1,800+ MCAT terms, defined.

An alcohol is an organic compound with a hydroxyl group (–OH) on a saturated sp³ carbon. Hydrogen bonding raises its boiling point. The most testable point is oxidation: primary gives an aldehyde or carboxylic acid, secondary gives a ket...

A carboxylic acid is an organic compound with the carboxyl group, –COOH. It is a weak acid, yet far more acidic than an alcohol because its carboxylate conjugate base is resonance-stabilized. Nearby electron-withdrawing groups increase i...

An aldehyde is an organic compound whose carbonyl group (C=O) carbon is bonded to one hydrogen and one R group (RCHO), at the end of a chain. Its electrophilic carbonyl carbon is usually more reactive toward nucleophiles than a ketone's,...

A ketone is an organic compound with a carbonyl group (C=O) whose carbon is bonded to two other carbon groups, R–CO–R′. The carbonyl carbon is electrophilic, so ketones undergo nucleophilic addition, and unlike aldehydes they resist mild...

An ester (RCOOR′) is a carboxylic acid derivative in which the acid's –OH is replaced by an alkoxy group (–OR′). Most testable: it forms from a carboxylic acid and an alcohol (Fischer esterification) and reverts by hydrolysis, and base h...

An amide is a carboxylic acid derivative, RC(=O)NR′₂, with nitrogen bonded directly to the carbonyl carbon. Resonance between the nitrogen lone pair and the C=O makes the C–N bond partially double, so amides are planar, non-basic, and th...

SN1 (substitution, nucleophilic, unimolecular) is a two-step reaction: the leaving group departs to form a carbocation, then the nucleophile attacks. The rate depends only on the substrate, and tertiary, allylic and benzylic substrates t...

Polar describes a bond or molecule with uneven electron sharing, giving partial positive and negative ends. A molecule is polar when its bond dipoles do not cancel, so geometry matters. Polar substances tend to dissolve in polar solvents.

SN2 means substitution, nucleophilic, bimolecular: a nucleophile attacks the carbon bearing a leaving group from the back side in one concerted step, inverting its configuration. Rate = k[substrate][nucleophile], and the reaction is fast...

Chromatography separates a mixture because each component partitions differently between a stationary phase and a moving mobile phase. In normal-phase systems (polar silica), less polar compounds stick less, travel farther and elute firs...

In organic chemistry, oxidation raises the oxidation state of a carbon: it gains bonds to oxygen or another more electronegative atom, or loses bonds to hydrogen. The most testable point is alcohol oxidation: primary → aldehyde → carboxy...

Enantiomers are stereoisomers that are non-superimposable mirror images, with every stereocenter inverted. They have identical physical properties in achiral settings, but rotate plane-polarized light equally in opposite directions and r...

Hydrogen bonding is an attraction between an H bonded to N, O, or F and a lone pair on another N, O, or F atom. It is usually the strongest intermolecular force between neutral molecules, so it raises boiling point, melting point and wat...

Diastereomers are stereoisomers that are not mirror images of each other. Unlike enantiomers, they generally differ in physical properties such as melting point, boiling point and solubility, so ordinary methods can separate them.

An anhydride (acid anhydride) is a carboxylic acid derivative with two acyl groups joined through one oxygen, RC(=O)–O–C(=O)R, formally two acids minus water. It is a reactive acylating agent: more reactive than esters and amides, less t...

The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure. In organic chemistry it mostly reflects intermolecular forces: stronger forces and larger molecules usually give a higher boiling point.

Resonance is the delocalization of π electrons or lone pairs over several atoms, shown by drawing Lewis structures that differ only in electron placement. The real molecule is a hybrid that is more stable than any single contributor, and...

Distillation separates liquids by boiling point: heating vaporizes mostly the more volatile component, which is condensed and collected. The lower-boiling component distills first; fractional distillation handles close boiling points, an...

A nucleophile is an electron-rich species that donates a lone pair or pi-bond pair to an electron-poor atom, usually carbon, forming a new bond. The testable point: nucleophilicity is a rate (kinetic) property, whereas basicity is thermo...

A carbonyl is a carbon double-bonded to oxygen (C=O). Oxygen's electronegativity makes the carbon partially positive and electrophilic, so nucleophiles attack it. Most testable: aldehydes, ketones, acids and their derivatives contain it,...

An acetal is a carbon bonded to two alkoxy (OR) groups, made from an aldehyde or ketone plus two alcohols with acid catalysis. Most testable: acetals are stable to base, nucleophiles and hydrides but hydrolyze in aqueous acid, so they se...

NMR (nuclear magnetic resonance) spectroscopy detects ¹H or ¹³C nuclei absorbing radio-frequency energy in a magnetic field, revealing the kinds of hydrogens or carbons in a molecule. Most testable: chemical shift gives the environment,...

Electronegativity is a relative measure of how strongly an atom pulls shared bonding electrons toward itself. It generally increases across a period and up a group, with fluorine highest, and a larger difference between bonded atoms make...

A bond is the attraction holding atoms together; in organic chemistry it usually means a covalent bond, a shared electron pair. A double bond is one σ plus one π, a triple is one σ plus two π, and the weaker π bond is the usual site of r...

A leaving group is the atom or group that departs with the bonding electron pair from the substrate in substitution and elimination reactions. Good ones are generally weak bases, the conjugate bases of strong acids (I⁻ > Br⁻ > Cl⁻ > F⁻,...

