1,800+ MCAT terms, defined.

An electrophile is an electron-pair acceptor: a species with a positive or partial positive charge, or an empty orbital, that bonds to a nucleophile's electron pair. On the MCAT, carbonyl carbons, carbocations and carbons bearing leaving...

The Cahn-Ingold-Prelog rules rank the four groups on a stereocenter by atomic number, working outward from the center until a difference appears. With the lowest priority group pointing away, 1 → 2 → 3 clockwise is R and counterclockwise...

Isomers are different compounds with the same molecular formula but different arrangements of atoms. The MCAT sorts them into constitutional isomers, which differ in connectivity, and stereoisomers, which share connectivity but differ in...

Infrared spectroscopy identifies functional groups by the infrared frequencies a molecule absorbs as its bonds stretch and bend. Peaks are read in wavenumber (cm⁻¹); the most testable features are the strong C=O peak near 1700 cm⁻¹ and t...

Nucleophilic substitution replaces a leaving group on an electrophilic carbon with a nucleophile. It runs by SN2 (one concerted step, inversion, rate depends on both reactants) or SN1 (carbocation intermediate, racemization, rate depends...

sp² describes an atom whose one s and two p orbitals mix into three equivalent hybrids, trigonal planar with angles near 120°. The leftover p orbital forms a π bond (or is empty in a carbocation). Testable: three electron groups (σ bonds...

Tautomerization is the rapid interconversion of two constitutional isomers that differ in the position of a hydrogen and a double bond, most often keto ⇌ enol. It is catalyzed by acid or base. For simple aldehydes and ketones the keto fo...

An enol has an OH group on a carbon of a C=C double bond (C=C–OH). It is a tautomer of a ketone or aldehyde. For most simple carbonyl compounds the keto form is strongly favored at equilibrium, though enol content rises with conjugation,...

E2 is a bimolecular elimination: a base removes a β-hydrogen in one concerted step as the leaving group departs, forming an alkene. The rate depends on both substrate and base, and the H and leaving group must be anti-periplanar.

An E1 reaction is a two-step, unimolecular elimination: the leaving group departs first to form a carbocation, then a weak base removes a β-hydrogen to form an alkene. Its rate depends only on the substrate, and it competes with SN1 thro...

Optical activity is the ability of a chiral substance to rotate the plane of plane-polarized light. Enantiomers rotate it by equal amounts in opposite directions, so a racemic mixture shows no net rotation. The sign (+ or −) is measured,...

Affinity chromatography purifies a molecule by its specific, reversible binding to a ligand fixed to the column beads. Components that don't bind wash through first, and the target is eluted later by free ligand or a change in pH or salt.

An aldol is a β-hydroxy aldehyde or ketone, formed when an enolate's α carbon attacks the carbonyl carbon of another molecule (aldol addition). Heating often dehydrates it to a conjugated α,β-unsaturated carbonyl (aldol condensation).

Affinity is the strength of attraction between a molecule and another substance, such as the stationary phase in chromatography. A compound with greater affinity for the stationary phase than for the mobile phase moves more slowly and el...

Stereochemistry is the study of how atoms are arranged in three dimensions and how that affects properties and reactions. Stereoisomers share connectivity but differ in spatial arrangement. Enantiomers differ in optical rotation; diaster...

Stereoisomers have the same molecular formula and the same atom-to-atom connectivity but different three-dimensional arrangements. They are either enantiomers (non-superimposable mirror images) or diastereomers (all other stereoisomers,...

Mass spectrometry ionizes a sample, separates the ions by mass-to-charge ratio (m/z), and plots their relative abundance. The molecular ion peak, usually at the highest m/z apart from small isotope peaks, gives the molecular mass. The ta...

An alkane is a saturated hydrocarbon: carbon and hydrogen joined by single bonds, with the formula CnH2n+2 when acyclic. It is nonpolar and held together by London dispersion forces, so boiling point rises with chain length and falls wit...

Nonpolar describes a bond with equal or nearly equal electron sharing, or a molecule with no net dipole. A molecule can be nonpolar even with polar bonds if their dipoles cancel by symmetry (CO₂, CCl₄). Nonpolar substances dissolve in no...

Size-exclusion (gel filtration) chromatography separates molecules by size on a column of porous beads. Large molecules cannot enter the pores, take a shorter path, and elute first; small molecules enter the pores and elute later. Bigges...

A hydroxyl is an –OH group covalently bonded to carbon, as in alcohols and phenols. It is polar and both donates and accepts hydrogen bonds, which raises boiling point and water solubility. Testable point: –OH is a poor leaving group unl...

Conformational isomers (conformers) are arrangements of one molecule that differ only by rotation about single bonds, so they interconvert without breaking any bond. Most testable: staggered forms are more stable than eclipsed, and in bu...

A protic solvent has a hydrogen on O or N (O–H, N–H), so it can donate hydrogen bonds; water and alcohols are examples. Most testable: polar protic solvents stabilize anions and carbocations, tending to favor SN1 and slow SN2 by weakenin...

The α-carbon is the carbon directly bonded to a carbonyl carbon. Its hydrogens are weakly acidic (pKa near 20 for a simple ketone) because losing H⁺ gives a resonance-stabilized enolate, so the α-carbon is usually the nucleophilic site i...

A free radical is an atom or molecule with an unpaired electron, usually made by homolytic bond cleavage from heat or light, and it is highly reactive. Radical stability runs 3° > 2° > 1° > methyl, so radical halogenation favors the more...

Keto names the carbonyl (C=O) form of a compound, most often the keto tautomer that interconverts with an enol (C=C–OH) by moving a proton and shifting a double bond. For simple aldehydes and ketones the keto form is usually strongly fav...

