 Alkenyl group

An alkenyl group is a monovalent carbon–hydrogen substituent derived from an alkene by removing one hydrogen atom. The attachment can be at a double-bonded carbon or elsewhere in the carbon skeleton. For an acyclic parent with one double bond, the group composition is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n-1}$.^[ref]

For example, a vinyl group attaches as $\mathrm{R}-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ and contributes ${\mathrm{C}}_2{\mathrm{H}}_3$, whereas an allyl group attaches as $\mathrm{R}-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ and contributes ${\mathrm{C}}_3{\mathrm{H}}_5$. The attachment atom in the first example belongs to the double bond; in the second it is adjacent to the double bond. A prenyl group is another alkenyl group, with five carbon atoms.^[ref]

The missing hydrogen identifies an attachment site in a larger molecule; it does not mean that the completed attached group must contain an unpaired electron. Formulae alone also do not specify connectivity. For $n=3$, the composition ${\mathrm{C}}_3{\mathrm{H}}_5$ does not by itself tell where the attachment or double bond lies. A structural formula supplies information that the atom count omits.