Question

Primary alcohols are oxidized by aqueous \(\mathrm{H}_{2}\) CrO \(\mathrm{O}_{4}\) to give carboxylic acids in a process that involves initial aldehyde formation, followed by conversion to an aldehyde hydirate that is further cxidized to the casboxylic acid.

Short answer

Answer: The step-by-step process includes 1) identifying the primary alcohol and oxidizing agent, 2) forming the aldehyde, 3) forming the aldehyde hydrate, and 4) further oxidizing to produce the carboxylic acid.

Step-by-step solution

Identify the Primary Alcohol and Oxidizing Agent

The primary alcohol is the starting compound that will undergo oxidation to eventually form carboxylic acid. In this case, the oxidizing agent is aqueous \(\mathrm{H}_{2}\mathrm{CrO}_{4}\), which will facilitate the oxidation of the primary alcohol.

Formation of the Aldehyde

In the first step of the reaction, the primary alcohol loses a hydrogen atom from its hydroxyl group (-OH) and a hydrogen atom from the adjacent carbon atom. This results in the formation of an aldehyde with a double bond between the carbon and oxygen atoms. The general reaction can be written as: $$ \textrm{R-CH}_{2}\textrm{OH} \xrightarrow[]{\mathrm{H}_{2}\mathrm{CrO}_{4}} \textrm{R-CHO} $$ Where R represents the rest of the alkyl group.

Formation of Aldehyde Hydrate

The aldehyde formed in the previous step undergoes further reaction with water, leading to the formation of an aldehyde hydrate. This is a result of the attack of a water molecule on the electrophilic carbon atom of the aldehyde, creating a new bond between the oxygen atom of water and the carbon atom of the aldehyde. This leads to the formation of a tetrahedral intermediate, in which the positive charge resides on the oxygen atom. After the transfer of a proton from one oxygen atom to another, we obtain the aldehyde hydrate. The general reaction can be written as: $$ \textrm{R-CHO} + \textrm{H}_{2}\textrm{O} \rightleftharpoons \textrm{R-CH(OH)\(_{2}\)} $$

Further Oxidation to the Carboxylic Acid

In the final step, the aldehyde hydrate undergoes oxidation by losing another hydrogen atom from the carbon atom adjacent to the -OH group and one of the hydrogen atoms bonded to the oxygen of the -OH group. This results in the formation of a double bond between the carbon and oxygen, leading to the carboxylic acid. The general reaction can be represented as: $$ \textrm{R-CH(OH)\(_{2}\)} \xrightarrow[]{\mathrm{H}_{2}\mathrm{CrO}_{4}} \textrm{R-COOH} $$ In conclusion, the process of oxidizing primary alcohols to carboxylic acids using aqueous \(\mathrm{H}_{2}\mathrm{CrO}_{4}\) involves the initial formation of an aldehyde, followed by its conversion to an aldehyde hydrate, and finally, its further oxidation to produce the carboxylic acid.

Key concepts

Primary Alcohol

Primary alcohols are organic compounds that have a hydroxyl group (-OH) attached to a carbon atom, which itself is only bonded to one other carbon atom or to no other carbon atoms. This type of alcohol is recognized for its ability to undergo oxidation reactions under appropriate conditions. The oxidation of a primary alcohol typically proceeds through the initial formation of an aldehyde.

• Structure: Primary alcohols have the general structure \( R-CH_2OH \), where \( R \) represents the rest of the molecule that isn't part of the functional group.
• Reactivity: Undergo oxidation to transform into carboxylic acids, but only under controlled conditions.

Reacting primary alcohols with strong oxidizing agents like aqueous \( \mathrm{H}_{2}\mathrm{CrO}_{4} \) leads to their transformation through an aldehyde intermediate into carboxylic acids. It's crucial to recognize that not all primary alcohols are alike; their reactivity may vary based on their surrounding molecular environment.

Carboxylic Acid

Carboxylic acids are a class of organic compounds characterized by their carboxyl group (-COOH). They are the final product when a primary alcohol undergoes oxidation. Known for their acidity, carboxylic acids have a range of applications from food additives to medicinal purposes.

• Structure: Carboxylic acids possess a general structural formula of \( R-COOH \), where \( R \) can be hydrogen or an alkyl group.
• Formation: They are formed when the aldehyde hydrate is further oxidized, allowing the addition of a double bond between a carbon and an oxygen within the carboxyl group.

In the oxidation of primary alcohols, the process involves removing hydrogen atoms to create double bonds that define the carboxyl functionality. This structural conversion gives carboxylic acids their characteristic acidic properties, such as the ability to donate a proton in reactions. The ability to convert alcohols into carboxylic acids is vital in synthetic organic chemistry, aiding the synthesis of more complex molecules.

Aldehyde Hydrate

An aldehyde hydrate is an intermediate compound formed during the oxidation of a primary alcohol. It participates in the transition from an aldehyde to a carboxylic acid. The formation of the hydrate is particularly significant because it stabilizes the intermediate state, making further oxidation possible.

• Intermediate Role: As a bridge between the aldehyde and carboxylic acid, aldehyde hydrates are crucial for the completion of the oxidation reaction.
• Structure: Formed when an aldehyde reacts with water, resulting in the general structure \( R-CH(OH)_2 \).
• Reactivity: Undergoes further oxidation to form a carboxylic acid, facilitated by an oxidizing agent.

The creation of aldehyde hydrates ensures the reaction proceeds, providing a necessary step that allows the completion of the oxidation process. This compound is less stable than its preceding aldehyde, owing to the extra hydroxyl groups present on the molecule. Thus, its conversion to carboxylic acid is a spontaneous step facilitated by the presence of the oxidizing agent.