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Migration of a neighboring C–H bonding pair onto an electron-deficient carbon, moving the carbocation site.

A 1,3-Diaxial interaction is the close nonbonded contact between axial groups separated by one ring atom in a chair. Replacing a carbon-bearing axial hydrogen by an oxygen changes this contact, so cyclohexane substituent preferences cannot be transferred unchanged to dioxanes.

Two ketone carbonyls separated by one carbon. The intervening C–H bonds are relatively acidic, and the enol can be stabilized by pi conjugation and an intramolecular hydrogen bond.

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.

An unsymmetrical epoxide with two methyl substituents at one ring carbon. Acidic opening can favour nucleophile attachment to that more substituted carbon.

A 2-Oxazoline has the five-membered ring $\mathrm{O}-\mathrm{C}=\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2$. A substituent at its imine carbon can be introduced by aromatic capture of an activated cyclic carbamate.

A benzene-fused five-membered lactone containing a gem-dimethyl-substituted saturated ring carbon. It can result from double methyl addition to one carbonyl of dimethyl phthalate followed by ring closure.

An acetonide is an isopropylidene acetal formed from a diol and acetone or an acetone equivalent. Formation of its O–CMe2 bonds leaves the original stereogenic carbon–oxygen bonds intact.

An anion formed by removing the terminal proton of an alkyne, $\mathrm{RC}\equiv {\mathrm{C}}^{-}$. It can act as a carbon nucleophile, for example in addition to a ketone.

The conjugated aldehyde $\mathrm{C}{\mathrm{H}}_2=\mathrm{CHCHO}$. A carbon nucleophile can add at its carbonyl carbon or, in a conjugate addition, at the terminal beta carbon. Atom mapping distinguishes the two products.

Aerobic respiration uses molecular oxygen as the final electron acceptor. For complete oxidation of glucose, the net material balance isThis records the overall conversion of glucose, molecular oxygen, carbon dioxide and water; it is not one enzyme-catalyzed step, and it omits the separate ATP accounting. For an original stoichiometry check, each side contains six carbon, twelve hydrogen and eighteen oxygen chemical atoms. If exactly $1\mathrm{mmol}$ of glucose undergoes this complete conversion, the equation requires $6\mathrm{mmol}$ of molecular oxygen and yields $6\mathrm{mmol}$ each of carbon dioxide and net water. This is an equation-based calculation, not an observed seed measurement.

The molecular oxygen is reduced to water, rather than being the source of the carbon dioxide's carbon. A formal redox electron balance, using acidic half-reaction notation, makes the roles explicit:Adding and cancelling gives the net equation above. Both half-reactions conserve chemical atoms and electric charge. The $24$ electrons balance a net transfer; they are carried through cellular intermediates, rather than released as a pool of free electrons. The final respiratory reduction can be written ${\mathrm{O}}_2+4{\mathrm{H}}^{+}+4e^{-}⟶2{\mathrm{H}}_2\mathrm{O}$. These equations are bookkeeping, not a tracing experiment that assigns every product oxygen chemical atom to its original molecule.^[ref]

Alcohols such as ethanol and glycerol contain hydroxy groups on carbon chemical atoms with only single covalent bonds.

An aldohexose is a six-carbon aldose: its open-chain reference form has an aldehyde group at C1. Glucose is an example.^[ref]

The unmodified open-chain skeleton has four stereocenters, C2–C5, giving $2^4=16$ stereoisomers, or eight enantiomer pairs. Fixing the C5 hydroxy group on the right leaves $2^3=8$ D forms; D-glucose's right/left/right/right pattern selects one. An aldohexose label therefore specifies less than a particular sugar name, and a molecular formula specifies less still. The existing glucose article checks its chemical atom inventory; monosaccharide ring-chain interconversion follows those same numbered carbon positions into a ring.

An aldopentose is a five-carbon aldose: its open-chain reference form has an aldehyde group at C1. Ribose is an example; deoxyribose is its 2-deoxy derivative.^[ref]

For the unmodified open-chain aldopentose skeleton, count one $\mathrm{CHO}$, three $\mathrm{CHOH}$ and one $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ group: ${\mathrm{C}}_5{\mathrm{H}}_{10}{\mathrm{O}}_5$. The three interior carbon positions, C2–C4, are stereocenters. Each allows two configurations, giving $2^3=8$ stereoisomers, grouped into four enantiomer pairs. The unlike chain ends prevent a meso compound identification. D-ribose selects the right/right/right hydroxy group pattern in the Fischer projection; the word aldopentose alone does not select that pattern. Compare the existing ribose and deoxyribose diagrams for the separate question of which oxygen is present.

An alkenyl aluminate has an anionic aluminium-containing unit attached to an alkene carbon. Intramolecular oxygen coordination can close a five-membered ring in directed propargylic alcohol reductions.

An alkyl group attached across metal centres. A bridging methyl can form a three-centre two-electron bond, rather than two full independent two-electron metal–carbon bonds.

An allene contains the unit $\mathrm{C}=\mathrm{C}=\mathrm{C}$ with two consecutive carbon–carbon covalent bonds of order two. Suitable terminal substituents give axial chirality.

An allenyl phosphine oxide contains an allene attached to a phosphine-oxide group. Attack at the central allene carbon can generate a carbanion on the adjacent phosphorus-bearing carbon, which is stabilized by the phosphoryl substituent.

The existence of different structural forms of the same chemical element. Diamond and graphite are crystalline examples for carbon; molecular oxygen and ozone illustrate that allotropy need not consist only of different packing arrangements of identical molecules.

The compound $\mathrm{C}{\mathrm{H}}_2=\mathrm{CHC}{\mathrm{H}}_2\mathrm{Br}$. nucleophilic substitution at its bromomethylene carbon introduces an allyl group without consuming the alkene.