers. Stereoisomers are divided into geometrical and optical isomers. Structural isomerism is commonly called constitutional isomerism. In these isomers functional groups and atoms in the molecules are connected in different ways. Chain isomerism is also called skeletal isomerism. Components of these isomers are attached in the form of branches. In positional isomers the position of functional groups or atoms is different. Functional isomerism we have studied above. In metamerism different alkyl chains are present. Tautomerism refers to the isomer of the compound which only differs in position of protons and electrons. In ring-chain isomerism one structure has an open chain structure whereas the other has ring structure. In stereoisomerism compounds have the same molecular formula but different orientation of atoms in three dimensional space. Geometric isomerism is known as cis-trans isomerism.

Note : Functional isomerism occurs when substances have the same molecular formula but different functional groups. So just check options and see if there is any functional group whose atoms are not present in any other functional group.