Question

Secondary alcohols can be oxidized to ketones ussing either \(\mathrm{PCC}_{\mathrm{a}} \mathrm{or} \mathrm{H}_{2} \mathrm{CrO}_{4}\),

Short answer

Answer: Chromium Hydroxide (\(\mathrm{Cr(OH)_{3}}\)) and water (H2O).

Step-by-step solution

Understanding the oxidation of secondary alcohols to ketones

Secondary alcohols have the general structure R2CHOH, where R can be an alkyl or aryl group. The alcohol group consists of two R groups, a carbon atom, and a hydrogen atom. The main aim of the reaction is to convert the alcohol group to a carbonyl group, which has a carbon atom double-bonded to an oxygen atom. When this conversion occurs, we get a ketone with the general structure R2CO.

Role of the oxidizing agents

PCC and H2CrO4 are both strong oxidizing agents that can facilitate the oxidation of secondary alcohols to ketones. They work by removing the hydrogen atom in the alcohol group and promoting the formation of the carbon-oxygen double bond.

The oxidation reaction using PCC

The balanced chemical equation for the oxidation of a secondary alcohol to ketone using PCC is given by: R2CHOH + \(\mathrm{CrO}_{3} \mathrm{Cl}^{-}\) (Pyridinium Chlorochromate) \(\rightarrow\) R2CO (Ketone) + \(\mathrm{CrO}_{2} \mathrm{Cl}^{-}\) + H2O During the reaction, the alcohol is dehydrogenated, and PCC is reduced to form CrO2Cl- and water as waste products.

The oxidation reaction using H2CrO4

The balanced chemical equation for the oxidation of a secondary alcohol to ketone using H2CrO4 is given by: R2CHOH + \(\mathrm{H}_{2} \mathrm{CrO}_{4}\) (Chromium Trioxide in Water) \(\rightarrow\) R2CO (Ketone) + \(\mathrm{Cr(OH)_{3}}\) + 3H2O In this reaction, the alcohol is dehydrogenated, and H2CrO4 is reduced to form Chromium Hydroxide and water as by-products. In conclusion, secondary alcohols can be successfully oxidized to ketones using either PCC or H2CrO4 as oxidizing agents. The reactions follow the same dehydrogenation process and involve the reduction of the oxidizing agents to their corresponding reduced forms.

Key concepts

PCC Oxidation

The process of converting secondary alcohols into ketones using Pyridinium Chlorochromate (PCC) is known as PCC oxidation. PCC is a milder and more selective oxidizing agent when compared to other chromic acid derivatives. This provides an advantage because it minimizes the risk of over-oxidation, where the substance could further oxidize to a carboxylic acid if not carefully controlled.

During PCC oxidation, the secondary alcohol undergoes a loss of hydrogen atoms (dehydrogenation) from the hydroxyl group and its adjacent carbon. This loss is facilitated by PCC, resulting in the formation of a ketone and a reduced form of chromium in the by-products. Given the mildness of PCC, it is often used in the synthesis of fine chemicals and complex organic molecules where overoxidation can be particularly problematic.

H2CrO4 Oxidation

An alternative to PCC for oxidizing secondary alcohols is the use of Chromic Acid, generally represented as H2CrO4. The process is typically referred to as H2CrO4 oxidation or Jones oxidation when chromium trioxide (CrO3) is dissolved in aqueous sulfuric acid. Unlike PCC, H2CrO4 is a very strong oxidizing agent and carries risks of overoxidation, making it less selective.

H2CrO4 oxidation transforms the alcohol into a ketone through the removal of two hydrogen atoms, similarly involving dehydrogenation. This reaction is more exothermic and vigorous, potentially leading to further oxidation products if the reaction conditions are not properly managed. The choice between PCC and H2CrO4 often comes down to the desired selectivity and the sensitivity of the alcohol substrate to stronger oxidizing conditions.

Ketone Synthesis

Ketone synthesis refers to the creation of ketones from various starting materials, like secondary alcohols. In organic synthesis, ketones are valuable due to their reactivity, making them key intermediates in the production of more complex organic compounds.

Methods of Ketone Synthesis

• Oxidation of secondary alcohols, via PCC or H2CrO4, as illustrated in the exercise.
• Friedel-Crafts Acylation, where an acyl group is introduced into an aromatic ring.
• From nitriles and organometallic reagents, such as Grignard reagents.

The versatility in methods offers chemists the flexibility to choose the most suitable pathway for ketone synthesis based on the substrate's complexity, sensitivity, and desired yield.

Oxidizing Agents in Organic Chemistry

Oxidizing agents are substances that can accept electrons during a chemical reaction and bring about the oxidation of another substance. In organic chemistry, these agents play a vital role by facilitating a wide array of reactions, including the transformation of alcohols into carbonyl compounds (like ketones and aldehydes).

Characteristics of Oxidizing Agents

• The ability to accept electrons and become reduced.
• High oxidation states, which they seek to lower via redox reactions.
• They often contain elements such as oxygen, chlorine or a heavy metal like chromium or manganese.

Choosing the appropriate oxidizing agent is crucial for the success of the reaction, as it dictates the reaction rate, yield, and selectivity. Common oxidizing agents, besides PCC and H2CrO4, include potassium permanganate (KMnO4), nitric acid (HNO3), and others, each with unique properties suitable for specific types of reactions.