Converting Primary Alcohols To Carboxylic Acids: A Simple Guide

Carboxylic acids are mainly prepared by the oxidation of various functional groups. The oxidation of primary alcohols is a common method for the synthesis of carboxylic acids. The complete equation for the conversion of a primary alcohol to a carboxylic acid is: \[3RCH_2OH + 2Cr_2O_7^{2-} + 16H^+ \rightarrow 3RCOOH + 4Cr^{3+} + 11H_2O\]. The primary alcohols are oxidized to carboxylic acids in two stages - first to an aldehyde and then to the acid. The oxidation of primary alcohols can be achieved using various methods, including potassium dichromate(VI) solution, pyridinium chlorochromate (PCC), and ruthenium complexes as catalysts. Additionally, carboxylic acids can be produced by the hydrolysis of acid chloride, esters, and nitriles.

Using potassium dichromate(VI) solution

To create carboxylic acid from a primary alcohol using potassium dichromate(VI) solution, you will need to follow a few important steps. Firstly, understand that the primary alcohol needs to be oxidised in two stages—first to an aldehyde and then to carboxylic acid.

The complete equation for the conversion of a primary alcohol to a carboxylic acid is:

$$3RCH_{2}OH + 2Cr_{2}O_{7}^{2-} + 16H^{+} \rightarrow 3RCOOH + 4Cr^{3+} + 11H_{2}O$$

The first step is to prepare the primary alcohol. The alcohol will be heated under reflux with an excess of the oxidising agent, which is a mixture of potassium dichromate(VI) solution and dilute sulfuric acid. This setup ensures that any aldehyde formed does not escape before it has time to be oxidised to carboxylic acid.

The second step involves the oxidation process. During the reaction, the potassium dichromate(VI) solution will turn from orange to green. This colour change is due to the reduction of dichromate(VI) ions to chromium(III) ions. The oxidation of the primary alcohol to an aldehyde is considered partial oxidation, while the further oxidation of the aldehyde to a carboxylic acid is an example of complete oxidation.

Finally, once the oxidation is complete, the mixture can be distilled. This process separates the carboxylic acid from the other components.

It is important to note that alternative methods exist to oxidise primary alcohols, such as using pyridinium chlorochromate (PCC) or Collins reagent as milder oxidising agents. However, the use of potassium dichromate(VI) solution is a common and effective approach to achieving this conversion.

Oxidation of primary alcohols

The oxidation of primary alcohols is a process that can be used to create carboxylic acids. This reaction involves the use of an oxidizing agent, typically a solution of sodium or potassium dichromate(VI) acidified with dilute sulfuric acid. The orange solution containing dichromate(VI) ions is reduced to a green solution containing chromium(III) ions during oxidation.

The complete equation for converting a primary alcohol to a carboxylic acid is:

$3RCH_2OH + 2Cr_2O_7^{2-} + 16H^+ \rightarrow 3RCOOH + 4Cr^{3+} + 11H_2O$

In this equation, "R" represents a hydrogen atom or a hydrocarbon group such as an alkyl group. The first stage of oxidation involves converting the primary alcohol to an aldehyde, which is then further oxidized to form the carboxylic acid. This two-stage process can be observed through colour changes in the solution.

To ensure the reaction proceeds to the carboxylic acid stage, an excess of the oxidizing agent is used. Heating under reflux, which involves heating the mixture in a flask with a vertically placed condenser, prevents any aldehyde formed from escaping before it has a chance to be oxidized further.

There are alternative methods for the oxidation of primary alcohols to carboxylic acids. One example is the use of pyridinium chlorochromate (PCC) as a catalyst in the presence of H5IO6 and acetonitrile. Another method involves a dehydrogenative reaction in the presence of hydroxide and a ruthenium complex catalyst. Additionally, an aerobic photooxidative cleavage of vicinal diols can yield carboxylic acids when using 2-chloroanthraquinone under photoirradiation with a high-pressure mercury lamp.

Acid chloride hydrolysis

The hydrolysis of acid chlorides is a fundamental reaction in organic chemistry, used in a variety of synthetic applications. It is a versatile reaction that can be used to prepare carboxylic acids, esters, and amides. The reaction occurs readily and does not require an acid or base catalyst. The mechanism of the reaction involves a nucleophilic attack by water on the carbonyl carbon of the acid chloride, forming a tetrahedral intermediate. This is followed by proton transfer and the expulsion of the chloride ion, resulting in the formation of the carboxylic acid.

