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Organic Reagents

By James Ashenhurst

Reagent Friday: Chromic Acid, H2CrO4

Last updated: January 29th, 2020 |

H2CrO4 (Chromic Acid, a.k.a. the protonated form of K2Cr2O7 / Na2Cr2O7 / K2CrO4 / Na2CrO4) As A Reagent In Organic Chemistry

In a blatant plug for the Reagent Guide, each Friday for the forseeable future here I will profile a different reagent that is commonly encountered in Org 1/ Org 2. 

Chromic Acid (H2CrO4) Is Equivalent To K2Cr2O7 + H2SO4 (Among Other Combinations)

Of all the reagents I encounter as a tutor in organic chemistry, I’m not sure there’s one that makes me want to rip my hair out more than chromic acid. I don’t have a lot of hair so I don’t say that lightly.

The thing is not that it is a particularly tricky reagent. It is, in fact, quite straightforward, once deciphered.

What makes it tricky is the vast number of different ways that textbooks (and instructors) show it being used in reactions.

chromic-acid-h2cro4-can-be-made-from-k2cr2o7-and-other-chromate-salts-with-acid

 Here’s the thing: Chromic acid, H2CrO4, is a strong acid and a reagent for oxidizing alcohols to ketones and carboxylic acids. For fairly mundane reasons owing primarily to safety and convenience,  chromic acid tends to be made in the reaction vessel as needed (through addition of acid to a source of chromium), rather than being dispensed from a bottle.

And that’s where the trouble begins. Choosing a source of chromium to make H2CrO4 from is a lot like choosing a favorite brand of bottled water. Beyond the packaging, they’re pretty much all the same. Depending on which textbook or instructor you have, however, you might see several different ways to do this, and it can be very confusing.

The key point is that Na2CrO4 (sodium chromate), Na2Cr2O7 (sodium dichromate), K2CrO4 (potassium chromate), K2Cr2O7 (potassium dichromate), and CrO3 (chromium trioxide) are all alike in one crucial manner: when they are combined with aqueous acid, each of them forms H2CrO4, and ultimately it’s H2CrO4 which does the important chemistry. Unfortunately I rarely see this point explained in textbooks. I remember this causing some confusion for me when I took the course. The K or Na ions present are just spectators.

H2CrO4 As A Reagent For The Oxidation Of Alcohols

Once H2CrO4 is formed, its reactions are pretty straightforward: it converts primary alcohols (and aldehydes) to carboxylic acids and secondary alcohols to ketones.

chromic-acid-oxidizes-primary-alcohols-to-carboxylic-acids-and-secondary-alcohols-to-ketones

Oxidation Of Primary Alcohols To Carboxylic Acids With Chromic Acid: How It Works

It does this through addition of the alcohol oxygen to chromium, which makes it a good leaving group; a base (water being the most likely culprit) can then remove a proton from the carbon, forming a new π bond and breaking the O-Cr bond.

mechanism-for-chromic-acid-oxidation-of-primary-alcohols-to-carboxylic-acids

I wish I could tell you that navigating this confusion  is ultimately rewarding due to the vast usefulness of H2CrO4 as a reagent. In fact, due to its high toxicity, chromic acid tends to find very little use in the organic chemistry laboratory outside of undergrad labs. There are far more useful reagents out there for performing these transformations.

P.S. You can read about the chemistry of chromic acid 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

The main oxidizing agents used for this transformation are Cr(VI) and Mn(VII), as shown below. The first two references use KMnO4.

