Chromyl chloride
Encyclopedia
Chromyl chloride is a chemical compound with the formula CrO2Cl2. This compound is an opaque dark blood-red liquid at room temperature and pressure. It is tetrahedral, somewhat like SO2Cl2
. CrO2Cl2 is similar to the most commonly encountered chromium(VI) derivative chromate
, [CrO4]2−; both are tetrahedral Cr(VI) compounds. However, they differ in both oxidizing power and in physical properties: one is a liquid and the other forms salts.
CrO2Cl2 is a neutral molecular species. This means that in the liquid and solid forms, the individual CrO2Cl2 entities interact purely via van der Waals bonding
. Such weak bonding leads to low melting and boiling points, which is related to the fact that it is a distillable liquid.
The diminished oxidizing power of [CrO4]2− vs. CrO2Cl2 can be ascribed to its anionic nature, which diminishes its electron affinity. Also, chloride is a poorer pi-donor ligand than is oxide.
, or the reaction with K2CrO4
with concentrated HCl, followed by H2SO4
as a dehydrating agent. The heavy CrO2Cl2 separates as an immiscible, dense liquid. It then can be separated using a separatory funnel or by simple distillation, which is normally performed. CrO2Cl2 can also be prepared using other chlorinating agents such as PCl5 and TiCl4
, or by addition of concentrated sulphuric acid to an intimate mixture of sodium chloride
and potassium dichromate, followed by gentle distillation.
, e.g. causing spontaneous combustion when dripped onto amorphous sulfur
; can also oxidize toluene
into benzaldehyde
. Its electrophilicity is demonstrated by its rapid hydrolysis
to give chromic and hydrochloric acid
s:
Its high reactivity toward water is further indicated by the fact that CrO2Cl2 fumes in moist air.
s, iodides and cyanides, so this test is therefore specific for chlorides. The test is related to the synthesis shown above, exposure of CrO42− to HCl.
s to aldehyde
s. Internal alkenes give alpha-chloroketones or related derivatives. It will also attack benzyl
ic methyl group
s to give aldehydes via the Étard reaction
.
Apart from this it can also be used for testing the absence of nitrate ions
, TiCl4
, and SO2Cl2
. Typical for other electrophilic chlorides, chlorocarbons are excellent solvents, especially dichloromethane
As a further practical complication, chromyl chloride attacks most greases.
and hexavalent chromium (CrVI)
Acute: HCl can be acutely lethal. Exposure to chromyl chloride vapour irritates the respiratory system
and severely irritates the eyes, and the liquid burns the skin and eyes. Ingestion would cause severe internal damage.
Chronic: CrVI can produce chromosomal aberrations and is a human carcinogen
via inhalation. Frequent exposure of the skin to chromyl chloride may result in ulceration.
Thus, CrO2Cl2 should be carefully handled in a well ventilated area. CrO2Cl2 is so aggressive that its storage can be problematic as it attacks rubber and most plastics as well as greases.
Sulfuryl chloride
Sulfuryl chloride is an inorganic compound with the formula SO2Cl2. At room temperature, it is a colorless liquid with a pungent odor. Sulfuryl chloride is not found in nature, as can be inferred from its rapid hydrolysis....
. CrO2Cl2 is similar to the most commonly encountered chromium(VI) derivative chromate
Chromate
Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...
, [CrO4]2−; both are tetrahedral Cr(VI) compounds. However, they differ in both oxidizing power and in physical properties: one is a liquid and the other forms salts.
CrO2Cl2 is a neutral molecular species. This means that in the liquid and solid forms, the individual CrO2Cl2 entities interact purely via van der Waals bonding
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
. Such weak bonding leads to low melting and boiling points, which is related to the fact that it is a distillable liquid.
The diminished oxidizing power of [CrO4]2− vs. CrO2Cl2 can be ascribed to its anionic nature, which diminishes its electron affinity. Also, chloride is a poorer pi-donor ligand than is oxide.
Preparation
CrO2Cl2 is prepared by the action of HCl on CrO3Chromic acid
The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...
, or the reaction with K2CrO4
Potassium chromate
Potassium chromate is a yellow chemical indicator used for identifying concentrations of chloride ions in a salt solution with silver nitrate...
with concentrated HCl, followed by H2SO4
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
as a dehydrating agent. The heavy CrO2Cl2 separates as an immiscible, dense liquid. It then can be separated using a separatory funnel or by simple distillation, which is normally performed. CrO2Cl2 can also be prepared using other chlorinating agents such as PCl5 and TiCl4
Titanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile...
