alkyl

Meaning: An alkyl group is the fragment left when one hydrogen is removed from an alkane. It is written R and bonds to the rest of the molecule through the carbon that lost the hydrogen. Names come from the parent alkane: methyl, ethyl, propyl, butyl. Branched isomers (isopropyl, isobutyl, sec-butyl, tert-butyl) differ in how that attachment carbon is bonded. Alkyl groups are nonpolar. Compared with hydrogen, they donate electron density weakly through induction and hyperconjugation, and they add steric bulk.

Equation: CnH2n+1, where n is the number of carbons (a dimensionless count). This is the formula of an acyclic alkyl group.

On the MCAT: Expect carbocation (and radical) stability rankings in SN1, E1 and alkene addition: tertiary > secondary > primary > methyl. Expect SN2 rates to run the opposite way (methyl > 1° > 2° >> 3°) because bulky alkyl groups block backside attack. Alkyl substituents also make a carboxylic acid slightly weaker, since electron donation destabilizes the conjugate base. Longer alkyl chains raise boiling point through stronger dispersion forces and lower water solubility.

Common confusion: Students treat alkyl and alkane as the same thing. An alkane is a complete, stable molecule. An alkyl group is a substituent with an open bonding site. The same alkyl groups that stabilize a carbocation in SN1 slow SN2 through steric hindrance.