alkyl group
/ AL-kil /
An alkyl group is an alkane with one hydrogen removed, leaving an open hand ready to bond to something else. Take methane (CH4), pull off one H, and you have a methyl group (CH3-); from ethane comes ethyl (CH2CH3-), and so on. Alkyl groups are the side branches you hang on a parent chain, and chemists abbreviate a generic one as 'R', the way algebra uses x.
The names follow the alkanes but swap the -ane ending for -yl: methane to methyl, propane to propyl. When the chain can attach in more than one way, the connection point matters and gets its own name. A three-carbon group bonded through an end carbon is propyl; bonded through its middle carbon it is isopropyl (also called 1-methylethyl). Four-carbon groups give butyl, sec-butyl, isobutyl, and tert-butyl, the last attaching through a carbon already holding three other carbons.
Alkyl groups are the vocabulary of substituents in IUPAC names — you cite them alphabetically as prefixes (ethyl before methyl) with their locants. They also carry chemical meaning: an alkyl group gently pushes electron density toward whatever it is attached to (the inductive and hyperconjugative donating effect), which is why more-substituted carbocations and radicals are more stable. So 'R' is not just bookkeeping; it subtly tunes reactivity everywhere it appears.
Isopropyl (CH3)2CH- and propyl CH3CH2CH2- have the same three carbons but attach differently — isopropyl through its central carbon, propyl through an end one.
Same atoms, different point of attachment — the connection defines which alkyl group it is.
For alphabetizing in a name, isopropyl files under 'i' but tert-butyl files under 'b' (the 'tert-' and 'sec-' prefixes are italicized and ignored, while 'iso' and 'neo' count).