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Organic Reagents

By James Ashenhurst

Reagent Friday: PCC (Pyridinium Chlorochromate)

Last updated: February 21st, 2020 |

Pyridinium Chlorochromate (PCC), A Useful Oxidant For Alcohols In Organic Chemistry

In a blatant plug for the Reagent Guide, each Friday  I profile a different reagent that is commonly encountered in Org 1/ Org 2. Version 1.2 just got released, with a host of corrections and a new page index. 

structure-of-pcc-pyridinium-chlorochromate

What it’s used for: PCC is a milder version of chromic acid. Essentially, what it does is oxidize alcohols one rung up the oxidation ladder, from primary alcohols to aldehydes and from secondary alcohols to ketones. Unlike chromic acid, PCC will not oxidize aldehydes to carboxylic acids.

Similar to or the same as: CrO3 and pyridine (the Collins reagent) will also oxidize primary alcohols to aldehydes.

Oxidation Of Primary Alcohols To Aldehydes With Pyridinium Chlorochromate (PCC) And Oxidation Of Secondary Alcohols To Ketones

Here are two examples of PCC in action. If you add one equivalent of PCC to either of these alcohols, you obtain the oxidized version. The byproducts (featured in grey) are Cr(IV) as well as pyridinium hydrochloride.

One has to be careful with the amount of water present in the reaction. If water is present, it can add to the aldehyde to make the hydrate, which could be further oxidized by a second equivalent of PCC were it present. This is not a concern with ketones, since there is no H directly bonded to C.

oxidation-of-primary-alcohols-to-aldehydes-and-secondary-alcohols-to-ketones-with-pcc-pyridinium-chlorochromate

Mechanism For The Oxidation Of Primary Alcohols To Aldehydes With  Pyridinium Chlorochromate

How does it work? Oxidation reactions of this sort are actually a kind of elimination reaction. We’re going from a carbon-oxygen single bond to a carbon-oxygen double bond. [See: Demystifying Alcohol Oxidations]  The elimination reaction can occur because we’re putting a good leaving group on the oxygen, namely the chromium, which will be displaced when the neighboring C-H bond is broken with a base.

mechanism-for-oxidation-of-primary-alcohol-with-pcc-to-give-aldehyde

The first step is attack of oxygen on the chromium to form the Cr-O bond. Secondly, a proton on the (now positive) OH is transferred to one of the oxygens of the chromium, possibly through the intermediacy of the pyridinium salt. A chloride ion is then displaced, in a reaction reminiscent of a 1,2 elimination reaction, to form what is known as a  chromate ester.  

The C-O double bond is formed when a base removes the proton on the carbon adjacent to the oxygen. [aside: I’ve drawn the base as Cl(-) although there are certainly other species which could also act as bases here (such as an alcohol). It is also possible for pyridine to be used as the base here, although only very low concentrations of the deprotonated form will be present under these acidic conditions.] The electrons from the C-H bond move to form the C-O bond, and in the process break the O-Cr bond, and Cr(VI) becomes Cr(IV) in the process (drawn here as O=Cr(OH)2 ).

Real life notes: If you end up using PCC in the lab,  don’t forget to add molecular sieves or Celite or some other solid to the bottom of the flask, because otherwise you get a nasty brown tar that is a real major pain to clean up.  The toxicity and mess associated with chromium (as popularized by this lady) has spurred the development of other alternatives like TPAP, IBX, DMP, and a host of other neat reagents you generally don’t learn about until grad school.

P.S. You can read about the chemistry of PCC and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF.

 


