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lecture 9 - Chemistry 140A Fall 2008(K Albizati Chapter 9...

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Chemistry 140A Fall 2008 (K. Albizati) 1 Chapter 9 Further Reactions of Alcohols and the Chemistry of Ethers Alcohols have a lot more reactivity modes many of which are mirrored in biological processes. We will look at more modes, like: Conversion to Alkoxides We briefly touched on the conversion of alcohols to alkoxides in Chapter 8. Alkoxides are fairly stable molecules. They are valuable as bases and as nucleophiles:
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Chemistry 140A Fall 2008 (K. Albizati) 2 It takes a base stronger than an alkoxide to completely deprotonate an alcohol, i.e., to drive this reaction completely to the right. Keep in mind…….. This is almost always an energetically favorable process. So……. In all cases, a stronger acid transfers a proton to a strong base to become a weaker acid-weaker base pair.
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Chemistry 140A Fall 2008 (K. Albizati) 3 Alkali metals can undergo an oxidation-reduction reaction to give alkoxides as well:
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Chemistry 140A Fall 2008 (K. Albizati) 4 Protonation of Alcohols to Give Alkyloxonium Ions We haven’t talked much about the basic properties of alcohols. The oxygen atom is weakly basic and can be protonated by strong acids to give alkyloxonium ions (oxonium ions): This essentially turns a poor leaving group ( - OH) into a fairly good one (H 2 O). Any small neutral molecule is a pretty good leaving group. This allows an important functional group transformation: conversion of alcohols into alkyl halides: This occurs via an S N 2-like Mechanism: Unfortunately this works well only with structurally simple 1 o alkyl halides and works well with HI and HBr but not so well with HCl because of the poorly nucleophilic chloride ion.
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