Welcome to Acyl Chlorides! The Super-Reactive Carboxylic Derivatives

Hello future Chemists! This chapter introduces you to a highly important, but extremely reactive, group of organic compounds: Acyl Chlorides (also sometimes called Acid Chlorides).

These molecules are the workhorses of organic synthesis, acting as fantastic starting materials to quickly create other functional groups like esters and amides, often under much milder conditions than their carboxylic acid counterparts. Don't worry about their scary reactivity — we will break down exactly why they are so powerful and how to control their reactions!

1. Understanding the Acyl Chloride Structure and Nomenclature

1.1 What is an Acyl Chloride?

Acyl chlorides are derivatives of carboxylic acids where the hydroxyl (-OH) group is replaced by a chlorine atom (-Cl).

  • Functional Group: The key functional group is the acyl group (\(R-C=O\)) bonded directly to a chlorine atom. It is written structurally as \(\text{RCOCl}\).
  • Analogy: Think of a carboxylic acid (\(\text{RCOOH}\)) as a normal battery. An acyl chloride (\(\text{RCOCl}\)) is the same battery but hooked up to a turbocharger. It holds a lot of potential chemical energy, making it much more reactive.

1.2 Naming Acyl Chlorides

Naming is straightforward. You take the name of the parent carboxylic acid, drop the -oic acid suffix, and replace it with -oyl chloride.

  • Carboxylic Acid: Ethanoic acid (\(\text{CH}_3\text{COOH}\))
  • Corresponding Acyl Chloride: Ethanoyl chloride (\(\text{CH}_3\text{COCl}\))
  • Carboxylic Acid: Propanoic acid
  • Corresponding Acyl Chloride: Propanoyl chloride

Key Takeaway: Acyl chlorides contain the \(\text{RCOCl}\) group and are named using the -oyl chloride suffix.

2. Preparation of Acyl Chlorides (Synthesis)

Acyl chlorides cannot be made easily using hydrochloric acid (HCl) reacting directly with the carboxylic acid, as the reaction is an equilibrium favouring the reactants. Instead, we use powerful chlorinating agents to achieve full substitution.

2.1 Reagents and Conditions

Acyl chlorides are prepared by reacting a carboxylic acid with one of three common phosphorus or sulfur halides:

  1. Phosphorus(V) chloride (\(\text{PCl}_5\))

    \(\text{RCOOH} + \text{PCl}_5 \rightarrow \text{RCOCl} + \text{POCl}_3 + \text{HCl}\)

  2. Phosphorus(III) chloride (\(\text{PCl}_3\)) (Requires heat)

    \(3\text{RCOOH} + \text{PCl}_3 \rightarrow 3\text{RCOCl} + \text{H}_3\text{PO}_3\)

  3. Thionyl chloride (\(\text{SOCl}_2\)) (The preferred method)

    \(\text{RCOOH} + \text{SOCl}_2 \rightarrow \text{RCOCl} + \text{SO}_2 (\text{g}) + \text{HCl} (\text{g})\)

Did You Know? Why is \(\text{SOCl}_2\) preferred?

The product mixture from \(\text{PCl}_5\) or \(\text{PCl}_3\) contains phosphorus compounds that are difficult to separate from the acyl chloride. However, when using thionyl chloride (\(\text{SOCl}_2\)), the by-products (\(\text{SO}_2\) and \(\text{HCl}\)) are both gases. They simply bubble out of the mixture, leaving a very pure acyl chloride product that requires less purification.

Key Takeaway: Acyl chlorides are typically synthesized from carboxylic acids using \(\text{SOCl}_2\), \(\text{PCl}_5\), or \(\text{PCl}_3\). The thionyl chloride method yields the purest product.

3. Explaining the Extreme Reactivity

Acyl chlorides are highly reactive towards nucleophiles (species that donate electron pairs). This reactivity is driven by two key factors, both related to the structure of the functional group:

3.1 The Highly Electrophilic Carbon Atom

The carbon atom in the carbonyl group (\(\text{C=O}\)) is already partially positive ($\delta+$) because oxygen is highly electronegative and pulls electrons away. In an acyl chloride, the chlorine atom (also highly electronegative) pulls even more electrons away from this central carbon.

  • The carbon atom becomes very strongly positive (\(\delta++\)) and is an excellent target for nucleophiles.