  1. Synthesis of a model depsipeptide segment of Luzopeptins (BBM 928), potent antitumor and antiretroviral antibiotics
    Marco A. Ciufolini and Shankar Swaminathan
    Tetrahedron Letters Volume 30, Issue 23, 1989, Pages 3027-3028
    DOI:     10.1016/S0040-4039(00)99393-6
    Step f in the synthesis (Scheme 1) is an oxidation of a primary alcohol to carboxylic acid using KMnO4.
  2. Stereocontrolled addition to a penaldic acid equivalent: an asymmetric of -β-hydroxy-L-glutamic acid
    Philip Garner
    Tetrahedron Letters Volume 25, Issue 51, 1984, 5855-5858
    DOI    : 10.1016/S0040-4039(01)81703-2
    The final step (g, 6 -> 7) in the synthesis in this paper is an oxidation of a primary alcohol to a carboxylic acid using KMnO4.The Jones oxidation, which uses chromic acid (CrO3 in H2SO4) is a common method for the oxidation of primary alcohols to carboxylic acids. The drawback is of course the production of stoichiometric amounts of chromium waste.
  3. Researches on acetylenic compounds. Part XIV. A study of the reactions of the readily available ethynyl-ethylenic alchohol, pent-2-en-4-yn-1-ol
    Sir Ian Heilbron, E. R. H. Jones and F. Sondheimer
    J. Chem. Soc., 1947, 1586-1590
    DOI:     10.1039/JR9470001586
  4. An Improved Procedure for the Oxidation of Alkynols to Alkynoic Acids
    C. Holland and N. W. Gilman
    Synth. Commun. 1974, 4, 203-210
    DOI: 10.1080/00397917408062073
  5. Useful procedures for the oxidation of alcohols involving pyridinium dichromate in aprotic media
    E.J. Corey, Greg Schmidt
    Tetrahedron Letters Volume 20, Issue 5, 1979, 399-402
    DOI: 10.1016/S0040-4039(01)93515-4
    Nobel Laureate Prof. E. J. Corey (Harvard) shows that PDC (pyridinium dichromate) in DMF can be used for the oxidation of primary alcohols to carboxylic acids.
00 General Chemistry Review
01 Bonding, Structure, and Resonance
02 Acid Base Reactions
03 Alkanes and Nomenclature
04 Conformations and Cycloalkanes
05 A Primer On Organic Reactions
06 Free Radical Reactions
07 Stereochemistry and Chirality
08 Substitution Reactions
09 Elimination Reactions
10 Rearrangements
11 SN1/SN2/E1/E2 Decision
12 Alkene Reactions
13 Alkyne Reactions
14 Alcohols, Epoxides and Ethers
15 Organometallics
16 Spectroscopy
17 Dienes and MO Theory
18 Aromaticity
19 Reactions of Aromatic Molecules
20 Aldehydes and Ketones
21 Carboxylic Acid Derivatives
22 Enols and Enolates
23 Amines
24 Carbohydrates
25 Fun and Miscellaneous
26 Organic Chemistry Tips and Tricks
27 Case Studies of Successful O-Chem Students

Comments

Comment section

41 thoughts on “Reagent Friday: Chromic Acid, H2CrO4”

    1. No, it should not, at least as far as oxidation is concerned. Prolonged heating of a tertiary alcohol with acid will lead to dehydration (elimination) but tertiary alcohols cannot be oxidized further.

      Reply

  2. What happens when we take methanol except 1° or 2° alcohol.
    I am trying to find out reaction related to oxidation of methanol but failed to find even a single one with any oxidising agent

    Reply
  3. Pingback: Degrees of Oxidation Using Methane as an Example - Quirky Science

  4. Yeah, pretty sure! In fact, I have the very question paper that contains the original question…
    I guess, it was an error then.
    Yeah, I get it.
    Thanks for helping me out!

    Reply

  5. Hey, my teacher says that Jones Reagent oxidizes aromatic primary alcohols to aldehydes and not carboxylic acids. Is that so? If it is, under what conditions (and with what substrates) do we get aldehyde/carboxylic acid?

    Reply

    1. Are you sure it’s Jones and not PCC?

      Jones is in an aqueous environment – any aldehyde will form a hemiacetal (via addition of water) and the hemiacetal can be further oxidized to the carboxylic acid…

      Reply

    1. No. Under very forcing conditions CrO3 can sometimes oxidize alkyl groups next to aromatic rings ( “benzylic oxidation” ) but it does not interact with alkenes/alkynes.

      Reply

  8. My professor actually says that chromic acid cannot chew all the way onto the akyl to form a carboxylic acid, so I do not know who to believe.

    Reply

  11. When listing the reagents that break down into chromic acid, you wrote KCrO4 instead of K2CrO4 for potassium chromate.

    Reply

  12. Hi–I have a problem in which the reagent is dichromic acid, not chromic acid. Does the mechanism differ? How are the products different? I cant find anything on a dichromic acid reaction.

