# Properties and reactions of carboxylic acid derivatives

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u26-properties-and-reactions-of-carboxylic/

This sub-topic covers the structure, physical properties and key chemical reactions of common carboxylic acid derivatives: acyl chlorides, esters, amides, and acid anhydrides. We focus on the core mechanism of nucleophilic acyl substitution.

**Prerequisites:** [Structure and reactions of carboxylic acids](https://www.owlsprep.com/study/cie-9701-u25-carboxylic-acids/); Nucleophilic substitution reaction mechanisms

## Learning objectives

- Identify and classify common classes of carboxylic acid derivatives
- Explain the trend in reactivity towards nucleophilic acyl substitution
- Write balanced equations for key reactions of carboxylic acid derivatives
- Predict products of hydrolysis, esterification and aminolysis reactions
- Identify unknown carboxylic acid derivatives from chemical test data

## Structure, Classification and Physical Properties

**Carboxylic acid derivative** — An organic compound derived from a carboxylic acid where the -OH group of the carboxyl group is replaced by another electronegative group. All derivatives retain the electrophilic acyl group.

*Example:* Ethanoyl chloride ($CH_3COCl$) and ethyl ethanoate ($CH_3COOCH_2CH_3$) are common carboxylic acid derivatives.

The four main classes of carboxylic acid derivatives tested in CIE A-Level are:

- Acyl chlorides (acid chlorides): general formula $RCOCl$
- Acid anhydrides: general formula $(RCO)_2O$
- Esters: general formula $RCOOR'$
- Amides: general formula $RCONH_2$, $RCONHR'$ or $RCONR'_2$

Boiling points depend on intermolecular forces: amides can form intermolecular hydrogen bonds via N-H groups, so they have the highest boiling points. Acyl chlorides and esters have no O-H or N-H bonds, so they cannot form intermolecular hydrogen bonds and have lower boiling points than carboxylic acids of similar molecular mass.

**Worked example:** Arrange the following in order of increasing boiling point: butanamide, butanoyl chloride, ethyl propanoate. Explain your answer.

1. Step 1: Identify the intermolecular forces for each compound
2. Butanamide (a primary amide) has N-H bonds, so it forms strong intermolecular hydrogen bonds, giving it the highest boiling point.
3. Both butanoyl chloride and ethyl propanoate cannot form intermolecular hydrogen bonds. Ethyl propanoate has a higher molecular mass than butanoyl chloride (same number of carbons), so stronger London dispersion forces.
4. Final order of increasing boiling point:
5. $$\text{butanoyl chloride} < \text{ethyl propanoate} < \text{butanamide}$$

## Reactivity Trend and Nucleophilic Acyl Substitution

**Nucleophilic acyl substitution** — A two-step reaction where a nucleophile attacks the electrophilic acyl carbon to form a tetrahedral intermediate, then the leaving group eliminates to reform the C=O double bond, resulting in substitution.

Reactivity towards nucleophilic acyl substitution depends on two key factors: 1) the partial positive charge on the acyl carbon (lower electron density = more reactive), 2) the stability of the leaving group (more stable leaving group = more reactive). The overall trend tested in exams is:

$$\text{Acyl chlorides} > \text{acid anhydrides} > \text{esters} > \text{amides}$$

This trend arises because chlorine is strongly electron-withdrawing (increasing acyl carbon partial charge) and chloride is a very stable leaving group. For amides, the amino group donates electron density via resonance (reducing acyl carbon partial charge) and $NH_2^-$ is a poor leaving group.

**Worked example:** Explain why propanoyl chloride reacts with methanol at room temperature, but methyl propanoate does not.

1. Step 1: Compare leaving group stability
2. Propanoyl chloride has a $Cl^-$ leaving group, which is a stable weak base and good leaving group. Methyl propanoate has a $CH_3O^-$ leaving group, which is a strong base and poor leaving group.
3. Step 2: Compare electrophilicity of the acyl carbon
4. The Cl atom in propanoyl chloride withdraws electron density, increasing the partial positive charge on the acyl carbon, making it more susceptible to nucleophilic attack by methanol. The methoxy group in methyl propanoate donates electron density via resonance, reducing the partial positive charge.
5. Conclusion: Propanoyl chloride is far more reactive, so reaction proceeds at room temperature.

> **Exam tip:** Always link reactivity to both leaving group stability and acyl carbon electrophilicity when explaining trends. Answers that only mention one factor usually lose full marks.

## Key Reactions of Carboxylic Acid Derivatives

All common reactions of carboxylic acid derivatives are variations of nucleophilic acyl substitution. The table below summarises the main products for each class of derivative:

| Derivative class | Acid hydrolysis product | Alkaline hydrolysis product | Reaction with alcohol | Reaction with ammonia |
| --- | --- | --- | --- | --- |
| Acyl chloride | Carboxylic acid + HCl | Carboxylate salt + HCl | Ester + HCl | Primary amide + NH₄Cl |
| Acid anhydride | 2 × Carboxylic acid | 2 × Carboxylate salt | Ester + Carboxylic acid | Primary amide + Carboxylic acid |
| Ester | Carboxylic acid + Alcohol | Carboxylate salt + Alcohol | No reaction (transesterification only) | No reaction |
| Amide | Carboxylic acid + Ammonium salt | Carboxylate salt + Ammonia | No reaction | No reaction |

**Worked example:** Write the balanced equation for the reaction of benzoic anhydride with excess aqueous sodium hydroxide, and name the organic product.

