4 General Scheme for the Nucleophilic Aliphatic Substitution:
5 Substitution Reactions. Nucleophilic aliphatic substitutions at the sp3 carbon are divided into the following sub-categories:1. Nucleophilic aliphatic substitution at the sp3 carbon, bimolecular (SN2). In this case the mechanism is often (but not always) a single step and the rate determining step involves both the nucleophile and substrate.2. Nucleophilic aliphatic substitution at the sp3 carbon, unimolecular (SN1). In this case the mechanism is multi-step and the rate determining step involves only the substrate.
7 The SN2 ReactionKinetics: Rate = k [RX] [Nu:-]. Both RX and Nu:- are involved in the rate-determining step.Nucleophile: Negatively-charged (strong nucleophile) work best; occasionally neutral nucleophiles can be used.Reactivity ofalkyl halides: CH3 > 1° > 2° due to steric effects that hinder backside attack. 3° halides do not react. Allyl (CH2=CH-CH2X) and benzyl (C6H5CH2X) halides are unusually reactive (comparable to or more reactive than methyl).
8 The SN2 ReactionSolvents: Wide variety can be used, but polar, aprotic solvents are favored and usually cause 2° halides to react by this mechanism.Rearrangements: Do not occurStereochemistry: Complete inversion of configuration via pentacoordinate carbon Transition State.
19 The SN1 ReactionKinetics: Rate = k [RX]. Only RX is involved in the rate-determining step.Nucleophile: Neutral (weak nucleophile) work best.Reactivity ofalkyl halides: 3° > 2° due to the relative stabilities of carbocations that form (electronic effects). Methyl and 1° halides do not react. Allyl and benzyl halides are unusually reactive due to the exceptional stabilities of the allyl and benzyl carbocations (resonance effects).
20 The SN1 ReactionStereochemistry: Partial racemization via intimate and solvent-separated ion pairs.Rearrangements: Occur if a more stable carbocation can form.Solvents: Wide variety can be used, but polar, protic solvents are favored and usually cause 2° halides to react by this mechanism.
25 Carbocations are involved in this mechanism, with some features of carbocations being : Stability: 3° > 2°. The more stable an intermediate is, the more easily it is formed; hence the relative reactivities of and halides. Methyl and 1° carbocations are seldom formed. Allyl and benzyl carbocations form readily because of resonance stabilization of the positive charge.Rearrangements: If possible, a carbocation may rearrange to a more stable one by an alkyl group shift or a hydride shift. The most common rearrangement is from a 2° to 3° ion. Primary carbocations, if formed (and they seldom are), could rearrange to either a 2° or 3° cation.
26 Example of an SN1 reaction of a 2° alkyl halide proceeding with carbocation rearrangement.
27 The Solvents 2Polar, protic solvents (good H-bonding donors and acceptors) (Favor SN1 and elimination).ROH, H2O, RNH2, liq. NH3, CH3CO2H
29 Elimination. The E2 Reaction The E2 reaction is a concerted reaction, with all bonds being made and broken at the same time.
30 Elimination. The E2 Reaction Kinetics: Rate = k [RX] [B:-]. Both RX and B:- are involved in the slow, rate-determining step.The base: Strong bases, such as KOH/alcohol or NaOCH2CH3 (NaOEt), are required.Reactivity ofalkyl halides: 3° > 2° > 1°. With a strong base, alkyl halides undergo elimination by the E2 reaction.Rearrangements: Do not occur
32 Elimination. The E2 Reaction Solvents: Reaction can occur in a variety of solvents. The use of a strong base is the driving force for elimination to occur.Reactants: 1) Must have one or more b hydrogens.2) When the eliminated H and the leaving group are anti to one another and in the same plane as the two carbons to which they are attached the elimination is fast. Otherwise, the gauche elimination is slow.Products: If two or more alkenes are produced, the one having the greater or greatest number of carbon-containing substituents attached to the C=C bond is usually the major product. The reason for this trend is based upon the stabilities of various alkenes, with the more or most stable alkene being formed more easily and thus is the major product.
37 Elimination. The E1 Reaction The E1 reaction occurs in several steps, with formation of the carbocation being involved in the first, rate-determining step
38 Elimination. The E1 Reaction Kinetics: Rate = k [RX]. Only RX is involved in the slow, rate-determining step.The base: Weakly basic substances, such as H2O, CH3OH, and CH3CH2OH can cause elimination to occur by the E1 reaction.Reactivity ofalkyl halides: 3° > 2°. 1° halides usually do not undergo E1 reaction. Note: If a strong base is present, E2 reaction occurs.
40 Elimination. The E1 Reaction Rearrangements: If possible, a carbocation may rearrange to a more stable one by an alkyl group shift or a hydride shift. The most common rearrangement is from a 2° to 3° ion. Primary carbocations, if formed (and they seldom are), could rearrange to either a 2° or 3° cation. Rearrangement occurs more rapidly than does the loss of a proton from a carbocation, so rearrangement products usually predominate.Solvents: Reaction is favored in polar, protic solvents.Reactants: 1) Must have one or more b hydrogens.2) Unlike the E2 reaction, no special geometry is required.