In which of the following, functional group isomerism is not possible?

Functional group isomerism occurs when compounds share the same molecular formula but possess different functional groups. Alcohols (R-OH) exhibit functional isomerism with ethers (R-O-R) [1]. Aldehydes (R-CHO) are functional isomers of ketones (R-CO-R) [1]. Cyanides (R-CN), also known as nitriles, exhibit functional isomerism with isocyanides (R-NC) [2]. However, alkyl halides (R-X, where X is a halogen) do not have a corresponding functional group isomer with the same molecular formula. While alkyl halides can exhibit chain isomerism or position isomerism (such as 1-chloropropane and 2-chloropropane), they lack a distinct functional group counterpart [1]. Therefore, functional group isomerism is not possible for alkyl halides. This distinction is fundamental in organic chemistry nomenclature and structural analysis [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Table 4.4 Nomenclature of organic compounds > p. 68
2. [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC6856803/