A chemical reaction interconverts chemical species, as defined by the [ref]. The consumed species are reactants; the formed species are reaction products. In ordinary chemical reactions, chemical atoms are rearranged while each chemical element's total atom count is conserved. For a complete chemical equation, net electric charge is conserved too.
For example, the balanced chemical equationdescribes conversion of molecular hydrogen and molecular oxygen into water. Both sides contain four hydrogen chemical atoms and two oxygen chemical atoms. Three reactant molecules become two reaction product molecules: atom counts are conserved, but molecule counts need not be. See Flowers et al., Chemistry 2e, balancing chemical equations.
The coefficients describe net consumption and formation, without asserting that three molecules collide in one step. A balanced chemical equation supplies quantitative constraints; it does not determine how quickly the chemical reaction occurs, or whether chemical equilibrium prevents complete conversion. Those questions need further information. Keeping these roles distinct makes stoichiometry useful even when the microscopic details are complicated.
An exothermic chemical reaction between a fuel and an oxidizing agent. Oxygen is a common oxidizer, but neither visible flame nor oxygen is required by every broader chemical use of the term.
The historical explanation that ordinary combustion and metallic calcination involve combination with oxygen, rather than release of phlogiston. It should not be extended into the false claim that every chemical acid contains oxygen.
Addition of water to a molecular species. A carbonyl compound can form a geminal diol by hydration, distinct from a solvent hydration shell.
Exchange of ionic partners, often used to replace a metal halide by an anionic ligand while an alkali-metal halide forms. Atom and charge balance distinguish it from an oxidative-addition reaction.
A concerted reorganization of bonding through a cyclic orbital interaction, including electrocyclic reactions. The participating orbitals and their electronic occupation determine the relevant symmetry selection rule.
A retro hetero-Diels-Alder reaction is the reverse of a Diels-Alder reaction involving a heteroatom-containing partner. For a six-membered ring with sequence , cleavage of and can give and . Whether the cycloreversion is concerted or proceeds through ionic intermediates depends on the substrate and conditions.
Orbital-symmetry rules associated with Robert Burns Woodward and Roald Hoffmann for concerted reactions. Ground-state and photochemically excited occupations can have different allowed stereochemical modes; an allowed mode does not determine every kinetic detail or guarantee a large reaction rate.
Interconversion of a conjugated pi system and a ring by formation or cleavage of one sigma bond at the pi-system termini, accompanied by pi-bond redistribution. Terminal rotation connects starting geometry with product stereochemistry.
Formation of a ring by concerted conversion of terminal pi-system overlap into a new sigma bond. Thermal four-pi closure is conrotatory, subject to the appropriate electronic-state assumptions.
Ring closure of a seven-carbon conjugated anion with eight pi electrons; thermal orbital-symmetry selection favors disrotatory motion.
For ordinary closed-shell systems, a thermal -electron electrocyclic process is conrotatory and a thermal -electron process is disrotatory. The corresponding simple photochemical rules are reversed. These rules relate terminal orbital phases and occupation to allowed concerted bond formation or cleavage.
Terminal rotation in opposite senses under a common viewing convention during an electrocyclic reaction.
Terminal rotation in the same rotational sense under a common viewing convention during an electrocyclic reaction. It is distinguished from disrotatory motion; looking from opposite ends without changing conventions can reverse an apparent clockwise label.
Formation of a three-membered carbon ring, for example by transferring a carbene-like carbon across an alkene.
Cyclopropanation between reactive groups tethered within one molecular framework. The tether constrains approach geometry, can form an additional ring simultaneously, and can influence which alkene face reacts.
An acid-promoted skeletal migration of a cyclohexadienone followed by rearomatization to a phenol. Carbonyl protonation activates the conjugated system; migration from a suitably placed quaternary carbon and subsequent proton loss can change ring connectivity while restoring aromaticity.
A para-tethered phenoxide can react through its carbon-nucleophilic resonance contributor. Intramolecular displacement of a tether leaving group at the para carbon creates a spiro ring and a cyclohexadienone. Subsequent acid-induced bond migration to a neighbouring carbon can expand the attached ring and restore the phenol. The two-stage sequence can therefore give a final substitution position different from the initial carbon attack.
Addition of Si–H across an unsaturated bond. For an alkene, the net products acquire one C–H and one C–Si bond. Alternative catalytic mechanisms can insert the alkene into either a metal–hydride or a metal–silyl bond; these alternatives should not be conflated.
A hydrosilylation cycle involving Si–H oxidative addition, alkene insertion into the metal–hydride bond, and C–Si reductive elimination. This defines the conventional version; insertion into a metal–silyl bond describes a modified alternative.
Interconversion of isomers without changing the overall molecular formula. Conformational interconversion can be kinetically slow when rotation is hindered.
Interconversion of constitutional forms by proton transfer accompanied by redistribution of pi bonds. Keto–enol interconversion is a familiar example.
Preferential formation of one product or reaction at one site. chemoselectivity, regioselectivity and stereoselectivity describe different comparisons.
Preferential reaction of one functional group or bond type in a molecule containing alternatives. It depends on reagents, conditions and reaction time rather than being an absolute property of a functional group.
Loss of carbon dioxide from a compound. Malonic-acid derivatives can decarboxylate through cyclic proton-transfer/bond-cleavage arrangements that generate a carbonyl-stabilized product.
Heating a substituted malonic acid can release carbon dioxide through a six-membered cyclic transition arrangement. The initial enol-like carboxylic-acid product tautomerizes to a substituted acetic acid.
A reaction that joins unsaturated components while forming a ring and new sigma bonds. Its transition-state entropy and activation volume depend on both molecular reorganization and the surrounding medium.
