1,800+ MCAT terms, defined.

A Brønsted-Lowry acid is a proton (H⁺) donor, and a Brønsted-Lowry base is a proton acceptor. The most testable point in organic chemistry is that a stronger acid has a more stable conjugate base, so a lower pKa means a stronger acid.

Steric hindrance is the slowing of a reaction, or the destabilizing of a conformation, because bulky groups crowd the space atoms need. Most testable: SN2 rates fall as the electrophilic carbon becomes more substituted (methyl > 1° > 2°...

Transmittance is the fraction of incident light that passes through a sample, T = I/I₀. In IR spectroscopy it is plotted against wavenumber, so absorption appears as a downward dip: the lower the transmittance, the more strongly the samp...

A bonding orbital is a molecular orbital formed when atomic orbitals overlap in phase, concentrating electron density between the nuclei. It lies lower in energy than the atomic orbitals that combined, so electrons placed in it stabilize...

A methyl ketone is a ketone with a CH₃ group on the carbonyl carbon, R–C(=O)–CH₃; acetone is the simplest. The testable point is the iodoform test: I₂ with NaOH gives a yellow CHI₃ precipitate, which most other ketones do not give.

NaOH is sodium hydroxide, a strong base that dissociates fully into Na⁺ and OH⁻. In organic reactions, OH⁻ acts as a nucleophile (SN2, ester hydrolysis) or as a base (E2, deprotonating acids), and the substrate and conditions decide whic...

An aldol condensation joins two aldehydes or ketones: an enolate's α-carbon attacks another carbonyl carbon to give a β-hydroxy carbonyl, which then loses H₂O to form a conjugated α,β-unsaturated carbonyl. The enolate partner must have a...

A nitrile is an organic compound with a carbon triple-bonded to nitrogen, R–C≡N (the cyano group). The most testable point is that SN2 attack by cyanide on an alkyl halide adds one carbon, and the nitrile can then be hydrolyzed to a carb...

A primary alcohol has its –OH on a carbon bonded to one other carbon (R–CH₂–OH), as in ethanol; methanol is usually grouped with them. The most testable point: oxidation gives an aldehyde with PCC but a carboxylic acid with strong oxidan...

A bond angle is the angle between two bonds that share a central atom. VSEPR and hybridization set it: sp³ about 109.5°, sp² about 120°, sp 180°. Lone pairs push bonding pairs closer, so angles in NH₃ and H₂O are somewhat smaller than 10...

Fractional distillation separates miscible liquids with close boiling points using a fractionating column, where repeated condensation and revaporization enrich the vapor in the more volatile component. The lower-boiling liquid distills...

Strain is the extra energy a molecule has because its geometry departs from the ideal, which makes it less stable. It comes from angle strain, torsional strain (eclipsing) and steric strain (crowding). Chair cyclohexane is nearly strain-...

A primary (1°) alcohol has its OH on a carbon bonded to just one other carbon, written R–CH₂OH. The most testable point is oxidation: PCC stops at the aldehyde, while strong oxidants such as Jones reagent or KMnO₄ carry it on to the carb...

The chair conformation is the puckered, lowest-energy shape of cyclohexane, with bond angles near 109.5° and all C–C bonds staggered. Ring flip swaps axial and equatorial positions, and bulky substituents are generally more stable equato...

The mobile phase is the liquid solvent or gas that flows through the stationary phase in chromatography and carries the sample along. Compounds that favor it over the stationary phase move faster; on polar silica, nonpolar compounds usua...

Vacuum distillation is distillation under reduced pressure, so the liquid boils at a lower temperature than its normal boiling point. The MCAT uses it for high-boiling or heat-sensitive compounds that could decompose if heated to their n...

Acid dissociation is the reversible loss of a proton from an acid, HA, to water or another base, giving the conjugate base A⁻ and H₃O⁺. Its extent is measured by Ka or pKa. In organic chemistry, a more stable conjugate base means a large...

Aldol addition is a reaction in which the enolate of one aldehyde or ketone attacks the carbonyl carbon of a second molecule, forming a β-hydroxy aldehyde or ketone. Most testable point: the new C–C bond joins the α-carbon of the nucleop...

A better leaving group departs more easily with the bonding electron pair because it is more stable afterward. Weak bases, the conjugate bases of strong acids (I⁻, Br⁻, tosylate), are generally good leaving groups, while strong bases lik...

A hydration reaction adds H and OH across a C=C double bond to make an alcohol. With aqueous acid it goes through a carbocation and follows Markovnikov's rule, so OH ends up on the more substituted carbon, and rearrangements are possible...

Conjugated π bonds are double or triple bonds separated by one single bond, so adjacent p orbitals overlap and the π electrons are delocalized over the whole chain. This lowers energy (extra stability) and shifts UV-Vis absorption to lon...

To tautomerize is to convert between tautomers, constitutional isomers that interconvert by moving a hydrogen and shifting a double bond, as in keto-enol. For simple aldehydes and ketones the keto form usually dominates, and the planar e...

A dehydration reaction loses water: either an alcohol eliminates H₂O to form an alkene (acid, heat), or two molecules join with H₂O released (condensation). Most testable: the more substituted alkene (Zaitsev product) usually dominates,...

A dioic acid is a dicarboxylic acid, named by adding "-dioic acid" to the alkane name, as in butanedioic acid, HOOC–CH₂–CH₂–COOH. Both carboxyl carbons are part of the chain count. Testable point: pKa1 is lower than pKa2, and a nearby se...

