Chem 140A Lecture Notes Winter 2011 Ch 7

Chem 140A Lecture Notes Winter 2011 Ch 7 - Chemistry 140A...

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Chemistry 140A Winter 2011 (K. Albizati) 1 Chapter 7 Further Reactions of Haloalkanes – Unimolecular Substitution and Elimination Reactions There are other fundamental reactions of haloalkanes which illustrate common reaction intermediates and pathways in organic chemistry. Solvolysis Reactions – (literally, cleavage by solvent) A reaction solvent is the liquid medium used to dissolve reactants to facilitate a chemical reaction. A solvolysis reaction involves the solvent as one of the reactants: When t-butyl bromide is placed in water, a fast reaction occurs, giving rise to an alcohol product. When water is the solvent the reaction is called a “hydrolysis.” Isopropyl bromide undergoes the same reaction but much more slowly: When methanol is the solvent, the product is methyl t-butyl ether: But there is something more here.
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Chemistry 140A Winter 2011 (K. Albizati) 2 Relative Reaction Rates These relative rates of reactions in a homologous series of alkyl bromides suggests that these reactions are not proceeding via an S N 2 mechanism. Instead the rate law for such reactions is found to be unimolecular: Rate = k[t-BuBr] in mol L -1 sec -1 This means that in the slowest step in the mechanism only t-BuBr is involved and that the nucleophilic species is not involved in the slowest step. A mechanism which is consistent with this rate law involves initial carbocation formation (heterolytic cleavage): The second step of the mechanism is a nucleophile-electrophile combination step:
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Chemistry 140A Winter 2011 (K. Albizati) 3 The third step is loss of a hydrogen ion (proton) to give the observed product: The first step must be the slow step and the subsequent two steps fast, to account for what is observed kinetically. So, the entire mechanism can be written as the individual steps: (Animated Mechanism 07010-01) We call this reaction an S N 1 reaction (substitution nucleophilic unimolecular) and the mechanism is an S N 1 mechanism.
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Winter 2011 (K. Albizati) 4 A closer look at carbonium ions: R R R Br R C R R carbonium ions are sp 2 hybridized and are trigonal planar, i.e., flat tetrahedral sp 3 hybridized R R R empty carbon 2p orbital Carbonium ions are not chiral. This has important implications later……. . Let’s take a look at the potential energy diagrams of S
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Chem 140A Lecture Notes Winter 2011 Ch 7 - Chemistry 140A...

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