A concerted cycloaddition of a conjugated diene and a dienophile that makes a six-membered ring with two new sigma bonds and one retained alkene. The internal diene atoms become the residual alkene atoms.
A common kinetic preference for an orientation with dienophile substituents directed beneath the diene pi system. Steric and orbital interactions compete, so endo preference is not an absolute rule.
A reagent combination that installs the two saturated carbons of an ethylene cycloaddition while allowing more favourable reactivity. A removable electron-withdrawing substituent can activate the dienophile and be removed later.
A preference for the cycloaddition orientation in which relevant dienophile substituents lie under the diene framework in the transition geometry. It is distinct from regiochemistry and is not an unconditional guarantee of one product.
The unsaturated partner that combines with a conjugated diene in a Diels–Alder reaction. Electron-withdrawing substituents often enhance reactivity in a normal-electron-demand process.
Mesylation installs a methanesulfonyl group on a heteroatom, often alcohol oxygen. An alcohol attacks electrophilic sulfur of a sulfonylating reagent, and base removes the O–H proton.
Hydroalumination adds an Al–H equivalent across an unsaturated bond. Syn and directed trans pathways give different alkenyl-aluminium geometries; protonolysis can preserve that stereochemical information.
Oxygen-directed anti addition of aluminium and hydrogen across an alkyne, giving a stereodefined vinylaluminium species.
Dihydroxylation installs hydroxyl groups on adjacent alkene carbons. osmium tetroxide gives a syn addition pathway through a cyclic osmate ester.
A cyclic osmate ester contains the newly formed carbon–oxygen bonds of an osmium-mediated alkene dihydroxylation. Both oxygen atoms attach on the same face; hydrolytic cleavage of metal–oxygen bonds releases the diol.
Transacetalization exchanges acetal alkoxy groups through acid-catalyzed cleavage and reformation. Protonated alkoxy departure gives an oxocarbenium species that a new alcohol can capture.
A syn addition attaches both new groups from the same face of an unsaturated unit. Formation of a cyclic osmate ester explains syn dihydroxylation.
An anti addition places the two newly attached groups on opposite faces of an unsaturated unit. Backside opening of a cyclic bromonium ion is a standard example.
Desilylation removes a silyl group. Attack by a nucleophile at silicon can pass through a pentacoordinate silicon arrangement before Si–O cleavage; the liberated oxygen may be trapped by a new electrophile.
In a standard oxidative addition, a bond X–Y is cleaved and both fragments become metal ligands, increasing formal metal oxidation state by two. A B–Br bond can give a metal–boryl bond and a bromide ligand without cleaving a borazine ring.
A metal-assisted pathway in which cleavage of a substrate C-H bond transfers H to an electrophilic ligand while new metal-carbon bonding forms. In directed Fischer-carbene insertion, it can precede carbon-carbon reductive elimination.
Beta-hydride elimination transfers hydrogen from the beta carbon of a metal-bound alkyl to the metal while forming an alkene. It can release a growing polymer chain and compete with migratory insertion. Its accessibility depends on geometry, available sites and bond energetics.
In migratory insertion, adjacent ligands combine while remaining attached to the metal in the product. In alkene insertion into M–R, R forms a bond to one alkene carbon while the other carbon acquires the M–C bond. metal oxidation state need not change.
Methyl abstraction transfers a metal-bound methyl group to an electrophile. A trityl cation can convert a neutral metallocene dimethyl into a cationic monomethyl, forming and opening a site for an alkene ligand.
Addition of a B–H bond across an unsaturated bond. For many ordinary alkenes, concerted syn addition places boron mainly on the less substituted carbon; subsequent oxidation can replace C–B by C–O with retention.
A hydride transfer moves hydrogen with its bonding electron pair from donor to acceptor. Reduced nicotinamide cofactors transfer hydride from their ring C4; this is not transfer of a hydrogen radical.
Conjugate addition attacks a beta position in a conjugated electrophile and redistributes pi bonding, often generating an enolate. proton transfer then supplies a saturated carbonyl product.
A nucleophile adds reversibly at the beta carbon to form an enolate, which is functionalized before elimination restores the alkene.
A 1,6-Conjugate addition attacks the delta carbon of an extended conjugated electrophile. The resulting extended enolate can be protonated at different sites; proton transfer or equilibration can favor a product retaining carbonyl–alkene conjugation.
A conjugate reduction adds a reducing equivalent at the beta position of a conjugated carbonyl system. Hydride delivery followed by protonation can reduce C=C while retaining C=O, unlike direct carbonyl-to-alcohol reduction.
Keto-enol tautomerism interconverts carbonyl and enol forms by proton transfer and pi-bond redistribution. Solvent-mediated relays avoid an unrealistic isolated intramolecular proton jump.
In a standard two-ligand reductive elimination, two metal-bound anionic fragments couple to form a bond while metal oxidation state decreases by two. The reacting ligands commonly need adjacent positions; favorable bond formation and metal reduction energetics determine whether the formal step actually occurs.
A dissociative substitution loses a ligand before binding the incoming replacement. A saturated 18-electron carbonyl can first form a 16-electron intermediate; the dissociation barrier and intermediate rearrangement influence rates and product geometry.
The carbonyl cis effect is preferential labilization of a ligand cis to another ligand in many six-coordinate metal carbonyls. Site preference in five-coordinate intermediates and transition states matters. Isotopic exchange can distinguish departure sites more directly than substitution-product geometry alone.
The susceptibility of a species or atomic site to accepting an electron pair from a nucleophile. It depends on structure, charge and reaction conditions.
An electrophile accepts an electron pair in forming a bond. Reaction selectivity depends on both the electrophile and its interacting nucleophile.
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 zirconium–hydrogen bond 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.
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