Chromic acid ester

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). 

The intermediate chromic acid ester, which is most probably formed on the more exposed non-benzylic alcohol, evolves by cleavage of a carbon-carbon bond, resulting in the formation of a ketone and a benzylic cation that yields a second ketone by deprotonation. 

Fig. 17.10. Mechanism of the Cr(VI) oxidation of alcohols to carbonyl compounds. The oxidation proceeds via the chromium(VI) acid ester A ("chromic acid ester") and yields chromium(IV) acid. The chromium(IV) acid may either disproportionate in an "inorganic" reaction or oxidize the alcohol to the hydroxy-substituted radical B. This radical is subsequently oxidized to the carbonyl compound by Cr(VI), which is reduced to Cr(V) acid in the process. This Cr(V) acid also is able to oxidize the alcohol to the carbonyl compound while it is undergoing reduction to a Cr(III) compound.

Steps 1 and 2 constitute an oxidation by the ionic pathway by Cr(VI), and steps 6 and 7 a similar oxidation by Cr(V), which is produced by an electron-transfer process. Either Cr(VI) (step 3) or Cr(IV) (step 4) [Cr(IV) is produced in step 2] may abstract a hydrogen and the resulting acyl radical is converted to carboxylic acid in step 5. Thus, chromium in three oxidation states is instrumental in oxidizing aldehydes. Still another possible process has been proposed in which the chromic acid ester decomposes as follows ... 

Chromic acid esters have been photolysed in aqueous solutions of potassium chromate in 40% alcohol (MeOH, EtOH, 2-PrOH) in the presence of aquoamine cobalt(iii) and the reaction appears to involve formation of Cr in the first step. It is also reported that Cr occurs as a short-lived species in the photoreduction of Cr in the liquid phase in rigid glasses it has been detected by e.s.r. However,... 

It should be underlined in conclusion that the choice of the means of modifying glue compositions with Cl is in the process of evolution. Towards this aim, additives such as guanidine chromate, some chromic acid esters and chromates, etc. that adsorb onto the glued surface and maintain the strength of the seam are used. They raise the resistance of the glued metal joint to the effects of seawater. 

No compound such as silicon chromate has been reported, but evidently silicon can be linked through oxygen to hexavalent chromium. A chromic acid ester of a silanol group was made by Schmidt and Schmidbaur (64), who prepared the tri-... 

Another example of the photoassisted substrate conversion due to a short-lived intermediate in the ground state is shown in figure 6. Chromic acid esters form chromium(V)/alkoxy radical pairs within the photochemical primary reaction. In the presence of such iron(III), cobalt(III), or copper(II) complexes which are able to interact coordinatively or by second sphere interactions with chromium(V) within the radical pair cage reoxidation to chromate(VI) occurs under simultaneous reduction of the metal complexes to corresponding iron(II), cobalt(II), and copper(I) species, respectively. Unfortunately, the efficiency of this photoassisted reaction is limited by... 

Fig. 6 Photoassisted reaction of chromic acid esters (R0Cr03 ) in the presence of cobalt(lll), iron(lll), or copper(ll) complexes.

Similarly to potassium permanganate, chromic acid oxidizes primary alcohols to aldehydes and secondary alcohols to ketones. With tertiary alcohols chromic acid esters are formed (32, 33) which are yellow to red. 

When heated in the presence of a carboxyHc acid, cinnamyl alcohol is converted to the corresponding ester. Oxidation to cinnamaldehyde is readily accompHshed under Oppenauer conditions with furfural as a hydrogen acceptor in the presence of aluminum isopropoxide (44). Cinnamic acid is produced directly with strong oxidants such as chromic acid and nickel peroxide. The use of t-butyl hydroperoxide with vanadium pentoxide catalysis offers a selective method for epoxidation of the olefinic double bond of cinnamyl alcohol (45). 

Meroquinenine, CgHjjOaN (meroquinene), formed by the oxidation of all four alkaloids and of cinchoninone or quininone and by the hydrolysis of quinenine or cinchenine (p. 489), crystallises from methyl alcohol in needles, m.p. 223-4° (dee.), [ajp -f- 27-5° (H2O). It gives a nitrosoamine, m.p. 67°, and a monoacetyl derivative, m.p. 110°, and can be esterified the ethyl ester hydrochloride has m.p. 165°. When oxidised by chromic acid it yields formic and cincboloiponic acids. On reduction with zinc dust and hydriodic acid, it adds on two atoms of hydrogen forming cincholoipon, CgH jOaN, and when heated with hydrochloric acid at 250-60° gives 3-ethyl-4-methylpyridine ()3-collidine). 