Decarboxylation is the loss of a carboxyl group as CO₂, leaving a C–H bond in its place. It occurs readily for β-keto acids and malonic acids on gentle heating, through a cyclic six-membered transition state and an enol, while most simpl...

Cyclohexane is a six-carbon ring of sp³ carbons, C₆H₁₂. It is puckered, not flat, and its chair conformation has almost no angle or torsional strain, so the chair is the most stable form. Bulky substituents generally prefer equatorial po...

Pyrophosphate (PPi, P₂O₇⁴⁻) is two phosphate groups joined by a phosphoanhydride bond. It is released when ATP is cleaved to AMP and when a nucleotide is added to DNA or RNA. Its hydrolysis to two Pi is strongly exergonic, so it helps dr...

A tosylate is the ester of an alcohol with p-toluenesulfonic acid (R–OTs). It converts a poor leaving group, OH⁻, into an excellent one, the tosylate anion, so the carbon can undergo SN2 (or E2) reactions. Tosylation keeps the carbon's c...

Trigonal planar describes a central atom with three electron groups and no lone pairs (AX₃), so all bond angles are about 120° in one plane. It goes with sp² hybridization, as in BF₃, carbocations, and the carbonyl carbon.

Ring strain is the extra potential energy of a cyclic molecule compared with a strain-free open-chain analog, arising from angle, torsional and steric strain. Cyclopropane is the most strained, while the cyclohexane chair is essentially...

Hydration is the addition of water (H and OH) across a C=C bond, turning an alkene into an alcohol. Acid-catalyzed hydration goes through a carbocation, so OH adds to the more substituted carbon (Markovnikov) and rearrangements are possi...

A geminal diol (gem-diol, or hydrate) has two OH groups on the same carbon. It forms when water adds to the C=O of an aldehyde or ketone. For most carbonyls the equilibrium favors the carbonyl, but small aldehydes and carbonyls with elec...

Keto-enol tautomerization is the rapid, acid- or base-catalyzed interconversion of a carbonyl compound and its enol by moving a proton and shifting the double bond. For simple aldehydes and ketones the keto form is usually favored at equ...

An aprotic solvent has no O–H or N–H bond, so it cannot donate hydrogen bonds. Polar aprotic solvents (DMSO, acetone, DMF, acetonitrile) generally favor SN2 because they leave anionic nucleophiles poorly solvated and more reactive.

Fischer esterification is the acid-catalyzed reaction of a carboxylic acid with an alcohol to form an ester and water, by nucleophilic acyl substitution. It is reversible, so excess alcohol or removal of water pushes it toward the ester...

Non-polar describes a bond or molecule with no net charge separation, because electrons are shared about equally or polar bond dipoles cancel by symmetry. Most testable: non-polar substances dissolve best in non-polar solvents (like diss...

Acidity is how readily a compound donates a proton (H⁺), measured by Ka or pKa. A lower pKa means a stronger acid. The most testable point is that acidity is judged by the stability of the conjugate base: the more stable A⁻ is, the stron...

A racemic mixture contains equal amounts of two enantiomers, written (±), so the optical rotations cancel and the mixture is optically inactive. It typically forms when a stereocenter is created from a planar, achiral intermediate, as in...

A mass spectrometer ionizes molecules, sorts the ions by mass-to-charge ratio (m/z), and records their relative abundance. The most testable point: the molecular ion peak (highest main m/z, apart from small isotope peaks) usually gives t...

An alkyne is a hydrocarbon with a carbon–carbon triple bond (one σ, two π), and its sp carbons give a linear geometry. Most testable: a terminal alkyne's sp C–H is far more acidic (pKa about 25) than alkene or alkane C–H, so a strong bas...

A diol has two hydroxyl (–OH) groups. Vicinal diols (OH on adjacent carbons) form from alkenes by syn addition with OsO₄ or by anti addition via an epoxide. Geminal diols (both OH on one carbon) are usually unstable hydrates of carbonyls.

A carbonyl group is a carbon double-bonded to oxygen (C=O). Oxygen pulls electron density, so the carbon is partially positive and attacked by nucleophiles. This is the core reactivity of aldehydes, ketones, carboxylic acids and their de...

Filtrate is the liquid that passes through the filter during filtration, carrying the solvent and dissolved solutes; the solid left on the filter is the residue. Whether the product is in the filtrate depends on the step, as in hot filtr...

Configurational isomers are stereoisomers with the same connectivity but a different fixed 3D arrangement, so interconverting them requires breaking a bond. They include enantiomers and diastereomers (including cis/trans). Rotation about...

HPLC (high-performance liquid chromatography) separates a mixture by pumping a liquid mobile phase at high pressure through a column packed with fine stationary-phase particles. Compounds that stick more strongly to the stationary phase...

A mesylate is the methanesulfonate group, CH₃SO₂O– (OMs), made from an alcohol and mesyl chloride. It turns a poor leaving group (OH) into an excellent one. Making it keeps the stereochemistry of the carbon, so a following SN2 gives net...

The Gabriel synthesis makes a primary amine from a primary alkyl halide: the phthalimide anion attacks by SN2, then hydrolysis or hydrazine releases R–NH₂. Its testable advantage is that it avoids the overalkylation seen when NH₃ is alky...

A lactone is a cyclic ester: the carbonyl carbon and the ester's single-bonded oxygen are both ring atoms (the C=O oxygen is outside the ring). It forms when a hydroxy acid esterifies intramolecularly, and five- and six-membered (γ and δ...