Sodium dichromate

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Sodium dichromate
Na2Cr2O7.png
Dichroman sodný.JPG
Identifiers
CAS number 10588-01-9 YesY
7789-12-0 (dihydrate)
PubChem 25408
ChemSpider 23723 YesY
EC number 234-190-3
UN number 3288
ChEBI CHEBI:39483 YesY
RTECS number HX7750000
HX7750000 (dihydrate)
Jmol-3D images Image 1
Properties
Molecular formula Na2Cr2O7
Molar mass 261.97 g/mol (anhydrous)
298.00 g/mol (dihydrate)
Appearance bright red
Odor odorless
Density 2.52 g/cm3
Melting point 356.7 °C (674.1 °F; 629.8 K)
Boiling point 400 °C (752 °F; 673 K) decomposes
Solubility in water 73 g/100 mL at 25 °C
Solubility in other solvents soluble in methanol, ethanol
Refractive index (nD) 1.661 (dihydrate)
Hazards
MSDS ICSC 1369
EU Index 024-004-00-7 (anhydrous)
024-004-01-4 (dihydrate)
EU classification Oxidant (O)
Carc. Cat. 2
Muta. Cat. 2
Repr. Cat. 2
Very toxic (T+)
Harmful (Xn)
Corrosive (C)
Dangerous for the environment (N)
R-phrases R45, R46, R60, R61, R8, R21, R25, R26, R34, R42/43, R48/23, R50/53,
S-phrases S53, S45, S60, S61
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazard OX: Oxidizer. E.g., potassium perchlorateNFPA 704 four-colored diamond
LD50 50 mg/kg
Related compounds
Other anions Sodium chromate
Sodium molybdate
Sodium tungstate
Other cations Potassium dichromate
Ammonium dichromate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Sodium dichromate is the inorganic compound with the formula Na2Cr2O7. Usually, however, the salt is handled as its dihydrate Na2Cr2O7·2H2O. Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compounds and materials based on chromium are prepared from this salt.[1] In terms of reactivity and appearance, sodium dichromate and potassium dichromate are very similar. The sodium salt is, however, around twenty times more soluble in water than the potassium salt (49 g/L at 0 °C) and its equivalent weight is also lower, which is often desirable.[2]

Production[edit]

Sodium dichromate is generated on a large scale from ores containing chromium(III) oxides. The ore is fused with a base, typically sodium carbonate, at around 1000 °C in the presence of air (source of oxygen):

2 Cr2O3 + 4 Na2CO3 + 3 O2 → 4 Na2CrO4 + 4 CO2

This step solubilizes the chromium and allows it to be extracted into hot water. At this stage, other components of the ore such as aluminium and iron compounds, are poorly soluble. Acidification of the resulting aqueous extract with sulfuric acid or carbon dioxide affords the dichromate:

2 Na2CrO4 + 2 CO2 → Na2Cr2O7 + 2 NaHCO3

The dichromate is isolated as the dihydrate by crystallization. In this way, many millions of kilograms of sodium dichromate are produced annually.

Since chromium(VI) is toxic, especially as the dust, such factories are subject to stringent regulations. For example, effluent from such refineries is treated with reducing agents to return any chromium(VI) to chromium(III), which is less threatening to the environment.[1] A variety of hydrates of this salt are known, ranging from the decahydrate below 19.5 °C (CAS# 13517-17-4 ) as well as hexa-, tetra-, and dihydrates. Above 62 °C, these salts lose water spontaneously to give the anhydrous material.

Reactions[edit]

Dichromate and chromate salts are oxidizing agents. For the tanning of leather, sodium dichromate is first reduced with sulfur dioxide.

In the area of organic synthesis,[2] this compound oxidizes benzylic and allylic C-H bonds to carbonyl derivatives. For example, 2,4,6-trinitrotoluene is oxidized to the corresponding carboxylic acid.[3] Similarly, 2,3-dimethylnaphthalene is oxidized by Na2Cr2O7 to 2,3-naphthalenedicarboxylic acid.[4]

Secondary alcohols are oxidized to the corresponding ketone, e.g. menthol to menthone;[5] dihydrocholesterol to cholestanone:[6]

3 R2CHOH + Cr2O72- + 2 H+ → 3 R2C=O + Cr2O3 + 4 H2O

Relative to the potassium salt, the main advantage of sodium dichromate is its greater solubility in water and polar solvents like acetic acid.

When heated strongly, sodium dichromate decomposes to form sodium chromate, chromium(III) oxide and oxygen: 4 Na2Cr2O7 → 4 Na2CrO4 + 2 Cr2O3 + 3 O2 [3][7]

Safety[edit]

Like all hexavalent chromium compounds, sodium dichromate is considered hazardous. It is also a known carcinogen.[8] Sodium Dichromate can be used in fluorene to fluorenone conversion.

References[edit]

  1. ^ a b Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi:10.1002/14356007.a07_067
  2. ^ a b Freeman, F. "Sodium Dichromate" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
  3. ^ a b Clarke, H. T.; Hartman, W. W. (1941), 2,4,6-Trinitrobenzoic Acid, Org. Synth. ; Coll. Vol. 1: 543 
  4. ^ Friedman, L. (1973), 2,3-Naphthalenedicarboxylic Acid, Org. Synth. ; Coll. Vol. 5: 810 
  5. ^ L. T. Sandborn (1929), l-Menthone, Org. Synth. 9: 59 ; Coll. Vol. 1: 340 
  6. ^ W. F. Bruce (1941), Cholestanone, Org. Synth. ; Coll. Vol. 2: 139 
  7. ^ Daintith, edited by John (2004). The Facts on File dictionary of organic chemistry. New York: Facts on file. p. 253. ISBN 0-8160-4928-9. 
  8. ^ ILO 1369 - Sodium Dichromate