Question

Show how cyclohexanecarboxylic acid could be synthesized from cyclohexane in good yield. O=C(O)C1CCCCC1

Short answer

Answer: The synthesis pathway for converting cyclohexane to cyclohexanecarboxylic acid involves three consecutive oxidation steps: 1. Oxidation of cyclohexane to cyclohexanol using potassium permanganate (KMnO₄). 2. Oxidation of cyclohexanol to cyclohexanone using pyridinium chlorochromate (PCC) or Jones reagent (CrO₃/H₂SO₄). 3. Oxidation of cyclohexanone to cyclohexanecarboxylic acid using potassium permanganate (KMnO₄) or Jones reagent (CrO₃/H₂SO₄) in an aqueous solution.

Step-by-step solution

Oxidation of Cyclohexane to Cyclohexanol

First, we need to convert cyclohexane to cyclohexanol. This can be achieved by using an oxidation reaction. A suitable oxidizing agent for this reaction is potassium permanganate (KMnO₄). The reaction proceeds with the following steps: 1. Add an excess of KMnO₄ in a solvent (e.g. water) to the cyclohexane at 0°C or lower. Make sure to maintain a slow rate of addition to prevent an exothermic reaction. 2. Stir the reaction mixture for several hours to ensure the formation of cyclohexanol. The reaction will produce cyclohexanol in good yield, as shown below: Cyclohexane + KMnO₄ --> Cyclohexanol

Oxidation of Cyclohexanol to Cyclohexanone

Once we have cyclohexanol, the next step is to oxidize it further to cyclohexanone. This can be done by using an oxidizing agent such as pyridinium chlorochromate (PCC) or Jones reagent (CrO₃/H₂SO₄). The oxidation reaction can be performed as follows: 1. Add the desired oxidizing agent to the reaction mixture containing cyclohexanol. 2. Stir the mixture, allowing the oxidation of cyclohexanol to occur, forming cyclohexanone. The reaction will generate cyclohexanone in good yield, as shown below: Cyclohexanol + PCC (or Jones reagent) --> Cyclohexanone

Oxidation of Cyclohexanone to Cyclohexanecarboxylic Acid

Finally, we need to oxidize the cyclohexanone to cyclohexanecarboxylic acid. This can be achieved using a strong oxidizing agent, such as potassium permanganate (KMnO₄) or Jones reagent (CrO₃/H₂SO₄) in an aqueous solution. The oxidation reaction can be carried out as follows: 1. Add the strong oxidizing agent to the reaction mixture containing cyclohexanone. 2. Stir the reaction mixture, allowing the oxidation of cyclohexanone to cyclohexanecarboxylic acid. The reaction will yield cyclohexanecarboxylic acid, as shown below: Cyclohexanone + KMnO₄ (or Jones reagent) --> Cyclohexanecarboxylic Acid By following these three consecutive oxidation steps, we have successfully synthesized cyclohexanecarboxylic acid from cyclohexane in good yield.

Key concepts

Oxidation Reactions in Organic Chemistry

Oxidation reactions are fundamental processes in organic chemistry where a molecule loses electrons, often resulting in an increase in the number of bonds to oxygen or a decrease in the number of bonds to hydrogen. In the context of syntheses, oxidation can convert hydrocarbons into functional groups such as alcohols, aldehydes, ketones, and carboxylic acids.

These reactions are not only pivotal in laboratory synthesis but also occur in biological systems and industrial applications. The choice of oxidizing agent and the specific conditions (like temperature and solvent) greatly influence the reaction outcome. Hence, understanding the behavior of different oxidizing agents is critical in planning and conducting organic synthesis effectively.

Potassium Permanganate Oxidation

Potassium permanganate (KMnO_4) is a powerful and commonly used oxidizing agent in organic chemistry. It's known for its distinctive purple color and its ability to oxidize a wide range of organic substrates. In dilute aqueous solutions, KMnO_4 can effect mild oxidations, such as converting alkenes to diols or alcohols to ketones or aldehydes.

In the synthesis of cyclohexanecarboxylic acid, it is employed in two critical steps: the conversion of cyclohexane to cyclohexanol and cyclohexanone to cyclohexanecarboxylic acid. The versatility of KMnO_4 allows for these transformations when used under carefully controlled conditions, such as low temperature and neutral or slightly basic pH, to avoid over-oxidation and maximize yield.

Pyridinium Chlorochromate (PCC)

Pyridinium chlorochromate (PCC) is a milder oxidizing agent as compared to potassium permanganate and is extensively used to oxidize primary alcohols to aldehydes, and secondary alcohols to ketones, without over-oxidizing to acids or esters. It's especially valuable when selective oxidation is required.

Using PCC for transforming cyclohexanol to cyclohexanone offers a high level of control, thus ensuring that the alcohol is oxidized only to the ketone stage, which is crucial for the precise synthesis of cyclohexanecarboxylic acid. PCC oxidation generally operates under anhydrous conditions to prevent the formation of side products.

Jones Reagent

The Jones reagent, consisting of chromium trioxide (CrO_3) in diluted sulfuric acid (H_2SO_4), is another robust oxidizing agent capable of transforming alcohols into ketones, aldehydes, or even carboxylic acids, depending on the reaction's specific conditions and the substrate used.

For the oxidation of cyclohexanol to cyclohexanone, and subsequently cyclohexanone to cyclohexanecarboxylic acid, Jones reagent can be used effectively. However, due to its high reactivity, careful control of the reaction conditions is essential to avoid over-oxidation. The Jones oxidation is usually performed in an acidic medium which can provide additional challenges such as acid-induced side reactions, so it is important that a chemist selects the appropriate conditions for the desired product.