Carboxylic Acid Derivatives Revision Notes for OCR A-Level Chemistry A

Carboxylic Acid Derivatives

Introduction to carboxylic acid derivatives

Carboxylic acid derivatives are a family of organic compounds that share an important structural feature called the acyl group. These compounds are called "derivatives" because they can all be converted back into their parent carboxylic acid through a reaction called hydrolysis (breaking down with water).

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Definition of Carboxylic Acid Derivatives

A derivative of a carboxylic acid is a compound that can be hydrolysed to form the parent carboxylic acid. This ability to regenerate the original acid is what unites this family of compounds.

The acyl group has the structure $\text{R-C=O}$ , where R represents an alkyl or aryl group. This carbonyl carbon ( $\text{C=O}$ ) is the reactive center in all these compounds.

There are four main types of carboxylic acid derivatives you need to know:

Esters ( $\text{R-COO-R'}$ )
Acyl chlorides ( $\text{R-COCl}$ )
Acid anhydrides ( $\text{R-CO-O-CO-R'}$ )
Amides ( $\text{R-CONH}_2$ or $\text{R-CONHR'}$ or $\text{R-CONR'R''}$ )

Each derivative contains the acyl group bonded to different atoms or groups. The identity of these attached groups determines the properties and reactivity of each derivative.

Esters

Structure and nomenclature

Esters contain the functional group $\text{-COO-}$ , where the acyl group is bonded to an oxygen atom, which is in turn bonded to an alkyl or aryl group. Esters are typically colorless liquids with pleasant, sweet, fruity smells. Many natural esters occur in fruits and flowers, giving them their characteristic aromas.

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Naming Esters: The Two-Part Approach

To name an ester, you need to identify both the alcohol part and the carboxylic acid part:

First, identify the alkyl chain attached to the single-bonded oxygen atom - this comes from the alcohol
Then identify the carboxylic acid part
Remove the "-oic acid" suffix from the carboxylic acid name and replace it with "-oate"
Put the alcohol part first, followed by the acid part

Memory tip: "Alcohol first, Acid last"

For example, when ethanoic acid reacts with methanol, it forms methyl ethanoate. The "methyl" part comes from methanol, and the "ethanoate" part comes from ethanoic acid.

Acyl chlorides

Structure and nomenclature

Acyl chlorides (also called acid chlorides) are highly reactive compounds where the $\text{-OH}$ group of a carboxylic acid has been replaced by a chlorine atom. They are extremely useful reagents in organic synthesis because they readily react with nucleophiles to form other carboxylic acid derivatives.

To name an acyl chloride:

Start with the name of the parent carboxylic acid
Remove the "-oic acid" suffix
Add "-oyl chloride"

For example, propanoic acid becomes propanoyl chloride.

The chlorine atom makes these compounds much more reactive than the parent carboxylic acids, which is why they are so useful in synthesis reactions.

Acid anhydrides

Structure and formation

An acid anhydride contains two acyl groups bonded together through an oxygen atom. The name "anhydride" means "without water", which gives us a clue about how these compounds are formed.

Acid anhydrides are produced when two carboxylic acid molecules combine with the loss of one water molecule. This is a condensation reaction. For example, two molecules of ethanoic acid can react to form ethanoic anhydride plus water.

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Understanding Acid Anhydride Structure

The structure of acid anhydrides may look complex, but remembering that it's essentially "two carboxylic acids without water" helps you understand and draw these compounds. Acid anhydrides are less reactive than acyl chlorides but can still be used to synthesize esters and amides, particularly in situations where acyl chlorides might be too reactive.

Amides

Amides contain a carbonyl group bonded directly to a nitrogen atom. The nitrogen may have hydrogen atoms attached (primary amide, $\text{R-CONH}_2$ ), one alkyl group (secondary amide, $\text{R-CONHR'}$ ), or two alkyl groups (tertiary amide, $\text{R-CONR'R''}$ ).

In this course, you'll mainly encounter primary and secondary amides formed from reactions of acyl chlorides with ammonia or primary amines. We'll look at these reactions in detail later.

Esterification

The esterification reaction

Esterification is the reaction between an alcohol and a carboxylic acid to produce an ester and water. This is a condensation reaction because a small molecule (water) is eliminated when two larger molecules combine.

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Reaction Conditions for Esterification

The reaction conditions are:

Heat under reflux (to maintain constant temperature while allowing the reaction to occur)
A few drops of concentrated sulfuric acid ( $\text{H}_2\text{SO}_4$ ) as a catalyst
The acid catalyst speeds up the reaction but isn't consumed

The general equation is:

$\text{Carboxylic acid} + \text{Alcohol} \rightleftharpoons \text{Ester} + \text{Water}$

Here's a specific example showing the formation of ethyl propanoate from propanoic acid and ethanol:

The ester name derives from both reactants - "ethyl" from ethanol and "propanoate" from propanoic acid.