(Advanced) References and Further Reading

  1. Pyridinium chlorochromate. An efficient reagent for oxidation of primary and secondary alcohols to carbonyl compounds
    E.J. Corey, J. William Suggs
    Tetrahedron Letters Volume 16, Issue 31, 1975, Pages 2647-2650
    DOI: 
    10.1016/S0040-4039(00)75204-X
    The original paper by Nobel Laureate Prof. E. J. Corey on the use of pyridinium chlorochromate as a mild oxidation reagent in organic synthesis.
  2. The story of the discovery of PCC was rather serendipitous, as explained by Prof. Suggs in this blog post: https://blogs.sciencemag.org/pipeline/archives/2005/10/11/the_old_stuff#comment-2123
  3. Pyridinium Chlorochromate: A Versatile Oxidant in Organic Synthesis
    Piancatelli, A. Scettri, M. D’Auria
    Synthesis 1982; 1982 (4): 245-258
    DOI: 
    10.1055/s-1982-29766
    Review on the applications of PCC in organic synthesis. Includes a discussion on the mechanism.
  4. Kinetics and Mechanism of the Oxidation of Alcohols by Pyridinium Chlorochromate
    Banerji Kalyan K.
    Bull. Chem. Soc. Jpn. 1978, 51 (9), 2732
    DOI: 10.1246/bcsj.51.2732
    A nice mechanistic study of PCC oxidation, and includes a probable mechanism of the reaction.
  5. Stoichiometry of the oxidation of primary alcohols with pyridinium chlorochromate. Evidence for a two-electron change
    Herbert C. Brown, C. Gundu Rao, and Surendra U. Kulkarni
    The Journal of Organic Chemistry 1979 44 (15), 2809-2810
    DOI: 1021/jo01329a051
    In this paper, Nobel Laureate H. C. Brown proves that PCC oxidations involve a transfer of 2 electrons from the Cr to the substrate. Therefore, one does not need to use an excess of PCC – 1 equivalent works fine.
  6. SYNTHESIS OF 1,2:4,5-DI-O-ISOPROPYLIDENE-D-erythro-2,3-HEXODIULO-2,6-PYRANOSE. A HIGHLY ENANTIOSELECTIVE KETONE CATALYST FOR EPOXIDATION
    Yong Tu, Michael Frohn, Zhi-Xian Wang, and Yian Shi
    Org. Synth. 2003 80, 1
    DOI: 
    10.15227/orgsyn.080.0001
    This tested procedure from Organic Syntheses uses PCC to make the chiral ketone catalyst for asymmetric epoxidation, known as ‘Shi epoxidation’ after its creator, Prof. Yian Shi (Colorado State).

 

Comments

Comment section

26 thoughts on “Reagent Friday: PCC (Pyridinium Chlorochromate)

  1. You’re website has been very helpful for preparing for my ochem final. Learning the concepts through your articles has bettered my understanding of the material. Thank you so much, this website condenses my entire quarter in a few pages!

  2. trying to get some images of PCC (pyridinium chlorochromate) in aqueous solution from ≈ 100 mM to 2 mM. Are they all brightly colored at these concentrations?

    1. It’s my own term and is admittedly not the most rigorous one. I use that because it oxidizes primary alcohols to aldehydes and stops there, in contrast to what I call, “strong” oxidants (like chromic acid) which oxidize primary alcohols to carboxylic acids.

  3. Hello James,

    This is very helpful. I have been looking for chemicals to oxidze primary alcohol to aldehydes only. It seems PCC may be a good choice. My question is that if I used carbohydrate, such as glucose, does it oxidize all other non-primary alcohol?

    Thanks,

    Jake

    1. Yes, you’d be wise to employ a glucose where the secondary alcohols have protecting groups. PCC is not selective enough for primary alcohols.

  4. Hi! Your website has been so, so helpful to me over the years during my chemistry undergraduate degree.

    Just wondering – I’ve seen PCC can oxidize activated C-H bonds in dihydrofurans to the corresponding lactone (http://reag.paperplane.io/00002426.htm#2). Do you know if any of the listed alternatives (DMP, TPAP etc) are also able to do this reaction?

    Thanks in advance!
    Kayla

  5. Sir I want to know one thing.i am having organic course in my college and my sir told that CH2Cl2 is used as solvent in PCC as it is non polar.However if DMF(dimethyl formamide) is used it gives directly carboxylic acid.i couldn’t find any explanation to that.Do you agree?

  6. I’ve got one question – what if PCC doesn’t work? Simply there is no oxidation. I do not use any additional base, only alcohol, pcc, and silicagel as a waste adsorbent. Maybe it requires additional portion of a base? What is a troubleshooting in such case? thanks!

    1. I have no idea of what your substrate is, but there are plenty of alternatives to PCC – Dess Martin, Swern, Parikh Doering, the list goes on and on. If you told me what your substrate is, that would help.

  7. I see many procedures for using PCC that perform the reaction in dichloromethane with addition of silica gel, but add diethyl ether before filtering off the silica. What is the purpose of the ether?

    1. The trick here is to try to extract your product from the chromate ester sludge. During filtration the silica is rinsed with a solvent. Using dichloromethane as a rinse solvent will make more of the highly colored sludge pass through the silica. If you use a non-chlorinated rinse solvent like diethyl ether you will pass your product through the silica but carry less of the chromate ester sludge. There’s nothing magical about diethyl ether, but it’s a solid choice.

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