Notes on Reaction Mechanisms, Acids/Bases, and Equilibria

Reaction mechanisms

Explanations of what happens in a reaction, detailing the step-by-step movement of electrons and the sequence of events.

Curved arrows

Symbols used in reaction mechanisms to show the movement of electrons from regions of higher electron density to lower electron density. A full-headed arrow indicates a two-electron move, while a half-headed arrow indicates a one-electron move.

Intermediates

Transient species formed during one step of a multistep reaction that are consumed in a subsequent step.

Lewis acid

A species that can accept an electron pair.

Lewis base

A species that can donate an electron pair.

Brønsted–Lowry acid

A proton (hydrogen ion) donor. All Brønsted–Lowry acids are also Lewis acids.

Brønsted–Lowry base

A proton (hydrogen ion) acceptor. All Brønsted–Lowry bases are also Lewis bases.

Electrophile

An electron-poor species (often a Lewis acid) that seeks electrons and is attacked by nucleophiles. Examples include carbocations and carbonyl carbons.

Nucleophile

An electron-rich species (often a Lewis base) that donates electrons and attacks electrophiles. Examples include carbanions and the lone pair of water.

Carbocation

A carbon atom with a positive charge (C⁺), typically sp² hybridized with a trigonal planar geometry, acting as a Lewis acid/electrophile. Usually formed via heterolysis.

Carbanion

A carbon atom with a negative charge (C⁻), typically sp³ hybridized with a tetrahedral geometry, acting as a Lewis base/nucleophile. Formed when bond electrons stay with carbon.

Heterolysis

The breaking of a covalent bond where both bonding electrons are retained by one of the atoms, resulting in the formation of ions (a cation and an anion).

Homolysis

The breaking of a covalent bond where each atom retains one of the bonding electrons, resulting in the formation of radicals.

Acid dissociation constant (K*a)

The equilibrium constant for the dissociation of an acid (HA) in water, given by the expression rac{[A^-][ ext{H}*3 ext{O}^+]}{[HA]}.

pKa

A measure of acid strength, defined as -\log K*a. A smaller pKa value indicates a stronger acid.

Predicting acid-base reaction direction

In an acid-base equilibrium, the reaction favors the formation of the weaker acid and the weaker base.

Periodic trend in acidity (halogen hydrides)

Acidity increases down a group in the periodic table for halogen hydrides (e.g., HF < HCl < HBr < HI) because atom size increases, bond length increases, and bond strength weakens, making proton donation easier.