Organic Chemistry

Organic Chemistry Glossary

Key terms from the Orgo 1 tracks of the Organic Chemistry catalog, defined and alphabetized. Search it, or jump by letter.

278 terms

A B C D E F G H I K L M N O P Q R S T U V Z

A

Acetylide alkylation: SN2 reaction of an acetylide with a primary alkyl halide that forms a new carbon–carbon bond.
Acetylide anion: The carbanion formed when a base such as NH2⁻ removes the acidic proton of a terminal alkyne (pKa about 25).
Achiral: Superimposable on its mirror image; often has a plane of symmetry.
Acid-catalyzed hydration: Addition of water across an alkene with a strong acid; follows Markovnikov's rule and can involve rearrangements.
Acid strength: How readily an acid gives up a proton; it is governed by how stable the conjugate base is.
Activation energy (Ea): The energy barrier that must be overcome for a reaction to proceed; written ΔG‡ when free energy is used.
Alcohol: A compound with an –OH group bonded to a carbon.
Aldehyde: A carbonyl compound in which the carbonyl carbon is bonded to at least one hydrogen.
Aldose: A sugar whose open-chain form contains an aldehyde.
Alkane: A hydrocarbon with only carbon–carbon single bonds; relatively unreactive, and its most common reaction is combustion.
Alkene: A hydrocarbon with a carbon–carbon double bond.
Alkyl dihalide: A compound with two halogens, which can undergo double elimination to give an alkyne.
Alkyl group: A substituent made of carbons and hydrogens, named by changing the alkane's –ane ending to –yl.
Alkyl halide: A compound in which a halogen is bonded to an sp³ carbon.
Alkyne: A hydrocarbon with a carbon–carbon triple bond.
Alkyne hydration: Addition of water to an alkyne; the enol product tautomerizes to a ketone, or to an aldehyde in hydroboration–oxidation of a terminal alkyne.
Allylic bromination: Bromination at an allylic C–H using NBS and light.
Allylic position: A carbon next to a C=C; its C–H bonds are weaker because the radical or cation formed is resonance stabilized.
Amide: A carbonyl compound in which the carbonyl carbon is bonded to a nitrogen.
Amine: A compound in which a nitrogen is bonded to carbon and has a lone pair.
Anti: An arrangement in which two groups on adjacent carbons are 180° apart.
Anti addition: Addition of two groups to opposite faces of a double bond.
Antibonding orbital: A higher-energy molecular orbital formed when atomic orbitals overlap out of phase; it is usually empty.
anti-Diol formation: Making a 1,2-diol with the two –OH groups on opposite faces, by opening an epoxide with water and acid.
Anti-Markovnikov addition: Addition in which the group adds to the less substituted carbon, as in hydroboration–oxidation or radical addition of HBr.
Anti-periplanar: The geometry in which the H and the leaving group lie in the same plane on opposite sides, required for E2.
Axial: A bond on a chair cyclohexane that points straight up or down, parallel to the ring's axis.

B

Backside attack: Approach of the nucleophile from the side opposite the leaving group, as in SN2.
Basicity: How readily a species takes a proton.
Bent geometry: A molecular geometry with two bonded atoms and one or two lone pairs on the central atom.
Bimolecular: A step in which two species are involved in the rate-determining step.
Bond dipole: The separation of partial charges (δ+ and δ−) along a polar bond, pointing toward the more electronegative atom.
Bonding orbital: A lower-energy molecular orbital formed when atomic orbitals overlap in phase; it usually holds the bonding electrons.
Bond-line structure: A simplified drawing in which each line is a bond, each bend or line end is a carbon, and the hydrogens on carbon are left out because they are implied.
Bond strength: The energy needed to break a bond; sigma bonds are stronger than pi bonds between the same two atoms.
Bridged ring: Two rings that share carbons but not a bond, as in norbornane; usually conformationally locked.
Brønsted–Lowry acid: A proton (H⁺) donor.
Brønsted–Lowry base: A proton (H⁺) acceptor.
Bulky base: A sterically large base that removes the most accessible hydrogen, giving the Hofmann product.

