Organic Reaction Mechanisms: How and Why Reactions Happen

Why Mechanisms Matter

A balanced chemical equation shows the starting materials and products of a reaction but reveals nothing about how the transformation occurs. The mechanism fills this gap by describing the electron movements, bond changes, and intermediate species that connect starting materials to products. Two reactions that look similar on paper might proceed through completely different mechanisms, leading to different stereochemical outcomes, different side products, and different responses to changes in reaction conditions.

Knowing the mechanism allows chemists to predict which conditions will accelerate a reaction, which will slow it, and which will redirect it toward different products. It explains why some reactions are stereospecific while others are not. It guides the design of new catalysts, the optimization of industrial processes, and the development of new synthetic strategies.

Curved Arrow Notation

Organic chemists use curved arrows to show electron movement during reactions. A full curved arrow (with a regular arrowhead) represents the movement of an electron pair. The arrow starts at the electron source (a lone pair, a bond, or a pi bond) and points to the electron destination (an atom or a bond being formed). A half-headed arrow (fishhook) represents the movement of a single electron, used in radical mechanisms.

Curved arrows always show electron flow, not atom movement. Electrons flow from electron-rich regions to electron-poor regions. A nucleophile (electron donor) is the source of the arrow; an electrophile (electron acceptor) is the target. In a typical bond-forming step, an arrow starts from the nucleophile lone pair and points to the electrophilic atom. In a bond-breaking step, an arrow starts from the bond being broken and points to the atom that will retain the electrons.

Proper curved arrow usage obeys conservation rules: every arrow must start from a source of electrons (lone pair, sigma bond, or pi bond), every arrow must end at an atom or between two atoms forming a new bond, and the total number of electrons must be conserved at every step. Charges must also balance. These rules constrain the possible mechanisms and make them testable predictions rather than arbitrary drawings.

Intermediates and Transition States

A reaction intermediate is a species that forms during the reaction but is not present in either the starting material or the product. Intermediates occupy energy minima on the reaction coordinate diagram. They are real, albeit short-lived, molecules or ions that can sometimes be detected or even isolated. Common intermediates in organic reactions include carbocations (positively charged carbons), carbanions (negatively charged carbons), radicals (species with unpaired electrons), and carbenes (neutral species with two nonbonding electrons on carbon).

A transition state is the highest-energy point on the pathway between a reactant and an intermediate, or between an intermediate and a product. Unlike intermediates, transition states cannot be isolated or directly observed because they exist for only a single molecular vibration (approximately 10^-13 seconds). Transition states are drawn with partial bonds (dashed lines) showing bonds in the process of forming or breaking. The energy of the transition state relative to the starting material determines the activation energy and thus the reaction rate.

The reaction coordinate diagram plots energy against reaction progress. Exergonic reactions have products lower in energy than reactants; endergonic reactions have higher-energy products. Each step in a multi-step mechanism has its own transition state. The rate-determining step is the step with the highest-energy transition state, because it acts as a bottleneck that controls the overall reaction rate.

Major Mechanism Types

Polar (ionic) mechanisms involve the movement of electron pairs between nucleophiles and electrophiles. Most of the reactions studied in introductory organic chemistry are polar mechanisms: nucleophilic substitution (SN1, SN2), elimination (E1, E2), electrophilic addition to alkenes, nucleophilic addition to carbonyls, and electrophilic aromatic substitution. In each case, the driving force is the attraction between regions of opposite charge or electron density.

Radical mechanisms involve species with unpaired electrons. They proceed through three phases: initiation (generation of radicals, usually by heat or UV light), propagation (chain reactions where radicals react with stable molecules to form products and regenerate radicals), and termination (destruction of radicals when two radicals combine). Halogenation of alkanes and radical polymerization are important radical reactions.

Pericyclic mechanisms involve the concerted reorganization of electrons in a cyclic transition state, with no intermediates and no charged species. Diels-Alder reactions, Cope rearrangements, Claisen rearrangements, and sigmatropic shifts are pericyclic reactions. Their stereochemistry and regiochemistry are governed by orbital symmetry rules (Woodward-Hoffmann rules) rather than by nucleophile-electrophile interactions.

Predicting Reaction Outcomes

To predict the products of a reaction mechanistically, start by identifying the nucleophile and electrophile (or the radical, or the diene and dienophile). Consider the stability of possible intermediates: more stable intermediates form faster. Tertiary carbocations are more stable than secondary, which are more stable than primary. Resonance-stabilized intermediates are favored over non-resonance-stabilized ones.

Consider steric effects: bulky groups slow down reactions that require close approach of reactants (like SN2) and favor reactions that proceed through less crowded pathways (like SN1 or E1). Consider electronic effects: electron-donating groups stabilize carbocations and destabilize carbanions, while electron-withdrawing groups do the opposite.

Finally, consider the thermodynamic and kinetic factors. The thermodynamic product is the most stable product. The kinetic product forms fastest. At low temperatures, kinetic control dominates and the faster-forming product predominates. At high temperatures, thermodynamic control dominates and the more stable product predominates. This distinction is particularly important in reactions like enolate alkylation and Diels-Alder reactions.