3.2 The Excellent Leaving Group

The chlorine atom leaves as a chloride ion, \(\text{Cl}^-\). The chloride ion is an exceptionally stable species, making it an excellent leaving group. This makes the substitution reaction energetically favourable and very fast.

Imagine the chlorine atom is a passenger on a plane. Since it's ready to jump off (stable as $\text{Cl}^-$), the plane (the molecule) reacts very quickly!

Quick Review Box: Reactivity of Carboxylic Derivatives

Acyl Chlorides (\(\text{RCOCl}\)) $\gg$ Carboxylic Acids (\(\text{RCOOH}\)) $\gg$ Esters (\(\text{RCOOR'}\)) $\gg$ Amides (\(\text{RCONH}_2\))
Acyl chlorides are the most reactive due to the highly polarised $\text{C}-\text{Cl}$ bond and the excellent $\text{Cl}^-$ leaving group.

Key Takeaway: Acyl chlorides are extremely reactive because the carbonyl carbon is highly electrophilic and the chloride ion is a superb leaving group.

4. Nucleophilic Addition-Elimination Reactions

All major reactions of acyl chlorides involve nucleophilic addition-elimination, where a nucleophile attacks the electrophilic carbonyl carbon, forms a tetrahedral intermediate (addition), and then the chloride ion is expelled (elimination).

Step-by-Step Mechanism Overview (Don't worry, we'll make this clear!)

  1. Nucleophilic Attack (Addition): The lone pair on the nucleophile (e.g., \(\text{H}_2\text{O}\), \(\text{ROH}\), \(\text{NH}_3\)) attacks the $\delta++$ carbonyl carbon. The $\pi$-electrons of the $\text{C=O}$ bond shift up onto the oxygen atom, forming a negative tetrahedral intermediate.
  2. Elimination of the Leaving Group: The lone pair on the oxygen shifts back down to reform the $\text{C=O}$ double bond. This kicks out the weak base, $\text{Cl}^-$, which is the excellent leaving group (Elimination).

This whole process happens rapidly, often at room temperature (R.T.), requiring no heat or catalyst.

4.1 Reaction with Water (Hydrolysis)

Acyl chlorides react violently with water, giving off steamy white fumes of hydrogen chloride gas (\(\text{HCl}\)).

  • Reagents: Water (\(\text{H}_2\text{O}\))
  • Conditions: Room temperature
  • Product: Carboxylic acid and hydrogen chloride gas.
  • Equation (Ethanoyl chloride example):
    \(\text{CH}_3\text{COCl} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{HCl}\)

4.2 Reaction with Alcohols (Esterification)

This is a fast and efficient method to produce esters.

  • Reagents: Alcohol (e.g., Ethanol, \(\text{C}_2\text{H}_5\text{OH}\))
  • Conditions: Room temperature
  • Product: Ester and hydrogen chloride gas.
  • Equation (Ethanoyl chloride + Ethanol):
    \(\text{CH}_3\text{COCl} + \text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{COOC}_2\text{H}_5 + \text{HCl}\)
  • Note: This is far quicker and more complete than reacting a carboxylic acid directly with an alcohol, which requires heating and an acid catalyst.

4.3 Reaction with Phenols (Esterification)

While alcohols and acyl chlorides react easily, phenols (\(\text{C}_6\text{H}_5\text{OH}\)) are slightly less nucleophilic due to the lone pair on the oxygen being partially delocalised into the benzene ring.

  • Reagents: Phenol
  • Conditions: Room temperature. (Often requires a base like \(\text{NaOH}\) to make the phenoxide ion, which is a better nucleophile, though the reaction with phenol itself at R.T. is sufficient for the syllabus.)
  • Product: Ester (specifically, a phenyl ester) and hydrogen chloride gas.
  • Equation (Benzoyl chloride + Phenol $\rightarrow$ Phenyl benzoate):
    \(\text{C}_6\text{H}_5\text{COCl} + \text{C}_6\text{H}_5\text{OH} \rightarrow \text{C}_6\text{H}_5\text{COOC}_6\text{H}_5 + \text{HCl}\)

4.4 Reaction with Ammonia (Amide Formation)

Ammonia acts as a nucleophile, resulting in the formation of a primary amide.