C

Cahn–Ingold–Prelog rules: The priority rules that rank groups by atomic number at the first point of difference; multiple bonds are counted as if they were duplicated single bonds.
Carbene: A neutral, highly reactive carbon with a lone pair and only six valence electrons; made in situ.
Carbenoid: A reagent, such as CH2I2 with Zn(Cu), that reacts like a carbene.
Carbocation: A carbon with a positive charge and only six valence electrons; sp² hybridized with an empty p orbital, and usually just an intermediate.
Carbocation rearrangement: The shift of a group to a neighboring carbocation to give a more stable one.
Carbocation stability: Increases with more alkyl groups: tertiary > secondary > primary > methyl.
Carbohydrate: One of the four main classes of biomolecules; its monomers are sugars with similar numbers of carbons and oxygens and about twice as many hydrogens.
Carbonyl: A carbon–oxygen double bond (C=O), found in many functional groups.
Carboxylic acid: A carbonyl compound in which the carbonyl carbon is bonded to an –OH group.
Catalyst: A substance that speeds up a reaction without being consumed and does not appear in the net equation.
Chain reaction: A reaction in which each step makes a new reactive species that continues the cycle.
Chair conformation: The lowest-energy shape of cyclohexane, with bond angles close to 109.5° and no ring strain.
Chemoselective: A reaction that affects only one functional group when several are present.
Chiral: Not superimposable on its mirror image.
Chiral center: A tetrahedral atom bonded to four different groups; also called an asymmetric center.
Cis: Two groups on the same side of a ring or double bond.
Cis–trans isomerism: Stereoisomerism in which two substituents lie on the same or opposite sides of a ring or double bond, giving compounds with different properties.
Click chemistry: A fast, selective reaction between an alkyne and an azide (R–N3) used for labeling in biological studies.
Combustion: The reaction of a compound with oxygen to give carbon dioxide and water; the most common alkane reaction.
Concerted reaction: A reaction in which bonds break and form at the same time in a single step.
Condensed structure: A shorthand that writes atoms in order with bonded groups next to each other, such as CH3CH2OH.
Conformational isomers (conformers): Different shapes of the same molecule that interconvert by rotation about single bonds.
Conjugate acid: The species formed when a base accepts a proton.
Conjugate base: The species left after an acid donates a proton.
Conjugate base stability: The main predictor of acidity: the more stable (weaker) the conjugate base, the stronger the acid. Resonance, atom size, electronegativity, inductive effects and hybridization all contribute.
Constitutional isomers: Compounds with the same molecular formula but different connections between atoms.
Covalent bond: A bond formed when two atoms share a pair of electrons.
Curved arrow: An arrow that shows the movement of an electron pair, starting at a lone pair, negative charge or bond and ending at an atom, positive charge or bond.
Cycloalkane: An alkane in which the carbons form a ring.
Cyclopropanation: Addition of a carbene or carbenoid across an alkene to form a cyclopropane.

D

D and L sugars: Labels showing the configuration of the bottom stereocenter of a Fischer projection; changing any other stereocenter gives a different sugar.
Degrees of unsaturation: The number of double bonds, triple bonds (counted as two) and rings in a molecule, found from its formula.
Dehydrohalogenation: Removal of HX from an alkyl halide by a base to form a double or triple bond.
Delocalized electrons: Electrons that are spread over several atoms instead of staying on one atom or in one bond.
Dextrorotatory and levorotatory: Describes a substance that rotates plane-polarized light clockwise (+) or counterclockwise (−).
Diastereomers: Stereoisomers that are not mirror images of each other; they have different physical properties.
Dihedral angle: The angle between two bonds on adjacent atoms, viewed along the bond between those atoms.
Dipole–dipole forces: Attractions between the partial positive and partial negative ends of polar molecules, such as those containing C=O or C–O bonds.
Dissolving-metal reduction: Reduction of an alkyne with Na or Li in liquid ammonia to give the trans alkene.
Double bond: A bond made of one sigma bond and one pi bond.

