Acyl Compounds: Structure and Properties Revision Notes | A Level AQA Chemistry

1. The Acyl Group: A Carbonyl with a Leaving Group

Carboxylic acids are useful, but they react slowly. Nature and synthetic chemists both need a way to make the carbonyl carbon far more electrophilic — and that is exactly what acyl compounds deliver.

The key structural feature is the acyl group, $\text{RCO}-$. Think of it as a carbonyl carbon ($\text{C=O}$) with one side bonded to a carbon chain and the other side bonded to a leaving group. That leaving group is what distinguishes the three acyl derivatives you need to know: acyl chlorides, acid anhydrides, and amides.

The carbonyl carbon in all three is electron-deficient because oxygen withdraws electron density through the $\text{C=O}$ bond. Attach an electronegative leaving group on the other side and you amplify that electrophilicity dramatically — which is why acyl chlorides react so vigorously with water, alcohols, and amines.

2. Structures of the Three Acyl Derivatives

Before comparing reactivity, you need to be confident drawing and naming each structure. The differences are subtle on paper but enormous in the lab.

In ethanoyl chloride ($\text{CH}_3\text{COCl}$), the chlorine is directly bonded to the carbonyl carbon. Chlorine is highly electronegative, so it withdraws electron density inductively from the already electron-poor carbonyl carbon. The result is an extremely reactive electrophilic centre.

In ethanoic anhydride, the bridging oxygen donates electron density into both carbonyl groups through lone-pair delocalisation. This partially reduces the electrophilicity of each carbonyl carbon compared with an acyl chloride — which is why anhydrides react more slowly and are safer to use in industrial processes.

In ethanamide, the nitrogen lone pair delocalises into the carbonyl $\pi$ system, creating partial double-bond character in the $\text{C}-\text{N}$ bond. This delocalisation makes amides far less reactive than acyl chlorides or anhydrides — the nitrogen is no longer a good leaving group because it has donated its electron density into the carbonyl.

3. Mechanisms of Nucleophilic Addition-Elimination

Acyl chlorides and acid anhydrides both react with nucleophiles by the same two-stage mechanism: nucleophilic addition to the carbonyl, followed by elimination of the leaving group. This is called nucleophilic addition-elimination (sometimes called acyl substitution).

The mechanism is worth understanding deeply because it explains every reaction of acyl compounds — with water, alcohols, ammonia, and amines — in one unified framework. Once you see the pattern, you can predict the product of any acyl compound reacting with any nucleophile.

The same two-stage logic applies when the nucleophile is an alcohol (giving an ester), ammonia (giving an amide), or a primary amine (giving an $N$-substituted amide). The nucleophile changes; the mechanism does not.

4. Reactions of Acyl Chlorides and Acid Anhydrides

Now that the mechanism is clear, it is worth cataloguing the key reactions systematically. Each reaction follows the same addition-elimination pattern, but the nucleophile and product change.

Notice that acyl chlorides and acid anhydrides give the same organic product in each case. The difference is the by-product: acyl chlorides release corrosive $\text{HCl}$ gas, whereas acid anhydrides release a carboxylic acid — far less hazardous. This is why ethanoic anhydride is preferred industrially for making aspirin (2-(ethanoyloxy)benzoic acid), even though it reacts more slowly.

5. Properties of Amides

Amides sit at the far end of the acyl compound reactivity spectrum. The nitrogen lone pair delocalises into the carbonyl $\pi$ system, creating a partial $\text{C}-\text{N}$ double bond and making the nitrogen a very poor leaving group. This delocalisation also has profound consequences for the physical properties of amides.

Primary and secondary amides ($\text{RCONH}_2$ and $\text{RCONHR}'$) can form intermolecular hydrogen bonds through their $\text{N}-\text{H}$ bonds and carbonyl oxygen. This gives them unexpectedly high melting points relative to their molar mass — ethanamide ($M_r = 59$) melts at $82\,^\circ\text{C}$, far higher than ethanoic acid ($M_r = 60$, melting point $17\,^\circ\text{C}$).

Amides are also the functional group found in peptide bonds, the linkages that hold amino acids together in proteins. Understanding amide bonding at A-level therefore connects directly to the biochemistry of proteins — though the mechanism of peptide bond formation is beyond this specification point.

