Chromium(III) chloride (also called chromic chloride) describes any of several
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s with the formula CrCl
3, where can be 0, 5, and 6. The
anhydrous compound with the formula CrCl
3 is a violet solid. The most common form of the trichloride is the dark green hexa
hydrate, CrCl
3. Chromium chlorides find use as
catalysts
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
and as precursors to dyes for wool.
Structure
Anhydrous chromium(III) chloride adopts the
YCl3 structure, with Cr
3+ occupying one third of the
octahedral
In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...
interstices in alternating layers of a pseudo-
cubic close packed lattice of Cl
− ions. The absence of cations in alternate layers leads to weak bonding between adjacent layers. For this reason, crystals of CrCl
3 cleave easily along the planes between layers, which results in the flaky (
micaceous) appearance of samples of chromium(III) chloride. If pressurized to 9.9
GPa
Grading in education is the process of applying standardized measurements for varying levels of achievements in a course. Grades can be assigned as letters (usually A through F), as a range (for example, 1 to 6), as a percentage, or as a numbe ...
it goes under a phase transition.
File:Chromium(III)-chloride-sheet-from-monoclinic-xtal-3D-balls-SF-overlay.png, Space-filling model of cubic close packing of chloride ions in the crystal structure of CrCl3
File:Chromium(III)-chloride-sheet-from-monoclinic-xtal-3D-balls.png, Ball-and-stick model
In chemistry, the ball-and-stick model is a molecular model of a chemical substance which displays both the three-dimensional position of the atoms and the bonds between them. The atoms are typically represented by spheres, connected by rods ...
of part of a layer
File:Chromium(III)-chloride-layers-stacking-from-monoclinic-xtal-3D-balls.png, Stacking of layers
Chromium(III) chloride hydrates
The hydrated chromium(III) chlorides display the somewhat unusual property of existing in a number of distinct chemical forms (isomers), which differ in terms of the number of chloride anions that are coordinated to Cr(III) and the
water of crystallization
In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions. In some contexts, water of crystallization i ...
. The different forms exist both as solids and in aqueous solutions. Several members are known of the series of
3−''z''(H2O)''n''">rCl3−''z''(H2O)''n''sup>''z''+. The common hexahydrate can be more precisely described as
2(H2O)4">rCl2(H2O)4l. It consists of the cation ''trans''-
2(H2O)4">rCl2(H2O)4sup>+ and additional molecules of water and a chloride anion in the lattice. Two other hydrates are known, pale green
2O)5">rCl(H2O)5l
2 and violet
2O)6">r(H2O)6l
3. Similar isomerism is seen with other chromium(III) compounds.
Preparation
Anhydrous chromium(III) chloride may be prepared by
chlorination Chlorination may refer to:
* Chlorination reaction
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transform ...
of
chromium metal directly, or indirectly by
carbothermic chlorination of
chromium(III) oxide
Chromium(III) oxide (or chromia) is an inorganic compound with the formula . It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.
Structure and properties
has the corundum ...
at 650–800 °C
:Cr
2O
3 + 3 C + 3 Cl
2 → 2 CrCl
3 + 3 CO
The hydrated chlorides are prepared by treatment of chromate with hydrochloric acid and aqueous methanol.
Reactions
Slow reaction rates are common with chromium(III) complexes. The low reactivity of the d
3 Cr
3+ ion can be explained using
crystal field theory Crystal field theory (CFT) describes the breaking of degeneracies of electron orbital states, usually ''d'' or ''f'' orbitals, due to a static electric field produced by a surrounding charge distribution (anion neighbors). This theory has been used ...
. One way of opening CrCl
3 up to substitution in solution is to reduce even a trace amount to CrCl
2, for example using
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
in
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
. This chromium(II) compound undergoes substitution easily, and it can exchange electrons with CrCl
3 via a
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
bridge, allowing all of the CrCl
3 to react quickly.
With the presence of some chromium(II), however, solid CrCl
3 dissolves rapidly in water. Similarly,
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
substitution reactions of solutions of
2(H2O)4">rCl2(H2O)4sup>+ are accelerated by chromium(II) catalysts.
