What defines an alkyl group?

An alkyl group is defined as a molecular fragment that results from the removal of one hydrogen atom from an alkane molecule. When you take an alkane, which is a saturated hydrocarbon containing only single bonds, and remove one of its hydrogen atoms, you create an alkyl group. This group retains the original carbon skeleton of the alkane but has one less hydrogen, allowing it to bond with other atoms or functional groups.

The significance of this definition lies in the fact that alkyl groups are often used as substituents in organic chemistry, forming the backbone for more complex organic molecules. Alkyl groups can vary in structure—ranging from simple groups like methyl (−CH₃) and ethyl (−C₂H₅) to larger, branched groups, depending on the number of carbon atoms and their arrangements.

The other choices do not accurately reflect the definition of an alkyl group as they either involve incorrect processes of hydrogen removal, pertain to different types of molecules, or describe unrelated chemical concepts. Thus, the answer correctly captures what an alkyl group truly represents in organic chemistry.