chapter9 - 9 Elimination Reactions Assigned Problems...

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9 Elimination Reactions Assigned Problems 9.1-20, 22-28 9.1 General reaction An elimination reaction occurs when a proton and leaving group are lost from adjacent carbons, as discussed above in 8.14. The reaction is termed a 1,2-elimination. If we number the carbon attached to the leaving group 1 the proton is lost from carbon 2. 9.2 Bimolecular Reaction The reaction of ethoxide ion with tert -butyl bromide in ethanol yields the elimination product 2-methylpropene. This reaction follows a second order rate law: Rate – k[EtO - ][t-BuBr] The rate law is consistent with a one-step or concerted mechanism and is typically referred to as a bimolecular elimination of E2 reaction. The E2 reaction often competes with the S N 2 reaction. The mechanism of the E2 reaction involves simultaneous breakage of the C-H and C-Br bonds and formation of the C=C and O-H bonds. Example Carbon-Deuterium bonds are stronger than C-H bonds. When the cleavage of a C-H(D) bond occurs during the rate limiting step of a reaction the reaction is slower for D than H. This is called a kinetic isotope effect. The presence of a kinetic isotope effect in E2 reactions supports concerted C-H and C-Br cleavages.
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9.3 Stereochemistry of the E2 Reaction Reactions that have several bonds are made a broken simultaneously usually have strict stereoelectronic requirements. This is due to the need for appropriate orbital overlap throughout the reaction. In the E2 reaction the sp 3 orbitals of the C-H and C-LG sigma bonds begin to form a π -bond and thus must be coplanar. There are two possible orientations for coplanarity –syn-periplanar and anti-periplanar. Figure 9.1 The syn-periplanar conformation has all of the bonds eclipsed while the anti-periplanar conformation has all of the bonds staggered, and is thus more stable. The preference for the
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This note was uploaded on 04/17/2009 for the course CHM 227 taught by Professor Rosen during the Fall '08 term at Rhode Island.

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chapter9 - 9 Elimination Reactions Assigned Problems...

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