alkyl halide

Meaning: An alkyl halide has the form R–X, where X is F, Cl, Br, or I attached to an sp³ carbon. Halogens are more electronegative than carbon, so the C–X bond is polar and the carbon is partially positive, which makes it an electrophile. Alkyl halides are classed as methyl, primary, secondary, or tertiary by how many carbons are bonded to the carbon bearing X. Halides leave as weakly basic X⁻ ions, so they are good leaving groups: I⁻ > Br⁻ > Cl⁻ > F⁻.

On the MCAT: Passages ask you to predict substitution versus elimination. Methyl and primary halides with a good nucleophile favor SN2 (one step, backside attack, inversion of configuration, rate = k[RX][Nu]). Tertiary halides favor SN1 (carbocation intermediate, racemization, rate = k[RX]) and E1, or E2 with a strong base. Secondary halides can follow either pathway depending on the nucleophile, base, and solvent. Polar aprotic solvents favor SN2; polar protic solvents favor SN1.

Common confusion: Fluorine is the most electronegative halogen but the worst halide leaving group, because F⁻ is the strongest base among the halides and the C–F bond is strong. Aryl and vinyl halides, with X on an sp² carbon, are not alkyl halides and generally do not undergo SN1 or SN2 readily.