09 Advanced Reaction Mechanisms

A framework for analyzing mechanisms

A plausible mechanism must explain both the product and the route by which it forms. Start by mapping the bonds formed and broken, changes in formal charge, and any migrating atoms or groups. Then propose the simplest pathway consistent with the reaction conditions.

A concerted pathway carries out related bond changes together in one step. A stepwise pathway proceeds through one or more intermediates, with each elementary step having its own transition state. Distinguishing these possibilities helps organize mechanistic evidence.

Test a proposed pathway from three complementary angles:

1. Orbital compatibility: Can the reacting orbitals overlap with suitable phase and orientation?

2. Stereochemistry: Does the proposed geometry account for the product's spatial arrangement?

3. Kinetics: Are the proposed steps consistent with measured rate laws and isotope effects?

A rate law is determined experimentally; it cannot be read directly from the balanced overall equation. No single type of evidence usually establishes a mechanism on its own.

Rearrangements and migrating groups

In a 1,2-shift, a group migrates with its bonding electron pair to a neighboring electron-deficient center. In a Wagner–Meerwein rearrangement, for example, a carbon–carbon bond migrates toward a carbocation, and the positive charge relocates as the carbon skeleton changes. The rearrangement may favor a product with greater charge stabilization or a more favorable framework. Apparent successive shifts need not occur in one concerted event: they can happen in separate steps, each with its own transition state.

The pinacol rearrangement illustrates how migration can accompany carbonyl formation. Under acidic conditions, a hydroxyl group is protonated and leaves as water. A neighboring group migrates as oxygen donates a lone pair to form a $\mathrm{C=O}$ bond. Predicting the product requires comparing which group migrates and which pathway can reach a favorable transition state and product.

The Hofmann rearrangement converts a primary amide to an amine while losing the carbonyl carbon. Its pathway links amide bromination, rearrangement to an isocyanate, and hydrolysis.

To assess migration stereochemistry, track the migrating group's bonds and spatial orientation. A stereospecific shift preserves a defined relationship between the starting-material geometry and the product. Loss of stereochemical information may instead suggest a longer-lived or more freely rotating intermediate. These observations constrain a mechanism, but usually do not establish it alone.

Pericyclic reactions and orbital symmetry

A pericyclic reaction reorganizes bonding electrons in a concerted process through a cyclic transition state, without discrete intermediates. Orbital symmetry determines which pathways are allowed. Here, “allowed” means that orbital symmetry permits a pathway; it does not guarantee that the reaction will be fast or favorable under every set of conditions.

Electrocyclic reactions

An electrocyclic reaction closes a conjugated chain to form a ring or opens a ring to form a conjugated chain. Its terminal orbitals rotate either conrotatorily, in the same direction, or disrotatorily, in opposite directions. In thermal reactions, a system with $4\pi$ electrons closes conrotatorily, while a system with $6\pi$ electrons closes disrotatorily. Photochemical excitation reverses these modes. Following substituent geometry through the permitted motion predicts relative product stereochemistry.

Cycloadditions

In a cycloaddition, pi systems combine to form a ring. In the thermal Diels–Alder reaction, the diene and dienophile interact suprafacially and form two bonds in one step. A suprafacial–suprafacial $[2+2]$ cycloaddition is thermally symmetry-disfavored but can be enabled photochemically.

Sigmatropic rearrangements

A sigmatropic rearrangement moves a sigma bond across a pi system. Thermal $[3,3]$ shifts, including the Cope and Claisen rearrangements, proceed suprafacially on the participating systems and often transfer stereochemical information predictably. A Claisen rearrangement of an allyl vinyl ether forms a new carbon–carbon bond and an unsaturated carbonyl product.

For a pericyclic stereochemistry problem, classify the reaction, identify the electron system, and determine whether the conditions are thermal or photochemical. Then apply the selection rule for that reaction class and follow the substituents through the permitted orbital motion. Conrotation and disrotation describe electrocyclic reactions; suprafacial and antarafacial describe participating systems in cycloadditions and sigmatropic shifts.

Testing mechanisms with kinetics and stereochemistry

Mechanisms must also agree with measured reaction rates. For an elementary bimolecular step, the rate depends on both reacting species. For an $\mathrm{S_N2}$ reaction, for example, the rate is proportional to both substrate and nucleophile concentrations:

An $\mathrm{S_N2}$ substitution gives inversion at the reacting stereocenter. A concerted $\mathrm{E2}$ elimination likewise depends on both substrate and base concentrations. Its usual anti-periplanar geometry aligns the breaking $\mathrm{C-H}$ and $\mathrm{C-X}$ bonds for orbital overlap and helps determine alkene stereochemistry.

Isotope substitution offers another mechanistic test. If replacing hydrogen with deuterium substantially slows a reaction, cleavage of the $\mathrm{C-H}$ bond is likely significantly involved in the rate-controlling transition state. This kinetic isotope effect supports a proposal, but it should be considered alongside rate laws, stereochemistry, and other evidence.

The strongest explanations connect structure to pathway: orbital overlap tests whether a concerted route is feasible, stereochemistry tests its geometry, and kinetics tests which species and bond changes are involved. A useful conclusion accounts for all these observations without treating any one of them as decisive by itself.