Pyridinium chlorochromate
Encyclopedia
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 a primary alcohol to the carboxylic acid
as does the Jones reagent. A disadvantage to using PCC is its toxicity. PCC was developed by Elias James Corey
and William Suggs in 1975.
Pyridinium dichromate is a similar oxidizing agent
, which has the advantage of being less acidic.
to a solution of one equivalent of chromic acid
and concentrated hydrochloric acid
:
Agarwal et al. presented an alternative synthesis that avoids the harmful side product chromyl chloride
(CrO2Cl2).
Chromium(VI) oxide is treated with pyridinium
chloride:
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 dichloromethane
. A sample reaction would be:
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.
In addition to simple oxidations of hydroxyl groups, rearrangements are possible. For example, tertiary alcohols cannot be oxidized directly. However, in the Babler oxidation, the chromate ester
formed with PCC and an allylic tertiary alcohol can isomerize via a [3,3]-sigmatropic reaction
before the carbonyl-forming oxidation step. Other common oxidants usually lead to simple dehydration rather than any oxidation reaction at tertiary hydroxyl centers.
Another notable oxidative reaction of PCC is its efficient conversion of unsaturated alcohols or aldehydes to cyclohexenone
s. This particular pathway is known as oxidative cationic cyclization.
, a known carcinogen
. Other methods for oxidizing alcohols using less toxic reagents have been introduced and are favored by green chemists
:
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
as does the Jones reagent. A disadvantage to using PCC is its toxicity. PCC was developed by Elias James Corey
Elias James Corey
Elias 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 agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant 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
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
to a solution of one equivalent of chromic acid
Chromic 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...
and concentrated 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....
:
- C5H5N + HCl + CrO3 → [C5H5NH][CrO3Cl]
Agarwal et al. presented an alternative synthesis that avoids the harmful side product chromyl chloride
Chromyl chloride
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 derivative chromate, [CrO4]2−; both are...
(CrO2Cl2).
Chromium(VI) oxide is treated with pyridinium
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
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 alcoholsAlcohol oxidation
Alcohol oxidation is an important organic reaction. Primary alcohols can be oxidized either to aldehydes or to carboxylic acids , while the oxidation of secondary alcohols normally terminates at the ketone stage...
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 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...
. 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
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
, and makes workup easier.
In addition to simple oxidations of hydroxyl groups, rearrangements are possible. For example, tertiary alcohols cannot be oxidized directly. However, in the Babler oxidation, the chromate ester
Chromate ester
A chromate ester is a chemical structure that contains a chromium atom in a +6 oxidation state that is connected via an oxygen linkage to a carbon atom. The Cr itself is in its chromate form, with several oxygens attached, and the Cr–O–C attachment makes this chemical group structurally similar to...
formed with PCC and an allylic tertiary alcohol can isomerize via a [3,3]-sigmatropic reaction
Sigmatropic reaction
A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular process. The name sigmatropic is the result of a compounding of the long-established sigma designation from single carbon-carbon...
before the carbonyl-forming oxidation step. Other common oxidants usually lead to simple dehydration rather than any oxidation reaction at tertiary hydroxyl centers.
Another notable oxidative reaction of PCC is its efficient conversion of unsaturated alcohols or aldehydes to cyclohexenone
Cyclohexenone
Cyclohexenone 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
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...
, a known 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...
. Other methods for oxidizing alcohols using less toxic reagents have been introduced and are favored by green chemists
Green chemistry
Green chemistry, also called sustainable chemistry, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances...
:
- DMSODimethyl sulfoxideDimethyl sulfoxide is an organosulfur compound with the formula 2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water...
-based oxidations (Swern oxidationSwern 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 periodinaneDess-Martin periodinaneDess–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, simplified workups,...
)