E

E1 reaction: Elimination, unimolecular: a two-step reaction through a carbocation, often competing with SN1 and favored at higher temperatures.
E2 reaction: Elimination, bimolecular: a one-step reaction in which a base removes a hydrogen as the leaving group departs.
Eclipsed conformation: A conformation in which the groups on adjacent carbons line up; higher in energy.
Electron configuration: A listing of which orbitals an atom's electrons occupy; for carbon it is 1s²2s²2p².
Electronegativity: An atom's tendency to attract the electrons in a bond. It increases toward the top and the right of the periodic table, and fluorine is the most electronegative element.
Electron geometry: The arrangement of all the electron groups (bonds and lone pairs) around a central atom.
Electron group: A lone pair or a bonded atom around a central atom; a double or triple bond counts as one group.
Electrophile: An electron-pair acceptor; it has a positive or partial positive charge.
Electrophilic addition: A reaction in which an electrophile adds to the pi bond of an alkene, usually through a carbocation.
Elimination reaction: A reaction that removes a leaving group and an adjacent hydrogen to form an alkene.
Enantiomeric excess (ee): A measure of how much more of one enantiomer a mixture contains: observed rotation divided by the specific rotation of the pure enantiomer, times 100%.
Enantiomers: Non-superimposable mirror-image stereoisomers; they have identical physical properties except for how they rotate plane-polarized light and how they act in chiral biological systems.
Endergonic: A reaction with a positive ΔG; products are higher in free energy than reactants.
Endothermic: A reaction that absorbs heat; ΔH is positive.
Enthalpy (ΔH): The heat change of a reaction at constant pressure; negative means exothermic and positive means endothermic.
Entropy (ΔS): A measure of disorder; reactions that increase disorder are favored by entropy.
Epoxidation: Conversion of an alkene to an epoxide with a peroxyacid, as a concerted syn addition.
Epoxide: A three-membered cyclic ether; strained, so a good electrophile.
Equatorial: A bond on a chair cyclohexane that points outward around the ring's edge.
Equilibrium: The state at which forward and reverse reactions occur at equal rates; ΔG = 0.
Ester: A carbonyl compound in which the carbonyl carbon is bonded to an –OR group.
Ether: A compound in which an oxygen is bonded to two carbons.
Exergonic: A reaction with a negative ΔG; products are lower in free energy than reactants.
Exothermic: A reaction that releases heat; ΔH is negative.
E/Z nomenclature: A system for naming alkene geometry using priorities: Z if the higher-priority groups are on the same side, E if they are on opposite sides. It is used for tri- and tetrasubstituted alkenes.

F

Fischer projection: A 2D drawing of a chiral molecule in which vertical bonds point away from the viewer and horizontal bonds point toward the viewer; common for sugars.
Fishhook arrow: A curved arrow with a half arrowhead that shows the movement of a single electron, used for radicals.
Fluoro-, chloro-, bromo-, iodo-: The prefixes used to name halogen substituents.
Formal charge: The charge an atom would carry if bonding electrons were shared equally: valence electrons minus lone-pair electrons minus the number of bonds.
Free radical: A species with an unpaired electron; unstable, reactive and does not follow the octet rule.
Functional group: A common group of atoms in a molecule that has characteristic chemical behavior; many reactions convert one functional group into another.
Fused ring: Two rings that share a bond.

G

Gauche: An arrangement in which two groups on adjacent carbons are 60° apart.
Gibbs free energy (ΔG): The energy that determines spontaneity: ΔG = ΔH − TΔS. A negative ΔG means a spontaneous reaction.

