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A chromate ester is a chemical structure that contains a chromium atom (symbol Cr) in a +6 oxidation state that is connected via an oxygen (O) linkage to a carbon (C) atom. The Cr itself is in its chromate form, with several oxygens attached, and the Cr–O–C attachment makes this chemical group structurally similar to other ester functional groups.
Oxidative cyclizations of olefinic alcohols to cyclic ethers may occur via [3+2], [2+2], [1] or epoxidation mechanisms. Insights into the mechanism is provided by structure-reactivity, implicating direct epoxidation by the chromate ester. [1] Subsequent epoxide opening and release of chromium leads to the observed products.
PCC is used as an oxidant.In particular, it has proven to be highly effective in oxidizing primary and secondary alcohols to aldehydes and ketones, respectively.The reagent is more selective than the related Jones' Reagent, so there is little chance of over-oxidation to form carboxylic acids if acidified potassium permanganate is used as long as water is not present in the reaction mixture.
Chromate salts contain the chromate anion, CrO 2− 4. Dichromate salts contain the dichromate anion, Cr 2 O 2− 7. They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.
The reaction proceeds through the formation of a chromate ester (1) from nucleophilic attack of the chlorochromate by the allylic alcohol. The ester then undergoes a [3,3]-sigmatropic shift to create the isomeric chromate ester (2). Finally, oxidation of this intermediate yields the α,β-unsaturated aldehyde or ketone product (3). [1]
Like many other oxidations of alcohols by metal oxides, the reaction proceeds via the formation of a mixed chromate ester: [3] [4] These esters have the formula CrO 3 (OCH 2 R) −. CrO 3 (OH) − + RCH 2 OH → CrO 3 (OCH 2 R) − + H 2 O. Like conventional esters, the formation of this chromate ester is accelerated by the acid.
An improved route involves the reaction of chromyl chloride and potassium chromate: [5] K 2 CrO 4 + CrO 2 Cl 2 → 2KCrO 3 Cl. The salt consists of the tetrahedral chlorochromate anion. The average Cr=O bond length is 159 pm, and the Cr-Cl distance is 219 pm. [6]
The mechanism for the polymerization process is the subject of much research, the central question being the structure of the active species, which is assumed to be an organochromium compound. [2] Robert L. Banks and J. Paul Hogan , both at Phillips Petroleum , filed the first patents on the Phillips catalyst in 1953.