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3 Jul 2008 22:58:45 IST

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migratory aptitude in beyer villeger and pinacol reaction

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sir i have few doubts in organic chemistry

1. why aldehyde do no give pinacol type reduction with Mg / water?

2. Migratory aptitude of groups in Beyer villeger oxidation including hydride and phenyl with t, s ,p alkyl groups.

3. why acid chlorides are not reduced by diborane while carboxylic acids can be reduced.

4. why alpha hydroxy ketones give tollen's test.

5. Nucleophilic substitution on ester are easy with respect to carboxylic acid while alkoxide ion is a poor leaving group than hydroxide ion.

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Madhusudan Chavan

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Joined: 9 Mar 2007

Posts: 2554

4 Jul 2008 17:38:47 IST

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2. Migratory aptitude of various groups : In case of unsymmetrical ketones, the group that migrates is the one which can more easily supply the electrons. The tendency of the groups to migrate from carbon to electron deficient oxygen follows the order :

tert-alkyl > sec-alkyl >phenyl > prim-alkyl > methyl.

Amongst aryl groups, the order is , p-methoxyphenyl > p-tolyl > phenyl > p-chlorophenyl.

The product actually formed during Baeyer-Villiger oxidation depends upon the migratory aptitude of groups. For example, the oxidation of actophenone with perbenzoic acid gives phenyl acetate and not methyl benzoate because phenyl group has a greater tendency to migrate to electron deficient oxygen than methyl group.

Similarly, Baeyer-Villiger oxidation of pinacolone gives tert.butly acetate and not methyl pivilate because migratory aptitude of tert. group is greater than that of methyl group.

1.Pinacol reaction : In presence of active metals such as magnesium (usually employed in the form of magnesium amalgam ) aldehydes and ketones undergo bimolecular reduction to form symmetrical glycols generally called pinacols. Reduction of aldehydes and ketones by dissolving metals produces alcohols. However, this reaction may also lead to the formation of 1,2-glycol.

I have not come across any evidence which specifically refers that this reaction is given only by ketones and not by aldehydes.

please put the other questions separately.

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