15s - Alkyl halides (R-X) undergo many substitution...

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Alkyl halides (R-X) undergo many substitution reactions. R–X + - OH R–OH (alcohols) R–X + - OR R–OR (ethers) R–X + - SR R–SR (thioethers) R–X + - CN R–CN (nitriles) R–X + :NH 3 R–NH 2 (amines) R–X + - C CR R–C CR (alkynes)
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Nucleophilic Substitution Nucleophile (Nu:) = Lewis base Substitution = two groups/atoms change place Two different mechanisms (S N 1/S N 2), depends on R X Creates a new carbon-nucleophile bond Nu: - + R X R Nu + :X -
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S N 2 S substitution N nucleophilic 2 bimolecular (2 species involved in rate-limiting step) Doubling the concentration of R-X or Nu: doubles the reaction rate. Rate = k [ R-X ] [ Nu: ]
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S N 2 Mechanism S N 2 is a concerted reaction bond forming and breaking happen in the same step The nucleophile always approaches C so that Nu C X are all in the same axis. transition state
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S N 2 Stereochemistry S N 2 reactions proceed with inversion of configuration. If the carbon with the halide is a chirality center, the stereochemistry changes.
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Alkyl Halides and S N 2 S N 2 reaction rates depend on alkyl halide substitution: (fastest) methyl > 1 ° > 2 ° 3 ° (slowest)
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Steric Hindrance As RXs become more substituted, it becomes more difficult for the nucleophile to displace the halogen. Steric hindrance - blocking by atoms/groups hinders or slows a reaction
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N 1 S substitution N nucleophilic 1 unimolecular (1 species involved in rate-limiting step) Doubling the concentration of
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This note was uploaded on 08/06/2011 for the course CHEM 221x taught by Professor Broyer during the Summer '11 term at USC.

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15s - Alkyl halides (R-X) undergo many substitution...

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