An organic reaction transforms organic compounds by changing their bonds, charge, or stereochemistry. Useful families include substitution reactions, addition reactions, elimination reactions, and rearrangements.
A reaction mechanism follows electron movement and intermediates rather than only comparing starting and final formulas. For instance, replacing a leaving group by a nucleophile can occur by a concerted pathway or through a carbocation intermediate; the two pathways predict different rate laws and stereochemical outcomes. Reaction classes organize transformations, while mechanisms explain how particular conditions produce them.
Reaction of a trialkyl phosphite with an alkyl halide to give a phosphonate and an alkyl halide byproduct. With a haloacetate it prepares a phosphonoacetate used in the Horner-Wadsworth-Emmons reaction.
Insertion of oxygen beside an aldehyde or ketone carbonyl, commonly with a peroxy acid. Cyclic ketones give ring-expanded lactones. The migrating group retains its configuration, but competing migrations can give different constitutional products.
Formation of a carbon–carbon bond at an anomeric carbon. In electrophilic aromatic substitution on a suitable heteroarene, carbon attack on an activated glycosyl donor is followed by deprotonation to restore aromaticity.
Addition of bromine and an oxygen nucleophile across an alkene to form a bromoether. Intramolecular attack on a bromonium ion can close a ring; tether length, charge distribution and anti opening determine the resulting connectivity.
A syn elimination in which oxidation of an organoselenide permits intramolecular beta-hydrogen removal and C–Se cleavage, producing an alkene.
A cycloaddition joining a three-atom 1,3-dipole to a two-atom unsaturated unit and forming a five-membered ring. The atom order of the dipole fixes which heteroatoms become adjacent.
Substitution of an alcohol using a phosphine and an azodicarboxylate reagent with a suitable acidic nucleophile. Carboxylate displacement normally inverts a stereogenic alcohol carbon; subsequent ester hydrolysis can unveil the inverted alcohol.
oxidation of an enol or enolate can generate a carbon-centred radical that adds intramolecularly to an alkene. Further oxidation or oxygen capture determines the final functional group.
A sequence in which electron transfer converts a radical intermediate into a cationic or anionic intermediate, enabling subsequent polar chemistry.
Intramolecular capture of an alkene-derived iodonium ion by an alcohol produces a cyclic ether and a carbon–iodine bond. Backside opening gives an anti relationship between the new C–O and C–I bonds.
A 1,2-carbon migration interconverts adjacent tertiary alpha-hydroxy carbonyl compounds. Formation of a new carbonyl at the original carbinol carbon accompanies conversion of the original carbonyl into an alcohol. Unlike dehydration, the net process preserves both oxygens.
oxidation of an alkyl halide or a related electrophile by dimethyl sulfoxide. An alkoxysulfonium intermediate undergoes base-promoted elimination to a carbonyl compound and dimethyl sulfide.
A 1,3-dipolar cycloaddition joining a nitrone to an alkene. Intramolecular tethering controls regioselectivity and relative approach; the nitrone nitrogen–oxygen bond can later be cleaved reductively.
A neighbouring tertiary amine displaces a leaving group to form a three-membered ammonium ring; attack at the original ring carbon cleaves its old C–N bond and incorporates the side-chain carbon into an expanded nitrogen ring.
Addition of an aldehydic C–H bond across an unsaturated bond, creating a ketone. Intramolecular alkene hydroacylation commonly constructs a five-membered ring via an acyl–metal hydride, alkene insertion and reductive elimination.
Replacement of an organic halide by hydrogen through a palladium catalytic cycle. oxidative addition, hydride delivery and reductive elimination can preserve the stereochemistry of an alkenyl fragment.
Addition of a bond between zirconium and hydrogen across an alkene or alkyne. Schwartz reagent commonly gives a syn addition and places zirconium at the less substituted terminus; reversible insertion can influence regioselectivity.
Palladium-catalyzed oxidation of an alkene to a carbonyl compound using water as the oxygen source in the usual catalytic cycle.
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.
Conversion of a carbonyl or related precursor into an alkene. Atom mapping and E/Z selectivity depend on the chosen reagent class.
Reaction of a phosphorus ylide with a carbonyl compound to form an alkene. The carbonyl carbon and ylidic carbon become the two alkene atoms; the oxygen is transferred to phosphorus.
