Alcohols, primary with chromic acid

Chromic acid (H2Cr04) is a good oxidizing agent and is fonned when solutions containing chromate (Cr04 ) or dichromate (Cr Oy ) are acidified. Sometimes it is possible to obtain aldehydes in satisfactory yield before they are further oxidized, but in most cases carboxylic acids are the major products isolated on treatment of primary alcohols with chromic acid. 

Benzyl alcohol readily undergoes the reactions characteristic of a primary alcohol, such as esterification and etherification, as well as halide formation. In addition, it undergoes ring substitution. In the presence of acid, polymerization is observed, and the alcohol can be thermally dehydrated to toluene [108-88-3], Catalytic oxidation over copper oxide yields benzaldehyde benzoic acid is obtained by oxidation with chromic acid or potassium permanganate. Catalytic hydrogenation of the ring gives cyclohexylmethanol [100-49-2]. 

A Cr(VI)-catalyst complex has been proposed as the reactive oxidizing species in the oxidation of frans-stibene with chromic acid, catalysed separately by 1,10-phenanthroline (PHEN), oxalic acid, and picolinic acid (PA). The oxidation process is believed to involve a nucleophilic attack of the olefinic bond on the Cr(VI)-catalyst complex to generate a ternary complex.31 PA- and PHEN-catalysed chromic acid oxidation of primary alcohols also is proposed to proceed through a similar ternary complex. Methanol- reacted nearly six times slower than methanol, supporting a hydride transfer mechanism in this oxidation.32 Kinetics of chromic acid oxidation of dimethyl and diethyl malonates, in the presence and absence of oxalic acid, have been obtained and the activation parameters have been calculated.33 Reactivity in the chromic acid oxidation of three alicyclic ketoximes has been rationalized on the basis of I-strain. Kinetic and activation parameters have been determined and a mechanism... 

Oxidation of 1-hexanol with chromic acid (sodium or potassium dichromate in aqueous sulfuric acid) yields hexanoic acid. Use of PDC or PCC in dichloromethane is not acceptable because those reagents yield aldehydes on reaction with primary alcohols. 

The chemical properties and uses of propargyl alcohol has three potentially reactive sites (1) a primary hydroxyl group (i.e., CH2OH), (2) a triple bond (-C=C-), and (3) an acetylenic hydrogen (-C=CH) that makes the alcohol an extremely versatile chemical intermediate. The hydroxyl group can be esterified with acid chlorides, anhydrides, or carboxylic acids, and it reacts with aldehydes or vinyl ethers in the presence of an acid catalyst to form acetals. At low temperatures, oxidation with chromic acid gives propynal or propynoic acid ... 

The procedure described here involves the metallation of dihydrofuran and subsequent alkylation with an alkyl iodide (bromides are much less reactive).5 The resulting substituted dihydrofuran, the intermediate postulated in the Larson procedure, is then treated with chromic acid to hydrolyze the enol ether and oxidize the resulting primary alcohol to the corresponding carboxylic acid as in the Larson procedure. The isolated oxidation product is of suitable purity for subsequent reactions, but if necessary, recrystallization from hexanes is readily accomplished. ... 

Selective oxidation of a primary alcohol. The final step in a recent synthesis of strophanthidin required selective oxidation of the primary hydroxyl group of stro-phanthidol (1) to an aldehyde group. Oxidations with chromic acid, N-haloamides,... 

Jones oxidation Oxidation of primary and secondary alcohols with chromic acid to give the corresponding carboxylic acids and ketones. 228... 

Jones reagent (1, 142-143). Primary allylic or benzyUc alcohols are oxidized in high yield with chromic acid in acetone. Thus cinnamyl alcohol is oxidized to cinnamaldehyde in 84% yield, and benzaldehyde is obtained from benzyl alcohol in 76% yield. Manganese dioxide has usually been used in such oxidations. 

Compound L must, therefore be an alcohol. It is indicated that L reacts with chromic acid to give M. This indicates that L must be either a primary or secondary alcohol since chromic acid is a strong oxidizing agent that readily oxidizes primary and secondary alcohols to... 

By oxidation of aldehydes and primary alcohols. Aldehydes can be oxidized to carboxylic acids with mild oxidizing agents such as Ag(NH3)2" HO (Section 16.12B). Primary alcohols can be oxidized with KMn04. Aldehydes and primary alcohols are oxidized to carboxylic acids with chromic acid (H2Cr04) in aqueous acetone (the Jones oxidation Section 12.4C). 

PCC is selective for the oxidation of primary alcohols to aldehydes. It is less reactive than the previously discussed oxidation with chromic acid in aqueous sulfuric acid, and the reaction is run stoichiometrically so that no PCC remains once all the alcohol molecules have been converted to aldehyde. PCC also has little effect on carbon-carbon double bonds or other easily oxidized functional groups. In the following example, geraniol is oxidized to geranial without affecting either carbon-carbon double bond ... 

We have shown that in aqueous chromic acid, a primary alcohol is oxidized first to an aldehyde and then to a carboxylic acid. In the second step, it is not the aldehyde that is oxidized, but rather the aldehyde hydrate formed by addition of a molecule of water to the aldehyde carbonyl group (hydration). An —OH of the aldehyde hydrate reacts with chromic acid to complete the oxidation of the aldehyde to a carboxylic acid. 

When a primary alcohol is oxidized with chromic acid, a carboxyhc acid is obtained. It is generally difficult to control the reaction to produce the aldehyde. 

Primary alcohols can be identified by oxidizing them to aldehydes with chromic acid or potassium permanganate in the presence of 2 n sulfuric acid. The aldehyde formed can be isolated by distillation and identified in the form of its dimedone derivative (see p. 230). Primary alcohols can be identified in this manner in the presence of secondary and tertiary alcohols, because dimedone condenses only with aldehydes, i.e., with products of a mild oxidation of primary alcohols. Carbonyl compounds can be isolated from an aqueous distillate by precipitating them with 2,4-dinitrophenyl-hydrazine (see p. 218). If paper chromatography cannot be used for identification (see p. 222), it is advisable to oxidize at least 500 mg of alcohol. 

This carboxylic acid can be prepared from the starting primary alcohol via oxidation with chromic acid ... 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

Jones oxidation is generally not useful for the transformation of primary alcohols into aldehydes. This is due to the equilibrium of the aldehydes with the corresponding hydrates in the aqueous media, leading to the subsequent oxidation of the aldehyde hydrates into carboxylic acids. In fact, kinetic studies support the assumption that chromic acid oxidizes aldehydes into carboxylic acids via the corresponding aldehyde hydrates.5... 

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