A hydrogen bond is an attraction between an H bonded to N, O, or F and a lone pair on another N, O, or F. It is an intermolecular force, so it raises boiling point and water solubility: an alcohol boils higher than an ether or alkane of...

An imine has a C=N double bond (R₂C=NR′) and is also called a Schiff base. It forms when an aldehyde or ketone condenses with ammonia or a primary amine, losing water. A secondary amine gives an enamine instead.

A hemiacetal is a carbon bonded to one −OH and one −OR group (plus two other substituents), formed when an alcohol adds to an aldehyde. Open-chain hemiacetals are usually unfavorable, but cyclic five- or six-membered ones, such as the ri...

An amine is an organic compound with a nitrogen atom bonded to one, two or three carbon groups (RNH₂, R₂NH, R₃N). The nitrogen lone pair makes amines weak bases and nucleophiles, so they are protonated to ammonium ions in acid. This driv...

A chiral molecule cannot be superimposed on its mirror image, so it exists as a pair of enantiomers. Usually this comes from a carbon bonded to four different groups, but a molecule with stereocenters and an internal mirror plane (a meso...

Saponification is base-promoted hydrolysis of an ester, classically a triglyceride with NaOH or KOH, giving glycerol and carboxylate salts (soap). The carboxylic acid is deprotonated as it forms, so the reaction is effectively irreversib...

Splitting is the division of a ¹H NMR signal into a multiplet by coupling to nonequivalent protons on neighboring atoms, usually the adjacent carbon. A proton with n equivalent neighbors usually gives n + 1 peaks (the n + 1 rule), so the...

A phenol is a compound with an OH group bonded directly to a benzene ring; C₆H₅OH is the parent. It is much more acidic than an aliphatic alcohol (pKa about 10 versus roughly 16 to 18) because phenoxide is resonance-stabilized, but it is...

A racemic mixture (racemate) is an equal, 50:50 mixture of two enantiomers. Their optical rotations cancel, so it shows zero net rotation and is optically inactive. It typically forms when a new stereocenter is made from achiral reactant...

Induction (the inductive effect) is the shift of electron density along sigma bonds toward or away from an electronegative or electropositive group. Electron-withdrawing groups stabilize nearby negative charge, so they strengthen acids,...

Extraction separates compounds by their different solubilities in two immiscible liquids, usually water and an organic solvent. In acid-base extraction, converting an acid or base to its charged salt moves it into the water layer. Neutra...

A functional group is a specific atom or group of atoms that gives an organic molecule its characteristic reactivity and properties, largely regardless of the rest of the molecule. Most testable: identify the group (carbonyl, hydroxyl, a...

sp³ describes an atom whose one s and three p orbitals mix into four equivalent hybrid orbitals pointing to the corners of a tetrahedron (about 109.5°). Carbon with four single bonds is sp³. The testable point: count electron domains; fo...

Chemical shift (δ) is the position of an NMR signal, in ppm relative to TMS at 0. Nuclei deshielded by nearby electronegative atoms or pi systems resonate at higher δ (downfield, to the left), and more shielded nuclei appear upfield.

A residue is the unit of an amino acid that remains in a peptide or protein after the elements of water are lost in forming peptide bonds. The most testable point is that a residue's side chain (R group) usually determines its role in fo...

A lone pair is two valence electrons held on one atom and not shared in a bond. It counts in VSEPR geometry and makes an atom a Lewis base or nucleophile. Most testable: lone pairs repel more than bonding pairs, so bond angles shrink (NH...

Polarity is an uneven sharing of electron density, giving partial charges (δ+ and δ−). A bond is polar when electronegativities differ, and a molecule is polar when its bond dipoles do not cancel. Polar molecules mix with polar solvents...

A quinone is a conjugated six-membered ring with two C=O groups, usually at the 1,4 (para) positions, made by oxidizing a hydroquinone. It is reversibly reduced by 2 e⁻ and 2 H⁺ to a hydroquinone, which is why ubiquinone (coenzyme Q) can...

A reducing agent donates electrons (or hydride) to another species and is itself oxidized. In organic chemistry, common ones are NaBH₄, LiAlH₄ and H₂ with a metal catalyst. The key test point: LiAlH₄ is stronger and reduces esters and ca...

IR spectroscopy identifies functional groups by the infrared frequencies a molecule absorbs as its bonds stretch and bend. Stronger bonds and lighter atoms absorb at higher wavenumber. The most testable peaks are a broad O–H near 3300 cm...

An alkene is a hydrocarbon with at least one carbon–carbon double bond (one σ and one π bond). The sp² carbons are planar, and the electron-rich π bond makes alkenes typically react by electrophilic addition, such as HX adding by Markovn...

A ketal is a carbon bonded to two OR groups and two carbon groups, R₂C(OR′)₂, made by reacting a ketone with two alcohols (or one diol) under acid catalysis. The most testable point: ketals resist base, nucleophiles and hydrides, so they...

Gas chromatography (GC) separates a vaporized mixture as an inert carrier gas moves it through a column coated with a stationary phase. Components that are more volatile and interact less with the stationary phase generally elute first,...

Nucleophilic addition is a reaction in which a nucleophile attacks the electrophilic carbon of a C=O (or other polar π bond), breaking the π bond and forming a tetrahedral intermediate. Aldehydes are generally more reactive than ketones.

A chiral center (stereocenter) is an atom, most often an sp³ carbon, bonded to four different groups, so the center has two possible configurations (R and S). A molecule with one such center is nonsuperimposable on its mirror image; n ce...