Transcription 12.01.12
Lecture 2B • 01/12/12 We covered three different reactions for converting alcohols into leaving groups. One was to turn an alcohol into an alkyl chloride, that was using thionyl chloride. Second reaction was using tosyl chloride; the primary difference between those two reactions is one of stereochemistry. An inversion of stereochemistry does occur if you use thionyl chloride, because it does affect the carbon-oxygen bond, but because forming a tosylate does not touch the carbon-oxygen bond, only the oxygen- hydrogen bond, there’s no change in stereochemistry there. We then saw phosphorus tribromide that reacts very similarly to the thionyl chloride; you get an alkyl bromide instead, but it also has inversion of configuration. The last reaction is not a new mechanism, it is just an Sn2 reaction, it’s called the Finkelstein reaction. Really this works, in a sense, off of Le Châtelier’s principle. In solution, in theory, sodium iodide can displace bromide, but sodium bromide can displace iodide, so you can have an Sn2 reaction that goes back and forth and back and forth and back and forth. Except, sodium iodide is somewhat soluble in acetone, while sodium chloride and sodium bromide are not. So, in fact, one of the things that we get out of this as a by-product is sodium bromide, which, again, is not soluble in acetone and therefore precipitates out and is no longer part of the reaction mixture, so there’s no reverse reaction possible. Because of this solubility trick, it allows this reaction to be pulled forward, which means you can get the alkyl iodide.
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