Figure: 09-00-02UN Title: Substitution versus elimination. Caption: In a substitution reaction, one group is replaced with another group. In an elimination reaction, the product is an alkene.

Figure: 09-00-03UN Title: E2 reaction. Caption: This is an example of an E2 reaction. This reaction is bimolecular. The mechanism shows that this reaction occurs in one step with the base removing the proton from a carbon that is adjacent to the carbon bonded to the halogen.

Figure: 09-00-04UN Title: The rate of an E2 reaction. Caption: The rate of an E2 reaction depends on the concentrations of both the alkyl halide and the hydroxide ion.

Figure: 09-00-05UN Title: Mechanism of the E2 reaction, I. Caption: The base removes a proton from a carbon that is adjacent to the carbon bonded to the halogen. As the proton is removed, the electrons that it shared with carbon move toward the adjacent carbon that is bonded to the halogen. As these electrons move toward the carbon, the halogen leaves, taking its bonding electrons with it.

Figure: 09-00-06UN Title: Mechanism of the E2 reaction, II. Caption: An elimination reaction is initiated by removing a proton from a b-carbon, which is called b-elimination.

Figure: 09-00-07UN Title: Relative reactivities of alkyl halides in E2 reactions. Caption: Alkyl iodides are the most reactive and alkyl fluorides are the least reactive in an E2 reaction.

Figure: 09-00-09UN Title: E2 elimination of 2-bromopropene to yield propene. Caption: An E2 reaction is regioselective. The proton can be removed from either b-carbon.

Figure: 09-00-10UN Title: Elimination reaction of 2-bromobutane to yield 2-butene and 1-butene. Caption: In the case of 2-bromobutane the two  carbons are not identical so three products can be formed. The major product will be the one that forms the more stable alkene.

Figure: 9.1 Title: Figure 9.1. Reaction coordinate diagram for an E2 reaction of 2-bromobutane and methoxide ion. Caption: The transition state has an alkene-like structure and any factors that stabilize an alkene will also stabilize the transition state.

Figure: 09-01-01.1UN Title: Transition states in the E2 reaction of 2-bromobutane. Caption: The more stable transition state will be formed.

Figure: 09-01-02UN Title: The reaction of 2-bromo-2-methylbutane with hydroxide yields 2-methyl-2-butene and 2-methyl-1-butene. Caption: The more substituted alkene is formed in a higher yield.

Figure: 09-01-03UN Title: E2 reaction of 2-chloropentane with hydroxide yields cis-2-pentene, trans-2-pentene, and 1-pentene. Caption: Zaitsev predicted that the more substituted alkene is obtained when a proton is removed from the b-carbon that is bonded to the fewest hydrogens.

Figure: 09-01-04UN Title: Relative reactivities of alkyl halides in an E2 reaction. Caption: A tertiary alkyl halide is more reactive than a secondary alkyl halide, which is more reactive than a primary alkyl halide.

Figure: 09-01-06UN Title: E2 reactions to form dienes. Caption: The major product of each reactant is the conjugated alkene because it is the most stable.

Figure: 09-01-07UN Title: Reaction of 2-bromo-2-methylbutane with tert-butoxide ion yields 2-methyl-2-butene and 2-methyl-1-butene. Caption: In the presence of a bulky base, the less stable alkene is the major product.

Figure: 09-01-09UN Title: Reaction of 2-iodobutane with tert-butoxide ion yields cis-2-butene, trans-2-butene, and 1-butene. Caption: If the alkyl halide is not sterically hindered and the base is only moderately hindered, the major product will be the more substituted alkene.

Figure: 09-01-14UN Title: Relative stabilities of carbocations. Caption: A tertiary carbocation is the most stable. A methyl cation is the least stable.

Figure: 09-01-15UN Title: Relative stabilities of carbanions. Caption: The tertiary carbanion is the least stable. The methyl anion is the most stable.

Figure: 09-01-28UN Title: Mechanism of the E1 reaction. Caption: The alkyl halide dissociates, forming a carbocation. The base removes a proton from the b-carbon.