H

Halogenation: Addition of Cl2 or Br2 across an alkene to give a vicinal dihalide by anti addition.
Halohydrin: A compound with a halogen and an –OH on adjacent carbons, formed when halogenation is done in water.
Halonium ion: A cyclic bromonium or chloronium intermediate formed in halogenation that blocks one face and gives anti addition.
Hammond postulate: The transition state resembles whichever neighbor (reactant, intermediate or product) is closer to it in energy.
Heat of hydrogenation: The heat released when an alkene adds H2; a smaller value means a more stable alkene.
Heteroatom: Any atom in an organic molecule other than carbon or hydrogen.
Heterogeneous catalyst: A catalyst in a different phase from the reactants, such as Pd/C.
Hofmann product: The less substituted alkene, favored when a bulky base removes the most accessible hydrogen; also called the anti-Zaitsev product.
HOMO: Highest Occupied Molecular Orbital; the highest-energy orbital that contains electrons.
Homolytic cleavage: Breaking a bond so that each atom keeps one electron, forming two radicals.
Hybridization: The mixing of atomic orbitals (s and p) to make new, equivalent hybrid orbitals that explain observed molecular geometry.
Hydride shift (1,2-): The movement of a hydrogen with its electron pair to an adjacent carbocation; the most common rearrangement.
Hydroboration–oxidation: A two-part hydration that adds –OH to the less substituted carbon of an alkene by syn addition, with no rearrangement.
Hydrocarbon: A compound made of only carbon and hydrogen.
Hydrogenation: Addition of H2 across a double or triple bond using a metal catalyst, giving syn addition.
Hydrogen bonding: An especially strong dipole–dipole attraction between a hydrogen on O, N or F and a lone pair on another O, N or F.
Hydrohalogenation: Addition of HCl, HBr or HI across an alkene to give an alkyl halide.
Hyperconjugation: Stabilization of a carbocation by overlap between neighboring sigma bonds and the empty p orbital.

I

Inductive effect: The pulling or pushing of electron density through sigma bonds because of differences in electronegativity.
Initiation: The step of a radical reaction in which radicals are first formed.
Intermolecular forces: Attractions between molecules that determine properties such as melting point, boiling point and solubility.
Internal alkyne: An alkyne with the triple bond inside the chain, with no hydrogen on the sp carbons.
Inversion of configuration: The flipping of a stereocenter's arrangement, as happens in an SN2 reaction.
Ion–dipole forces: Attractions between an ion and the oppositely charged end of a polar molecule.
Ionic bond: A bond formed by the attraction between a cation and an anion, typical when the electronegativity difference between the atoms is large.
IUPAC nomenclature: The systematic naming system developed by the International Union of Pure and Applied Chemistry, based on the carbon chain and the functional groups present.

K

Ka: The acid dissociation constant; a larger Ka means a stronger acid.
Kekulé structure: A Lewis structure in which every bonding pair is drawn as a line.
Ketone: A carbonyl compound in which the carbonyl carbon is bonded to two carbons.
Ketose: A sugar whose open-chain form contains a ketone.
Kinetics: The study of how fast reactions happen and by what pathway.

L

Leaving group: An atom or group that departs with a pair of electrons; good leaving groups are stable, weakly basic ions.
Lewis acid: An electron-pair acceptor.
Lewis base: An electron-pair donor.
Lewis structure: A drawing that shows the atoms, bonds and lone pairs of a molecule.
Lindlar catalyst: A poisoned palladium catalyst that stops hydrogenation at the cis alkene.
Linear geometry: The arrangement of two electron groups at 180°.
Localized electrons: Electrons held on one atom or in one bond; they cannot take part in resonance.
London dispersion forces: Weak attractions caused by temporary, induced dipoles; present between all molecules and strongest for large, nonpolar ones such as hydrocarbons.
Lone pair: A pair of valence electrons that is not shared in a bond.
LUMO: Lowest Unoccupied Molecular Orbital; the lowest-energy orbital that has no electrons.

M

Markovnikov's rule: In adding HX to an alkene, the halogen goes to the more substituted carbon because that gives the more stable carbocation.
mCPBA: meta-Chloroperoxybenzoic acid; the most common reagent for epoxidation.
Mechanism arrow rules: Curved arrows start at a lone pair, negative charge or bond and end at an atom, positive charge or bond; a lone pair moving to an atom forms a bond, and a bond moving away breaks it.
Mechanistic step: One of the individual steps of a mechanism, each with its own rate and transition state.
Meso compound: A compound that has chiral centers but is achiral overall because of an internal plane of symmetry.
Methyl, ethyl, propyl, butyl: The one-, two-, three- and four-carbon alkyl groups, abbreviated Me, Et, Pr and Bu.
Methyl shift (1,2-): The movement of a methyl group to an adjacent carbocation; less common than a hydride shift.
Molecular geometry: The arrangement of only the atoms around a central atom; lone pairs still shape it but are not part of the description.
Molecular orbital: An orbital formed when atomic orbitals combine in a bond; it extends over the bonded atoms.
Molecular orbital theory: A model in which atomic orbitals combine into molecular orbitals that are spread over the whole molecule.
Monomer: A small molecule that links with others to form a polymer.

