1,800+ MCAT terms, defined.

An oxidizing agent is the species that takes electrons from another species, so it is itself reduced. In organic chemistry, it converts alcohols to carbonyl compounds or acids. Testable point: the oxidant is the one reduced, and its stre...

An alkyl group (R) is a saturated hydrocarbon substituent, CnH2n+1, made by removing one hydrogen from an alkane (methyl, ethyl, propyl). Alkyl groups donate electron density weakly, so they stabilize carbocations: 3° > 2° > 1° > methyl.

Ammonia (NH₃) is a small neutral molecule with a lone pair on nitrogen, so it acts as a weak base and as a nucleophile. Its most testable organic use is reacting with electrophiles such as alkyl halides to make amines, a reaction that of...

Phosphoric acid (H₃PO₄) is a weak triprotic acid whose conjugate bases, the phosphates, form esters and anhydrides in ATP and the DNA backbone. Testable point: its three pKa values (about 2, 7 and 12) mean H₂PO₄⁻ and HPO₄²⁻ are the main...

An enamine is a compound with a nitrogen bonded directly to a C=C double bond (N–C=C). It forms from an aldehyde or ketone and a secondary amine with loss of water. Its β carbon is nucleophilic, like an enol or enolate; primary amines us...

Transesterification is the conversion of one ester into another by swapping the alkoxy group (OR′) for a different alcohol's OR″, under acid or base catalysis. It is a nucleophilic acyl substitution, and an excess of the new alcohol driv...

Acetic acid, CH₃COOH (ethanoic acid), is a two-carbon carboxylic acid and a weak acid (pKa about 4.76) that ionizes only partly in water. Its conjugate base, acetate, is resonance stabilized, and this explains the acidity and the titrati...

Unsaturated describes an organic compound with at least one pi bond (C=C or C≡C), so it has fewer hydrogens than the saturated alkane of the same carbon count. Each pi bond or ring lowers the hydrogen count by two, which the degrees-of-u...

Zaitsev's rule says that in an elimination reaction (E1, or E2 with a small base) the major alkene is usually the more substituted, more stable one. A bulky base such as tert-butoxide often favors the less substituted (Hofmann) alkene in...

Tetrahedral describes an atom with four groups or electron domains arranged at about 109.5°, as in an sp³ carbon. Most testable: a planar sp² carbonyl carbon becomes tetrahedral when a nucleophile adds, forming the tetrahedral intermediate.

In ¹H NMR, a proton (or set of equivalent protons) with n equivalent neighboring protons on the adjacent atom is usually split into n + 1 peaks. Testable point: an ethyl group gives a triplet (CH₃) and a quartet (CH₂). Equivalent protons...

A phosphodiester is a phosphate group esterified to two alcohol groups, R–O–P(O)(O⁻)–O–R′. In DNA and RNA, phosphodiester bonds join the 3′ OH of one nucleotide to the 5′ phosphate of the next. They form the covalent, negatively charged...

Recrystallization purifies a solid by dissolving it in a minimum of hot solvent and cooling slowly, so the desired compound crystallizes while most impurities stay in solution. Key point: the ideal solvent dissolves the compound well whe...

Plane-polarized light has its electric field oscillating in a single plane, made by passing unpolarized light through a polarizing filter. Chiral, optically active compounds rotate that plane; the direction (+ clockwise, − counterclockwi...

Ion exchange chromatography separates charged molecules by their attraction to oppositely charged groups fixed on a resin. Testable point: a cation exchange resin is negatively charged and holds positive analytes, and the reverse for an...

Peptide bond formation joins the carboxyl group of one amino acid to the amino group of another, forming an amide bond and releasing H₂O (a condensation, or nucleophilic acyl substitution). Resonance gives the bond partial double-bond ch...

An aldol reaction joins two aldehydes or ketones: the enolate (from the α carbon) attacks the carbonyl carbon of a second molecule, forming a β-hydroxy carbonyl. The testable point is that the new C–C bond links an α carbon to a carbonyl...

A lactam is a cyclic amide: the N–C=O group sits inside a ring. It forms when an amino acid's amine attacks its own carboxyl group and water is lost. Ring size is named by Greek letter (β = 4-membered, γ = 5, δ = 6), and the strained β-l...

Net charge is the sum of the positive and negative charges on a molecule at a given pH, set by which ionizable groups are protonated. Key test point: a molecule is net positive when pH is below its pI, zero at the pI, and net negative ab...

Chemical equilibrium is the state of a reversible reaction in which forward and reverse rates are equal, so concentrations stay constant (not necessarily equal). In organic chemistry it governs esterification, acetal formation and tautom...

An elimination reaction removes a leaving group and a hydrogen from adjacent carbons, forming a C=C π bond (for example, alkyl halide → alkene). E2 is one concerted step with a strong base; E1 goes through a carbocation. The more substit...

Acetone (propanone, CH₃COCH₃) is the simplest ketone. It is a polar aprotic solvent: it accepts hydrogen bonds at its carbonyl oxygen but has no O–H or N–H to donate. Its most testable role is favoring SN2 reactions by leaving nucleophil...

An electron acceptor is a species that takes up electrons. In organic chemistry it usually means an electrophile (Lewis acid) that accepts an electron pair from a nucleophile to form a bond. Look for a partial positive atom or an empty o...

Aliphatic describes organic compounds, or parts of molecules, that are not aromatic. They can be straight, branched, or ring-shaped (alicyclic), and saturated or unsaturated. Alkanes, alkenes, alkynes and cyclohexane are aliphatic; benze...

A cyanohydrin has an –OH group and a nitrile (–C≡N) on the same carbon. It forms when cyanide ion adds to the carbonyl carbon of an aldehyde or ketone. Testable point: the product has one more carbon, and its nitrile can be hydrolyzed to...

