In the context of structural isomerism in organic chemistry, for which of the following classes of compounds is functional group isomerism not possible?

Introduction / Context:
This question focuses on the concept of functional group isomerism, an important type of structural isomerism in organic chemistry. Functional group isomers are compounds that have the same molecular formula but possess different functional groups, leading to different chemical properties. You are asked to identify the class of compounds among alcohols, aldehydes, alkyl halides and cyanides for which such functional group isomerism cannot occur. Understanding which functional groups can give rise to functional group isomers helps in predicting possible structures from a given molecular formula in many exam problems.

Given Data / Assumptions:
Alcohols can have functional group isomers that are ethers with the same molecular formula.
Aldehydes can have functional group isomers that are ketones with the same molecular formula.
Cyanides (nitriles) can have functional group isomers that are isocyanides (isonitriles) with the same molecular formula.
Alkyl halides contain a carbon halogen bond but do not have a commonly associated alternative functional group with identical molecular formula that differs only by functional group type.
We assume simple monofunctional compounds without additional functional groups.

Concept / Approach:
Functional group isomerism occurs when the same molecular formula can be written in such a way that the functional group changes, for example alcohol versus ether or aldehyde versus ketone. The key idea is that the molecular formula is the same, but the arrangement of atoms leads to a different functional group. For alcohols, an example is C2H6O, which can be written as ethanol (an alcohol) or as dimethyl ether (an ether). For aldehydes, C3H6O can represent propanal (an aldehyde) or propanone (a ketone). For cyanides, a molecular formula like C2H3N can represent acetonitrile or methyl isocyanide. In contrast, alkyl halides R–X, where X is a halogen, do not have a corresponding set of common functional group isomers with the same simple molecular formula and a different functional group classification.

Step-by-Step Solution:
Step 1: Consider alcohols. A classic example is C2H6O, which can exist as CH3CH2OH (ethanol, an alcohol) and CH3OCH3 (dimethyl ether, an ether). These are functional group isomers, so alcohols clearly show functional group isomerism. Step 2: Consider aldehydes. For instance, C3H6O can represent CH3CH2CHO (propanal, an aldehyde) or CH3COCH3 (propanone, a ketone). Aldehydes therefore exhibit functional group isomerism with ketones. Step 3: Consider cyanides. A formula like C2H3N can correspond to CH3CN (acetonitrile, a nitrile) or CH3NC (methyl isocyanide, an isonitrile). Thus cyanides show functional group isomerism with isocyanides. Step 4: Now consider alkyl halides, which have the general formula CnH2n+1X, where X is a halogen. There is no common distinct functional group that shares exactly the same simple molecular formula and differs only by a reorganisation that produces a different functional group category. Step 5: Alkyl halides can have chain isomers or position isomers, but they do not exhibit a pair of structures with different functional groups and identical molecular formula in the way that alcohols, aldehydes and cyanides do. Therefore functional group isomerism is not possible for alkyl halides.

Verification / Alternative check:
To verify, you can systematically write down molecular formulas for each class and attempt to draw another structure with a different functional group but the same formula. For alcohols, ethers serve as the corresponding functional group isomers. For aldehydes, the paired functional group is ketones. For cyanides, the alternative functional group is isocyanides. However, when you take an alkyl halide like C2H5Cl, there is no other stable organic compound with this exact molecular formula that would be placed into a different functional group category, such as an alcohol, ether or nitrile, without changing the overall formula. This confirms that alkyl halides lack functional group isomerism under standard classification schemes.

Why Other Options Are Wrong:
Alcohols are wrong as an answer because they clearly show functional group isomerism with ethers, for example ethanol and dimethyl ether for C2H6O.
Aldehydes are wrong because they exhibit functional group isomerism with ketones, such as propanal and propanone for C3H6O.
Cyanides are wrong because they show functional group isomerism with isocyanides, as seen in the nitrile and isonitrile forms of the same molecular formula.

Common Pitfalls:
Students often confuse different types of isomerism. Chain isomerism, positional isomerism and functional group isomerism may all occur separately, and it is important not to assume that every class of compounds can show every type of isomerism. Another common mistake is thinking that changing the position of the halogen in an alkyl halide is a type of functional group isomerism; in fact, this is positional isomerism, because the functional group carbon halogen bond remains the same. Keeping the definitions of each isomerism type clear helps to answer such questions correctly.

Final Answer:
Functional group isomerism is not possible for Alkyl halides, so that is the correct choice.