Reaction Mechanism · aiwiki.page

A reaction mechanism is a detailed account of how a chemical reaction proceeds from reactants to products. It describes the elementary events involved, the breaking and formation of chemical bonds, and the structures and properties of intermediates and transition states. Unlike an overall chemical equation, which records the net transformation, a mechanism explains the molecular process underlying it. An acceptable mechanism must agree with reaction stoichiometry, measured rates, and other experimental evidence; several mechanisms may remain compatible with incomplete data. (goldbook.iupac.org)

Elementary steps and intermediates

An elementary reaction is a single chemical event within a mechanism. A reaction may consist of one elementary step or a sequence of steps. Adding the step equations and cancelling species that occur on both sides must reproduce the overall equation. A reaction intermediate is formed during the process and subsequently consumed, so it normally disappears from the net equation. (openstax.org)

The molecularity of an elementary step counts its participating reactant entities. Unimolecular steps involve one entity, bimolecular steps two, and termolecular steps three. These entities can be molecules, atoms, or ions. Molecularity characterizes an elementary event, not an arbitrary overall reaction assembled from several events. (openstax.org)

For example, an elementary step $A+B\rightarrow I$ has a mass-action rate expression $v=k[A][B]$ . Here $k$ is the rate constant and brackets denote molar concentrations. However, the coefficients of an overall reaction generally do not determine its rate law, because intermediate steps may produce a different concentration dependence. (openstax.org)

Electron movement and structural representation

In organic chemistry, mechanisms are commonly represented using structural formulas and curved arrows. A full-headed curved arrow indicates the redistribution of an electron pair; a half-headed, or fishhook, arrow indicates the movement of one electron. The arrow begins at the electrons’ source and points toward their destination, rather than indicating the physical movement of an atom. (openstax.org)

In polar reactions, a nucleophile supplies an electron pair to an electrophile. Bond cleavage may be heterolytic, with both bonding electrons assigned to one fragment, or homolytic, with one electron assigned to each fragment. These representations distinguish electron-pair processes from processes involving unpaired electrons and specify the proposed sequence of bond changes. They are mechanistic models, not direct recordings of electron trajectories. (openstax.org)

Transition states and energy barriers

A transition state is the configuration associated with passage across a reaction barrier. Bonds may be partially formed or broken in this configuration. It is distinct from an intermediate: an intermediate can occur between successive elementary steps, whereas a transition state represents passage through an individual step. Reaction-energy diagrams depict these barriers and help distinguish single-step from multistep pathways. (openstax.org)

The activation energy describes the temperature dependence of a rate constant in the Arrhenius equation. Free-energy descriptions instead compare reactants with the transition state through an activation Gibbs free energy, $\Delta G^\ddagger$ . These quantities concern reaction kinetics rather than the overall thermodynamic favorability of product formation: a favorable transformation can still face a substantial reaction barrier. (openstax.org)

Kinetics and rate control

Chemical kinetics connects a proposed mechanism with measurable changes in concentration over time. Elementary rate expressions are combined to obtain predictions for the overall reaction. Intermediate concentrations may require elimination through additional relationships, such as a rapidly established chemical equilibrium preceding a slower product-forming step. The assumptions used in such a derivation form part of the model and must be appropriate to the experimental conditions. (openstax.org)

A rate-determining step is often introduced as the slow step controlling a reaction. More precisely, it is an elementary step whose rate constant influences the overall rate more strongly than other rate constants. This definition avoids treating “slowest” as a universal comparison independent of concentrations and reversibility. Where several steps exert comparable influence, assigning one uniquely controlling step is not justified. (goldbook.iupac.org)

Example: bimolecular nucleophilic substitution

The S<sub>N</sub>2 reaction illustrates how structural and kinetic evidence jointly support a mechanism. Hydroxide reacts with bromomethane according to

\mathrm{OH^-+CH_3Br\rightarrow CH_3OH+Br^-}.

Its rate depends on both reactants:

v=k[\mathrm{OH^-}][\mathrm{CH_3Br}].

The mechanism consists of one concerted step: the nucleophile approaches carbon opposite the departing group, while the new carbon–oxygen bond forms and the carbon–bromine bond breaks. There is a transition state but no intervening intermediate. (openstax.org)

For an appropriate stereogenic carbon, this backside approach produces inversion of configuration. Agreement between the predicted concentration dependence and the observed spatial rearrangement provides stronger support than either observation alone. The example also demonstrates that a reaction equation, rate law, and structural mechanism supply different kinds of information about the same transformation. (openstax.org)

Experimental evidence and catalysis

Mechanistic proposals are tested against multiple observations, including product identity, reaction rates, and stereochemical outcomes. Isotopic substitution supplies another probe: a kinetic isotope effect is a change in a rate constant caused by replacing an atom with one of its isotopes. In multistep reactions, the observed effect can reflect a rate-controlling step or an equilibrium preceding it, so interpretation requires the complete kinetic context. (goldbook.iupac.org)

Catalysis changes the available mechanism by providing an alternative pathway with more favorable kinetic barriers. A catalyst may participate in intermediate-forming steps but is regenerated during the cycle. Catalyzed and uncatalyzed pathways need not contain the same number of elementary steps, even though they produce the same net transformation. Consequently, a mechanistic description must specify whether catalytic species participate and how their regeneration occurs. (openstax.org)