A carboxylate ion (RCOO⁻) is the conjugate base formed when a carboxylic acid loses its proton. Resonance delocalizes the negative charge over both oxygens, which explains the acidity of carboxylic acids and why carboxylate salts dissolv...

Inorganic phosphate (Pi) is free phosphate, PO₄³⁻ and its protonated forms such as HPO₄²⁻ and H₂PO₄⁻, not bonded to carbon. It is released when ATP or a phosphate ester is hydrolyzed, and its stability relative to the reactant helps make...

Inversion (Walden inversion) is the flip in three-dimensional arrangement at a stereocenter during an SN2 reaction, because the nucleophile attacks from the side opposite the leaving group. The most testable point: a chiral substrate giv...

The suffix -oic acid names a carboxylic acid (R–COOH) in IUPAC nomenclature: the alkane name drops its final -e and takes -oic acid, as in ethanoic acid (acetic acid). The carboxyl carbon is C-1 of the chain, and the group outranks other...

Configuration is the fixed three-dimensional arrangement of groups around a stereocenter (R or S) or a double bond (E or Z, cis or trans). It changes only if a bond is broken and re-formed, unlike conformation, which changes by rotation...

A hydrocarbon is a compound made only of carbon and hydrogen, such as an alkane, alkene, alkyne or arene. It is nonpolar and held together by London dispersion forces, so boiling point rises with chain length and falls with branching.

NMR (nuclear magnetic resonance) spectroscopy uses a magnetic field and radio waves to probe the environments of nuclei such as ¹H and ¹³C. Most testable: chemical shift, integration and splitting reveal the type, number and neighbors of...

Nucleophilicity is how fast a species donates an electron pair to an electrophilic atom, usually carbon; it is kinetic, unlike basicity. It often tracks basicity, but not always: in polar protic solvents I⁻ > F⁻, while in polar aprotic s...

E/Z isomerism is geometric isomerism about a C=C bond, named by ranking the two groups on each sp2 carbon by Cahn-Ingold-Prelog priority. Higher-priority groups on the same side give Z; on opposite sides give E. E and Z isomers are diast...

An elimination reaction removes H and a leaving group from adjacent carbons to form a C=C double bond. E2 is one concerted step that needs a strong base; E1 goes through a carbocation. The more substituted alkene is usually the major pro...

An electronegative atom strongly attracts the shared electrons in a covalent bond. Electronegativity increases across a period and up a group (F is highest), and unequal sharing makes a polar bond with δ− on the more electronegative atom.

R/S configuration is the Cahn–Ingold–Prelog label for the absolute arrangement of four different groups around a stereocenter. Rank the groups by priority, point the lowest away from you, and trace 1 → 2 → 3: clockwise is R, counterclock...

Carbonic acid (H₂CO₃) is a weak diprotic acid: a carbonyl carbon bearing two OH groups, formed when CO₂ dissolves in water. It loses a proton to give bicarbonate. Most testable: it is the acid member of the CO₂/bicarbonate buffer, so mor...

The Rf (retention factor) is the distance a compound travels divided by the distance the solvent front travels in thin-layer or paper chromatography. It is between 0 and 1. On a normal-phase silica plate, less polar compounds usually hav...

Protein electrophoresis separates proteins by their movement through a gel in an electric field. In SDS-PAGE, SDS gives proteins a similar charge per mass, so they separate by size and smaller ones travel farther toward the positive elec...

Keto-enol tautomerism is the rapid equilibrium between a carbonyl compound (keto form, C=O) and an enol (C=C–OH), which differ in the position of a hydrogen and a double bond. For simple aldehydes and ketones the keto form is usually str...

LiAlH₄ is lithium aluminum hydride, a strong hydride (H⁻) donor that reduces carbonyl compounds. It reduces aldehydes, ketones, esters, and carboxylic acids to alcohols, and amides and nitriles to amines. Unlike NaBH₄, it reduces esters...

Chirality is the property of a molecule that cannot be superimposed on its mirror image. It usually arises from a stereocenter, such as an sp³ carbon with four different groups. Testable point: a chiral molecule has an enantiomer, but a...

Nitrogen nucleophiles, such as ammonia and amines, attack electrophiles with the lone pair on nitrogen. Neutral amines form amides with acyl derivatives, imines or enamines with carbonyls, and alkylated amines by SN2; amide N and ammoniu...

Nucleophilic acyl substitution is a reaction where a nucleophile adds to a carbonyl carbon, then the leaving group departs, swapping the group on an acyl compound. Reactivity follows leaving-group ability: acyl chloride > anhydride > est...

Peptide chemistry covers how amino acids link through amide (peptide) bonds, formed when a carboxyl group and an amine condense and release water. Most testable: resonance gives the peptide bond partial double-bond character, so it is pl...

Spectroscopy is the use of how molecules absorb electromagnetic radiation at specific energies to work out their structure. The MCAT's main types are IR (functional groups), ¹H NMR (hydrogen environments) and UV-vis (conjugation). Photon...

A Newman projection views a molecule straight down one C–C bond: the front carbon is a point where three bonds meet, and the back carbon is a circle. It is used to compare conformations. Staggered is lower in energy than eclipsed, and in...

An internal plane of symmetry is an imaginary mirror plane that cuts a molecule into two halves that are reflections of each other. A molecule with one is achiral, so a compound with stereocenters and an internal plane is meso and optica...

UV spectroscopy measures how much ultraviolet light (about 200–400 nm in a standard UV-Vis instrument) a sample absorbs as electrons jump from filled to empty orbitals. Most testable: more extended conjugation means a smaller energy gap...