Chromic acid generation

The hydroxyl in cassaic acid is secondary because oxidation of the acid with chromic acid generates a diketo acid (m.p. 238-239°, [a] —164.5° in ethanol) which did not give positive reactions for aldehydes and which formed a methyl ester (m.p. 129-130°), a dioxime (m.p. 130-132°), and a disemicarbazone (m.p. 290°). Ruzicka and Dalma (19) have suggested that the saturated acid, free of oxygen-containing substituents, from which cassaic acid is derived, be named cassanic acid. The diketo acid obtainable... 

In some cases the acid is generated via hydride reduction to the alcohol, followed by chromic acid oxidation to the acid. The latter reaction also proceeds without appreciable racemization12. 

Scheme 34 shows the synthesis of the bc portion (336), which possessed three of the nine asymmetric centers present in cobyric acid. Retrosynthesis determined that (336) could be obtained, via sulfide contraction, from the two intermediates (337) and (338). Ring c was synthesized from (+)-camphor quinone (not shown). Ring b (337) was obtained from 8-methyl-j8-acetylacrylic acid (339), the two adjacent chiral centers being generated in the required relative orientation by a Diels-Alder cycloaddition with butadiene in the presence of tin(IV) chloride. Fractional crystallization served to resolve the diastereomeric a-phenethylamine salts derived from them, eventually affording the compound (340). Oxidation with chromic acid cleaved the double bond in (340) and one of the newly generated... 

The alcohol (1) is transformed into a chromic acid ester (2), which evolves to an aldehyde or a ketone (3). When an aldehyde is generated, it can react with water to form the hydrate (4) that can evolve as in Equation below,5 resulting in the formation of an acid (5). 

No use of an organic solvent is found in this oxidation, in which chromic acid is generated by the action of sulfuric acid on soditun dichromate. 

Interestingly, in Jones oxidation, chromic acid is almost always generated from chromium trioxide while in the rest of the chromic acid oxidations, sodium or potassium dichromate are almost exclusively used. This seems to be the result of an irrational tradition originated since the reagents... 

The main inconvenience was the lifetime of the electrode, which was lowered by gradual passivation with compounds present in urine. However, the use of an optimised mercury film, high urine dilutions and short accumulation times minimised this effect. Surface fouling was also alleviated by fibre regeneration along lmin in concentrated chromic acid. The same fibre could normally be used for five different urine analysis runs involving approximately 35 measurements before it had to be replaced. This problem had not been noticed when the mercury film was generated in situ. 

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... 

Cyclopentanone is prepared by oxidation of cyclopentanol. Any one of a number of oxidizing agents would be suitable. These include PDC or PCC in CH2C12 or chromic acid (H2Cr04) generated from Na2Cr207 in aqueous sulfuric acid. 

In the early 1970s, surface modification of most polymers was achieved using redox initiators. Ce+4-induced initiation was employed to achieve surface grafting of acrylamide onto LDPE film [117]. The film was first oxidised by chromic acid and then reduced with diborane to form a hydroxyl-rich surface which was then used to initiate graft polymerisation of acrylamide using Ce+4/HN03. The mechanism of chromic-acid-facilitated surface oxidation of LDPE surface is shown in Scheme 6a and that of free-radical generation is represented in Scheme 6b. 

Scheme 6 a The mechanism of chromic acid facilitated surface oxidation of LDPE. b Free-radical generation on surface-oxidised LDPE... 

Nitric acid (cone.) Acetic acid, aniline, chromic acid, acetone, alcohol, or other flammable liquids, hydrocyanic acid, hydrogen sulfide, or other flammable gases, nitratable substances copper, brass, or any heavy metals (or will generate nitrogen dioxide/nitrous fumes)... 

All of the chromium reagents produce by-products and washings that contain hazardous chromium salts and must be collected as hazardous waste. In many cases, simple oxidants such as household bleach (sodium hypochlorite, NaOCl) can accomplish the same oxidations as chromic acid without using heavy metals, and without generating hazardous waste. Oxidations using sodium hypochlorite involve mildly acidic or basic conditions that may be better than chromic acid for acid-sensitive compounds. 

In chemical oxidation or reduction the redox reagent and the substrate often form a covalent or ionic bond, for example, an ester in chromic acid oxidation [8], a sulfonium methylide in the Swern oxidation [9], cyclic esters in the svn dihydroxylation with OSO4 [10], or in the selenium dioxide oxidation of ketones and aldehydes [11]. In electrochemical processes the substrate must diffuse from the bulk of the solution to the electrode and compete there with other components of the electrolyte by competitive adsorption for a position at the electrode surface [12]. The next step is then generation of the reactive intermediate by electron transfer at the electrode that reacts with a low activation energy to the products. In chemical oxidations or reductions one finds a reductive or oxidative elimination of the intermediate with a higher activation energy. 

Wine (total) Digestion of sample with concentrated HN03 and chromic acid addition of hydroxylamine chlorhydrate to cold flask transferring to mercury/hydride generator addition of SnCI2 in H2S04 to reduce purging to detector AAS (flameless) o> CO r 95-107 Cacho and Castells 1989... 

The rinse water from a chrome plating operation has an average total chromium concentration (as Cr) of 20mg/L 450,000 gal/yr of this rinsewater is generated as a process wastewater. What is the value of the chromium ( /yr) as chromic acid (H2Cr04) if it is recovered from the rinse water. Chromic acid costs 3.75/lb. 

Metal Salts and Metallic Salt Complexes Various metallic cations can be encountered Co, Fe, Ni, Cu, Cr, and so on [77]. For example, the decomposition of azido-pentaamine cobalt (III) yields an initiating azide radical the reductive interaction between the chromic acid and a suitable monomer such as acrylamide generates a radical on the monomer (10.37) [78]. Other systems are described in Ref. [79]. 

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