Figure: 09-01-29UN Title: Orbital picture of a carbocation stabilized by hyperconjugation. Caption: Hyperconjugation drains electron density from the C-H bond, thereby weakening it.

Figure: 09-01-30UN Title: E1 elimination of 2-chloro-2-methylbutane with water yields 2-methyl-2-butene and 2-methyl-1-butene. Caption: The major product is generally the more substituted alkene.

Figure: 9.2 Title: Figure 9.2. Reaction coordinate diagram for the E1 reaction of 2-chloro-2-methylbutane. Caption: The major product is the more substituted alkene because its greater stability causes the transition state leading to its formation to be more stable.

Figure: 09-02-01UN Title: Relative reactivities of alkyl halides in an E1 reaction. Caption: A tertiary benzylic alkyl halide is the most stable in an E1 reaction, whereas the vinyl alkyl halide is the least stable.

Figure: 09-02-02UN Title: Relative reactivities of alkyl halides in an E1 reaction. Caption: In an E1 reaction, the alkyl iodide is the most reactive and the alkyl fluoride is the least reactive.

Figure: 09-02-03UN Title: Reaction of 3-chloro-2-methyl-2-phenylbutane with methanol gives 2-methyl-3-phenyl-2-butene. Caption: Since the E1 reaction forms a carbocation intermediate, rearrangement may occur to form a more stable carbocation. The secondary carbocation rearranges to give a tertiary benzylic cation.

Figure: 09-02-04UN Title: Reaction of 5-bromo-2-heptene with methanol to give 2,4-heptadiene. Caption: The secondary carbocation formed initially undergoes a 1,2-hydride shift to become a more stable secondary allylic cation.

Figure: 09-02-05UN Title: Elimination from an alkyl halide to form an alkene. Hydrohalogenation of an alkene to make an alkyl halide. Caption: The elimination requires a base ro remove a proton from the carbocation to form the alkene. The addition reaction requires an acid to react with the nucleophilic alkene to form the carbocation.

Figure: 09-02-15UN Title: Elimination reaction of 2-bromopropane gives cis-2-pentene, trans-2-pentene, and 1-pentene. Caption: E2 and E1 reactions are regioselective and stereoselective.

Figure: 09-02-16UN Title: Eclipsed and staggered conformations of the H and X in an alkyl halide. Caption: Syn-periplanar is an eclipsed conformation. Anti-periplanar is a staggered conformation.

Figure: 09-02-17UN Title: Sawhorse projections of the alkyl halide. Caption: Syn elimination is front-side attack. Anti elimination is back-side attack.

Figure: 09-02-18UN Title: An E2 reaction is stereoselective. Caption: More of one stereoisomer is formed than the other.

Figure: 09-02-19UN Title: Cis and trans products in an E2 reaction. Caption: E2 is stereoselective. If the reactant has two hydrogens bonded to the b-carbon, both the E and Z products will be formed.

Figure: 09-03-01UN Title: E1 elimination of 3-bromo-2,2,3-trimethylpentane with ethoxide yields (E)-3,4,4-trimethyl-2-pentene and (Z)-3,4,4-trimethyl-2-pentene. Caption: The E isomer is formed in the greater yield.

Figure: 09-03-02UN Title: If only one b-hydrogen is present, then only one conformer is formed. Caption: Only one conformer would be present to allow anti elimination.

Figure: 09-03-06UN Title: Calculation of equilibrium constant for an E2 reaction. Caption: The reaction is faster if the Keq is large and slower if Keq is small.

Figure: 09-03-07UN Title: Elimination of 3-chloro-3,4-dimethylhexane to form (E)-3,4-dimethyl-3-hexene and (Z)-3,4-dimethyl-3-hexene. Caption: The major product has the bulkiest groups in the trans positions.

Figure: 13 Title: Problem 13a -- solved. Give the major product formed when the alkyl halide undergoes an E1 reaction, II. Caption: Predict the regiochemistry of the reaction.