N

NBS: N-bromosuccinimide; supplies a low concentration of Br2 for allylic bromination without adding across the alkene.
Net dipole moment: The vector sum of all the bond dipoles in a molecule; it decides whether the molecule as a whole is polar.
Newman projection: A drawing that looks straight down one carbon–carbon bond to show the groups on the front and back carbons.
Nitrile: A functional group with a carbon–nitrogen triple bond (C≡N); it often behaves like a carbonyl.
Nonpolar covalent bond: A covalent bond in which the electrons are shared equally or nearly equally, as between two identical atoms or in a C–H bond.
Nucleophile: An electron-pair donor that attacks an electron-poor center; it has a lone pair, a negative charge or a pi bond.
Nucleophilicity: How readily a species attacks an electrophile; increases with negative charge, polarizability and (in polar aprotic solvents) basicity.
Number of stereoisomers (2ⁿ): A molecule with n chiral centers can have at most 2ⁿ stereoisomers.

O

Octahedral geometry: The electron geometry of six electron groups at 90° angles.
Octet rule: The tendency of atoms to bond so that they have eight valence electrons around them (two for hydrogen).
Optical activity: The ability of a chiral substance to rotate the plane of plane-polarized light.
Orbital: A region around a nucleus where an electron is likely to be found.
Organic chemistry: The study of carbon and carbon-containing compounds.
Organic synthesis: Using a series of reactions to turn a simple starting material into a more complex target molecule.
Osmium tetroxide (OsO4): A toxic, expensive reagent that adds two –OH groups to the same face of an alkene; used in catalytic amounts.
Oxidation: An increase in oxidation number: losing electrons or forming bonds to oxygen.
Oxidative cleavage: Breaking both the pi and sigma bond of a C=C by forming new C–O bonds.
Oxymercuration–demercuration: A Markovnikov hydration of an alkene using a mercury(II) reagent that avoids carbocation rearrangements.
Ozonolysis: Cleavage of an alkene's double bond with ozone to give carbonyl compounds.

P

Parent chain: The longest continuous carbon chain in a molecule, which sets the base name.
Partially condensed structure: A drawing that shows some bonds as lines while writing other groups in condensed form.
Peroxide: A compound with a weak O–O single bond that breaks easily to form radicals.
pH versus pKa: If the pH is below an acid's pKa, the acid is mostly protonated; if the pH is above the pKa, it is mostly deprotonated.
Pi (π) bond: A bond formed by side-to-side overlap of unhybridized p orbitals; it prevents rotation about the bond.
pKa: The negative log of Ka; a lower pKa means a stronger acid, and each unit of difference is a tenfold difference in acidity.
Plane of symmetry: A plane that divides a molecule into two mirror-image halves; its presence means the molecule is achiral.
Plane-polarized light: Light whose waves vibrate in only one plane.
Polar aprotic solvent: A polar solvent that cannot hydrogen bond, such as acetonitrile, DMF and DMSO; speeds SN2.
Polar covalent bond: A covalent bond in which the electrons are shared unequally because the two atoms differ moderately in electronegativity.
Polarizability: How easily an atom's electron cloud can be distorted; larger atoms such as iodine are more polarizable.
Polar protic solvent: A polar solvent that can hydrogen bond, such as water, ethanol and methanol; slows SN2 and speeds SN1.
Polymer: A large molecule made of repeating monomer units.
p orbital: A dumbbell-shaped orbital; each energy level from n = 2 up has three, oriented along the x, y and z axes.
Potassium permanganate (KMnO4): A strong oxidizer whose dark purple color fades in the presence of an alkene, making it a test for alkenes.
Primary carbon: A carbon bonded to one other carbon.
Primary, secondary and tertiary alcohols and alkyl halides: Classified by the carbon bearing the –OH or halogen: primary if that carbon has one or no other carbons, secondary if two, tertiary if three.
Priority group: The highest-ranked functional group in a molecule; it must be in the parent chain, gets the lowest number and sets the suffix.
Prochiral: Describes a molecule that is not chiral but becomes chiral after a single reaction.
Propagation: A step in which one radical is consumed and a new radical is formed, keeping the chain going.

