Comparison of SN1 and SN2 Reactions

Comparison of SN1 and SN2 Reactions

Let’s compare what we know about the SN1 and SN2 Reactions and reactions, the organize this material into a brief table.

(1) Effect of the Nucleophile on SN1 and SN2 Reactions

– The nucleophile takes part in the slow step (the only step) of the SN2 reaction but not in the slow step of the SN1.

– Therefore, a strong nucleophile promotes the SN2 but not the SN1.

– Weak nucleophiles fail to promote the SN2 reaction; therefore, reactions with weak nucleophiles often go by the SN1 mechanism if the substrate is secondary or tertiary.

SN1 : Nucleophile strength is unimportant (usually weak).

SN2 : Strong nucleophiles are required

(2) Effect of the Substrate on SN1 and SN2 Reactions

– The structure of the substrate (the alkyl halide) is an important factor in determining which of these substitution mechanisms might operate.

– Most methyl halides and primary halides are poor substrates for SN1 substitutions because they cannot easily ionize to high-energy methyl and primary carbocations.

– They are relatively unhindered, however, so they make good SN2 substrates.

– Tertiary halides are too hindered to undergo SN2 displacement, but they can ionize to form tertiary carbocations.

– Tertiary halides undergo substitution exclusively through the SN1 mechanism.

– Secondary halides can undergo substitution by either mechanism, depending on the conditions.

SN1 substrates : 3° > 2°   (1° and CH3X are unlikely)

SN2 substrates : CH3X > 1° < 2°   (3° is unsuitable)

– If silver nitrate (AgNO3) is added to an alkyl halide in a good ionizing solvent, the silver ion removes the halide ion to give a carbocation.

– This technique can force some unlikely ionizations, often giving interesting rearrangements.

(3) Effect of the Solvent on SN1 and SN2 Reactions

– The slow step of the SN1 reaction involves formation of two ions.

– Solvation of these ions is crucial to stabilizing them and lowering the activation energy for their formation.

– Very polar ionizing solvents such as water and alcohols are needed for the SN1.

– The solvent may be heated to reflux (boiling) to provide the energy needed for ionization.

– Less charge separation is generated in the transition state of the SN2 reaction.

– Strong solvation may weaken the strength of the nucleophile because of the energy needed to strip off the solvent molecules.

– Thus, the SN2 reaction often goes faster in less polar solvents if the nucleophile will dissolve.

– Polar aprotic solvents may enhance the strength of weak nucleophiles.

SN1 : Good ionizing solvent required.

SN2 : May go faster in a less polar solvent.

(4) Kinetics of SN1 and SN2 Reactions

– The rate of the SN1 reaction is proportional to the concentration of the alkyl halide but not the concentration of the nucleophile. It follows a first-order rate equation.

– The rate of the SN2 reaction is proportional to the concentrations of both the alkyl halide [R-X] and the nucleophile [Nuc: ]. It follows a second-order rate equation.

SN1 rate = kr [R-X]

SN2 rate = kr [R-X] [Nuc: ]

(5) Stereochemistry of SN1 and SN2 Reactions

– The SN1 reaction involves a flat carbocation intermediate that can be attacked from either face. Therefore, the SN1 usually gives a mixture of inversion and retention of configuration.

– The SN2 reaction takes place through a back-side attack, which inverts the stereochemistry of the carbon atom. Complete inversion of configuration is the result.

SN1 stereochemistry : Mixture of retention and inversion; racemization.

SN2 stereochemistry : Complete inversion.

(6) Rearrangements on SN1 and SN2 Reactions

– The SN1 reaction involves a carbocation intermediate. This intermediate can rearrange, usually by a hydride shift or an alkyl shift, to give a more stable carbocation.

– The SN2 reaction takes place in one step with no intermediates. No rearrangement is possible in the SN2 reaction.

SN1: Rearrangements are common.

SN2: Rearrangements are impossible.

SUMMARY: Nucleophilic Substitutions

Comparison of SN1 and SN2 Reactions

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