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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.

The specific five-carbon prenyl group is the alkenyl group 3-methylbut-2-en-1-yl, with connectivityHere $\mathrm{R}$ is the rest of the attached molecule; the prenyl group itself has composition ${\mathrm{C}}_5{\mathrm{H}}_9$. The ChEBI structural record uses SMILES *CC=C(C)C, with the asterisk marking the attachment point.^[ref]

The count is instructive: the attachment-end $\mathrm{C}{\mathrm{H}}_2$ contributes one carbon and two hydrogen atoms; $\mathrm{CH}$ contributes one and one; the next double-bonded carbon contributes no hydrogen; and the two methyl groups contribute two carbon and six hydrogen atoms. Together these give five carbon and nine hydrogen atoms. The double bond is one bond away from the attachment atom, connecting the prenyl group with the broader allyl group pattern.

Attaching the group through oxygen gives $\mathrm{R}-\mathrm{O}-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}=\mathrm{C}(\mathrm{C}{\mathrm{H}}_3{)}_2$, a prenyl ether; attachment through another atom gives a different linkage. The same carbon fragment therefore does not specify the whole functional group environment. In biological prenylation, the terminology can also encompass longer isoprenoid-derived attachments, so the particular transferred group must be stated.