Lecture 15 - polarity think back to our discussion on...

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How do you choose between S N 1 and S N 2? 1. primary alkyl halides are ONLY involved in S N 2 reactions and tertiary alkyl halides are ONLY involved in S N 1 reactions. 2. The real problem is choosing S N 2 vs S N 1 for secondary alkyl halides, benzylic alkyl halides and allylic alkyl halides. Consider: a) rate laws S N 1 rate [alkyl halide] S N 2 rate [alkyl halide][nucleophile] Therefore if you increase the concentration of nucleophile this will increase the rate of S N 2 and have no effect on rate of S N 1. b) the better the nucleophile, the faster the rate of the S N 2 reaction (again, no effect on S N 1) e.g. OH - favors S N 2 while H 2 O favors S N 1 c) solvent polarity: must consider the polarity of solvent. The more polar the solvent the more stabilized the S N 1 TS will be. When considering solvent
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Unformatted text preview: polarity think back to our discussion on dipoles and hydrogen bonding. One measure of a solvents polarity or ability to solvate a charged species is the dielectric constant ( ). For example, consider H 2 O: In general, we can classify solvents as protic or aprotic. If a solvent is capable of engaging in hydrogen bonding (must have an N-H or O-H bond) then it is considered protic, if not it is aprotic. Examples of protic solvents: H 2 O CH 3 OH CH 3 CH 2 OH CH 3 CH 2 CH 2 OH Examples of aprotic solvents 2-propanone dichloromethane ethoxy ethane hexane Lets consider the effect the solvent may have on both S N 2 and S N 1 reactions. S N 2 reaction coordinate diagram: S N 1 reaction coordinate diagram:...
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Lecture 15 - polarity think back to our discussion on...

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