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Physical Properties
• Alcohols, ethers and epoxides exhibit dipole-dipole interactions
because they have a bent structure with two polar bonds
...
Thus,
alcohols are more polar than ethers and epoxides
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Preparation of Alcohols, Ethers, and Epoxides
• Alcohols and ethers are both common products of
nucleophilic substitution
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• In theory, unsymmetrical ethers can be synthesized in
two different ways; in practice, one path is usually
preferred
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• Alkoxides can be prepared from alcohols by a BrØnstedLowry acid—base reaction
...


• NaH is an especially good base for forming alkoxide
because the by-product of the reaction, H2, is a gas that
just bubbles out of the reaction mixture
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• In halohydrins, an intramolecular version of the
Williamson ether synthesis can occur to form epoxides
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This is called a
rearrangement
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• A 1,2-shift can convert a less stable carbocation into a
more stable carbocation
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Rearrangements can occur whenever a
carbocation is formed as a reactive intermediate
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2° Carbocation A rearranges
to the more stable 3° carbocation by a 1,2-hydride shift,
whereas carbocation B does not rearrange because it is 3°
to begin with
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• The mechanism of this reaction also consists of two
parts: conversion of the OH group into a better leaving
group, and nucleophilic cleavage by Br¯ via an SN2
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reaction
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• An alkyl tosylate is composed of two parts: the alkyl group R,
derived from an alcohol; and the tosylate (short for ptoluenesulfonate), which is a good leaving group
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• Alcohols are converted to tosylates by treatment with ptoluenesulfonyl chloride (TsCl) in the presence of
pyridine
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• Tosylate is a good leaving group because its conjugate
acid, p-toluenesulfonic acid (CH3C6H4SO3H, TsOH) is a
strong acid (pKa = -7)
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Thus, the
C—O bond of the alcohol is not broken when tosylate is
formed
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• Generally, alkyl tosylates are treated with strong nucleophiles
and bases, so the mechanism of substitution is SN2, and the
mechanism of elimination is E2
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• We now have another two-step method to convert an alcohol
to a substitution product: reaction of an alcohol with TsCl and
pyridine to form a tosylate (step 1), followed by nucleophilic
attack on the tosylate (step 2)
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• Step 2 is an SN2 reaction, so it proceeds with inversion of
configuration because the nucleophile attacks from the
backside
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Example:

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Figure 9
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HBr and HI are strong acids that are also sources of good
nucleophiles (Br¯ and I¯ respectively)
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• The mechanism of ether cleavage is SN1 or SN2, depending on
the identity of R
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With methyl or 1° R groups, the C—O bond is
cleaved by an SN2 mechanism
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The
methyl group undergoes nucleophilic substitution by an SN2
mechanism, resulting in the cleavage of the second C—O
bond
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• Epoxides do contain a strained three-membered ring
with two polar bonds
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• The reaction occurs readily with strong nucleophiles
and with acids like HZ, where Z is a nucleophilic atom
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• In step 2 the alkoxide is protonated with water to generate a
neutral product with two functional groups on adjacent atoms
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With these strong nucleophiles, the
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reaction occurs by an SN2 mechanism
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Nucleophilic attack of ¯OCH3 occurs from the backside at
either C—O bond, because both ends are similarly
substituted
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e
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Optically inactive starting materials give optically inactive
products!
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• Acids HZ that contain a nucleophile Z also open epoxide rings by
a two-step sequence
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Figure 9
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• Note that the site selectivity of these two reactions is exactly
opposite

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