SN1 and SN2 Reactions • Do not occur with vinyl halides or aryl halides. • Consider the carbocation formed for SN1. • Consider the backside attack for SN2.
Elimination Reactions • The substitution reaction mechanisms you have learned are just part of the picture. • In the SN1 and SN2 reactions, the species that acts as a nucleophile may also act as a base and abstract a proton. This causes the elimination of HX and the formation of an alkene. • An elimination reaction can occur along with the substitution reaction.
Elimination B:- is a species acting as a base. Since HX is lost, this particular reaction is called a dehydrohalogenation.
E1 Reactions • E1 = elimination, unimolecular • Rate = k[substrate] (a first order process) • The rate-limiting step is the formation of the carbocation, the same as for SN1 (that’s why E1 competes).
E1 Reaction Mechanism • Step 1: The substrate forms a carbocation intermediate (rate-limiting step). • Step 2: Methanol acts as a base and removes H+(fast step).
E1 Reaction Profile rate = k[(CH3)3Br] -EA(step 1)/RT k = Ae
E1 Occurs with and Competes with SN1 • When bromocyclohexane is heated with methanol, two products are possible. • Can you draw the mechanism that leads to each product?
E1 Reactions • E1 reactions are exothermic. • E1 reactions occur in at least two steps and compete with SN1 reactions. • The first step is slow.It is the formation of the carbocation intermediate. • The second step is fast. It is the abstraction of H+ from the carbocation by the base.
Factors Affecting E1 Reactions • Structure of the substrate • Can a stable carbocation be formed? • Strength of the base • Nature of the leaving group • The solvent in which the reaction is run. • Must be able to stabilize the carbocation and the LG (which is usually an ion).
Factors Affecting E1 Reactions - Structure of the Substrate • The most important factor influencing the rate of E1 reactions is the stability of the carbocation formed. • Stability of carbocation: 3° > 2° >1° • Relative rates for E1: 3°>2°≈1°(resonance)
Factors Affecting E1 Reactions - Strength of the Base • The rate is not much affected by the strength of the base. Weak bases will work.
Factors Affecting E1 Reactions - the Leaving Group • The LG should be good.
Factors Affecting E1 Reactions - Solvent Effects • The solvent must be capable of dissolving both the carbocation and the leaving group. • E1 reactions require highly polar solvents that strongly solvate ions. • Typical solvents: water, an alcohol, acetone (to help the alkyl halide to dissolve), or a mix.
Rearrangements in E1 Reactions • The carbocation can undergo a structural rearrangement to produce a more stable species. • hydride shift (~H) • methyl shift (~CH3) • If ionization would lead to a 1° carbocation, look for a rearrangement to occur. • Ionization rates can be increased by the addition of reagents such as AgNO3 (how?); however, Ag is not cheap.
A Hydride Shift Can Occur in Either SN1 or E1 Reactions • What would the E1 products be? CH3OH ~H -H+ -Br - SN1
A Methyl Shift Can Occur in Either SN1 or E1 Reactions • What would the E1 product be? CH3OH ~CH3 -H+ SN1
Zaitsev’s Rule • When two or more elimination products are possible, the product with the more substituted double bond will predominate.
Zaitsev’s Rule • When two or more elimination products are possible, the product with the more substituted double bond will predominate. • Alkyl groups are electron-donating and contribute electron density to the πbond. • Bulky groups can be more widely separated.
E1 Reactions - Summary • The structure of the carbocation is the most important factor: • Relative rates for E1: 3°>2°. • The base is typically weak or moderate in strength. • The LG should be good. • The solvent should be polar and protic to stabilize the carbocation and LG. • Products can exhibit rearrangements and will follow Zaitsev’s Rule.