Allylic and Benzylic Reactivity: Carbocations, Radicals, Anions, and SN2 Reactions

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Allylic and Benzylic Reactivity

Introduction

Allylic and benzylic positions in organic molecules are characterized by their proximity to double bonds (allylic) or aromatic rings (benzylic). These positions exhibit unique reactivity due to resonance stabilization, which influences the behavior of carbocations, radicals, anions, and the rates of substitution and elimination reactions.

Allylic and Benzylic Positions

Definitions and Structural Features

Allylic group: A group attached to a carbon atom adjacent to a carbon-carbon double bond.
Benzylic group: A group attached to a carbon atom adjacent to a benzene ring or substituted benzene ring.
These positions are unusually reactive, and certain reactions occur preferentially or exclusively at these sites.

Structures of allylic and benzylic positions

Reactions Involving Allylic and Benzylic Carbocations

Resonance Stabilization of Carbocations

Carbocations at allylic and benzylic positions are stabilized by resonance, which delocalizes the positive charge over multiple atoms, lowering the energy of the intermediate and increasing reactivity.

Allylic carbocation: The positive charge is delocalized over two terminal carbons.
Benzylic carbocation: The positive charge is delocalized over the aromatic ring, resulting in several resonance structures.

Resonance structures of the allyl cation Resonance structures of the benzyl cation

MO Theory and Charge Distribution

Molecular orbital (MO) theory and electrostatic potential maps (EPM) confirm the delocalization of positive charge in allylic and benzylic carbocations, as predicted by resonance structures.

Electrostatic potential maps of allyl and benzyl cations

Comparison of Solvolysis Rates

The enhanced stability of allylic and benzylic carbocations leads to much faster SN1 solvolysis rates for allylic and benzylic alkyl halides compared to non-allylic/non-benzylic analogs. This is due to the lower activation energy required to form the stabilized carbocation intermediate.

Alkyl Chloride	Relative Rate (SN1)
tert-butyl chloride	1.0
tert-cumyl chloride	620
benzhydryl chloride	200*
trityl chloride	>600,000

*In 80% aqueous ethanol.

Resonance-stabilized carbocation from tert-cumyl chloride tert-butyl chloride ionization

Effect of Electron-Donating Substituents

Electron-donating groups (EDGs), especially in the ortho or para positions on the aromatic ring, further stabilize benzylic carbocations and enhance reaction rates. For example, a para-methoxy group increases the solvolysis rate dramatically.

Effect of para-methoxy group on solvolysis rate

Delocalization Involving Heteroatoms

Delocalization of lone pairs from heteroatoms (such as oxygen) into the carbocation center further stabilizes the intermediate. This occurs via overlap of the oxygen 2p orbital with the carbon 2p orbitals of the carbocation.

Overlap of oxygen and carbon 2p orbitals

Regioisomer Formation

For allylic carbocations, the positive charge is shared between two terminal carbons, allowing for the formation of regioisomeric products after reactions such as hydration.
For benzylic carbocations, regioisomer formation does not occur because only aromatic products are stable; non-aromatic products are not favored.

Reactions Involving Allylic and Benzylic Radicals

Resonance Stabilization of Radicals

Allylic and benzylic radicals are stabilized by resonance, similar to carbocations. The unpaired electron is delocalized over the π system, making these radicals more stable and easier to generate than non-allylic/non-benzylic radicals.

Resonance structures of the allyl radical Resonance structures of the benzyl radical

Bond Dissociation Energies

The bond dissociation energy (BDE) for removing an allylic or benzylic hydrogen is significantly lower than for non-allylic/non-benzylic hydrogens, reflecting the increased stability of the resulting radicals.

Bond dissociation energies for allylic and non-allylic hydrogens

Generation of Benzylic and Allylic Radicals

Benzylic hydrogens are readily abstracted during radical halogenation reactions, such as bromination with Br2 under light.

Benzylic bromination of cumene Initiation step of benzylic bromination Propagation step of benzylic bromination

Allylic Halogenation and Competing Reactions

Allylic halogenation can occur via radical substitution, but addition to the double bond is a competing reaction. Careful control of reaction conditions is required to favor substitution over addition.

Addition vs. substitution in allylic bromination Addition product in allylic bromination Substitution product in allylic bromination

N-Bromosuccinimide (NBS) in Allylic and Benzylic Bromination

NBS is a reagent that allows for controlled, low-concentration bromination at allylic and benzylic positions, minimizing unwanted addition reactions.

NBS bromination of cyclohexene

Mechanism of NBS Bromination

Initiation: Homolytic cleavage of the N–Br bond generates a bromine atom.
Propagation: (1) Bromine atom abstracts an allylic hydrogen, (2) HBr reacts with NBS to produce Br2, (3) Allylic radical reacts with Br2 to form the brominated product.

Initiation step of NBS bromination Propagation steps of NBS bromination

Reactions Involving Allylic and Benzylic Anions

Resonance Stabilization of Anions

Allylic and benzylic anions are stabilized by resonance, which delocalizes the negative charge over the π system. This stabilization is reflected in their lower pKa values compared to non-allylic/non-benzylic analogs.

Resonance structures of the allyl anion Resonance structures of the benzyl anion

pKa Values and Acidity

The enhanced stability of allylic and benzylic anions is reflected in their pKa values. For example, the pKa of propene is ~43, and that of toluene is ~41, both much lower than ordinary alkanes (pKa ~55).

Deprotonation of propene (pKa ~43) Deprotonation of toluene (pKa ~41)

Allylic Grignard Reagents and Rearrangement

Grignard reagents formed from allylic halides exhibit rapid equilibration, with the MgBr group moving between two partially negative carbons. This is an example of allylic rearrangement, where two distinct structures are in equilibrium (not resonance structures).

Equilibration of allylic Grignard reagents Hybrid structure of allylic carbanion Allylic rearrangement

Formation of Grignard Reagents from Different Alkyl Halides

Different, but related, alkyl halides can produce the same Grignard reagent due to allylic rearrangement.

Formation of Grignard reagents from different alkyl halides

Product Mixtures from Unsymmetrical Allylic Grignard Reagents

Unsymmetrical allylic Grignard reagents produce a mixture of products, regardless of which alkyl halide is used to form the reagent.

Mixture of products from unsymmetrical allylic Grignard reagent

Elimination and Substitution at Allylic and Benzylic Positions

E2 Eliminations

E2 elimination reactions are favored over SN2 at allylic and benzylic positions due to the stabilization of the developing carbanion character in the transition state. Factors favoring elimination include branching at the α- or β-carbon and greater acidity of β-hydrogens.

E2 elimination at benzylic position

Summary of Factors Favoring E2 over SN2

Branching at the α-carbon
Branching at the β-carbon
Greater acidity of β-hydrogens

SN2 Reactions at Allylic and Benzylic Positions

SN2 reactions are also enhanced at allylic and benzylic positions due to stabilization in the transition state, often attributed to overlap of 2p orbitals.

SN2 reaction at allylic position SN2 reaction at benzylic position SN2 reaction at non-allylic/non-benzylic position

Transition State Stabilization in SN2 Reactions

The rate enhancement in SN2 reactions at allylic and benzylic positions is credited to stabilization in the transition state, where p-orbital overlap allows for delocalization of charge.

Transition state stabilization in SN2 reactions

Additional info: This summary omits allylic and benzylic oxidation and biosynthesis of terpenes and steroids, as directed by the source material. All images included are directly relevant to the explanations provided and reinforce key mechanistic and structural concepts.