Pyridinium chlorochromate (PCC) is a yellow-orange

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantiti ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a '' chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betw ...

₅H₅NHsup>+ rO₃Clsup>−. It is a

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

used primarily for

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of se ...

s to form

carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...

s. A variety of related compounds are known with similar reactivity. PCC offers the advantage of the selective oxidation of alcohols to aldehydes or ketones, whereas many other reagents are less selective.

Structure and preparation

PCC consists of 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 oft ...

cation, ₅H₅NHsup>+, and a tetrahedral chlorochromate anion, rO₃Clsup>−. Related salts are also known, such as 1-butylpyridinium chlorochromate, ₅H₅N(C₄H₉)CrO₃Cl] and potassium chlorochromate. PCC is commercially available. Discovered by accident, the reagent was originally prepared via addition of

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 ...

into a cold 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 ...

in concentrated

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. It is a component of the gastric acid in the digesti ...

: :C₅H₅N + HCl + CrO₃ → ₅H₅NHCrO₃Cl] In one alternative method, formation of

chromyl chloride Chromyl chloride is the inorganic compound with the formula CrO2Cl2. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal complexes. Preparation Chromyl chloride can be prepared by th ...

(CrO₂Cl₂) fume during the making of the aforementioned solution was minimized by simply changing the order of addition: a cold solution of pyridine in concentrated hydrochloric acid was added to solid chromium trioxide under stirring.

Uses

Oxidation of alcohols

PCC is used as an

oxidant An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an ox ...

. In particular, it has proven to be highly effective in oxidizing 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 bon ...

s, respectively. The reagent is more selective than the related Jones' Reagent, so there is little chance of over-oxidation to form

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 if acidified

potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution. Potassium permanganate is widely used in the c ...

is used as long as water is not present in the reaction mixture. A typical PCC oxidation involves addition of an alcohol to a suspension of PCC in

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 w ...

. The general reaction is: :2 ₅H₅NHCrO₃Cl] + 3 R₂CHOH → 2 ₅H₅NHl + Cr₂O₃ + 3 R₂C=O + 3 H₂O For example, the

triterpene Triterpenes are a class of chemical compounds composed of three terpene units with the molecular formula C30H48; they may also be thought of as consisting of six isoprene units. Animals, plants and fungi all produce triterpenes, including squale ...

lupeol was oxidized to lupenone: : Example_of_PCC_oxidation_of_secondary_alcohol

Babler oxidation

With tertiary alcohols, the chromate ester formed from PCC can isomerize ''via'' a ,3sigmatropic reaction and following oxidation yield an enone, in a reaction known as the Babler oxidation: Dauben oxidation en

This type of oxidative transposition reaction has been synthetically utilized, ''e.g.'' for the synthesis of

morphine Morphine is a strong opiate that is found naturally in opium, a dark brown resin in poppies (''Papaver somniferum''). It is mainly used as a pain medication, and is also commonly used recreationally, or to make other illicit opioids. There ...

. Using other common oxidants in the place of PCC usually leads to dehydration, because such alcohols cannot be oxidized directly.

Other reactions

PCC also converts suitable unsaturated alcohols and aldehydes to cyclohexenones. This pathway, an oxidative

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

ic cyclization, is illustrated by the conversion of (−)-

citronellol Citronellol, or dihydrogeraniol, is a natural acyclic monoterpenoid. Both enantiomers occur in nature. (+)-Citronellol, which is found in citronella oils, including ''Cymbopogon nardus'' (50%), is the more common isomer. (−)-Citronellol is w ...

to (−)- pulegone. PCC also effects allylic oxidations, for example, in conversion of dihydrofurans to furanones.

Related reagents

Other more convenient or less toxic reagents for oxidizing

s include

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 ...

, which is used in Swern and Pfitzner–Moffatt oxidations, and hypervalent iodine compounds, such as the Dess–Martin periodinane.

Safety

One disadvantage to the use of PCC is its toxicity, which it shares with other

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 ...

compounds.

References

External links

IARC Monographs Supplement 7, Chromium and Chromium Compounds

National Pollutant Inventory, Chromium(VI) Compounds Fact Sheets
{{Chromium compounds Chromates Oxidizing agents Pyridinium compounds IARC Group 1 carcinogens