Alcohols, primary dichromate

ANSWER Carboxylic acids can be made from oxidative ozonolysis of alkenes, by oxidation of primary alcohols using dichromate under aqueous conditions, and by oxidation of aldehydes. 

By the controlled oxidation of primary alcohols with a solution of potassium or sodium dichromate in dilute sulphuric acid. To avoid the further oxidation to the corresponding acid, the aldehyde is removed as rapidly as possible by distillation through a fractionating column, for example ... 

By oxidation of primary alcohols with alkaline potassium permanganate solution or with a dichromate and dilute sulphuric add, for example ... 

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. 

Oxidation of primary alcohols to aldehydes (Section 15.10) Pyridinium dichromate (PDC) or pyridinium chloro-chromate (PCC) in anhydrous media such as dichloromethane oxidizes primary alcohols to aldehydes while avoiding overoxidation to carboxylic acids. 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

Primary and secondary alcohols undergo oxidation when reacted with a mixture of acidified potassium dichromate (VI) and dilute sulphuric acid. Primary alcohols are oxidised in the sequence... 

An effective oxidation of a,p-unsaturated primary alcohols and benzylic alcohols, but not saturated alcohols, to the corresponding aldehydes has been achieved using polymer-supported quaternary ammonium dichromate [14],... 

Quinolinium dichromate (QDC) oxidations of primary and secondary alcohols both proceed via a cyclic chromate ester. Acrylonitrile polymerization was observed in the oxidation of several para- and meffl-substituted benzaldehydes to the corresponding benzoic acids by quinolinium chlorochromate (QCC). QCC oxidations of diphenacyl sulfide and of aromatic anils have been studied. 

The primary alcohol or aldehyde is heated under reflux with acidified potassium dichromate solution. For example ... 

Primary alcohols are readUy oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMn04) in neutral, acidic or alkaline media or by potassium dichromate (K2Cr207) and chromium trioxide (CrOg) in acidic media (Jones reagent). 

The oxidation of alcohols to the corresponding aldehydes, ketones or acids certainly represents one of the more important functional group transformations in organic synthesis and there are numerous methods reported in the literature (1-3). However, relatively few methods describe the selective oxidation of primary or secondary alcohols to the corresponding aldehydes and ketones and most of them traditionally use a stoichiometric terminal oxidant such as chromium oxide (4), dichromate (5), manganese oxide (6), and osmium or ruthenium oxides as primary oxidants (7). 

The remaining three steps are accomplished without purification of the intermediate products. The secondary hydroxy group is protected by acetylation and the benzyl ether is removed by hydrogenolysis to provide a primary alcohol. The alcohol is oxidized to a carboxylic acid by ruthenium(III) chloride or pyridinium dichromate. This method has been applied to the synthesis of various enzyme inhibitors containing the 1-hydroxyethylene isostere. 

Conditions that do permit the easy isolation of aldehydes in good yield by oxidation of primary alcohols employ various Cr(VI) species as the oxidant in anhydrous media. Two such reagents are pyridinium chlorochromate (PCC), C5H5NH+ ClCr03, and pyridinium dichromate (PDC), (C5H5NH)22+ Cr2072- both are used in dichlorome thane. 

Oxidation-Reduction in Blood Analysis Demonstrating the Reaction in a Breathalyzer," J. Chem. Educ., Vol. 67,1990, 263. The oxidation of a primary alcohol by the orange dichromate ion is shown to first form an aldehyde, then a carboxylic acid, and green chromium(III) ion. The use of this reaction, principles of spectrometry, and gas laws in a commercial device for measuring blood-alcohol content are discussed. 

A copper-chromium oxide on pumice catalyst has particular value for the dehydrogenation of primary and secondary alcohols to the corresponding carbonyl compounds (see Section 5.6.1, p. 581). Dissolve 10.4g of barium nitrate (AnalaR) in 280 ml of water at about 80 °C and add to this hot solution 87 g of copper(n) nitrate trihydrate (AnalaR) stir the mixture and heat until a homogeneous solution results. Prepare a solution of 50.4 g of recrystallised ammonium dichromate in a mixture of 200 ml of water and 75 ml of concentrated ammonia solution (d 0.880). To the ammonium chromate solution at 25-30 °C add the hot (80 °C) nitrate solution in a thin stream with stirring. Allow the mixture to cool and filter off the yellowish-brown precipitate with suction press with a glass stopper and suck as dry as possbile. Transfer the... 

The oxidation of secondary alcohols with sodium dichromate in dilute sulphuric acid gives acceptable yields of ketones since these do not normally undergo extensive further oxidation under the reaction conditions (cf. Section 5.7.1, p. 587, the oxidation of primary alcohols to aldehydes). 

Differentiation between primary, secondary and tertiary alcohols. The three classes of alcohols differ in their behaviour on oxidation with hot acidic dichromate solution. Primary alcohols yield aldehydes and secondary alcohols... 

To an ice-cold mixture of 1.0 ml of concentrated sulphuric acid and 5 ml of saturated aqueous potassium dichromate solution, add 2 ml of the alcohol or its concentrated aqueous solution. If the alcohol is not miscible with the reagent, shake the reaction mixture vigorously. After 5 minutes, dilute with an equal volume of water, distil and collect the first few ml of the aqueous distillate in a test tube cooled in ice. (Aldehydes and ketones are volatile in steam.) Test a portion of the distillate for a carbonyl compound with 2,4-dinitrophenylhydrazine reagent (p. 1218). If a solid derivative is obtained, indicating that the compound was a primary or secondary alcohol, test a further portion with SchifFs reagent (p. 1291) to distinguish between the two possibilities. The derivative may be recrystallised the m.p. may give a preliminary indication of the identity of the alcohol. 

The experimental data for oxidation of benzyl alcohol,1 aliphatic primary and secondary alcohols,2 and cholesterol3 with cetyltrimethylammonium (CTA) dichromate indicated that the reactions occur in a reverse micelle system produced by the oxidant. Michaelis-Menten-type kinetics were observed with respect to the reductants. The product of the oxidation of cholesterol depends on the solvent. In dichloromethane, the product is 7-dehydrocholesterol, whereas with dichloromethane containing acetic acid the product is 5-cholesten-3-one. A low kinetic isotope effect, k /ku = 2.81, was observed in the oxidation of methanol- this, combined with the rate data and the reverse solvent isotope effect [ (H20)/fc(D20) = 0.76], suggests that these reactions... 

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. 

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