, or by addition of concentrated sulphuric acid to an intimate mixture of sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
and potassium dichromate, followed by gentle distillation.
- K2Cr2O7 + 4NaClSodium chlorideSodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
+ 6H2SO4Sulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
→ 2CrO2Cl2 + 2KHSO4 + 4NaHSO4Sodium bisulfateSodium bisulfate, also known as sodium hydrogen sulfate , is an acid salt. It is a dry granular product that can be safely shipped and stored. The anhydrous form is hygroscopic. Solutions of sodium bisulfate are acidic, with a 1M solution having a pH of Sodium bisulfate, also known as sodium...
+3H2OWaterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
Chemical properties
CrO2Cl2 is highly electrophilic and an aggressive oxidizing agentOxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
, e.g. causing spontaneous combustion when dripped onto amorphous sulfur
Allotropes of sulfur
There are a large number of allotropes of sulfur. In this respect, sulfur is second only to carbon.The most common form found in nature is yellow orthorhombic α-sulfur, which contains puckered rings of . Chemistry students may have seen "plastic sulfur"; this is not an allotrope but a mixture of...
; can also oxidize toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
into benzaldehyde
Benzaldehyde
Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...
. Its electrophilicity is demonstrated by its rapid hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
to give chromic and hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
s:
- CrO2Cl2 + 2H2O → H2CrO4 + 2HCl
Its high reactivity toward water is further indicated by the fact that CrO2Cl2 fumes in moist air.
Chromyl chloride test for chloride
The chromyl chloride test entails heating a sample suspected of containing chloride with potassium dichromate and concentrated sulfuric acid. If chloride is present, chromyl chloride is formed and red fumes of CrO2Cl2 are evident. If there is no chloride present, no red fumes are produced. No analogous compounds are formed with fluorides, bromideBromide
A bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...
s, iodides and cyanides, so this test is therefore specific for chlorides. The test is related to the synthesis shown above, exposure of CrO42− to HCl.
Reagent for oxidation of alkenes
Depending on solvent, CrO2Cl2 oxidizes terminal alkeneAlkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s to aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s. Internal alkenes give alpha-chloroketones or related derivatives. It will also attack benzyl
Benzyl
In organic chemistry, benzyl is the term used to describe the substituent or molecular fragment possessing the structure C6H5CH2-. Benzyl features a benzene ring attached to a CH2 group.-Nomenclature:...
ic methyl group
Methyl group
Methyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...
s to give aldehydes via the Étard reaction
Etard Reaction
The Étard reaction is a chemical reaction that involves the direct oxidation of an aromatic or heterocyclic bound methyl group to an aldehyde using chromyl chloride. It is named for the French chemist Alexandre Léon Étard...
.
Apart from this it can also be used for testing the absence of nitrate ions
Compatible solvents
CrO2Cl2 is such an aggressive reagent that solvents must be chosen judiciously. In light of its high reactivity toward water, CrO2Cl2 can be expected to decompose upon exposure to alcohols, similar to the behavior of other highly electrophilic chlorides such as VOCl3Vanadium oxytrichloride
Vanadium oxytrichloride is the inorganic compound with the formula VOCl3. This distillable liquid hydrolyzes readily in air and is a strong oxidant. It is used as a reagent in organic synthesis.-Properties:...
, TiCl4
Titanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile...
, and SO2Cl2
Sulfuryl chloride
Sulfuryl chloride is an inorganic compound with the formula SO2Cl2. At room temperature, it is a colorless liquid with a pungent odor. Sulfuryl chloride is not found in nature, as can be inferred from its rapid hydrolysis....
. Typical for other electrophilic chlorides, chlorocarbons are excellent solvents, especially dichloromethane
Dichloromethane
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
As a further practical complication, chromyl chloride attacks most greases.
Safety considerations
CrO2Cl2 reacts with water to release hydrochloric acid (HCl)Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
and hexavalent chromium (CrVI)
Hexavalent chromium
Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately of hexavalent chromium were produced in 1985...
Acute: HCl can be acutely lethal. Exposure to chromyl chloride vapour irritates the respiratory system
Respiratory system
The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...
and severely irritates the eyes, and the liquid burns the skin and eyes. Ingestion would cause severe internal damage.
Chronic: CrVI can produce chromosomal aberrations and is a human carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...
via inhalation. Frequent exposure of the skin to chromyl chloride may result in ulceration.
Thus, CrO2Cl2 should be carefully handled in a well ventilated area. CrO2Cl2 is so aggressive that its storage can be problematic as it attacks rubber and most plastics as well as greases.