The pyridinium dichromate (PDC) or Cornforth reagent is a
pyridinium
Pyridinium refers to the cation . It is the conjugate acid of pyridine. Many related cations are known involving substituted pyridines, e.g. picolines, lutidines, collidines. They are prepared by treating pyridine with acids.
As pyridine is ...
salt of
dichromate with the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
5H5NH">5H5NHsub>2
2O7">r2O7 This compound is named after the Australian-British chemist Sir
John Warcup Cornforth (b. 1917) who introduced it in 1962. The Cornforth reagent is a strong oxidizing agent which can convert
primary and secondary
alcohols to
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s and
ketone
In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...
s respectively. In its chemical structure and functions it is closely related to other compounds made from
hexavalent chromium
Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the ...
oxide, such as
pyridinium chlorochromate
Pyridinium chlorochromate (PCC) is a yellow-orange salt with the formula 5H5NHsup>+ rO3Clsup>−. It is a reagent in organic synthesis used primarily for oxidation of alcohols to form carbonyls. A variety of related compounds are known with simil ...
and
Collins reagent
Collins reagent is the complex of chromium(VI) oxide with pyridine in dichloromethane. This metal-pyridine complex, a red solid, is used to oxidize primary alcohols to the corresponding aldehydes and secondary alcohols to the corresponding keton ...
. Because of their toxicity, these reagents are rarely used nowadays.
Synthesis and properties
The Cornforth reagent is prepared by slow addition of a concentrated aqueous solution of
chromium trioxide
Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.
This compound is a dark-purple ...
to
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 d ...
. The reaction may cause explosion, which is avoided by thoroughly dissolving the trioxide in water and cooling the solution by ice. The product is filtered, washed with acetone and dried, yielding an orange powder. The powder is stable in air, not particularly hygroscopic and has an almost neutral
pH that facilitates its handling; it is only slightly acidic owing to the presence of pyridinium cations. The Cornforth reagent is readily soluble in water,
dimethylformamide
Dimethylformamide is an organic compound with the formula ( CH3)2NC(O)H. Commonly abbreviated as DMF (although this initialism is sometimes used for dimethylfuran, or dimethyl fumarate), this colourless liquid is miscible with water and the majo ...
and
dimethyl sulfoxide
Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds a ...
(DMSO). It is poorly soluble in
acetone
Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour.
Acetone is miscib ...
and chlorinated organic solvents, such as
dichloromethane
Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...
, and forms
suspensions
In chemistry, a suspension is a heterogeneous mixture of a fluid that contains solid particles sufficiently large for sedimentation. The particles may be visible to the naked eye, usually must be larger than one micrometer, and will eventually ...
.
[Steven V. Le]
Oxidation
Elsevier, 1992, p. 272
Applications
The Cornforth reagent is a strong oxidizing agent which can convert primary alcohols to aldehydes and secondary alcohols to ketones, both as a solution or suspension. This application was first mentioned in 1969, but fully developed only in 1979 by E. J. Corey and G. Schmidt. They mentioned that reaction of ''saturated'' primary alcohols with PDC, using dimethylformamide as solvent, results in oxidation to carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s rather than aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s. However, no oxidation to carboxylic acids occurs on allylic and benzylic primary alcohols.
:
The oxidation is usually carried out at ambient conditions, in nearly neutral pH conditions, in dimethylformamide or dichloromethane or their mixture. The choice of solvent or their ratio affects the reaction rate; in particular, higher content of dimethylformamide results in stronger oxidation. The slow oxidation rate for some alcohols can be accelerated by the addition of molecular sieve
A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecu ...
s, organic acids or acetic anhydride
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a col ...
or of their combinations. The acceleration by molecular sieves works best when their pore diameter is about 0.3 nm, and it is apparently unrelated to their water absorption capability. Among organic acids, acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
, pyridinium trifluoroacetate
Trifluoroacetic acid (TFA) is an organofluorine compound with the chemical formula CF3CO2H. It is a structural analogue of acetic acid with all three of the acetyl group's hydrogen atoms replaced by fluorine atoms and is a colorless liquid with a ...
or pyridinium tosyl
In organic chemistry, a toluenesulfonyl group (tosyl group, abbreviated Ts or Tos) is a univalent functional group with the chemical formula –. It consists of a Toluene, tolyl group, –, joined to a sulfonyl group, ––, with the open vale ...
ate can be added, the first one being most efficient and easiest to remove. The achieved acceleration is remarkable, but the reaction inevitably turns from neutral (pH) to acidic. Comparable acceleration is achieved with acetic anhydride, which is used in sugar and nucleoside
Nucleosides are glycosylamines that can be thought of as nucleotides without a phosphate group. A nucleoside consists simply of a nucleobase (also termed a nitrogenous base) and a five-carbon sugar (ribose or 2'-deoxyribose) whereas a nucleotide ...
chemistry. Reaction acceleration depends not only on the additives but also on their form, so all reagents are preferred dry and freshly prepared, and PDC and molecular sieves should be finely ground. The disadvantage of the accelerators is that they may simultaneously promote several oxidation routes thereby reducing the selectivity of the reaction.[
In its chemical structure and functions, the Cornforth reagent is closely related to other pyridinium salts of hexavalent chromium oxide, such as ]pyridinium chlorochromate
Pyridinium chlorochromate (PCC) is a yellow-orange salt with the formula 5H5NHsup>+ rO3Clsup>−. It is a reagent in organic synthesis used primarily for oxidation of alcohols to form carbonyls. A variety of related compounds are known with simil ...
yHCrO3Cl] and to pyridine complexes such as the Collins reagent
Collins reagent is the complex of chromium(VI) oxide with pyridine in dichloromethane. This metal-pyridine complex, a red solid, is used to oxidize primary alcohols to the corresponding aldehydes and secondary alcohols to the corresponding keton ...
, CrO3·2Py in dichloromethane and the Sarett oxidation, Sarret reagent, CrO3·2Py in pyridine.[
]
Safety issues
The Cornforth reagent is very toxic to aquatic life and may cause long-term damage to the environment if released in large amounts. It irritates skin and mucous membranes and may induce allergic reactions; it is carcinogenic. The maximum allowable concentration varies between 0.01 and 0.1 mg·m−3 in air depending on the country. Because it contains hexavalent chromium
Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the ...
, it is a suspected carcinogen, and as a strong oxidant, pyridinium dichromate promotes fires, releasing carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
, carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
and toxic metal smoke. The fire can be extinguished by water or CO2.[
]
See also
*Oxidation with chromium(VI)-amine complexes Oxidation with chromium(VI) complexes involves the conversion of alcohols to carbonyl compounds or more highly oxidized products through the action of molecular chromium(VI) oxides and salts. The principal reagents are Collins reagent, PDC, and PCC. ...
References
{{Chromates and dichromates
Reagents for organic chemistry
Dichromates
Pyridinium compounds
Oxidizing agents