Producing acyl chlorides from carboxylic acids

Acyl chlorides can be prepared by reacting carboxylic acids with several chlorinating agents. The following examples feature ethanoic acid, but the reactions are applicable to other carboxylic acids as well.

1. Phosphorus(III) chloride (PCl₃)

Carboxylic acids react with PCl₃ when heated to yield acyl chlorides. For example:

3CH₃COOH + PCl₃ ➔ 3CH₃COCl + H₃PO₃

1. Phosphorus(V) chloride (PCl₅)

At room temperature, carboxylic acids react with PCl₅ to produce acyl chlorides. For example:

CH₃COOH + PCl₅ ➔ CH₃COCl + POCl₃ + HCl

1. Thionyl chloride (SOCl₂)

SOCl₂ reacts with carboxylic acids at room temperature, forming acyl chlorides. For example:

CH₃COOH + SOCl₂ ➔ CH₃COCl + SO₂ + HCl

In these reactions, the -OH group of the carboxylic acid is replaced by a -Cl atom, yielding the acyl chloride. The by-products (H₃PO₃, POCl₃, HCl, SO₂) are easily separated from the desired acyl chloride product.

Oxidation reactions of carboxylic acids

Under normal conditions, most carboxylic acids do not undergo further oxidation.

However, there are exceptions:

• Methanoic acid (HCOOH) is susceptible to oxidation by mild oxidising agents, such as Fehling’s solution, Tollens’ reagent, acidified K₂Cr₂O₇ or acidified KMnO₄ yielding carbon dioxide and water:

HCOOH + [O] ➔ CO₂ + H₂O

• Ethanedioic acid (HOOCCOOH) can be oxidised by stronger oxidising agents, such as warm acidified potassium manganate(VII) (KMnO₄), to also produce carbon dioxide and water:

HOOCCOOH + [O] ➔ 2CO₂ + H₂O

Producing benzoic acid by side chain oxidation

Benzoic acid can be produced by oxidising the methyl group of methylbenzene (an alkylbenzene). The benzene ring activates the alkyl side chain, allowing it to be oxidised.

The reaction involves heating methylbenzene with alkaline potassium manganate(VII) solution (KMnO₄). This oxidises the methyl group to a carboxylate ion (-COO^-). Subsequent addition of a dilute acid protonates the carboxylate, yielding benzoic acid.

The equation for this reaction is:

Electron-withdrawing groups increase acidity

Incorporating electron-withdrawing groups, such as chlorine, on the carbon atom adjacent to the carboxyl group leads to an increase in the acidity of carboxylic acids:

1. These groups pull electrons away from the O–H bond, weakening it and encouraging the release of H⁺ ions.
2. They also stabilise the negative charge on the carboxylate ion, making the dissociation process more favourable.

The impact of substituting hydrogen atoms with chlorine atoms on acidity is illustrated in the table below:

The addition of highly electronegative chlorine atoms reduces the K_a value and enhances the acid's strength.

Trichloroethanoic acid is the strongest acid due to the significant electron-withdrawing effect of its three chlorine atoms.

Carboxylic acids are more acidic than alcohols and phenols

Carboxylic acids are found to be stronger acids than both alcohols and phenols.

This difference in acid strength is due to two main factors:

1. The C=O double bond draws electron density away from the O–H bond, making it weaker and more prone to dissociation.

2. Delocalisation of electrons within the carboxylate ion stabilises the negative charge, further facilitating dissociation.

Phenols exhibit greater acidity than alcohols but are still less acidic than carboxylic acids.

The following table compares their acid strengths:

Carboxylic acids as weak acids

Carboxylic acids, such as ethanoic acid (CH₃COOH), exhibit typical acid behaviour when dissolved in water:

• They dissociate partially to release H⁺_(aq) ions:
• CH₃COOH_(aq) ⇌ CH₃COO^-_(aq) + H⁺_(aq)
• They react with bases, like NaOH, to form salts (known as carboxylates) and water:
• CH₃COOH_(aq) + NaOH_(aq) ➔ CH₃COO^- Na⁺_(aq) + H₂O_(l)

However, carboxylic acids are considered weak acids because the equilibrium of the dissociation reaction lies significantly to the left, indicating that most of the acid molecules remain undissociated:

K_a (ethanoic acid) = 1.7 x 10^-5 mol dm^-3 at 25 °C

The lower the K_a value, the weaker the acid.

Carboxylic Acids

This lesson covers: 

1. Carboxylic acids as weak acids
2. Comparison of acid strength between carboxylic acids, phenols, and alcohols
3. Effect of electronegative substituents on carboxylic acid strength
4. Synthesis of benzoic acid
5. Oxidation reactions of carboxylic acids
6. Synthesis of acyl chlorides