E2 is bimolecular elimination: in one concerted step, a strong base removes a β-hydrogen 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 NMR spectrum plots radio-frequency absorption against chemical shift (δ, ppm) for nuclei such as ¹H or ¹³C in a magnetic field. Each set of equivalent nuclei gives one signal, and its position, area and splitting reveal structure; n e...

Nuclear spin is an intrinsic angular momentum of some nuclei, such as ¹H and ¹³C, that makes them act like tiny magnets. In a magnetic field its spin states differ in energy, and NMR detects the radio-frequency energy absorbed when nucle...

The retro-aldol reaction is the reverse of aldol addition: a β-hydroxy aldehyde or ketone splits at the bond between its α and β carbons into two carbonyl compounds, with an enolate as the departing group. Glycolysis uses it when aldolas...

Beer's law says a solution's absorbance of light is directly proportional to the concentration of the absorbing species and to the path length. The most testable point is that doubling concentration doubles absorbance (A = εlc), a linear...

A halide here is an organic compound with a halogen (F, Cl, Br, I) bonded to carbon, such as an alkyl halide R–X or an acyl halide RCOX. The most testable point is leaving-group ability: I⁻ > Br⁻ > Cl⁻ > F⁻, so R–I reacts fastest in subs...

Benzoic acid, C₆H₅COOH, is a benzene ring bearing a carboxyl group; it is a weak acid (pKa about 4.2). Its conjugate base, benzoate, is resonance-stabilized and water soluble, so aqueous base pulls it out of an organic layer in an extrac...

Electron density is how much electron probability sits in a region of a molecule. It is greater near electronegative atoms, lone pairs, and π systems, and lower near electron-withdrawing groups. Electron-rich sites tend to act as nucleop...

Hydrophilic ("water-loving") groups are polar or charged functional groups, such as −OH, −NH₂, −COOH and −COO⁻, that interact favorably with water through hydrogen bonds or ion–dipole forces. Adding them generally raises a molecule's sol...

A molecular vibration is the repeating stretching or bending of bonds about their equilibrium positions, with quantized energies. It is what IR spectroscopy detects: a vibration absorbs IR when it changes the dipole moment, and stronger...

Acrylic acid is propenoic acid, CH₂=CH–COOH, the simplest carboxylic acid with a C=C conjugated to the C=O (an α,β-unsaturated acid). It is slightly more acidic than propanoic acid, and its alkene can undergo addition or polymerization.

An alkyl chain is a saturated, nonpolar hydrocarbon group of single-bonded carbons and hydrogens (methyl, ethyl, propyl) attached to the rest of a molecule. Longer chains generally increase hydrophobicity and London dispersion forces, so...

An amine is a nitrogen with a lone pair bonded to hydrogen and/or alkyl or aryl groups (RNH₂, R₂NH, R₃N). The lone pair makes it a base and a nucleophile, so below the pKa of its conjugate acid (about 10–11 for simple alkylamines) it exi...

Ammonia (NH₃) has a nitrogen lone pair; an amine replaces one or more H with alkyl or aryl groups. Both are weak bases and nucleophiles that form ammonium ions with acid, and delocalizing the lone pair (aniline, amides) lowers basicity.

An axial bond is a bond on a chair cyclohexane carbon that points straight up or down, parallel to the ring's axis, alternating from carbon to carbon. Axial groups suffer 1,3-diaxial strain, so bulky groups usually prefer equatorial posi...

A benzyl group is C₆H₅CH₂−, a benzene ring attached through a CH₂ carbon (abbreviated Bn). That CH₂ is the benzylic position, where cations and radicals are resonance stabilized by the ring, so benzylic sites are unusually reactive.

A carbon center is a particular carbon atom identified by its role in a structure or reaction: electrophilic (electron poor, attacked by nucleophiles), nucleophilic (electron rich), or chiral (four different groups). Most testable: nucle...

The carbonate ion, CO₃²⁻, is a planar, resonance-stabilized anion whose −2 charge is delocalized over three oxygens. The most testable point is that all three C–O bonds are identical, with bond order 4/3 and a length between a single and...

The coupling constant J is the spacing, in hertz, between adjacent peaks within an NMR multiplet. It measures spin–spin splitting between hydrogens on neighboring carbons. J is set by the molecule, not the spectrometer, so it stays the s...

A cyclic diketone is a ring bearing two ketone C=O groups, such as 1,3-cyclohexanedione. Open-chain diketones also appear in intramolecular aldol closure: a 1,4-diketone typically closes to a five-membered ring and a 1,5-diketone to a si...

An enolate anion is the resonance-stabilized conjugate base formed when a base removes an α hydrogen from a carbonyl compound; the negative charge is shared by the α carbon and the oxygen. It reacts as a carbon nucleophile, which is the...

An equatorial bond on a chair cyclohexane points outward around the ring's "equator", roughly in the ring's average plane, rather than straight up or down like an axial bond. Substituents are usually more stable equatorial because that a...

A formyl group is –CHO, a carbonyl carbon bonded to one hydrogen; it is the functional group of an aldehyde. Testable point: it is easily oxidized to a carboxylic acid and reduced to a primary alcohol, and it is named with the prefix for...

Fractional crystallization separates the components of a solid mixture by their different solubilities: as a solution cools or evaporates, the least soluble component generally crystallizes first and is filtered off. Diastereomers can be...

A methyl carbon has no other carbons attached; a primary (1°) carbon has one. Methyl and primary alkyl halides are usually the best SN2 substrates because backside attack is unhindered, and they generally avoid SN1 because their carbocat...

The retardation factor (Rf) in thin-layer or paper chromatography is the distance a compound travels divided by the distance the solvent front travels. It is between 0 and 1. On normal-phase silica, a more polar compound sticks to the pl...