This chapter turns to readily accessible cationic carbon species, carbons polarized to be electron deficient and serve as electrophiles in C–C bond formations in metabolism. Three major categories include (1) the carbonyl groups of aldehydes, ketones, and acyl thioesters, (2) allylic cations available from early dissociation of the C–OPP bond in 2-prenyl-pyrophosphate substrates, and (3) oxocarbenium ions at C₁ of glucosyl and ribosyl moieties attached to diphosphate groups. A fourth set of carbons that behave as electron deficient centers in S_N² reactions are the three substituents on the sulfonium atom of S-adenosylmethionine. Conjugated olefins also react as electrophiles.