The Jones oxidation, also known as the Jones-Sarett oxidation, is a chemical reaction used to oxidize primary and secondary alcohols to carboxylic acids and ketones, respectively. It was developed independently by Sir Ewart Jones and Robert L. Sarett in the 1930s.
The Jones oxidation typically employs chromic acid (H2CrO4) or chromate salts (e.g., sodium or potassium dichromate) in an acidic solution. The reaction proceeds via the formation of chromate esters, which undergo further oxidation to yield the desired carbonyl compounds. The chromic acid acts as the oxidizing agent in the reaction.
The general reaction scheme for the Jones oxidation of primary alcohols to carboxylic acids is:
���2�� (����ℎ��)+[�]→����� (���������� ����)RCH2​OH (alcohol)+[O]→RCOOH (carboxylic acid)
For secondary alcohols, the oxidation yields ketones:
�2���� (����ℎ��)+[�]→�2�� (������)R2​CHOH (alcohol)+[O]→R2​CO (ketone)
The mechanism of the Jones oxidation involves the formation of chromate esters, which then undergo elimination of chromium oxide to yield the carbonyl compounds.
While the Jones oxidation is effective, it does have some drawbacks, including the toxicity of chromium compounds and the potential for over-oxidation of sensitive functional groups. As a result, alternative oxidation methods have been developed, such as the use of safer and more selective reagents like pyridinium chlorochromate (PCC) or Dess-Martin periodinane (DMP). These alternatives offer milder reaction conditions and improved functional group tolerance. Nonetheless, the Jones oxidation remains an important reaction in organic synthesis, particularly in cases where its specific reactivity is advantageous.
The Jones-Sarett oxidation, also known simply as the Jones oxidation, proceeds through several steps. It involves the use of chromic acid (H2CrO4) or chromate salts (e.g., sodium or potassium dichromate) in an acidic solution. Here’s a simplified mechanism for the oxidation of a primary alcohol to a carboxylic acid:
- Formation of Chromate Ester:
- The primary alcohol (RCH2OH) reacts with chromic acid or a chromate salt in the presence of acid (usually sulfuric acid) to form a chromate ester intermediate.
- The reaction begins with protonation of the alcohol oxygen, making it a better leaving group.
- The protonated alcohol then attacks the chromium atom of the chromate ion, forming a chromate ester intermediate.
���2��+�2���4→���2�−���3�+�2�RCH2​OH+H2​CrO4​→RCH2​O−CrO3​H+H2​O
- Oxidative Cleavage:
- The chromate ester undergoes an oxidative cleavage reaction, where the chromium(VI) species oxidizes the carbon attached to the hydroxyl group.
- This cleavage results in the formation of a carboxylic acid and a chromium(III) species.
���2�−���3�→�����+��(��)3RCH2​O−CrO3​H→RCOOH+Cr(OH)3​
- Regeneration of Chromic Acid:
- The chromium(III) species formed in the previous step is then reoxidized back to chromic acid in the presence of oxygen from the air or through further reaction with a chromium(VI) species in the solution.
- This regeneration step ensures the continuous availability of chromic acid for further oxidation reactions.
4��(��)3+3�2→4���42−+6�2�4Cr(OH)3​+3O2​→4CrO42−​+6H2​O
Overall, the Jones-Sarett oxidation converts a primary alcohol to a carboxylic acid through the action of chromic acid or chromate salts in an acidic environment. The mechanism involves the formation of a chromate ester intermediate followed by oxidative cleavage to yield the desired carboxylic acid product.
The Jones-Sarett oxidation, named after Sir Ewart Jones and Robert L. Sarett, is a valuable method for converting primary and secondary alcohols into carboxylic acids and ketones, respectively. This oxidation reaction finds applications in both laboratory and industrial settings. Here are some of its key applications:
- Synthesis of Carboxylic Acids: The Jones-Sarett oxidation is commonly employed to convert primary alcohols into carboxylic acids. This transformation is particularly useful in organic synthesis for the preparation of various carboxylic acid derivatives, such as esters, amides, and acid chlorides. Carboxylic acids are essential building blocks in pharmaceuticals, agrochemicals, and materials science.
- Conversion of Secondary Alcohols to Ketones: Secondary alcohols undergo oxidation to form ketones using the Jones-Sarett reagent. Ketones are versatile intermediates in organic chemistry and are utilized in the synthesis of pharmaceuticals, fragrances, and fine chemicals. This oxidation reaction allows for the selective transformation of secondary alcohols without affecting other functional groups present in the molecule.
- Oxidative Cleavage: The Jones-Sarett oxidation can be employed for the oxidative cleavage of carbon-carbon bonds in certain substrates. This reaction can be useful in the synthesis of complex molecules by cleaving specific bonds to generate key intermediates.
- Purification of Alcohols: In addition to its use as an oxidation reagent, chromic acid (the active oxidizing species in the Jones-Sarett oxidation) can be employed for the purification of alcohols. Treatment of alcohols with chromic acid leads to the formation of chromate esters, which can then be hydrolyzed to yield pure alcohols.
- Teaching and Research: The Jones-Sarett oxidation is commonly taught in organic chemistry courses as a classic example of alcohol oxidation. It serves as a fundamental reaction for students to understand the principles of oxidation-reduction reactions and their applications in organic synthesis. Furthermore, it continues to be a subject of research interest for developing more efficient and environmentally friendly oxidation methods.
Despite its effectiveness, the Jones-Sarett oxidation does have limitations, including the use of toxic chromium reagents and the potential for over-oxidation. As a result, alternative oxidation methods have been developed, such as those employing safer and more selective reagents like pyridinium chlorochromate (PCC) or Dess-Martin periodinane (DMP). However, the Jones-Sarett oxidation remains a valuable tool in the toolbox of synthetic chemists for specific applications where its reactivity is advantageous.