With molten
alkali metal chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
s such as
potassium chloride, CrCl
3 gives salts of the type M
3CrCl
6 and K
3Cr
2Cl
9, which is also octahedral but where the two chromiums are linked via three chloride bridges.
The hexahydrate can also be dehydrated with
thionyl chloride
Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...
:
:CrCl
3 + 6 SOCl
2 → CrCl
3 + 6 SO
2 + 12
HCl
Complexes with organic ligands
CrCl
3 is a
Lewis acid, classified as "hard" according to the
Hard-Soft Acid-Base theory. It forms a variety of adducts of the type
3L3">rCl3L3sup>''z'', where L is a
Lewis base. For example, it reacts with
pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a ...
() to form the
pyridine complex:
:CrCl
3 + 3 C
5H
5N → CrCl
3(C
5H
5N)
3
Treatment with
trimethylsilylchloride in THF gives the anhydrous THF complex:
:CrCl
3 + 12 Me
3SiCl → CrCl
3(THF)
3 + 6 (Me
3Si)
2O + 12 HCl
Precursor to organochromium complexes
Chromium(III) chloride is used as the precursor to many
organochromium compounds, for example
bis(benzene)chromium, an analogue of
ferrocene:
:
Phosphine complexes derived from CrCl
3 catalyse the trimerization of
ethylene to
1-hexene.
Use in organic synthesis
One niche use of CrCl
3 in
organic synthesis is for the ''in situ'' preparation of
chromium(II) chloride
Chromium(II) chloride describes inorganic compounds with the chemical formula, formula Chromium, CrChlorine, Cl2(H2O)n. The anhydrous solid is white when pure, however commercial samples are often grey or green; it is hygroscopic and readily disso ...
, a reagent for the reduction of
alkyl halide
The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely us ...
s and for the synthesis of (''E'')-alkenyl halides. The reaction is usually performed using two
mole
Mole (or Molé) may refer to:
Animals
* Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America
* Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
s of CrCl
3 per mole of
lithium aluminium hydride, although if aqueous acidic conditions are appropriate
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
and
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
may be sufficient.
:
Chromium(III) chloride has also been used as a Lewis acid in organic reactions, for example to catalyse the nitroso
Diels-Alder reaction.
Dyestuffs
A number of chromium-containing dyes are used commercially for wool. Typical dyes are
triarylmethanes consisting of ortho-hydroxylbenzoic acid derivatives.
[Thomas Gessner and Udo Mayer "Triarylmethane and Diarylmethane Dyes" in ]Ullmann's Encyclopedia of Industrial Chemistry
''Ullmann's Encyclopedia of Industrial Chemistry'' is a major reference work related to industrial chemistry by Chemist Fritz Ullmann, first published in 1914, and exclusively in German as "Enzyklopädie der Technischen Chemie" until 1984.
Hist ...
2002, Wiley-VCH
Wiley-VCH is a German publisher owned by John Wiley & Sons. It was founded in 1921 as Verlag Chemie (meaning "Chemistry Press": VCH stands for ''Verlag Chemie'') by two German learned societies. Later, it was merged into the German Chemical Soci ...
, Weinheim.
Precautions
Although
trivalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.
Description
The combining capacity, or affinity of an ...
chromium is far less poisonous than
hexavalent, chromium salts are generally considered toxic.
References
Further reading
*''Handbook of Chemistry and Physics'', 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
*''The Merck Index'', 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
*J. March, ''Advanced Organic Chemistry'', 4th ed., p. 723, Wiley, New York, 1992.
*K. Takai, in ''Handbook of Reagents for Organic Synthesis, Volume 1: Reagents, Auxiliaries and Catalysts for C-C Bond Formation'', (R. M. Coates, S. E. Denmark, eds.), pp. 206–211, Wiley, New York, 1999.
External links
International Chemical Safety Card 1316(anhydr. CrCl
3)
(CrCl
3·6H
2O)
*
ttps://www.cdc.gov/niosh/npg/npgd0140.html NIOSH Pocket Guide to Chemical Hazardsbr>
IARC Monograph "Chromium and Chromium compounds"
{{Chlorides
Chromium(III) compounds
Chlorides
Metal halides
Coordination complexes