  • Reagents: Concentrated Aqueous Ammonia (\(\text{NH}_3\))
  • Conditions: Room temperature
  • Product: Amide and hydrogen chloride. (Since the product mixture contains HCl, usually two moles of $\text{NH}_3$ are required: one to react with the acyl chloride, and one to neutralise the resulting $\text{HCl}$ to form ammonium chloride.)
  • Equation (Ethanoyl chloride + Ammonia):
    \(\text{CH}_3\text{COCl} + 2\text{NH}_3 \rightarrow \text{CH}_3\text{CONH}_2 + \text{NH}_4\text{Cl}\)

4.5 Reaction with Amines (Amide Formation)

Primary (\(\text{RNH}_2\)) and secondary (\(\text{R}_2\text{NH}\)) amines also react to form amides (secondary and tertiary amides, respectively).

  • Reagents: Primary or Secondary Amine (e.g., Methylamine, \(\text{CH}_3\text{NH}_2\))
  • Conditions: Room temperature
  • Product: An N-substituted amide and hydrogen chloride. (Again, two moles of amine are used — one to react, one to neutralise the acid.)
  • Equation (Ethanoyl chloride + Methylamine):
    \(\text{CH}_3\text{COCl} + 2\text{CH}_3\text{NH}_2 \rightarrow \text{CH}_3\text{CONHCH}_3 + \text{CH}_3\text{NH}_3^+\text{Cl}^-\)

Key Takeaway: Acyl chlorides are highly versatile reagents that react rapidly with nucleophiles (\(\text{H}_2\text{O}\), alcohols, phenols, \(\text{NH}_3\), amines) at room temperature via the nucleophilic addition-elimination mechanism to form carboxylic acids, esters, or amides.

5. Relative Hydrolysis: Acyl Chlorides vs. Other Chlorinated Compounds

The syllabus requires you to compare the ease of hydrolysis (reaction with water) of acyl chlorides, alkyl chlorides (halogenoalkanes), and halogenoarenes (aryl chlorides, like chlorobenzene).

5.1 Comparing Reactivity Towards Hydrolysis

Hydrolysis involves replacing the chlorine atom with an -OH group (a nucleophilic substitution). The rate depends heavily on how easily the C-Cl bond breaks.

Order of Reactivity:

Acyl Chloride $\gg$ Alkyl Chloride $\gg$ Halogenoarene

5.2 Detailed Explanation

(a) Acyl Chlorides (\(\text{RCOCl}\)) - Extremely Reactive

As discussed above, acyl chlorides react instantly with water at R.T.

  • Reason 1: High Electrophilicity: The carbonyl carbon is strongly positive (\(\delta++\)).
  • Reason 2: Mechanism: They undergo the rapid addition-elimination mechanism, where the initial attack is highly favourable.
(b) Alkyl Chlorides (Halogenoalkanes, $\text{R}-\text{Cl}$) - Moderate Reactivity

Alkyl chlorides react slowly with water, requiring heating under reflux with aqueous alkali (e.g., \(\text{NaOH}\)) to speed up the process.

  • Reason 1: Polarity: The carbon atom is only slightly positive ($\delta+$) as it is bonded only to the slightly more electronegative chlorine.
  • Reason 2: Mechanism: They undergo slower nucleophilic substitution (\(\text{SN}1/\text{SN}2\)) which does not benefit from the strong pull of the carbonyl oxygen.
(c) Halogenoarenes (Aryl Chlorides, e.g., Chlorobenzene) - Very Low Reactivity

Halogenoarenes are virtually inert and do not react with water, even under harsh conditions.

  • Reason 1: Orbital Overlap/Delocalisation: The lone pair on the chlorine atom overlaps with the delocalised $\pi$-electron system of the benzene ring.
  • This overlap introduces partial double bond character to the \(\text{C}-\text{Cl}\) bond.
  • Result: The \(\text{C}-\text{Cl}\) bond is much shorter and stronger than in alkyl chlorides or acyl chlorides, making it extremely difficult to break. Nucleophilic attack is strongly inhibited.

Memory Aid: Think of the chlorine atom being 'stuck' in the benzene ring due to the double-bond character. It can't escape easily!

Key Takeaway: Acyl chlorides are the most reactive because of the strong electrophilic nature of the carbonyl carbon and the easy addition-elimination mechanism. Halogenoarenes are the least reactive because the \(\text{C}-\text{Cl}\) bond is strengthened by partial double bond character due to resonance with the benzene ring.


You have successfully covered acyl chlorides! Focus on understanding the addition-elimination mechanism and why acyl chlorides are such powerful synthetic tools!