Arrow pushing

Arrow pushing, or electron pushing, is a method for illustrating how organic reactions happen step by step. It was first devised by Sir Robert Robinson. The technique involves drawing curved arrows on the reactant structures in a chemical equation to show the reaction mechanism. These arrows trace the movement of electrons as old bonds break and new bonds form, giving a visual sense of where electrons come from (nucleophiles), where they are drawn to (electrophiles), and where they end up (leaving groups). This shows the overall direction of electron flow. Arrow pushing never directly depicts the motion of atoms; instead, it shows the movement of electron density, which indirectly reveals how atoms shift.

Arrow pushing is also used to describe how formal positive and negative charges are redistributed across organic molecules through resonance. However, unlike its use in actual reactions, applying arrow pushing to draw alternative resonance forms is purely a bookkeeping tool for tracking formal charges and electrons. In this case, no electrons—or electron density—actually move; it is simply a way to understand how electrons are arranged in a delocalized system.

The method has been extended to inorganic chemistry, especially for s- and p-block elements, and works well for hypervalent compounds.

**Notation**

The use of curved arrows to show electron flow in reaction mechanisms was introduced by Sir Robert Robinson in 1922. Organic chemists employ two types of arrows within molecular structures to describe electron movements. Single-barbed arrows indicate the path of a single electron, while double-barbed arrows show the movement of an electron pair. The tail of the arrow is placed at an electron source—either a lone pair on an atom or a bond between atoms, where electron density is relatively high. The head points toward an electron sink, an area of relatively low electron density.

When a bond breaks, electrons leave the bond site; this is shown by a curved arrow pointing away from the bond, with its head aimed at the next unoccupied molecular orbital. Electrons can transfer to a specific atom or to a single (sigma) bond, turning it into a double (pi) bond, but the arrow always points to a specific atom because electrons always move to a new atom when pushed. Bond formation is represented by a curved arrow pointing between two species.

For clarity, it is best to draw arrows starting from a lone pair or a σ or π bond and ending at a position that can accept an electron pair, so the reader knows exactly which electrons are moving and where they go. Bonds break where a corresponding antibonding orbital gets filled. Some authorities allow the simplification that an arrow can start at a formal negative charge that corresponds to a lone pair. However, not all formal negative charges imply a lone pair (for example, the boron in F₄B⁻), so this usage requires caution.

**Breaking of bonds**

A covalent bond in an organic molecule consists of a pair of two electrons, called an electron pair. Organic reactions proceed through the sequential breaking and forming of such bonds. Chemists recognize two ways a bond can break: homolytic cleavage and heterolytic cleavage.

**Homolytic bond cleavage**

Homolytic cleavage splits the electron pair of a bond, causing the bond to break. This is shown by two single-barbed curved arrows pointing away from the bond. The result is that each atom formerly joined by the bond retains a single unpaired electron, denoted by a dot. The movement of a single electron is indicated by a curved arrow often called a fish hook. These single-electron species are free radicals. Heat or light is needed to supply enough energy for this process.

For example, ultraviolet light causes the chlorine–chlorine bond to break homolytically. The electron pair splits, shown by two fish-hook arrows pointing from the bond toward each chlorine atom. After the reaction, both chlorine atoms are left with a single unpaired electron. This is the initiation stage of free radical halogenation.

**Heterolytic bond cleavage**

Heterolytic cleavage occurs when the electron pair from a bond moves entirely to one of the atoms that were joined. The bond breaks, forming a negatively charged species (an anion) and a positively charged species (a cation). The anion keeps the electrons from the bond, while the cation loses them. The anion usually forms on the more electronegative atom (for example, atom B), because that atom attracts electrons more strongly, leading to its negative charge.

**Acid–base reactions**

A Lewis acid–base reaction happens when a molecule with a lone electron pair (a base) donates its electrons to an electron-pair acceptor (an acid). This is shown with a curved arrow pointing from the nonbonding electron pair to the electron acceptor. In a Brønsted–Lowry acid–base reaction, arrows are used the same way and help indicate the attacking proton.