A Wittig reaction with that replaces a carbonyl oxygen by . On a cyclic ketone it installs an exocyclic methylene without inserting a new ring atom.
Conditions favouring a Z alkene in a Wittig reaction, often using a nonstabilized ylide under kinetic, salt-controlled conditions. Isolation or verification is still required when complete stereoselectivity is not established.
Formation of a carbon–nitrogen bond by carbonyl condensation followed by imine/iminium reduction. Conditions and amine identity determine the substitution pattern at nitrogen.
Installation of an exocyclic or terminal methylene at the alpha position of a carbonyl compound. Hydroxymethylation/dehydration or Mannich-base elimination can provide examples.
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.
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.
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.
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.
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.
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.
Formation of a new benzene ring by joining carbon fragments, often followed by elimination and tautomerization that establish aromaticity.
A nitroalkane carbon can join the two ends of a five-carbon conjugated ketone framework. carbon–carbon bond formation, thiolate displacement and nitrite elimination can build a substituted phenol. The nitro-bearing carbon, rather than the nitro nitrogen, enters the aromatic ring.
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.
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.
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.
Fragmentation of an activated oxime by cleavage of a carbon–carbon bond anti to its N–O leaving group, yielding a nitrile and a carbon electrophile or a product derived from it. Stabilization of a developing carbocation can favour fragmentation over migration. It differs from the usual Beckmann rearrangement, which inserts nitrogen into the migrating carbon–carbon bond and yields an amide after hydrolysis.
A carbon substituent opposite the activated N–O leaving group migrates onto nitrogen as the N–O bond cleaves. The oxime geometry therefore determines which bond is replaced by a carbon–nitrogen bond.
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.
Replacement of a carbon-bound bromine by hydrogen. In suitable bromophenols, acid-promoted ipso protonation and reductive trapping of departing bromine provide a plausible polar mechanism.
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.
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.
The conventional addition orientation in which the incoming hydrogen goes to the alkene carbon already bearing more hydrogens, in the ordinary applicable unsymmetrical-alkene setting.
An addition whose regiochemical outcome is opposite to the conventional Markovnikov orientation. hydroboration followed by oxidation places OH at the less substituted alkene carbon.
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.
The Weitz-Scheffer epoxidation of an electron-poor alkene uses hydroperoxide addition followed by intramolecular closure and O–O bond cleavage. A planar achiral enone in an achiral environment can give a racemic epoxide pair.
A Robinson annulation combines a Michael addition with an intramolecular aldol reaction and dehydration. Tracking the atoms shows which carbonyl survives and which new bond closes the six-membered ring.
An elimination reaction removes groups from a substrate while forming a new multiple bond. Removing a hydrogen and a leaving group from neighboring carbons can produce an alkene.
An E2 elimination combines bond cleavage and bond formation in one elementary step; an E1 elimination passes through an intermediate, usually a carbocation. These alternatives predict different rate laws and stereochemical requirements. In a concerted elimination, orbital alignment can make an antiperiplanar conformation decisive, linking product formation to molecular geometry.
Beta-elimination, also called 1,2-elimination, removes groups from adjacent atoms while forming a new multiple bond between those atoms. The IUPAC definition allows the two central atoms to be elements other than carbon, although carbon–carbon examples are common. It describes the overall connectivity change, rather than specifying one unique reaction mechanism.[ref]
The net dehydration of ethanol illustrates the topology:One carbon loses a hydrogen and its neighbor loses the oxygen-containing group; the carbon pair becomes the double bond of ethene. Both sides contain two carbons, six hydrogens and one oxygen. The balanced equation describes the net conversion, without asserting that unactivated ethanol changes spontaneously or that all bond changes occur in one step.
The positional label beta must be separated from the mechanistic label in E2 elimination. Both an E1 elimination and an E2 elimination can produce a beta-elimination transformation. Their differing intermediates, rate laws and geometric requirements are further information beyond the atom-connectivity pattern.
Loss of a hydrogen halide to form a carbon–carbon multiple bond. Along a chlorinated polymer backbone, repeated elimination can create unsaturated or conjugated segments rather than exclusively changing ester side groups.
A concerted elimination in which a base removes a beta hydrogen as a leaving group departs and a pi bond forms. Orbital alignment and substrate structure affect the allowed pathway.
antiperiplanar conformation elimination in a cyclohexane chair with axial leaving group and axial beta hydrogen.
