1,800+ MCAT terms, defined.

Formic acid (HCOOH, methanoic acid) is the simplest carboxylic acid, with an H where other carboxylic acids have an alkyl group. It is a weak acid, but stronger than acetic acid (pKa about 3.75 versus 4.76), because no electron-donating...

Silica is silicon dioxide (SiO₂). As silica gel it is the polar stationary phase in TLC and column chromatography. With a less polar eluent, polar compounds stick to it more strongly and move more slowly (lower Rf) than nonpolar ones.

The molecular ion (M⁺•) is the radical cation formed when a molecule loses one electron in mass spectrometry. Its m/z equals the molecular mass, so it is usually the highest-m/z major peak apart from isotope peaks, though it can be weak...

In organic chemistry, ppm (parts per million) is the unit of chemical shift (δ) in NMR spectroscopy. It reports how far a signal lies from the reference (TMS, 0 ppm) as a fraction of the spectrometer frequency. Higher ppm means downfield...

The carboxyl group is –COOH: a carbonyl carbon (C=O) also bonded to an OH. It defines carboxylic acids and is a weak acid because the carboxylate left after losing H⁺ is resonance stabilized. Most testable: pKa is typically near 4–5, so...

An antibonding orbital is a molecular orbital formed by out-of-phase overlap of atomic orbitals. It has a node between the nuclei and lies higher in energy than the parent orbitals, so electrons in it weaken the bond and lower the bond o...

Vacuum (suction) filtration separates an undissolved solid from a liquid by drawing the mixture through filter paper in a Büchner funnel, with lower pressure in the flask below. It is faster than gravity filtration, and it is the usual w...

Silica gel is porous silicon dioxide (SiO₂) with polar Si–OH groups on its surface. It is the usual polar stationary phase in TLC and column chromatography, so polar compounds stick more strongly and move slowly (low Rf), while nonpolar...

HPLC separates a mixture by pumping a liquid mobile phase at high pressure through a column of fine stationary-phase particles; compounds attracted more to the stationary phase elute later. Testable point: in normal phase the least polar...

A Michael addition is the conjugate (1,4) addition of a nucleophile, usually a stabilized enolate, to the β carbon of an α,β-unsaturated carbonyl, forming a new C–C bond. Test point: attack is at the β carbon, not the carbonyl carbon, th...

Relative configuration is the arrangement of groups at a stereocenter compared with a reference compound, as in D/L labeling against glyceraldehyde, rather than an R/S assignment. If no bond to the stereocenter is broken, its configurati...

A vicinal diol (glycol) has two OH groups on adjacent carbons, as in ethylene glycol. The most testable point is how it forms from an alkene: OsO₄ or cold, dilute KMnO₄ gives syn addition, while epoxide formation then ring opening gives...

The carbonyl carbon is the carbon double-bonded to oxygen (C=O). Oxygen pulls electron density away, so this sp² carbon is partially positive and electrophilic. Nucleophiles attack it in nucleophilic addition (aldehydes, ketones) and nuc...

Ortho (o-) describes two substituents on adjacent carbons of a benzene ring, a 1,2 relationship. Most testable: electron-donating groups and halogens direct an incoming electrophile to the ortho and para positions, while strongly electro...

Gauche is a staggered conformation in which two substituents on adjacent carbons are 60° apart in a Newman projection. It is slightly higher in energy than anti (180°) because of steric strain, so anti is the most stable conformer of but...

Spin-spin coupling is the interaction between nonequivalent hydrogens on neighboring carbons in ¹H NMR that splits one signal into a multiplet. By the n + 1 rule, n equivalent neighboring hydrogens give n + 1 peaks; equivalent hydrogens...

An alkyl halide (haloalkane) is R–X, with a halogen (F, Cl, Br, I) bonded to an sp³ carbon. The polar C–X bond makes that carbon electrophilic, so alkyl halides undergo SN1, SN2, E1 and E2 reactions. Leaving-group ability runs I⁻ > Br⁻ >...

A hemiketal is a carbon bonded to one OH, one OR, and two carbon groups, formed when an alcohol adds to a ketone. The reaction is reversible, and the intramolecular version gives the ring form of ketoses such as fructose. A second alcoho...

The N-terminus is the end of a peptide or protein that has a free α-amino group (–NH₃⁺ at physiological pH). Sequences are written and numbered from the N-terminus (left) to the C-terminus (right), so the first residue listed is the N-te...

A conjugated system has p orbitals on adjacent atoms overlapping in a continuous chain, usually alternating single and double bonds, so π electrons are delocalized. The most testable point: conjugation stabilizes a molecule, and longer c...

Eclipsed describes a conformation about a C–C single bond in which substituents on adjacent carbons line up (dihedral angle 0° in a Newman projection). It is a high-energy form because of torsional strain, plus steric strain for bulky gr...

A mirror image is the reflection of a molecule. If the reflection cannot be superimposed on the original, the molecule is chiral and the pair are enantiomers. Most testable: enantiomers have opposite configuration at every stereocenter,...

Vinylic describes the sp² carbons of a C=C double bond and the atoms (H, halogen) attached directly to them. Most testable: vinylic halides generally resist SN1 and SN2, unlike allylic halides, and vinylic H appear downfield in ¹H NMR.

Acid-base separation is a liquid-liquid extraction that moves acidic or basic compounds from an organic solvent into water by converting them to ionic salts, leaving neutral compounds behind. Testable point: NaHCO₃ extracts carboxylic ac...