Important characteristics of esterification

The key features of esterification you need to remember:

The reaction is reversible, shown by the $\rightleftharpoons$ symbol
The position of equilibrium typically doesn't favor products completely
Esters often have pleasant, fruity odors and occur naturally in fruits
The molecular formula of ethyl propanoate is $\text{C}_5\text{H}_{10}\text{O}_2$

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Exam Tip: Writing Esterification Equations

When writing esterification equations, always include the reaction conditions (heat, $\text{H}_2\text{SO}_4$ catalyst) and remember to write it as a reversible reaction with the $\rightleftharpoons$ symbol.

Hydrolysis of esters

Hydrolysis means "breaking down with water." Esters can be hydrolyzed using either acidic or alkaline conditions, but these two methods give different results.

Acid hydrolysis

In acid hydrolysis, the ester is heated under reflux with dilute aqueous acid. The acid acts as a catalyst. This reaction is simply the reverse of esterification:

$\text{Ester} + \text{Water} \rightleftharpoons \text{Carboxylic acid} + \text{Alcohol}$

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Worked Example: Acid Hydrolysis of an Ester

Key points about acid hydrolysis:

It's a reversible reaction (equilibrium established)
The products are a carboxylic acid and an alcohol
An acid catalyst is required
The reaction needs heating under reflux

Alkaline hydrolysis (saponification)

Alkaline hydrolysis uses aqueous hydroxide ions ( $\text{OH}^-$ ) instead of acid. This reaction is also called saponification and is irreversible.

When an ester reacts with hydroxide ions:

A carboxylate ion (not a carboxylic acid) is formed
An alcohol is also produced
The reaction goes to completion (irreversible)

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Worked Example: Alkaline Hydrolysis

For example, with sodium hydroxide solution as the alkali, sodium ethanoate salt is formed:

$\text{CH}_3\text{COOCH}_3 + \text{NaOH} \rightarrow \text{CH}_3\text{COO}^-\text{Na}^+ + \text{CH}_3\text{OH}$

Notice the single arrow ( $\rightarrow$ ) indicating this is an irreversible reaction.

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Key Difference Between Acid and Alkaline Hydrolysis

Acid hydrolysis produces a carboxylic acid (reversible reaction, $\rightleftharpoons$ )

Alkaline hydrolysis produces a carboxylate salt (irreversible reaction, $\rightarrow$ )

Don't confuse these two types of hydrolysis in your exam. Remember that alkaline hydrolysis produces an ionic salt, not a neutral carboxylic acid, which is why it's irreversible.

Acyl chlorides and their reactions

Preparation of acyl chlorides

Acyl chlorides can be prepared directly from their parent carboxylic acids using thionyl chloride ( $\text{SOCl}_2$ ). The reaction must be carried out in a fume cupboard because the products $\text{SO}_2$ and $\text{HCl}$ are harmful gases that are evolved during the reaction.

The general equation is:

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

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Worked Example: Preparing Propanoyl Chloride

Propanoic acid reacts with thionyl chloride to form propanoyl chloride:

$\text{CH}_3\text{CH}_2\text{COOH} + \text{SOCl}_2 \rightarrow \text{CH}_3\text{CH}_2\text{COCl} + \text{SO}_2(g) + \text{HCl}(g)$

Notice that two gaseous products are evolved, making this a vigorous reaction that must be performed with proper safety precautions.

Why acyl chlorides are so reactive

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Understanding the High Reactivity of Acyl Chlorides

Acyl chlorides are extremely reactive compounds because:

The chlorine atom is a good leaving group
The carbonyl carbon ( $\text{C=O}$ ) is electron-deficient ( $\delta^+$ )
They readily react with nucleophiles (electron-rich species)
They maintain the $\text{C=O}$ double bond through the reaction

This high reactivity makes acyl chlorides excellent reagents for synthesizing other carboxylic acid derivatives like esters and amides with high yields.

Reaction with alcohols to form esters

Acyl chlorides react vigorously with alcohols at room temperature to produce esters. No catalyst is needed, unlike in esterification with carboxylic acids.

The general equation is:

$\text{RCOCl} + \text{R'OH} \rightarrow \text{RCOOR'} + \text{HCl}$

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Worked Example: Forming an Ester from an Acyl Chloride

Ethanoyl chloride reacts with propan-1-ol to form propyl ethanoate and hydrogen chloride gas.

Advantages over direct esterification:

Much faster reaction
Goes to completion (not reversible)
No catalyst needed
Higher yields

Reaction with phenols to form esters

Carboxylic acids are not reactive enough to form esters with phenols directly. However, acyl chlorides and acid anhydrides are sufficiently reactive to react with phenols to produce phenyl esters.

