1,800+ MCAT terms, defined.

Ultraviolet spectroscopy measures how strongly a sample absorbs UV light (about 200–400 nm) as electrons jump from π or nonbonding orbitals to π* orbitals. Most testable: the more extended the conjugation, the smaller the energy gap and...

An α-hydrogen is a hydrogen on the carbon next to a carbonyl group (the α-carbon). It is unusually acidic for a C–H because the enolate left after deprotonation is resonance-stabilized, so a carbonyl compound needs an α-H to form an enol...

An electron-withdrawing group pulls electron density toward itself through sigma bonds (inductive effect) or through pi bonds (resonance). It stabilizes nearby negative charge, so it usually makes acids stronger and bases weaker, and it...

An aryl group is a substituent made by removing one hydrogen from an aromatic ring, so it attaches through a ring carbon; phenyl, C₆H₅−, is the simplest, and Ar is the general abbreviation. Testable point: aryl halides generally resist S...

Constitutional (structural) isomers have the same molecular formula but different atom-to-atom connectivity, so they are different compounds with different properties. The key test: if the bonding order differs, they are constitutional;...

In organic chemistry, recombination is the joining of two free radicals into a stable molecule with a new covalent bond. It is a termination step of a radical chain reaction, since two radicals are consumed and none are produced. In biol...

A carbonyl compound with a C=C between the α and β carbons, so C=C–C=O is conjugated. Resonance makes the β carbon electrophilic, so nucleophiles can add at the carbonyl carbon (1,2) or the β carbon (1,4, conjugate or Michael addition)....