6. Purity Testing of Organic Compounds

After synthesising an acyl compound — or any organic product — you need to verify its identity and purity. AQA requires you to know two distinct approaches: one for pure organic solids and one for pure organic liquids.

The underlying principle in both cases is that a pure substance has a sharp, reproducible physical property. Impurities disrupt intermolecular forces and cause measurable deviations from the expected value.

In practice, a melting point apparatus is used: a small amount of the solid is placed in a capillary tube, heated slowly, and the temperature at which the solid first begins to melt and the temperature at which it is fully liquid are both recorded. A pure sample gives a range of $1-2\,^\circ\text{C}$; an impure sample may melt over $5-10\,^\circ\text{C}$ or more.

Comparing the measured melting point against the literature value also helps confirm identity. If the melting point matches the literature value and the range is sharp, you have strong evidence that the compound is both pure and correctly identified.

For a liquid, thin-layer chromatography (TLC) can complement boiling point measurement: a pure compound gives a single spot on a TLC plate, whereas a mixture gives multiple spots. The $R_f$ value of the spot can be compared with an authentic sample run alongside it on the same plate.

7. Exam Technique and Mark-Winning Strategies

Acyl compound questions on AQA papers tend to test three things: structural recognition, mechanism writing, and the ability to compare reactivity with justification. Each of these has specific language that the mark scheme rewards.

Worked example: Writing a Full Mechanism for Acyl Chloride + Ethanol

Draw the mechanism for the reaction of ethanoyl chloride with ethanol, $\text{C}_2\text{H}_5\text{OH}$, and name the organic product. [4 marks]

1. Nucleophile: oxygen lone pair on $\text{C}_2\text{H}_5\text{OH}$. Electrophile: carbonyl carbon of $\text{CH}_3\text{COCl}$. — The carbonyl carbon is electron-deficient due to the $\text{C=O}$ bond and the inductive withdrawal by $\text{Cl}$. Always identify these before drawing arrows. [B1 — identifying the nucleophilic site]
2. Curly arrow from O lone pair of $\text{C}_2\text{H}_5\text{OH}$ to the carbonyl carbon. Simultaneously, curly arrow from $\text{C=O}$ pi bond to the oxygen of the carbonyl, forming a tetrahedral intermediate with $\text{O}^-$ and $\text{C}-\text{Cl}$ still intact. — The pi bond breaks heterolytically — both electrons move onto the more electronegative oxygen. The intermediate is tetrahedral at the former carbonyl carbon. [M1 — correct curly arrow from nucleophile to carbonyl carbon]
3. Curly arrow from $\text{C}-\text{Cl}$ bond to $\text{Cl}$, regenerating the $\text{C=O}$ bond and releasing $\text{Cl}^-$. — The $\text{C}-\text{Cl}$ bond breaks heterolytically, with both electrons going to chlorine. The carbonyl reforms. [M1 — correct curly arrow showing departure of Cl^-]
4. $\text{Cl}^-$ removes a proton from the $\text{O}-\text{H}$ group on the intermediate, generating $\text{HCl}$ and the ester product: ethyl ethanoate, $\text{CH}_3\text{COOC}_2\text{H}_5$. — The proton transfer step regenerates $\text{HCl}$ as a by-product. The organic product is named as alkyl acylate: ethyl ethanoate. [A1 — correct product name and HCl identified]

Answer: Organic product: ethyl ethanoate, $\text{CH}_3\text{COOC}_2\text{H}_5$. By-product: $\text{HCl}$. Mechanism type: nucleophilic addition-elimination.

When an exam question asks you to compare acyl chlorides and acid anhydrides, the mark scheme typically awards one mark for reactivity (acyl chlorides react faster/more vigorously) and one mark for the reason (chloride is a better leaving group than ethanoate, or the carbonyl carbon in acyl chlorides is more electrophilic). A third mark is often available for the safety/industrial context.

For purity questions, the command word is usually describe or explain. If asked to describe the test, state the observable: the temperature at which melting begins and ends, and whether the range is sharp. If asked to explain, add the scientific principle: impurities disrupt the crystal lattice, lowering and broadening the melting point.

Compare structure to property carefully: ethanamide forms a more extensive hydrogen-bond network than ethanoic acid dimers, so its melting point is higher because more intermolecular interactions must be disrupted across the solid lattice.