The rate of hydrolysis is influenced by several factors, including temperature, solvent, and the presence of a base. The reaction typically occurs in a polar solvent, such as water or methanol, and the rate increases with higher temperatures as it provides more energy for the reaction. The presence of a base, such as pyridine or triethylamine, can accelerate the reaction by removing the proton from the water molecule.

The overall reaction scheme for the hydrolysis of an acid chloride can be summarized as follows:

Acid chloride + Water → Carboxylic acid + By-products (e.g., HCl)

The hydrolysis of acid chlorides is a valuable tool in organic synthesis, allowing for the preparation of various organic compounds, including carboxylic acids, which are fundamental building blocks in chemistry.

Pyridinium chlorochromate (PCC) catalysis

Pyridinium chlorochromate (PCC) is a commercially available, stable reagent that efficiently oxidizes a wide variety of alcohols to carbonyl compounds. It is a milder version of chromic acid and is often used as an alternative to other oxidizing agents such as potassium permanganate (KMnO4) and sodium dichromate (Na2Cr2O7).

The primary use of PCC is in the oxidation of primary and secondary alcohols to aldehydes and ketones, respectively. This is a one-rung oxidation, and unlike chromic acid, PCC does not further oxidize aldehydes to carboxylic acids. This makes PCC a useful reagent when the desired product is an aldehyde, as over-oxidation to a carboxylic acid is avoided.

The mechanism of PCC oxidation involves a transfer of two electrons from chromium to the substrate. The first step is the attack of the alcohol oxygen on the chromium atom to form the Cr-O bond. This is followed by the transfer of a proton from the (now positive) OH group to one of the oxygens of chromium, possibly through the pyridinium salt. A chloride ion is then displaced, forming a chromate ester with a C-O double bond. The byproducts of this reaction are Cr(IV) and pyridinium chloride.

PCC is soluble in many organic solvents, particularly dichloromethane at room temperature. It is important to control the amount of water present in the reaction as water can add to the aldehyde product, forming a hydrate that could be further oxidized by a second equivalent of PCC. This is not a concern with ketones, as there is no H directly bonded to C.

In addition to its use in alcohol oxidation, PCC also converts suitable unsaturated alcohols and aldehydes to cyclohexenones through an oxidative cationic cyclization pathway. It also facilitates allylic oxidations, such as the conversion of dihydrofurans to furanones.

Ruthenium complex catalysts

The oxidation of primary alcohols to carboxylic acids is an important industrial reaction used in the synthesis of bulk and fine chemicals. Traditional methods of this process involve the use of stoichiometric amounts of toxic oxidizing agents, which generate stoichiometric waste.

A greener approach to this reaction utilises water as the oxygen source and solvent. This method is made possible by the use of a ruthenium catalyst, a pincer complex in which a ruthenium centre is clamped by a bipyridine-based claw. This bipyridine-based ruthenium complex, known as 2 (BIPY-PNN)Ru(H)(Cl)(CO), catalyses the oxidation of primary alcohols to carboxylate in basic aqueous solution.

The catalytic mechanism involves four steps:

• Dehydrogenation of alcohol to aldehyde: This step involves the transfer of oxygen from water to the alcohol, releasing hydrogen.
• Coupling of aldehyde and water to form the gem-diol: This step involves the cleavage of water by the catalyst, followed by concerted hydroxyl and hydrogen transfer.
• Dehydrogenation of gem-diol to carboxylic acid
• Deprotonation of carboxylic acid to carboxylate anion under base: The formation of the carboxylate anion occurs via direct deprotonation of the carboxylic acid.

This ruthenium-catalysed process offers several advantages, including high catalytic activity, stability, and a wide substrate scope. It also represents a safer and cleaner approach to the synthesis of carboxylic acids, as it does not require the use of toxic oxidising agents.

Another ruthenium complex catalyst used in this reaction is the facial ruthenium-CPP complex, generated in situ from [Ru(COD)Cl2]n and a hybrid N-heterocyclic carbene (NHC)-phosphine-phosphine ligand (CPP). This catalyst system also exhibits high catalytic activity, stability, and a high turnover number.

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