Question

Which of the following is an alkyl group?
A. Ethyl
B. Ethane
C. Ethyne
D. None of the above

Answer 1

Hint: The general form of an alkyl group is \[{{C}_{n}}{{H}_{2n+1}}\]. Now, try and apply this framework to each of the above options to see which one fits this formula and is thus the answer.

Complete step-by-step answer:
Let us first define what an alkyl group is and its significance in Organic Chemistry.
In organic chemistry, an alkyl substituent is an alkane missing one hydrogen atom. The term alkyl is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of \[{{C}_{n}}{{H}_{2n+1}}\]1. Similarly, a cycloalkyl group is derived from a cycloalkane by removal of a hydrogen atom from a ring and has the general formula \[{{C}_{n}}{{H}_{2n-1}}\] Typically an alkyl is a part of a larger molecule. In structural formula, the symbol R is used to designate a generic (unspecified) alkyl group.
Commonly in Chemistry, alkyl is a group, a substituent, that is attached to other molecular fragments. For example, A dialkyl ether is an ether with two alkyl groups.
With this characterisation in mind, let us now analyse each of the given options.
We find that only ethyl(${{C}_{2}}{{H}_{5}}-$) of the given options fits the characterisation of an alkyl group whereas ethane(${{C}_{2}}{{H}_{6}}$) and ethyne (${{C}_{2}}{{H}_{2}}$) are an alkane and an alkyne respectively.
Therefore, the answer to this question is a).

Note: Usually, alkyl groups are attached to other atoms or groups of atoms. Free alkyls occur as free radical, as anions, or as cations. The cations are called carbocations. The anions are called carbanions.