lecture 9

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

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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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Chemistry 140A Fall 2008 (K. Albizati) 5 2 o and 3 o halides react similarly………………. . but probably proceed via carbonium ion intermediates in a solvolysis-like mechanism: Of course, if a carbonium ion is formed there is the possibility of elimination to form an alkene via an E1-like process. Under the right conditions an alcohol can be dehydrated to form an alkene:
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Chemistry 140A Fall 2008 (K. Albizati) 6 This occurs via a mechanism involving an alcohol protonation step, loss of water to give a carbonium ion and then an E1-like alkene forming step: This works well in some simple cases, usually where a symmetrical carbonium ion can form and only one alkene can result: Unfortunately, carbonium ions have high and varied chemical reactivity and in unsymmetrical cases (especially involving 2 o carbonium ions) rearrangements of the C and H bonds in the skeleton can occur.
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Chemistry 140A Fall 2008 (K. Albizati) 7 Carbonium Ion Rearrangements The alcohol below was subjected to HBr in an attempt to form the corresponding alkyl bromide via S N 1 nucleophilic substitution: This is a result of a common mode of reactivity of carbonium ions called a hydride shift: Essentially, a hydrogen atom moves (“shifts”) from one carbon to an adjacent carbon, moving with the electrons in the C-H bond to form a new, generally more stable, carbonium ion. In this case, a 2 o carbonium ion becomes 3 o by a hydride shift. The entire mechanism of the above reaction can now be written:
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This note was uploaded on 07/25/2009 for the course CHEM 140A taught by Professor Whiteshell during the Spring '04 term at UCSD.

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

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