Org.Prb.Help

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Pyridinium Chlorochromate (PCC)

This reagent is used in methylene chloride solution. It effects the oxidation of 1º-alcohols to aldehydes and 2º-alcohols to ketones. It is a milder oxidant than Jones' reagent, and the absence of water in the reaction mixture avoids the further oxidation of aldehydes to carboxylic acids.

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Aqueous Chromic Acid (Jones' Reagent)

This reagent is often prepared by dissolving sodium dichromate (or chromium trioxide) in aqueous sulfuric acid. It readily oxidizes 1º and 2º-alcohols, usually in acetone solution. The oxidation of 1º-alcohols proceeds to the aldehyde, but because water adds to aldehydes to give a "hydrate" derivative, the oxidation continues on to give the corresponding carboxylic acid. Oxidation of 2º-alcohols stops at the ketone product. Because this reagent is strongly acidic, it cannot be used with acid-sensitive compounds.

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Lead Tetraacetate and Periodic Acid

These reagents function similarly to cleave vicinal glycols to aldehydes and ketones, depending on their alkyl substitution. Pb(OAc)4 is usually used in benzene or acetic acid solution; HIO4 is used in aqueous solvent systems.

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Ozonolysis

Ozone (O3) rapidly attacks double and triple bonds, generating reactive intermediates called ozonides. Because these ozonide products are often explosively unstable, they are not isolated but are decomposed either reductively (Zn dust in hydroxylic solvents) or oxidatively (by hydrogen peroxide or other oxidants). Depending on the alkyl substitution of the double bond, alkenes give aldehydes and/or ketones on reductive work-up, and carboxylic acids and/or ketones on oxidative work-up. Alkynes give carboxylic acids. Single carbon fragments generally end up as carbon dioxide when an oxidative work-up is used.

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Hydration

The addition of water to alkenes and alkynes is acid-catalyzed. Alkenes give alcohol products, which may be predicted by the Markovnikov Rule. The addition of water to alkynes also follows the Markovnikov Rule, but additionally requires a mercuric salt as a catalyst. The addition product from an alkyne is an "enol". Enols are generally unstable, rearranging immediately to a more stable carbonyl tautomer.

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Hydroboration

Diborane (B2H6) as its monomer borane (BH3) adds to double and triple bonds, giving alkyl boranes. This addition does not follow the Markovnikov Rule, so when the boron is replaced by a hydroxyl group through alkaline hydrogen peroxide treatment the product is a regioisomer of the acid catalyzed hydration product. The product from an alkyne treated by this sequence is an "enol". Enols are generally unstable, rearranging immediately to a more stable carbonyl tautomer.

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