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8A2009lecture15 - Lecture 15 O r g a n i c C h e m i s tr y...

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Lecture 15 Organic Chemistry Brief 8A Which Memorial day picnic food contains a compound that participates in an S N 2 reaction with DNA, potentially leading to cancer?? A. Potato salad B. Beer C. Steak, grilled, well-done D. Veggies and dip Nucleophilic Substitution Reactions Nu + R-X R-Nu + X - Nu + R-X R-Nu + X - General Reaction: + CH 3 -Cl + Cl - OH CH 3 -OH Example: Two Types of Nucleophilic Substitution Reactions S N 1 S N 2 Substitution Nucleophilic Rate-determining step 1 is unimolecular 2 is bimolecular
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S N 2 Mechanism Nu C L R R R L R R R Nu C Nu R R R + L - Mechanism consistent with observations… (1) Nu/ Alkyl Halide involved in transition state (2) Me/Primary substrate react fastest (3) Change in stereochemistry at C L = leaving group (halide in alkyl halide ) S N 2 Mechanism Energy coordinate diagram -Steric crowding in TS raises the energy of the TS making the barrier for the reaction larger, and therefore slowing the rate of the reaction. Methyl substrate Secondary substrate Factors influencing S N 2 Reactions (1) Nature of the leaving group More stable anions are better leaving groups. More stable anions are the conjugate bases on strong acids. e.g. I - > Br - > Cl - > F - (recall halide trend is due to size, not electronegativity) E.g. CH 3 -I will react faster than CH 3 -Cl with the same Nu C-F bonds are very unreactive-- one reason why used in Teflon (--CF 2 CF 2 -- polymer).
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Factors influencing S N 2 Reactions (1) Nature of the leaving group More stable anions are better leaving groups. More stable anions are the conjugate bases on strong acids.
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