Sodium dichromate chromic acid preparation

A. (+)-lBomenthone. Into a 1-L, three-necked, round-bottomed flask equipped with a mechanical stirrer, condenser, thermometer and dropping funnel, are placed 54.6 g (0.35 mol) of (+)-isomenthol (Note 1) and 350 mL of ether. A solution of chromic acid, prepared by mixing 56.7 g (0.23 mol) of potassium dichromate and 31 S3 mL (0.58 mol) of 98% sulfuric acid and diluting to 200 mL with water, is added dropwise to maintain the reaction temperature at 25°C. The mixture is stirred for a further 2 hr. The ether layer is separated and the aqueous phase is extracted twice with 100-mL portions of ether. The combined ether extracts are washed with saturated sodium bicarbonate solution, dried over magnesium sulfate, and evaporated under... 

In aqueous solution. Use of a solution of chromic acid prepared from dilute sulfuric acid and sodium dichromate dihydrate is illustrated by the oxidation of hydroquinone to quinone, isolated by a tedious process in 86-92% yieid. ... 

Benzoquinone ( quinone ) is obtained as the end product of the oxidation of aniline by acid dichromate solution. Industrially, the crude product is reduced with sulphur dioxide to hydroquinone, and the latter is oxidised either with dichromate mixture or in very dilute sulphuric acid solution with sodium chlorate in the presence of a little vanadium pentoxide as catalyst. For the preparation in the laboratory, it is best to oxidise the inexpensive hydroquinone with chromic acid or with sodium chlorate in the presence of vanadium pent-oxide. Naphthalene may be converted into 1 4-naphthoquinone by oxidation with chromic acid. 

Potassium and ammonium dichromates are generally made from sodium dichromate by a crystallization process involving equivalent amounts of potassium chloride or ammonium sulfate. In each case the solubiHty relationships are favorable so that the desired dichromate can be separated on cooling, whereas the sodium chloride or sulfate crystallizes out on boiling. For certain uses, ammonium dichromate, which is low in alkaH salts, is required. This special salt may be prepared by the addition of ammonia to an aqueous solution of chromic acid. Ammonium dichromate must be dried with care, because decomposition starts at 185°C and becomes violent and self-sustaining at slightly higher temperatures. 

Terephthalic acid has been obtained from a great many /)-disubstituted derivatives of benzene or cyclohexane by oxidation with permanganate, chromic acid, or nitric acid. The following routes appear to have preparative value from />-toluic acid, />-methylacetophenone,2 or dihydro-/)-tolualdehyde by oxidation with permanganate from f>-cymene by oxidation with sodium dichromate and sulfuric acid from />-dibromobenzene or from /i-chloro- or -bromobenzoic acid by heating at 250° with potassium and cuprous cyanides and from />-dibromo-benzene, butyllithium, and carbon dioxide. ... 

Chromic acid is usually prepared by adding chromium(VI) oxide (CrCb) or sodium dichromate (Na2Cr207) to aqueous sulfuric acid. 

Laboratory oxidation of alcohols most often is carried out with chromic acid (H2Cr04), which usually is prepared as required from chromic oxide (Cr03) or from sodium dichromate (Na2Cr207) in combination with sulfuric acid. Ethanoic (acetic) acid is a useful solvent for such reactions ... 

It is also possible to prepare a chromic acid solution by treating sodium dichromate (Na2Cr207) or potassium dichromate (K C Ot) with sulfuric acid. Consequently, sodium14 and potassium15 dichromate can be used, instead of chromium trioxide, in Jones oxidations. 

Notes. (1) The chromic acid solution may be prepared as follows. Dissolve lOOg (0.33 mol) of sodium dichromate dihydrate in 300 ml of water and slowly add 134g (73 ml, 1.34 mol) of concentrated sulphuric acid (98%, d 1.84). Cool the solution and dilute to 500 ml with water in a graduated flask. 

The preparation of potassium dichromate (Preparation 61) illustrated how chromic oxide, Cr203, can be oxidized to a chromate in which chromium exists as Cr03. For the preparation of chromic alum, it might seem as if chromic oxide or the natural chromite should yield chromic sulphate directly on treatment with sulphuric acid. This is impossible, however, because both of these substances are very resistant to the action of acids. Practically, they yield only to the action of alkaline oxidizing agents, which convert them into a chromate. Therefore potassium, or sodium, dichromates are always the products made directly from the mineral, and these serve as the materials from which other compounds of chromium are prepared. To make chromic alum from potassium dichromate it is necessary to reduce the chromium to the state of oxidation in which it originally existed in the mineral, and to add sufficient sulphuric acid to form the sulphates of potassium and... 

The chromic acid reagent is prepared by dissolving sodium dichromate (Na2Cr207) in a mixture of sulfuric acid and water. The active species in the mixture is probably chromic acid, H2Cr04, or the acid chromate ion, HC1O4. Adding chromium trioxide (C1O3) to dilute sulfuric acid achieves the same result. 

In some cases, the removal of organic residues from glassware requires the use of a chromic acid wash (prepared by adding 100 ml of concentrated sulfuric acid slowly and with constant stirring to a solution of 5 g of sodium dichromate in 5 ml of water). In this situation, it is important to thoroughly rinse the glassware in deionized water so as to remove any trace of chromium, particularly if the latter species is part of the analytical scheme. 

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