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FA2008 - CHEM 140A (Substitution and Elimination Reactions)

FA2008 - CHEM 140A (Substitution and Elimination Reactions)...

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D. D. Grove Substitution and Elimination Reactions CH 222 Page 1 of 23 Substitution/Elimination Reactions Nucleophilic Substitution Reactions Basic S N 2 Reaction CH 3 CH 2 I + KBr CH 3 CH 2 Br + K I acetone Note that the iodide is substituting for the bromide. The iodide is the nucleophile and the bromide the leaving group . The organic compound where reaction takes place is the substrate . Kinetics of Reaction s double [CH 3 CH 2 Br], observe doubling of rate s double [KI], observe doubling of rate, which leads to Rate = k[CH 3 CH 2 Br][KI] k = rate constant This suggests a biomolecular mechanism, one in which two molecules collide and then react. Understand and be able to illustrate what the above statements mean. Stereochemistry Substitution occurs with inversion of configuration. CH 3 CH 3 CH 2 H I CH 3 CH 2 CH 3 H HS HS optically active optically active Na Na I inversion of configuration Stereoelectronic Requirement The nucleophile must approach from the backside of the C-I bond. The reason for this has to do with molecular orbital considerations. Transition State CH 3 CH 3 CH 2 H I HS partial bonds transition state for S N 2 reaction Reacting carbon is sp 2 hybridized in TS. Note: TS is charged. * - * - The S N 2 reaction is a concerted process: bond-making and bond- breaking occur simultaneously. Potential Energy Diagram This is a potential energy diagram that describes the energy change during the reaction. Note that this is an exothermic process with a single maximum, the transition state. Be able to draw and label these diagrams. Consequences of Stereoelectronic Requirement (Rate Profile) H C C H 3 L H H C C H 3 L CH 3 CH 3 C C H 3 L CH 3 H C H L H > > > 1 o 2 o 3 o methyl decreasing rate fastest slowest PE reaction coordinate sm product ) G
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D. D. Grove Substitution and Elimination Reactions CH 222 Page 2 of 23 Substitution/Elimination Reactions For practical purposes, S N 2 reactions do not occur with 3 E substrates; backside attack is sterically hindered. Here is what a space-filling model of methyl iodide looks like. Note how the backside of the reacting carbon is exposed. the back side of methyl iodide is open to attack by a nucleophile. iodide leaving group Methyl iodide tert-Butyl iodide backside of carbon is sterically hndered. Notice that the back side of the reactive carbon in tert -butyl iodide is now sterically hindered by the three methyl groups; indeed, one can just barely see the back of the reactive carbon atom. An incoming nucleophile has a difficult time getting into close enough to the reactive carbon to actually react. Cyclic Substrates The table below gives the relative rates of reaction of cyclic alkyl bromides with lithium iodide in acetone. Alkyl Group
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FA2008 - CHEM 140A (Substitution and Elimination Reactions)...

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