week 2 - 5 - Chemistry S-20ab Chemistry E-2a Lecture 7 Week...

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Oxidation of Alcohols Alcohols can be oxidized with various oxidizing agents. The most common are various derivatives of CrO 3 . We can make the following general observations: For primary alcohols: OH CrO 3 ("dry") O H OH CrO 3 ("wet") O OH (Where else have you seen an “option” for forming aldehydes versus forming carboxylic acids?) For secondary alcohols: O OH CrO 3 What do you think will happen for tertiary alcohols? OH CrO 3 ??? Reading : Sections 10.5 and 10.6 Chemistry S-20ab Week 3 132
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Oxidation of Alcohols: Mechanisms Draw curved-arrow mechanisms for each of the following steps in the oxidation of isopropanol: OH + CrO 3 O CrO 3 H O CrO 3 H O + H 2 CrO 3 + H 3 O + H + H 3 O + Reading : Section 10.6 Chemistry S-20ab Week 3 133
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Oxidation of Primary Alcohols to Carboxylic Acids In water, the oxidation of primary alcohols proceeds beyond the aldehyde stage and ends up with a carboxylic acid. Provide a mechanism for these steps: Initial oxidation (same as on previous page): CrO 3 OH O H Subsequent reactions: O H + H 3 O + H OH HO + H + OH + CrO 3 O CrO 3 H H H HO HO O CrO 3 H O + H 2 CrO 3 + H 3 O + H HO HO + H 3 O + Reading : Section 10.6 Chemistry S-20ab Week 3 134
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Reduction of Aldehydes and Ketones Aldehydes and ketones can be reduced to alcohols using NaBH 4 . We will not discuss the mechanism of this reaction at this point; you should simply be aware that this reaction gives exactly the reverse of the oxidation reactions discussed above: Interconversion of primary alcohols and aldehydes : OH CrO 3 ("dry") O H O H OH NaBH 4 Interconversion of secondary alcohols and ketones : O O NaBH 4 OH OH CrO 3 Reading : Section 19.8 (no, that’s not a typo!) Chemistry S-20ab Week 3 135
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Chemical and Stereochemical Equivalence The following molecule has many hydrogen atoms. Let’s take a look at various pairs of hydrogen atoms and ask: are they the same or not the same ? The best way to do so is to perform a substitution test . If the two compounds are entirely different, the groups are constitutionally inequivalent. If the two compounds are diastereomers, the groups are diastereotopic. If the two compounds are enantiomers, the groups are enantiotopic. If the two compounds are identical, the groups are homotopic. Let’s look at some examples: Br Br Br Reading : Section 10.8 Chemistry S-20ab Week 3 136
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Br Br Br Br Reading : Section 10.8 Chemistry S-20ab Week 3 137
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Thiols vs. Alcohols: Similarities The following reactions show some of the similarities between thiols and alcohols. Can you draw curved-arrow mechanisms for each step?
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