Pyridinium chlorochromate

Pyridinium chlorochromate (PCC) is a yellow-orange salt with the formula [C₅H₅NH]⁺[CrO₃Cl]⁻. It is a reagent in organic synthesis used primarily for oxidation of alcohols to form carbonyls. 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.^[1]

Pyridinium chlorochromate

Chemical structure of pyridinium chlorochromate
Ball-and-stick model of the pyridinium cation Ball-and-stick model of the chlorochromate anion
Names
IUPAC name Pyridinium chlorochromate
Other names PCC; Corey-Suggs reagent
Identifiers
CAS Number	26299-14-9 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:176795
ChemSpider	10608386 Y
ECHA InfoCard	100.043.253
EC Number	247-595-5
PubChem CID	129695875
UNII	DTV5HU1N27 Y
CompTox Dashboard (EPA)	DTXSID40893953
InChI InChI=1S/C5H5N.ClH.Cr.3O/c1-2-4-6-5-3-1;;;;;/h1-5H;1H;;;;/q;;+1;;;-1 N Key: LEHBURLTIWGHEM-UHFFFAOYSA-N N InChI=1/C5H5N.ClH.Cr.3O/c1-2-4-6-5-3-1;;;;;/h1-5H;1H;;;;/q;;+1;;;-1/rC5H5N.ClCrO3/c1-2-4-6-5-3-1;1-2(3,4)5/h1-5H;/q;-1/p+1 Key: LEHBURLTIWGHEM-YOEUSAHMAN
SMILES C1=CC=[NH+]C=C1.[O-][Cr](=O)(=O)Cl
Properties
Chemical formula	C5H6ClCrNO3
Molar mass	215.56 g/mol
Appearance	yellow-orange solid[1]
Melting point	205 °C (401 °F; 478 K)
Solubility in other solvents	soluble in acetone, acetonitrile, THF
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Toxic, oxidizer, carcinogenic, strong environmental pollutant
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H272, H317, H350, H410
Precautionary statements	P201, P221, P273, P280, P302+P352, P308+P313
NFPA 704 (fire diamond)	[2] 2 1 2 OX
Safety data sheet (SDS)	external SDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Pyridinium chlorochromate in a vial

Structure and preparation

PCC consists of a pyridinium cation, [C₅H₅NH]⁺, and a tetrahedral chlorochromate anion, [CrO₃Cl]⁻. Related salts are also known, such as 1-butylpyridinium chlorochromate, [C₅H₅N(C₄H₉)][CrO₃Cl] and potassium chlorochromate.

PCC is commercially available. Discovered by accident,^[3] the reagent was originally prepared via addition of pyridine into a cold solution of chromium trioxide in concentrated hydrochloric acid:^[4]

C₅H₅N + HCl + CrO₃ → [C₅H₅NH][CrO₃Cl]

In one alternative method, formation of toxic chromyl chloride (CrO₂Cl₂) fumes during the making of the aforementioned solution were 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.^[5]

Uses

Oxidation of alcohols

PCC is used as an oxidant. In particular, it has proven to be highly effective in oxidizing primary and secondary alcohols to aldehydes and ketones, respectively. The reagent is more selective than the related Jones' Reagent, so there is little chance of over-oxidation to form carboxylic acids if acidified potassium permanganate 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.^[6][7][8] The general reaction is:

2 [C₅H₅NH][CrO₃Cl] + 3 R₂CHOH → 2 [C₅H₅NH]Cl + Cr₂O₃ + 3 R₂C=O + 3 H₂O

For example, the triterpene lupeol was oxidized to lupenone:^[9]

Babler oxidation

Main article: Babler oxidation

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

The Babler-Dauben oxidation of cyclic tertiary allylic alcohols to cyclic enones using PCC.

This type of oxidative transposition reaction has been synthetically utilized, e.g. for the synthesis of morphine.^[10]

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 cationic cyclization, is illustrated by the conversion of (−)-citronellol to (−)-pulegone.

PCC also effects allylic oxidations, for example, in conversion of dihydrofurans to furanones.^[1]

Related reagents

Other more convenient or less toxic reagents for oxidizing alcohols include dimethyl sulfoxide, 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 compounds.

See also

• Oxidation with chromium(VI)-amine complexes