An electrophilic substitution replaces an atom or group on a substrate through reaction with an electrophile. The major organic example is electrophilic aromatic substitution, in which an aromatic carbon-hydrogen bond is replaced while the aromatic system is ultimately restored.

In aromatic nitration, attack on an electrophilic nitrogen species forms a nonaromatic intermediate, followed by loss of a proton. The final restoration of aromaticity helps explain substitution rather than net addition. The reacting substrate and the identity of the electrophile determine the actual mechanism and site selectivity.

Regioselectivity is preferential reaction at one of several constitutionally distinct sites or orientations. It concerns which bonds form where, rather than the configuration of the resulting stereocenters.

For example, adding a reagent across an unsymmetrical alkene can place its two components in either orientation, producing different constitutional isomers. Electronic effects, steric interactions, and the catalyst determine the product ratio. A reaction can be strongly regioselective but weakly enantioselective, since site preference and handedness are independent questions.

Alkylation of a malonate ester enolate, followed by ester hydrolysis and decarboxylation, produces a substituted carboxylic acid. Two sequential alkylations replace the two original active-methylene hydrogens.

Replacement of a carbon-bound halogen by hydrogen. The mechanism and selectivity depend on the reducing agent, substrate and medium; ordinary aryl nucleophilic substitution need not be involved.

Activated oximes rearrange to amides, with migration of the carbon substituent anti to the leaving group. A cyclic ketoxime can therefore expand a ring to a lactam.

Oxidative cleavage of an alkene by ozone, followed by workup. A reductive workup can yield carbonyl compounds; sodium borohydride can reduce the resulting aldehydes and ketones to alcohols.

carbon acylation of an ester enolate followed by deprotonation of the beta-dicarbonyl product. ketone-enolate acylation by an acyl chloride is a closely related acyl-substitution strategy, although it is not the classical ester self-condensation.

Formation of a new benzene ring by joining carbon fragments, often followed by elimination and tautomerization that establish aromaticity.

Formation of a carbonyl accompanies migration of an adjacent C–H or C–C bond and departure of a leaving group. An alkoxide-assisted hydride migration can convert a suitably aligned halohydrin into a ketone.

Intramolecular assistance by a group near a reacting centre. Temporary bridging can alter rate, connectivity, stereochemistry or apparent retention; an intermediate with equivalent paths can erase an original stereochemical distinction.

Addition of a nitroalkane-derived nitronate to an aldehyde or ketone gives a beta-nitro alcohol after protonation. Subsequent dehydration is possible under some conditions but is a separate step.

electrophilic activation of an alkene by mercury(II), followed by nucleophilic opening of a bridged mercurinium ion. water usually attacks the more substituted carbon; subsequent demercuration replaces C–Hg with C–H.

Replacement of a carbon–mercury bond by a carbon–hydrogen bond in a synthetic sequence. Its detailed mechanism need not be identical to that of the preceding oxymercuration.

Net loss of water. A nitroaldol reaction can first give an alcohol; eliminating water from that product is a separate reaction rather than an automatic part of the addition mechanism.

Installation of an exocyclic or terminal methylene at the alpha position of a carbonyl compound. Hydroxymethylation/dehydration or Mannich-base elimination can provide examples.

Formation of a carbon–nitrogen bond by carbonyl condensation followed by imine/iminium reduction. Conditions and amine identity determine the substitution pattern at nitrogen.

Conversion of a carbonyl or related precursor into an alkene. Atom mapping and E/Z selectivity depend on the chosen reagent class.

Conversion of a primary or secondary nitro-derived nitronate into a carbonyl compound by suitable hydrolytic conditions. The nitro-bearing carbon becomes an aldehyde carbon for a primary nitroalkane or a ketone carbon for a secondary one.

A three-atom dipolar electronic system participating in a [3+2]-cycloaddition.

Ring formation accompanied by elimination of water.