Q

Quaternary carbon: A carbon bonded to four other carbons.

R

Racemic mixture: An equal 50:50 mixture of two enantiomers; it shows no net rotation of plane-polarized light.
Radical addition of HBr: Addition of HBr to an alkene in the presence of peroxides, giving the anti-Markovnikov product.
Radical halogenation: Replacement of a C–H hydrogen on an alkane with Cl or Br through a radical chain mechanism.
Radical initiator: A compound, such as a peroxide, that easily forms radicals to start a chain reaction.
Radical polymerization: Building an alkene polymer by a radical chain reaction with initiation, propagation and termination steps.
Radical stability: Radicals follow the same stability order as carbocations: tertiary > secondary > primary > methyl.
Rate-determining step: The slowest step in a mechanism, which controls the overall rate.
Rate law: An equation that relates the rate of a reaction to the concentrations of reactants; the rate law of the slow step matches the overall one.
R configuration: A chiral center whose priorities 1→2→3 run clockwise when the lowest-priority group points away from the viewer.
Reaction conditions: The temperature, time, pressure and other circumstances needed for a reaction.
Reaction coordinate diagram: A graph of energy versus reaction progress that shows reactants, products, intermediates, transition states and activation energies.
Reaction intermediate: A species formed in one step and consumed in the next, shown as a valley between two peaks.
Reaction mechanism: The step-by-step description of how reactants turn into products, drawn with curved arrows.
Reagent: A substance added to a reaction that provides atoms that end up in the product.
Reduction: A decrease in oxidation number: gaining electrons or breaking bonds to oxygen.
Regioselective: A reaction that gives mostly one constitutional isomer when more than one is possible.
Resonance: The delocalization of electrons across more than one atom, so that no single Lewis structure describes the molecule.
Resonance hybrid: The combined, more accurate picture of a molecule that blends all its resonance structures.
Resonance structure: One of several Lewis structures that differ only in the placement of electrons and together describe a molecule; also called a resonance contributor.
Retrosynthesis: Planning a synthesis by working backward from the target to simpler starting materials.
Ring expansion: A rearrangement in which a carbocation next to a small ring enlarges the ring by one carbon.
Ring flip: The interconversion of the two chair forms of cyclohexane, which swaps axial and equatorial positions.
Ring strain: The extra energy of a ring caused by bond angles that differ from the ideal 109.5° and by eclipsing interactions.

