Chromic acid, aldehydes from, with alcohols

Direct oxidation of primary trialkylboranes to aldehydes and of secondary tri-alkylboranes to ketones, without isolation of the alcohol, is possible with pyri-dinium chlorochromate (PCC) or aqueous chromic acid (formed from Na2Cr207 and H2S04). For example, 1-octene was converted directly to octanal using disi-amylborane, followed by oxidation with PCC (5.25). 

Sometimes the over-oxidation we were trying to avoid in oxidizing alcohols to aldehydes is actually the reaction you want. It s best done with an aqueous solution of Cr(Vl) or Mn(Vll). Acidic or basic aqueous potassium permanganate is often a good choice. From alcohols in acidic solution the mechanism follows very much the lines of the chromic acid mechanism from aldehydes, the mechanism is very similar. 

Acetaldehyde [75-07-0] (ethanal), CH CHO, was first prepared by Scheele ia 1774, by the action of manganese dioxide [1313-13-9] and sulfuric acid [7664-93-9] on ethanol [64-17-5]. The stmcture of acetaldehyde was estabhshed in 1835 by Liebig from a pure sample prepared by oxidising ethyl alcohol with chromic acid. Liebig named the compound "aldehyde" from the Latin words translated as al(cohol) dehyd(rogenated). The formation of acetaldehyde by the addition of water [7732-18-5] to acetylene [74-86-2] was observed by Kutscherow] in 1881. 

Controlled oxidation of a primary alcohol with a mixture of sulfuric and chromic acids gives the corresponding aldehyde. In the preparation of low-molecular-weight aldehydes, an aqueous medium is used and the product is removed by steam distillation, thus preventing further oxidation. This procedure is well illustrated by the preparation of propion-aldehyde (49%) and isovaleraldehyde (60%). Certain benzyl alcohols are dissolved in aqueous acetic acid for chromic acid oxidation. Ole-finic aldehydes are produced by a rapid low-temperature (5-20°) oxidative procedure, as illustrated by the preparation of 2-heptenal (75%) from 2-heptenol. Aldehyde ethers such as methoxyacetaldehyde and ethoxy-acetaldehyde have been prepared by the chromic acid oxidation of the corresponding alcohols in 17% and 10% yields, respectively. ... 

Phenylacetaldehyde. BenzeneacetatdeHyde a-loluic aldehyde a-tolualdehyde Hyacinthin. C(H,0 mol wt 120.14. c 79.97%, H 6.71%, O 13.32%. C(HyCH,CHO. Prepd by oxidizing phenylethyl alcohol with chromic acid. High-yield synthesis from styrene oxide or styrene glycol G. Paparatto. G. Gregorio, Tetrahedron Letters 29, 1471 (1988). 

Although primary alcohols are first oxidized to aldehydes, the aldehydes are further oxidized to carboxylic acids. The ability of chromic acid to oxidize aldehydes but not ketones is taken advantage of in a test that uses chromic acid to distinguish between aldehydes and ketones (see Experiment 52D). Secondary alcohols are oxidized to ketones, but no further. Tertiary alcohols are not oxidized at all by the reagent hence, this test can be used to distinguish primary and secondary alcohols from tertiary alcohols. Unlike the Lucas test, this test can be used with all alcohols regardless of molecular weight and solubility. 

Thus, the chromic acid reagent gives a clear-cut distinction between primary and secondary alcohols and aldehydes on the one hand and tertiary alcohols and ketones on the other. Aldehydes may be distinguished from primary and secondary alcohols by means of Schiff s, Tollens s, Benedict s (Sec. 23.4), and Fehling s tests, and primary and secondary alcohols of lower molar mass may be differentiated on the basis of their rates of reaction with concentrated hydrochloric acid containing zinc chloride—the Lucas reagent (Sec. 25.11B). 

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. 

Nitro alcohols from the condensation of aromatic aldehydes with sodium salts of nitroparaffins are oxidized to a-nitro ketones with chromic-acetic acids, as illustrated by the preparation of a-nitroacetophenone, CjHsCOCHjNOj (80%). ... 

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