An elimination mechanism involving ionization to a carbocation before beta deprotonation forms the alkene. Its limiting unimolecular rate law assumes ionization controls the rate.
In an E1cb elimination, deprotonation first produces a conjugate-base intermediate, followed by leaving-group loss. Dehydration of an appropriate aldol product can form a conjugated enone.
The aldol reaction forms a carbon-carbon bond when an enol or enolate adds to an aldehyde or ketone. The initial addition product is a beta-hydroxy carbonyl compound, containing the combined carbon frameworks.
For example, acetaldehyde can add to another acetaldehyde molecule to give 3-hydroxybutanal. Subsequent loss of water gives an alpha,beta-unsaturated carbonyl compound, the condensation product. Keeping addition and dehydration distinct explains why an aldol reaction need not itself eliminate water. The reaction is useful for constructing longer carbon skeletons with predictable functional groups.
Addition of an enolate or enol to an aldehyde or ketone, giving a beta-hydroxy carbonyl compound. Subsequent dehydration is a distinct step and gives an aldol condensation.
An aldol reaction between two reactive sites of the same molecule. A tethered enol or enolate attacks a carbonyl group to close a ring; the tether, conformation and competing chemical reactions influence the product.
An aldol addition organized through a boron enolate and a coordinated carbonyl partner. Enolate geometry and auxiliary-controlled facial selection jointly determine product stereochemistry.
A six-membered chair-like organization of a metal enolate and carbonyl compound used to rationalize aldol stereoselectivity.
A six-membered chair-like transition-state model for an aldol-type bond-forming step. It predicts relative stereochemistry by comparing substituent placement and nonbonded interactions, but must be adapted to the actual enamine or metal-enolate catalyst.
An enamine formed reversibly between proline and an enolizable carbonyl compound attacks an electrophilic carbonyl. The carboxylic acid group can orient and activate the electrophile; hydrolysis releases the aldol product and regenerates proline.
An aldol addition followed by water elimination, commonly producing a conjugated carbonyl compound. Identifying which carbonyl supplies the enolate distinguishes constitutional products.
A Michael addition makes a bond at the beta carbon of a conjugated electrophile through conjugate addition. An enamine can supply a carbon nucleophile; subsequent hydrolysis restores its original carbonyl group.
Conjugate addition of a thiol-derived sulfur nucleophile to an activated alkene. The sulfur substituent is installed at the beta carbon of an enone, not at its carbonyl carbon.
A sequence in which one conjugate addition generates or exposes a nucleophile for another conjugate addition. Atom mapping distinguishes successive bond-forming steps from a concerted cycloaddition.
Conjugate addition involving a nitroalkene electrophile or a nitronate nucleophile. Identifying which partner supplies the carbon nucleophile is essential for following a tandem reaction.
A nucleophilic addition forms a bond by attack of a nucleophile on an electrophilic multiple bond, reducing its bond order. A common case is attack on the carbon of a carbonyl group, with its pi electrons displaced toward oxygen.
Protonation of the resulting oxygen-centered intermediate can give an alcohol. Unlike nucleophilic substitution, the basic addition step does not expel a leaving group from the attacked carbon. Carbonyl polarization and orbital geometry explain both why carbon is attacked and why the incoming bond follows a preferred approach direction.
A Grignard reaction uses an organomagnesium reagent, conventionally written , as a carbon-based nucleophile or base. Addition to an aldehyde or ketone, followed by protonation of the alkoxide product, forms an alcohol with a new carbon-carbon bond.
The reagent is not simply a free carbanion: its bonding, aggregation, and coordination depend on its environment. Proton donors can consume it by an acid-base reaction instead of the intended carbonyl addition. This competition makes functional-group compatibility central to understanding its synthetic role.[ref]
Two organomagnesium additions to a suitable ester give a tertiary alcohol after work-up. The first addition–elimination produces a ketone, which accepts the second carbon substituent.
Addition of an organomagnesium reagent to carbon dioxide followed by protonation gives a carboxylic acid with one additional carbon. A free acid cannot remain present during a subsequent Grignard addition without consuming reagent.
The Bürgi-Dunitz angle describes the oblique approach of a nucleophile toward a carbonyl carbon, conventionally measured relative to C=O. A typical trajectory is near , with the optimum depending on the system.
