Chemistry · Ch 12 — Organic Chemistry – Some Basic Principles and Techniques

Electron Movement in Organic Reactions

12.7.3

Electron Movement in Organic Reactions

Organic reactions involve the breaking and forming of covalent bonds. Both processes depend on the movement of electrons — either as pairs or as single electrons. Understanding how to represent this movement is essential for predicting reaction mechanisms.

Curved-Arrow Notation

The movement of electron pairs in organic reactions is shown using curved-arrow notation. A curved arrow begins at the point from which an electron pair is shifted and ends at the location to which that pair moves. The arrowhead points to the new position of the electron pair.

Note

The curved arrow represents the movement of electrons, not atoms. It shows how bonding changes due to electronic redistribution during the reaction.

Three common patterns for shifting an electron pair are:

From a $\pi$ bond to an adjacent bond position — the electron pair moves from a multiple bond into a bonding position between two atoms.
From a $\pi$ bond to an adjacent atom — the electron pair moves from a multiple bond onto a single atom, creating a lone pair or a negative charge.
From an atom to an adjacent bond position — a lone pair on an atom moves to form a new bond with an adjacent atom.

Movement of a Single Electron

When only one electron moves (as in free-radical reactions), a different arrow is used. A single barbed curved arrow, often called a "fish-hook" arrow or half-headed arrow, indicates the movement of a single electron.

Watch out

Do not confuse the full-headed curved arrow (for electron pairs) with the half-headed fish-hook arrow (for single electrons). Using the wrong arrow in a mechanism is a common error.

Examples of Electron Movement

Example 1: Formation of methanol from hydroxide ion and bromomethane

The hydroxide ion ( $\mathrm{HO}^-$ ) has a lone pair of electrons on oxygen. This lone pair attacks the carbon atom of bromomethane ( $\mathrm{CH_3Br}$ ), forming a new $\mathrm{C-O}$ bond. Simultaneously, the $\mathrm{C-Br}$ bond breaks, and the electron pair from that bond moves to the bromine atom, forming bromide ion.

\mathrm{HO}^- + \mathrm{CH_3-Br} \longrightarrow \mathrm{CH_3OH} + \mathrm{Br}^-

The curved arrow starts at the lone pair on oxygen and ends at the carbon atom. A second curved arrow starts at the $\mathrm{C-Br}$ bond and ends at the bromine atom.

Example 2: Dissociation of chloromethane into free radicals

When chloromethane ( $\mathrm{CH_3Cl}$ ) undergoes homolytic cleavage, the $\mathrm{C-Cl}$ bond breaks such that each atom gets one electron from the bond. This is a single-electron movement.

\mathrm{CH_3-Cl} \longrightarrow \mathrm{\cdot CH_3} + \mathrm{\cdot Cl}

Here, a fish-hook arrow (half-headed) is used to show the movement of a single electron from the bond to each atom.

Tip

In exam problems, always check whether the reaction involves heterolytic cleavage (electron-pair movement, full-headed arrow) or homolytic cleavage (single-electron movement, fish-hook arrow). The type of arrow tells you the mechanism.

Key Points to Remember

Curved arrows show the flow of electrons, not the movement of atoms. …

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