6.2 How Organic Reactions Occur: Mechanisms

Reaction Mechanisms and Bond Cleavage

Process of Chemical Transformations

A reaction mechanism is a step-by-step account of how reactants become products. It shows you exactly which bonds break, which bonds form, and in what order, along with how electrons move at each stage.

Every individual step involves breaking and/or making chemical bonds. Bond-breaking removes electrons from an existing bond, while bond-making brings electrons together to form a new one. For example, in a single step you might see a C–H bond break while a C–Br bond forms simultaneously.

Along the way, mechanisms often reveal intermediate species that exist briefly between steps. Common intermediates include carbocations (carbon bearing a positive charge), carbanions (carbon bearing a negative charge), and radicals (atoms with an unpaired electron). Identifying these intermediates is critical because they determine what products you'll get.

Why does this matter? If you understand the mechanism of one reaction, you can predict the outcome of similar reactions. This same logic is what drives synthetic chemistry, from designing drug molecules to building new materials.

Heterolytic vs. Homolytic Bond Cleavage

There are two fundamentally different ways a covalent bond can break, and the distinction shapes everything about the reaction that follows.

Heterolytic cleavage splits the bond unevenly. One atom takes both bonding electrons, producing an ion pair:

• The atom that gains the electron pair becomes a negatively charged anion (e.g., $Br^-$)
• The atom that loses the electron pair becomes a positively charged cation (e.g., a carbocation, $R^+$)

Heterolytic cleavage is far more common in organic chemistry and drives polar reactions like $S_N1$, $S_N2$, $E1$, and $E2$.

Homolytic cleavage splits the bond evenly. Each atom keeps one electron, producing two radicals, each with an unpaired electron (e.g., $Cl\cdot$, an alkyl radical $R\cdot$).

Homolytic cleavage is less common overall but shows up in specific contexts like free-radical halogenation and polymerization.

Polar vs. Radical Reactions

The type of bond cleavage determines which of two major reaction categories you're dealing with.

Polar reactions arise from heterolytic cleavage. Key features:

• Electrons move in pairs, typically from a nucleophile (electron-rich species) to an electrophile (electron-poor species)
• Charged intermediates form along the pathway (carbocations, carbanions)
• These reactions dominate organic chemistry because so many organic molecules contain polarized bonds, such as $C-O$, $C-N$, and $C-X$ bonds (where $X$ is a halogen)
• A leaving group, a stable species that departs with the bonding electrons, is often involved
• Examples: nucleophilic substitution, elimination, and electrophilic addition reactions

Radical reactions arise from homolytic cleavage. Key features:

• Electrons move one at a time; each fragment keeps a single electron
• Neutral but highly reactive radical intermediates form (e.g., $HO\cdot$, $C_6H_5\cdot$)
• Less common than polar reactions because radicals are harder to control and generally less stable
• Still important in specific areas: halogenation of alkanes, polymerization of alkenes, and biological processes like lipid peroxidation

Reaction Energy Profile

A reaction coordinate diagram maps the energy changes that occur as a reaction proceeds. The x-axis tracks reaction progress and the y-axis tracks potential energy. Reactants sit on the left, products on the right, and any intermediates or transition states appear in between.

The transition state is the highest-energy arrangement of atoms along the pathway. Think of it as the top of an energy hill the reaction must climb over. You can't isolate a transition state; it's a fleeting geometry, not a stable species.

Activation energy ($E_a$) is the energy difference between the reactants and the transition state. It represents the minimum energy input needed for the reaction to proceed. A large $E_a$ means a slow reaction; a small $E_a$ means a fast one.

For multi-step reactions, each step has its own transition state and activation energy. The rate-determining step is the slowest step, which is usually the one with the highest $E_a$. This single step controls the overall rate of the entire reaction, so when you're analyzing kinetics, focus there first.