On reduction with sodium amalgam the acid adds on two atoms of hydrogen the resulting amorphous aeid yields a crystalline dimethyl ester, C2iH2gOgN2, colourless prisms, m.p. 143-7° dec.). On hydrogenation in presenee of platinic oxide as catalyst 2 mols, of hydrogen are absorbed to form the acid, CjgHjgOgNj, colourless prisms, [ajo - -17-7° (HjO). On oxidation with chromic acid the Cjg acid is converted into Wieland s Ci7 acid and Hanssen s Cjg acid. ... 

Ketohydroxycassanic acid, C20H32O4, has also been used for another mode of degradation by Ruzicka, Dalma and Scott (1941). On oxidation by chromic acid in acetic acid it yields diketocassanic acid, C20H30O4, m.p. 225°, [a]u ° — 44° (EtOH), which forms a methyl ester, m.p. 108°, (EtOH), and is reduced by sodium amyloxide at 220° to cassanic acid, C20H34O2, m.p. 224°, [a]f - - 3° (CHCI3), which on selenium dehydrogenation also yields 1 7 8-trimethylphenanthrene. 

Hydroxy-20-cyanohydrins can be oxidized to 3-ketones in good yield with chromic acid, and the osmate ester of the unsaturated nitrile is also stable to this oxidant. " After hydrolysis of the osmate ester, the new 17-hydroxy-20-cyanohydrin which is presumably formed cannot be isolated, but loses hydrogen cyanide during the hydrolysis, and only the 17a-hydroxy-20-ketone is obtained. 

Note 3. Although in principle the chromate ester can be formed directly from the 18-iodo-l 8,20-ether with silver chromate, hydrolysis and oxidation with aqueous chromic acid sulfurc acid is equally efficient. 

The mechanisms by which transition-metal oxidizing agents convert alcohols to aldehydes and ketones are complicated with respect to their inorganic chemistry. The organic chemistry is clearer and one possible mechanism is outlined in Figure 15.4. The key intennediate is an alkyl chromate, an ester of an alcohol and chromic acid. 

By oxidation with chromic acid, this is converted into cyclohexanone-3-carboxylic acid, in which the —CH. OH— group is converted into the —CO— group. This is converted into its ethyl ester and treated with magnesium methyl iodide, and the product, on hydrolysis, yields l-methyl-cyclohexane-l-ol-3-carboxylic acid, which is converted byhydro-bromic acid into 1-bromo-l - methyl - cyclohexane - 3 - carboxylic acid. When this is digested with pyridine, hydrobromic acid is eliminated and yields l-methyl-A -cyclohexane-3-carboxylic acid of the formula—... 

Dextro-dihydroverbenol melts at 58° C. and boils at 218° C. it yields an acetic ester, the odour of which recalls that of bornyl acetate. Dextro-dihydroverbenone is produced by the oxidation of the above alcohol by means of chromic acid, or by the reduction of verbenone by means of hydrogen in presence of colloidal palladium. It boils at 222° C. (Djj 0-9685 [a]o + 52-1 9° 1-47535 molecular refraction 44 45) and gives... 

The method used is described by Drysdale, Stevenson, and Sharkey.4 The methyl ester of butadienoic acid has not been described previously, but the free acid contaminated by 2-bu-tynoic acid has been prepared by Wotiz, Matthews, and Lieb 5 by carbonation of propargylmagncsium bromide. Ethyl butadienoate has been prepared by Eglinton, Jones, Mansfield, and Whiting by alkali-catalyzed isomerization of ethyl 3-butynoate prepared from 3-butynol by chromic acid oxidation and esterification. 

Strong support for the cyclic ester intermediate comes from the measurement of the relative rates of oxidation by chromic acid of cis- and // fl/w-l,2-dimethyl-1,2-cyclopentanediol. In water and in 90 % acetic acid is 17,000 and... 

It was found by Chatterji and Mukherjee that the rate law for the oxidation of formaldehyde indicated that the chromic acid was esterified by the aldehyde hydrate formed, although they did not succeed in isolating the ester.The hypothesis of ester formation seems to be supported by the experience that the rate of reaction is increased by addition of pyridine. 

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