Pyridinium chlorochromate is a reddish orange solid reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. Pyridinium chlorochromate, or
PCC, will not fully oxidize the alcohol to the
carboxylic acidCarboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -COH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids are called...
as does the Jones reagent. A disadvantage to using PCC is its toxicity. PCC was developed by
Elias James CoreyElias James Corey is an American organic chemist. In 1990 he won the Nobel Prize in Chemistry "for his development of the theory and methodology of organic synthesis", specifically retrosynthetic analysis...
and William Suggs in 1975.
Pyridinium dichromate is a similar
oxidizing agentAn oxidizing agent can be defined as either:#a chemical compound that readily transfers oxygen atoms, or#a substance that gains electrons in a redox chemical reaction...
, which has the advantage of being less acidic.
The original preparation by Corey involves adding one equivalent of
pyridinePyridine is a simple aromatic heterocyclic organic compound with the chemical formula C
5H
5N used as a precursor to agrochemicals and pharmaceuticals, and is also an important solvent and reagent. It is structurally related to benzene, wherein one CH group in the aromatic...
to a solution of one equivalent of
chromium(VI) trioxideChromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and...
and concentrated
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
:
-
- C5H5N + HCl + CrO3 → [C5H5NH][CrO3Cl]
Agarwal et al.
Pyridinium chlorochromate is a reddish orange solid reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. Pyridinium chlorochromate, or
PCC, will not fully oxidize the alcohol to the
carboxylic acidCarboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula -COH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids are called...
as does the Jones reagent. A disadvantage to using PCC is its toxicity. PCC was developed by
Elias James CoreyElias James Corey is an American organic chemist. In 1990 he won the Nobel Prize in Chemistry "for his development of the theory and methodology of organic synthesis", specifically retrosynthetic analysis...
and William Suggs in 1975.
Pyridinium dichromate is a similar
oxidizing agentAn oxidizing agent can be defined as either:#a chemical compound that readily transfers oxygen atoms, or#a substance that gains electrons in a redox chemical reaction...
, which has the advantage of being less acidic.
Preparation
The original preparation by Corey involves adding one equivalent of
pyridinePyridine is a simple aromatic heterocyclic organic compound with the chemical formula C
5H
5N used as a precursor to agrochemicals and pharmaceuticals, and is also an important solvent and reagent. It is structurally related to benzene, wherein one CH group in the aromatic...
to a solution of one equivalent of
chromium(VI) trioxideChromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and...
and concentrated
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
:
-
- C5H5N + HCl + CrO3 → [C5H5NH][CrO3Cl]
Agarwal et al. presented an alternative synthesis that avoids the harmful side product
chromyl chlorideChromyl 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...
(CrO
2Cl
2).
Chromium(VI) oxide is treated with
pyridiniumPyridine is a simple aromatic heterocyclic organic compound with the chemical formula C
5H
5N used as a precursor to agrochemicals and pharmaceuticals, and is also an important solvent and reagent. It is structurally related to benzene, wherein one CH group in the aromatic...
chloride:
-
- [C5H5NH+]Cl− + CrO3 → [C5H5NH][CrO3Cl]
Properties and uses
PCC is primarily used as an oxidant. In particular, it has proven to be highly effective in oxidizing primary and secondary alcohols to aldehydes and ketones, respectively. Rarely does over-oxidation occur (whether intentionally or accidentally) to form carboxylic acids. A typical PCC oxidation involves addition of the alcohol to a suspension of PCC in
dichloromethaneDichloromethane is the organic compound with the formula CH
2Cl
2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. More than 500,000 tons were produced in 1991. Although it is not miscible with water, it is miscible with many organic...
. A sample reaction would be:
-
- C5H5NHCrO3Cl + R2CHOH → C5H5NHCl + H2CrO3 + R2C=O
In practice the chromium byproduct deposits with pyridine as a sticky black tar, which can complicate matters. Addition of an inert adsorbent such as crushed molecular sieves or silica gel allows the sticky byproduct to adsorb to the surface, and makes workup easier.
PCC is also remarkable for its high selectivity. For example, when oxidizing tertiary
allylAn allyl group is an alkene hydrocarbon group with the formula H2C=CH-CH2-. It is made up of a vinyl group, CH2=CH-, attached to a methylene -CH2. For example allyl alcohol has the structure H2C=CH-CH2OH. Another example of a simple...
alcohols, unsaturated
aldehydeAn aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom , is called the aldehyde group...
s are observed as the sole product. This reaction is known as the Babler oxidation. Otherwise such oxidations commonly afford dienes as by-products resulting from dehydration.
Another notable oxidative reaction of PCC is its efficient conversion of unsaturated alcohols or aldehydes to
cyclohexenoneCyclohexenone is an organic compound which is a versatile intermediate used in the synthesis of a variety of chemical products such as pharmaceuticals and fragrances...
s. This particular pathway is known as oxidative cationic cyclization.
Controversy
PCC is controversial as it contains
chromium(VI)Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via conversion to sodium dichromate. Approximately of hexavalent chromium were produced in 1985. Other hexavalent chromium compounds are chromium...
. Other methods for oxidizing alcohols using less toxic reagents have been introduced and are favored by
green chemistsGreen chemistry, also called sustainable chemistry, is a chemical philosophy encouraging the design of products and processes that reduce or eliminate the use and generation of hazardous substances. Whereas environmental chemistry is the chemistry of the natural environment, and of pollutant...
:
- DMSO
Dimethyl sulfoxide is the chemical compound with the formula
2SO. It was first synthesized in 1866 by the Russian scientist Alexander Zaytsev, who reported his findings in a German chemistry journal in 1867...
-based oxidations (Swern oxidationThe Swern oxidation, named after Daniel Swern, is a chemical reaction whereby a primary or secondary alcohol is oxidized to an aldehyde or ketone using oxalyl chloride, dimethyl sulfoxide and an organic base, such as triethylamine...
, Moffatt oxidation)
- hypervalent iodine based oxidation (such as the Dess-Martin periodinane
Dess-Martin periodinane is a chemical reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. This periodinane has several advantages over chromium- and DMSO-based oxidants that include milder conditions, shorter reaction times, higher yields, and simplified workups...
)
External links