Pyridinium chlorochromate

Pyridinium chlorochromate

Names
IUPAC name Pyridinium chlorochromate
Other names PCC
Identifiers
CAS Number	26299-14-9 N
ChemSpider	10608386 N
Jmol 3D model	Interactive image
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 O=[Cr](=O)([O-])Cl.[nH+]1ccccc1
Properties
Chemical formula	C5H5NHClCrO3
Molar mass	215.56 g/mol
Appearance	orange crystalline powder
Melting point	205 °C (401 °F; 478 K)
Solubility in other solvents	soluble in dichloromethane, benzene, diethyl ether, acetone, acetonitrile, THF
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Pyridinium chlorochromate (PCC) is a red–orange salt with the formula C₅H₅NH[CrO₃Cl]. It is a reagent in organic synthesis used primarily for the oxidation of alcohols to form carbonyls. A variety of related compounds are known with similar reactivity. Although no longer widely used, PCC offered the advantage of the selective oxidation of alcohols to aldehydes, whereas many other reagents were less selective. Disadvantages to using PCC are the tedious reaction workup and its toxicity. The chemical was first synthesized and used by E.J. Corey and J. William Suggs.

Preparation and structure

The original preparation, which was discovered by accident, involves the reaction of pyridine with chromium trioxide and concentrated hydrochloric acid:

C₅H₅N + HCl + CrO₃ → [C₅H₅NH][CrO₃Cl]

Alternative syntheses have been developed.

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

Properties and 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. Rarely does over-oxidation occur (whether intentionally or accidentally) to form carboxylic acids, unlike the related Jones reagent. A typical PCC oxidation involves addition of the alcohol to a suspension of PCC in dichloromethane. A sample reaction would be:

C₅H₅NH⋅CrO₃Cl + R₂CHOH → C₅H₅NHCl + H₂CrO₃ + R₂C=O

In practice the chromium byproduct deposits with pyridine as a sticky black tar, which can complicate workup. 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.

Other reactions

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 oxidative reaction of PCC is its conversion of unsaturated alcohols or aldehydes to cyclohexenones. This pathway, an oxidative cationic cyclization, is illustrated by the conversion of (−)-citronellol to (−)-pulegone. PCC also effects allylic oxidations, e.g. for the conversion dihydrofurans to the lactones.

Related reagents

Other reagents for oxidizing alcohols using more convenient or less toxic reagents include DMSO-based oxidations (Swern oxidation, Moffatt oxidation) and hypervalent iodine based oxidation (such as the Dess–Martin periodinane).

See also

• Oxidation with chromium(VI)-amine complexes

External links

• IARC Monograph "Chromium and Chromium compounds"
• History of PCC
• National Pollutant Inventory - Chromium VI and compounds fact sheet