1. Step 1: Recall the structure of benzoic anhydride: $(C_6H_5CO)_2O$
2. Step 2: Alkaline hydrolysis cleaves the anhydride into two carboxylate groups, which are deprotonated under alkaline conditions.
3. Step 3: Balance the equation, accounting for the excess sodium hydroxide:
4. $$(C_6H_5CO)_2O + 2NaOH \rightarrow 2C_6H_5COONa + H_2O$$
5. The organic product is sodium benzoate.

## Identification and Synthesis of Derivatives

Carboxylic acid derivatives are common intermediates in organic synthesis questions. A simple chemical test to distinguish acyl chlorides from other derivatives is reaction with water: only acyl chlorides are reactive enough to produce steamy white fumes of HCl at room temperature.

> **tip**
>
> When planning syntheses, acyl chlorides are the preferred starting material for making esters and amides because they are reactive enough to react at room temperature without needing a catalyst, unlike carboxylic acids.

**Worked example:** An unknown derivative X reacts with water to produce steamy fumes and a carboxylic acid. Reaction of X with ethanol produces ethyl benzoate. Identify X.

1. Step 1: Steamy fumes from reaction with water confirms X is an acyl chloride, as only acyl chlorides are reactive enough to produce HCl fumes at room temperature.
2. Step 2: Ethyl benzoate has the structure $C_6H_5COOCH_2CH_3$. The acyl group in the product comes from the starting acyl chloride.
3. Step 3: Replace the ethoxy group in ethyl benzoate with Cl to get X.
4. X is benzoyl chloride, with formula $C_6H_5COCl$.

## Common pitfalls

- **Wrong:** Writing a carboxylic acid as the product of alkaline ester hydrolysis instead of a carboxylate salt
  - Why it fails: Alkaline conditions fully deprotonate any carboxylic acid formed, so the carboxylate salt is the final product
  - Correct: Always write the neutralised carboxylate salt (e.g. $RCOO^-Na^+$) as the product of alkaline hydrolysis
- **Wrong:** Stating that amides are more reactive than esters because they have higher boiling points
  - Why it fails: Boiling point depends on intermolecular forces, which has no relation to chemical reactivity towards nucleophilic acyl substitution
  - Correct: Remember the reactivity order: acyl chlorides > acid anhydrides > esters > amides
- **Wrong:** Drawing nucleophilic acyl substitution as a one-step direct displacement of the leaving group
  - Why it fails: The reaction proceeds via a tetrahedral intermediate after the nucleophile adds to the carbonyl group
  - Correct: Always draw the two-step mechanism: addition to form tetrahedral intermediate, then elimination of the leaving group to reform C=O
- **Wrong:** Claiming esters can form hydrogen bonds between their own molecules
  - Why it fails: Esters have no O-H or N-H bonds, so they cannot donate hydrogen bonds, only accept them from water
  - Correct: State that esters have only dipole-dipole interactions and London dispersion forces between their molecules
- **Wrong:** Forgetting that acid anhydrides produce two organic products per mole in reactions with nucleophiles
  - Why it fails: Acid anhydrides have two acyl groups per molecule, so one reacts and one remains as a carboxylic acid product
  - Correct: Always account for both organic products when writing equations for acid anhydride reactions

## Cheatsheet

| Topic | Key Exam Fact |
| --- | --- |
| Reactivity order | Acyl chloride > acid anhydride > ester > amide |
| Acid ester hydrolysis | Carboxylic acid + alcohol, reversible |
| Alkaline ester hydrolysis | Carboxylate salt + alcohol, irreversible |
| Acyl chloride + alcohol | Ester + HCl, reacts at room temperature |
| Acyl chloride test | Steamy HCl fumes with cold water |
| Hydrogen bonding | Only amides can form intermolecular hydrogen bonds |

## What's next

Mastering the properties and reactions of carboxylic acid derivatives is critical for tackling organic synthesis questions, which make up a large portion of the higher-mark questions in CIE A-Level Chemistry papers 2 and 3. Nucleophilic acyl substitution is a core mechanism that reappears in topics like amino acids, proteins, and polyester synthesis, so building a strong foundation here will help you tackle more complex content later. Next, you will explore specific reactions of esters including hydrolysis, and learn how esters are the core component of natural fats and oils. This content also links directly to full organic synthesis planning, where you will combine reactions of carboxylic acids and their derivatives to build target organic molecules.

- [Acylation reactions](https://www.owlsprep.com/study/cie-9701-u26-acylation-reactions/)
- [Organic synthesis](https://www.owlsprep.com/study/cie-9701-u27-overview/)
- [Synthetic route planning](https://www.owlsprep.com/study/cie-9701-u27-synthetic-route-planning/)

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