S

Saturated hydrocarbon: A hydrocarbon with no double or triple bonds, holding the maximum number of hydrogens.
Sawhorse projection: A drawing of a carbon–carbon bond seen from an angle, with the groups on each carbon shown.
s character: The percentage of a hybrid orbital that comes from the s orbital; more s character holds electrons closer to the nucleus.
S configuration: A chiral center whose priorities 1→2→3 run counterclockwise when the lowest-priority group points away from the viewer.
Secondary carbon: A carbon bonded to two other carbons.
Sigma (σ) bond: A bond formed by head-on overlap of orbitals along the line between two nuclei; it allows rotation, and the first bond between any two atoms is always sigma.
SN1 reaction: Substitution, Nucleophilic, unimolecular: a two-step reaction that goes through a carbocation, so stereochemistry is lost.
SN2 reaction: Substitution, Nucleophilic, bimolecular: one step in which the nucleophile attacks as the leaving group departs.
Solvent: The medium a reaction runs in; it usually does not become part of the product.
s orbital: A spherical orbital; each energy level has one.
Specific rotation: A standard measure of a compound's optical activity, adjusted for concentration and path length.
sp³ hybridization: The mixing of one s and three p orbitals into four equal orbitals; gives tetrahedral geometry and 25% s character.
sp² hybridization: The mixing of one s and two p orbitals into three equal orbitals, leaving one unhybridized p orbital that forms a pi bond; gives trigonal planar geometry and 33% s character.
sp hybridization: The mixing of one s and one p orbital into two equal orbitals, leaving two unhybridized p orbitals; gives linear geometry and 50% s character.
Spontaneous reaction: A reaction that is favorable and has a negative ΔG; it says nothing about how fast it goes.
Staggered conformation: A conformation in which the groups on adjacent carbons are as far apart as possible; lower in energy.
Stereocenter: An atom where swapping two groups gives a different stereoisomer; includes chiral centers and double-bond cis/trans positions.
Stereoisomers: Compounds with the same connections between atoms but a different arrangement in space.
Stereoselective: A reaction that gives mostly one stereoisomer when more than one is possible.
Stereospecific: A reaction in which each stereoisomer of the starting material gives a different stereoisomer of the product because of the mechanism.
Steric hindrance: Slowing of a reaction because large groups block the approach to the reactive site.
Steric strain: Extra energy from groups being forced too close together.
Strong base: A species such as KOtBu, LDA, NaH, OH⁻ or NH2⁻ that readily removes protons and so favors elimination.
Strong nucleophile, weak base: A group such as I⁻, Br⁻, Cl⁻, CN⁻ or HS⁻ that favors substitution over elimination.
Substituent: An atom or group attached to the parent chain; named with an –yl ending for carbon groups.
Substitution reaction: A reaction in which a nucleophile replaces a leaving group.
Substrate: The compound being acted on in a reaction, such as the alkyl halide in a substitution.
Sulfonate: A good leaving group made by converting an –OH into a sulfonate ester, such as a tosylate.
Syn addition: Addition of two groups to the same face of a double bond.
syn-Diol formation: Making a 1,2-diol with the two –OH groups on the same face, using OsO4 with a co-oxidant such as NMO.

T

Tautomerization: A rapid, reversible shift of a proton and a double bond; an enol converts to the more stable carbonyl form.
Terminal alkyne: An alkyne with the triple bond at the end of the chain, so it has an acidic C–H.
Termination: A step in which two radicals combine and end the chain.
Tertiary carbon: A carbon bonded to three other carbons.
Tetrahedral geometry: The arrangement of four electron groups at about 109.5°.
Thermodynamics: The study of the energy changes and the stability of reactants and products; it tells whether a reaction is favorable.
Thiol: A compound with an –SH group bonded to a carbon.
Torsional strain: Extra energy from eclipsing interactions between bonds on adjacent atoms.
Trans: Two groups on opposite sides of a ring or double bond; trans isomers are usually more stable than cis.
Transition state: The highest-energy point of a reaction step, shown as a peak on an energy diagram; it cannot be isolated.
Trigonal bipyramidal geometry: The electron geometry of five electron groups, with three in a plane and two along the axis.
Trigonal planar geometry: The flat arrangement of three electron groups at about 120°.
Trigonal pyramidal geometry: A molecular geometry with three bonded atoms and one lone pair on a central atom with four electron groups, as in ammonia.
Triple bond: A bond made of one sigma bond and two pi bonds.

U

Unimolecular: A step in which only one species is involved in the rate-determining step.

V

Valence bond theory: A model in which a bond forms when atomic orbitals, each holding one electron, overlap.
Valence electrons: The outermost electrons of an atom, which are the ones involved in bonding.
Vinyl alcohol (enol): An –OH on a carbon of a C=C; unstable and quickly converts to a carbonyl compound.
VSEPR theory: Valence Shell Electron Pair Repulsion theory; predicts geometry by assuming electron groups spread as far apart as possible to minimize repulsion.

Z

Zaitsev's rule: In an elimination, the more substituted alkene is the major product.