Figure: 09-03-14UN Title: Elimination reactions of substituted cyclohexanes. Caption: For two groups on a cyclohexane ring to be parallel, both must be in the axial position.

Figure: 09-03-15UN Title: The less stable conformation of cyclohexane undergoes an E2 reaction. Caption: The H and Cl are parallel if the Cl is in the axial position, so an E2 reaction can occur.

Figure: 09-03-17UN Title: E2 reaction of neomenthyl chloride. Caption: For neomenthyl chloride, the most stable conformation undergoes an E2 reaction.

Figure: 09-03-18UN Title: E2 reaction of menthyl chloride. Caption: The less stable conformation of menthyl chloride undergoes an E2 reaction.

Figure: 09-03-23UN Title: For trans-1-chloro-2-methylcyclohexane, the hydrogen that is removed from the b-carbon must be in the axial position. Caption: The b-carbon has a hydrogen that has the lowest number of hydrogens attached, but it is not in the axial position.

Figure: 09-03-24UN Title: For E1 reactions of alkyl cyclohexanes, the groups do not have to be in the axial positions. Caption: An intermediate carbocation is formed.

Figure: 09-03-36UN Title: Primary alkyl halides under SN2 and E2 conditions. Caption: A primary alkyl halide can undergo both a substitution or an elimination reaction when reacted with a good nucleophile such as the methoxide ion. The SN2 product is favored in the case of a primary alkyl halide.

Figure: 09-03-37UN Title: Sterically hindered primary alkyl halides in SN2 and E2 reactions. Caption: The nucleophile will have a difficult time getting to the back side of the -carbon. Elimination is the favored reaction.

Figure: 09-03-38UN Title: Reaction of a sterically hindered nucleophile/base with a primary alkyl halide. Caption: The sterically hindered nucleophile will have a difficult time getting to the back side of the -carbon. Elimination is the favored reaction.

Figure: 09-03-39UN Title: Secondary alkyl halide under SN2/E2 conditions. Caption: A secondary alkyl halide can undergo both substitution and elimination under these conditions.

Figure: 09-03-40UN Title: The relative amounts of substitution and elimination products are affected by temperature. Caption: A higher temperature favors elimination.

Figure: 09-03-41UN Title: Tertiary alkyl halides under SN2/E2 conditions. Caption: A tertiary alkyl halide is the most reactive under E2 conditions and the least reactive under SN2 conditions.

Figure: 09-03-43UN Title: SN1 versus E1 conditions. Caption: In an SN1 or E1 reaction, a poor nucleophile or base is used.

Figure: 9.1 Title: Table 9.1. Effect of the steric properties of the base on the distribution of products in an E2 reaction. Caption: For a tertiary alkyl halide, a bulky base produces a higher yield of the less substituted alkene.

Figure: 9.2 Title: Table 9.2. Products obtained from the E2 reaction of methoxide ion and 2-halohexanes. Caption: Hydrogen fluoride produces the highest yield of 1-hexene; hydrogen iodide gives the highest yield of 2-hexene.

Figure: 9.3 Title: Table 9.3. Summary of the reactivity of alkyl halides in elimination reactions. Caption: Primary alkyl halides undergo E2. Secondary and tertiary alkyl halides undergo E1 and E2.

Figure: 9.4 Title: Table 9.4. Stereochemistry of substitution and elimination reactions. Caption: Products are given for SN1, SN2, E1, and E2 reactions.

Figure: 9.5 Title: Table 9.5. Relative reactivities of alkyl halides. Caption: In an SN2 reaction, the reactivity of alkyl halides follows the trend 1º > 2º > 3º. For SN1, E1, and E2 reactions, the reactivity of alkyl halides follows the trend 3º > 2º > 1º.

Figure: 9.6 Title: Table 9.6. Summary of the products expected in substitution and elimination reactions. Caption: Primary alkyl halides react by an SN2 mechanism. Secondary and tertiary alkyl halides can undergo substitution and elimination.