Chapter 8 Lectures

Chapter 8 Lectures - Chapter 8 Nucleophilic Substitution...

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Chapter 8 Nucleophilic Substitution Reactions of Alkyl Halides Organic compounds in which an sp 3 hybridized carbon is bonded to an electronegative atom or group, called the leaving group , can undergo two types of reactions shown below: leaving group Y R X H R substitution Y + X (chapter 8) R elimination + H Y + X (chapter 9) In substitution reactions the leaving group is displaced by an entering atom or group (Y functions as a nucleophile). In elimination reactions (discussed in the next chapter) the leaving group is eliminated along with a proton from an adjacent carbon (Y functions as a base). This chapter focuses on substitution reactions of alkyl halides (i.e., leaving group = halide).
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Two Types of Nucleophilic Substitution Reactions – S N 2 and S N 1 There are two important types for the nucleophilic substitution reaction which are classified as S N 2 or S N 1 where the number (1 or 2) indicates whether the reaction rate depends on the concentration of both (for S N 2) or just one (for S N 1) of the reacting species. The two reaction types are summarized below, highlighting their differences: X Nu + X Nu δ δ δ + Nu + X S N 2 (concerted, bimolecular) transition state X Nu + X S N 1 (stepwise, unimolecular) + Nu + Nu slow fast k 1 k 1 carbocation intermediate Key Features Reaction Kinetics Stereochemistry Relative Rates S N 2 Rate = k 1 [R-X][Nu ] Inversion 1° > 2° > 3° S N 1 Rate = k 1 [R-X] Racemization 3° > 2° > 1°
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The reaction coordinate diagrams below correspond to the one-step (concerted) and two-step (stepwise) mechanisms of the S N 2 and S N 1 reactions. Note that the rate of the S N 1 reactions depends only on the alkyl halide (both concentration and reactivity) since only the alkyl halide is involved in the rate-determining step. R–X R–X E Rxn Coord Rxn Coord X Nu δ δ δ + + + Nu + Nu R–Nu + X R–Nu + X + Nu rate-determining step fast step + X E S N 2 Reaction S N 1 Reaction unchanged
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Nucleophilic Substitution Draw the substitution products for the following substitution reactions: OH CH 3 O Na I CH 3 S NH 3 Na CN P(CH 3 ) 3 CH 3 Cl I Br Br CH 3 Br CH 3 I CH 3 Br CH 3 CN I CH 3 NH 3 + Br CH 3 OH Cl OCH 3 I I Br SCH 3 Br Br CH 3 P(CH 3 ) 3 +
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The rate of an S N 2 reaction depends on several factors including the structure of the alkyl halide starting material, the leaving group, the nucleophile and the solvent. 1. The reaction rate is dependent on the concentration of two reactants (i.e., it is second order), BOTH the alkyl halide and the nucleophile. This means that both reactants are involved in the transition state of the RDS (i.e., a bimolecular TS) 2. The reaction rate shows dependence on the alkyl halide structure (see p.348-350): methyl halide > 1° halide > 2° halide > 3° halide no S N 2 reaction occurs at sp 2 carbons 3. The reaction of an alkyl halide with a halogen bonded to a carbon stereocenter results in inversion of configuration, i.e, this is a stereospecific reaction (see p.351): OH Br H Et Me S HO H Et Me R
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This note was uploaded on 04/21/2010 for the course CHEM 2261 taught by Professor Crowe during the Fall '08 term at LSU.

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Chapter 8 Lectures - Chapter 8 Nucleophilic Substitution...

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