The Bürgi-Dunitz trajectory describes the preferred approach of a nucleophile toward a carbonyl carbon, usually at an obtuse angle to the carbon-oxygen bond, often near . It is a geometrical trend, not a fixed angle obeyed by every reacting system.
An oblique approach balances favorable interaction with the carbonyl antibonding orbital against repulsion from occupied orbitals. Crystallographic observations of close contacts motivated this picture, linking static molecular structures with a possible path of nucleophilic addition. Steric constraints and the other substituents can modify the accessible trajectory.
A nucleophilic substitution replaces a leaving group by a nucleophile. In a concerted SN2 reaction, bond formation and departure occur in the same elementary step, commonly with inversion at the attacked stereocenter.
An SN1 reaction instead forms an intermediate before capture, often a carbocation, so rearrangement and a different stereochemical outcome may be possible. Substrate structure, solvent, leaving-group ability, and nucleophile properties influence which pathway is available. The substitution product alone does not uniquely identify the mechanism.
An atom or group departing with the bonding electron pair in a heterolytic substitution. Its stability, basicity and environment affect reactivity; the substrate geometry also controls access to the substitution pathway.
Non-geminal substitution puts incoming groups on different central atoms. On a phosphazene ring, it still permits cis/trans relative configurations and does not by itself specify one stereoisomer.
In geminal substitution, two incoming groups replace substituents on the same central atom. In cyclotriphosphazenes, gives a geminal pair. Primary amino groups can enable a base-catalyzed proton-abstraction/chloride-elimination route that is unavailable after substitution by a secondary amine.
Nucleophile addition to an acyl carbonyl followed by leaving-group elimination, often through a tetrahedral reaction intermediate.
Formation of an ester from an acyl component and an alcohol. Intramolecular esterification of a hydroxy acid makes a lactone. Fischer esterification is the acid-catalysed carboxylic-acid/alcohol case.
Base-promoted bimolecular acyl substitution: nucleophile addition gives a tetrahedral intermediate, then a leaving group departs from the acyl carbon.
Formation of a lactone by intramolecular esterification or acyl substitution. Favourable ring sizes can help trap an intermediate, but do not make the product universally unreactive.
Acid-catalysed ester formation from a carboxylic acid and an alcohol, proceeding through carbonyl addition and elimination with net water formation.
Breaking an ester linkage can proceed by different bonds and mechanisms. Acyl–oxygen and alkyl–oxygen cleavage should be distinguished when explaining protecting-group selectivity.
Breaking the bond between an acyl carbon and the ester alkoxy oxygen. Ordinary hydroxide-promoted ester saponification follows addition–elimination through a tetrahedral intermediate.
Breaking the bond between an ester oxygen and its alkyl substituent. Acid removal of a tert-butyl group can follow this route without requiring water or a tetrahedral acyl-addition intermediate.
Exchange of an ester alkoxy group through reaction with an alcohol, commonly using acid or base catalysis. Excess exchanging alcohol can shift the chemical equilibrium.
For exchange of closely related ester and alcohol groups, an excess alcohol reservoir and dilution of the outgoing alcohol can favour conversion through mixture chemical potentials. Equal molecular stoichiometry does not itself imply a positive standard entropy change; a reaction quotient is needed.
A limiting unimolecular nucleophilic substitution mechanism with rate-determining ionization, commonly producing a carbocation, followed by nucleophile capture. Its ideal rate law is first order in substrate.
In an addition–elimination pathway, a nucleophile attacks an aromatic carbon and forms a Meisenheimer complex; leaving-group loss restores aromaticity. Electron-withdrawing groups ortho or para to the attacked site can stabilize the addition intermediate.
An intramolecular nucleophilic aromatic substitution that transfers an aryl group between sites in one molecule.
An intramolecular aryl transfer to a carbon nucleophile. A tethered carbon anion attacks an aromatic ipso carbon and displaces the original oxygen-, nitrogen- or sulfur-based attachment.
Vicarious nucleophilic substitution replaces aromatic hydrogen using a nucleophile that carries its own leaving group. In nitroarenes, reversible carbanion addition gives a sigma-H adduct; base-induced elimination of the hydrogen and the nucleophile-borne leaving group, followed by protonation, gives substitution without expelling hydride.
A sigma-H adduct forms by nucleophilic addition at an aromatic carbon that initially bears hydrogen. Both the original hydrogen and the incoming nucleophile remain attached at that carbon before the rearomatization steps.
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