A pure crystalline solid usually melts over a narrow range (about 1–2 °C) near its literature value. Impurities typically lower the melting point and broaden the range, as in freezing point depression. A mixed melting point with an authe...

Recrystallization (recrystallisation) purifies a solid by dissolving it in a minimum of hot solvent and cooling, so the compound crystallizes while most impurities stay dissolved. Most testable: the solvent should dissolve the compound w...

In organic chemistry, oxidation of a carbon means gaining bonds to oxygen (or another more electronegative atom) or losing bonds to H; reduction is the reverse. Most testable: alcohol oxidation, where PCC stops at the aldehyde and strong...

Specific rotation, [α], is the observed rotation of plane-polarized light by a chiral compound, scaled to a 1 dm path and 1 g/mL concentration so it is a characteristic constant. Enantiomers have equal magnitude and opposite sign; a race...

In NMR, a deshielded nucleus has less surrounding electron density, so it feels more of the applied magnetic field and absorbs at higher frequency. It appears downfield (left, larger δ in ppm). Electronegative neighbors and π systems typ...

A nucleophile attacks when it donates an electron pair (a lone pair or π bond) to form a new bond to an electron-poor atom, the electrophile, commonly a carbon with a partial positive charge. Testable point: the bond forms at the electro...

Diethyl bromomalonate, BrCH(CO₂Et)₂, is diethyl malonate with one α-hydrogen replaced by Br. Unlike diethyl malonate, which is the nucleophile once deprotonated, its C–Br carbon is an electrophile that can undergo SN2 displacement of Br⁻.

The Strecker synthesis makes an α-amino acid from an aldehyde, ammonia and cyanide: an α-aminonitrile forms first, then acid hydrolysis turns the nitrile into COOH. With achiral reagents the product is a racemic mixture, a frequent MCAT...

An acyl group is R–C(=O)–, a carbonyl carbon bonded to an R group and with one open bond to attach elsewhere. It is the unit transferred in nucleophilic acyl substitution, and reactivity of acyl derivatives tracks how good the leaving gr...

A resonance structure is one of two or more valid Lewis structures for the same species that differ only in where π electrons and lone pairs are placed, not in atom positions. The real molecule is a resonance hybrid, and delocalization m...

Formaldehyde (methanal, H₂C=O) is the simplest aldehyde: its carbonyl carbon carries two hydrogens and no alkyl group. With little steric hindrance and no electron-donating alkyl groups, it is a highly reactive electrophile toward nucleo...

A reagent is a substance added to a starting material (the substrate) to cause a specific reaction, and it is usually consumed. The testable point: a reagent is used up, unlike a catalyst, and its identity (such as NaBH₄ versus LiAlH₄) d...

PBr₃ is phosphorus tribromide, a reagent that converts primary and secondary alcohols into alkyl bromides. It works by SN2, so the carbon bearing the OH is inverted if it is a stereocenter, and carbocation rearrangements are generally av...

An NMR spectrum plots the radio-frequency absorption of nuclei (usually ¹H) in a magnetic field against chemical shift in ppm. Signal count shows distinct hydrogen environments, position shows deshielding, area shows number of H, and spl...

Chemoselectivity is a reagent's preference for reacting with one functional group over another in the same molecule. The most testable case: NaBH₄ typically reduces aldehydes and ketones but not esters, while the stronger LiAlH₄ reduces...

SOCl₂ is thionyl chloride, a reagent that converts carboxylic acids to acid chlorides and alcohols to alkyl chlorides. The OH becomes a good leaving group, and SO₂ and HCl leave as gases. Testable point: with a chiral alcohol and pyridin...

Retention time is the time from sample injection to the detector peak of a compound in chromatography (GC or HPLC). The stronger a compound's attraction to the stationary phase, the longer its retention time, so predict elution order fro...

Absolute configuration is the true three-dimensional arrangement of the four groups on a stereocenter, labeled R or S by Cahn-Ingold-Prelog priority rules. Most testable: the R/S label comes from structure and does not predict whether ro...

Absorbance (A) measures how much light a sample absorbs at a given wavelength: A = log₁₀(I₀/I) = εbc. It is unitless and, for dilute solutions, directly proportional to concentration and path length. Higher A means less transmitted light.

To react nucleophilically is to act as a nucleophile: an electron-rich species donates a lone pair or pi bond to form a new bond to an electron-poor atom (the electrophile). Curved arrows start at the electron-rich site and point to the...

Simple distillation separates liquids by boiling point: the more volatile component vaporizes first, condenses in a condenser, and is collected as distillate. It works well when boiling points differ widely (roughly 25 °C or more); close...

Ethers are organic compounds with an oxygen bonded to two carbon groups, R–O–R′. They cannot hydrogen-bond to each other, so they boil far lower than alcohols of similar mass, and open-chain ethers are fairly unreactive, which makes them...

In organic chemistry, R is a generic label for a carbon-containing group (usually alkyl) attached to a functional group, so R–OH means any alcohol. The testable point is that R's size and electron-donating character change reactivity. In...

Free rotation is the twisting of one part of a molecule relative to another about a single (σ) bond, which interconverts conformations at room temperature. Double bonds, rings and partial double bonds (such as the amide C–N bond) restric...

Planar means the atoms of a group lie in one flat plane. In organic chemistry this is the geometry around sp² centers (trigonal planar, bond angles near 120°), such as alkene and carbonyl carbons, carbocations, benzene and amide bonds. M...

Torsional strain is the extra energy a molecule has when bonds on adjacent atoms are eclipsed instead of staggered, because the bonding electron pairs repel. Testable point: around a C–C single bond, staggered conformations are lowest in...