    Reply

  13. Hello sir!
    Can u please help me by telling that what would happen if chromic acid is added to phenol??

    Reply

  14. You say that chromic acid converts primary alcohols (and aldehydes) to carboxylic acids and secondary alcohols to ketones.

    But here’s what my teacher’s powerpoint says: Methanol gives formaldehyde. A primary alcohol gives an aldehyde. A secondary alcohol gives a ketone. A tertiary alcohol does not oxidize.

    Reply

    1. Well, the first oxidation product of a primary alcohol will be the aldehyde.

      Usually it doesn’t stop there. What will then happen is that the aldehyde will add water to form a hydrate and this will be further oxidized to give a carboxylic acid.

      Reply

    1. The reaction is usually run in acetone or other organic solvent. Whether or not an aldehyde is water soluble would depend on the length of its alkyl chain. After butyraldehyde, the water solubility tends to decrease significantly.

      Reply

  18. So…on a practice exam I’m currently taking, two back-to-back questions are as follows:

    2-Phenylethanol yields what acid upon treatment with COLD chromic acid?

    and

    2-Phenylethanol yields what acid upon treatment with HOT chromic acid or permanganate?

    I know both the answers, but I don’t understand the DIFFERENCE in cold and hot chromic acid. I did a Google search and, not unexpectedly, I returned nada.

    Reply

    1. Jamaal: I think the issues is that, if done carefully, chromic acid only oxidizes primary alcohols to aldehydes, but with harsher conditions, it oxidizes them to carboxylic acids. So with cold temperatures, limited chromic acid, and short reaction times, it is possible to get an aldehyde as a product, but with heat, excess chromate, and longer reactions, the carboxylic acid is the usual product.

      Reply

  19. Hi, Can you please help me with a related question? What is the reaction when you mix acetic acid and chromic acid? I have encountered this problem in my class and I have no examples showing what happens. Thanks!

    Reply

    1. i think there will be no positive results since chromic acid is used to oxidize ketone aldehyde, alcohol to carboxylic acid and since acetic acid is belong to carboxylic acid group there will no reaction

      Reply

  20. in my textbook (Bruice), the OH-group attacks the proton instead of the keton-group, and then the alcohol does a SN2 reaction on the protonated chromic acid, with water as the leaving group. But I think your mechanism is more correct, because the pKa of a keton is slightly higher than the pKa of the alcohol. So the keton (more basic) wants to share its electrons more than the alcohol and that’s why the keton is protonated instead of the alcohol functional group.
    Is this a good explanation?

    Reply

    1. I couldn’t see my own comments, and that’s why I wrote it again. now there’s two of it…. I’m so sorry, I didn’t want to spam this page.

      Reply

    2. I’m a fan of Grossman’s statement that in the context of introductory courses we should emphasize *reasonable* reaction mechanisms rather than the absolutely most correct one. I think both mechanisms described are reasonable. The “SN2 on the metal” has the advantage of being simpler to draw. The mechanism I drew is probably more consistent with experiment.
      As for the byproduct I think I dropped an arrow where I should have shown net loss of OH from the chromium to get HOCr(O)OH (H2CrO3). This is the species that loses water to get CrO2.

      Reply

  21. but I don’t see where the CrO2 comes from (the leaving group) because when the leaving group leaves the alcohol molecule (that becomes a keton), you have H3CrO4- and when water leaves from this molecule, HCrO3- remains? I don’t know how the CrO2 molecule is formed.

    Reply

  22. In my textbook, the oxidation with the chromic acid is the following: H2CrO4 adds a proton on one of its OH-groups, and then the primary/secondary alcohol does an SN2 reaction while H2O is the leaving group.
    But I think your mechanism is more correct, because if you look at chromic acid, it has both keton groups and alcohol groups. pKa for ketones is about 20-24 and for alcohols 17. So ketones want to share their electrons more than alcohols, because they are more basic. So the proton will add to the keton group instead of adding to the alcohol group.

    Is this a correct explanation?

    Reply

  23. Hey,

    It seems the carbon chain on the primary alcohol got one element longer during the first step of the oxidation mechanism. Is this wrong, or am I simply missing something here?

    Great blog, by the way. It’s a pleasure to learn ochem from someone who takes his time explaining things for a change!

    Reply

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