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Important Synthetic Route

Neither reaction requires an acid catalyst. For example, ethanoyl chloride reacts with phenol to form phenyl ethanoate plus $\text{HCl}$ .

This is an important synthetic route because it's the only practical way to make esters from phenols.

Reaction with water to form carboxylic acids

When water is added to an acyl chloride, a violent reaction occurs with the evolution of dense, steamy hydrogen chloride fumes. A carboxylic acid is formed.

The general equation is:

$\text{RCOCl} + \text{H}_2\text{O} \rightarrow \text{RCOOH} + \text{HCl}$

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Worked Example: Acyl Chloride + Water

$\text{CH}_3\text{COCl} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{HCl}$

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Safety Warning

This reaction is so vigorous that you should never add water directly to an acyl chloride. The steamy fumes produced are toxic and corrosive $\text{HCl}$ gas.

Reaction with ammonia to form primary amides

Ammonia can act as a nucleophile by donating its lone pair of electrons. When acyl chlorides react with ammonia, primary amides are formed.

Two moles of ammonia are needed for each mole of acyl chloride:

One molecule acts as the nucleophile to form the amide
The second molecule neutralizes the $\text{HCl}$ produced, forming ammonium chloride ( $\text{NH}_4\text{Cl}$ )

The equation is:

$\text{RCOCl} + 2\text{NH}_3 \rightarrow \text{RCONH}_2 + \text{NH}_4\text{Cl}$

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Worked Example: Forming a Primary Amide

Ethanoyl chloride reacts with excess ammonia to produce ethanamide (a primary amide) and ammonium chloride.

Reaction with primary amines to form secondary amides

Primary amines ( $\text{RNH}_2$ ) also act as nucleophiles and react with acyl chlorides in the same way as ammonia. However, the product is a secondary amide because the nitrogen atom is attached to two carbon atoms.

Again, two moles of amine are required:

$\text{RCOCl} + 2\text{R'NH}_2 \rightarrow \text{RCONHR'} + \text{R'NH}_3^+\text{Cl}^-$

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Worked Example: Forming a Secondary Amide

Ethanoyl chloride reacts with methylamine to form N-methylethanamide (a secondary amide) plus methylammonium chloride.

Note: The "N-" prefix in the name indicates that the methyl group is attached to the nitrogen atom, not to a carbon in the main chain.

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Remember the Stoichiometry

Always remember that two equivalents of ammonia or amine are needed - one to form the amide and one to neutralize the $\text{HCl}$ produced.

Reactions of acid anhydrides

Acid anhydrides undergo similar reactions to acyl chlorides with alcohols, phenols, water, ammonia, and amines. The key difference is that acid anhydrides are less reactive than acyl chlorides, making them useful when a gentler reagent is needed.

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Worked Example: Acid Anhydride Reaction

Ethanoic anhydride can react with phenol to form phenyl ethanoate:

$2(\text{CH}_3\text{CO})_2\text{O} + \text{C}_6\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{COOC}_6\text{H}_5 + \text{CH}_3\text{COOH}$

Notice that one of the products is ethanoic acid, not $\text{HCl}$ as with acyl chlorides. This makes the reaction less vigorous and easier to control in some laboratory preparations.

Mechanism of nucleophilic addition-elimination

The reactions of acyl chlorides with nucleophiles all follow the same mechanism called nucleophilic addition-elimination. This is an important mechanism that combines features of both carbonyl chemistry and halogenoalkane chemistry.

Step 1: Addition

The nucleophile (such as water, ammonia, or an alcohol) has a lone pair of electrons. This lone pair is attracted to and donated to the $\delta^+$ carbon atom in the $\text{C=O}$ group of the acyl chloride.

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Key Events in the Addition Step

A dative covalent bond forms between the nucleophile and the carbonyl carbon
The $\pi$ -bond of the $\text{C=O}$ group breaks
A negatively charged intermediate is formed
The shape around the carbon changes from trigonal planar to tetrahedral

Step 2: Elimination

In the elimination step:

A lone pair of electrons on oxygen reforms the $\text{C=O}$ double bond
This causes the chloride ion ( $\text{Cl}^-$ ) to leave as a leaving group
A proton ( $\text{H}^+$ ) is also lost to complete the elimination
The shape returns to trigonal planar around the carbon

For simplicity, the loss of $\text{Cl}^-$ and $\text{H}^+$ are often shown together in the second step.

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Why It's Called Nucleophilic Addition-Elimination

The mechanism is called nucleophilic addition-elimination because:

First, the nucleophile adds to the carbonyl carbon
Then, the chloride ion is eliminated

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Shape Changes During the Mechanism

The carbon atom changes from trigonal planar (120° bond angles) in the reactant, to tetrahedral (109.5° angles) in the intermediate, back to trigonal planar in the product.

Exam tip: When drawing mechanisms, always show:

Curly arrows starting from lone pairs or bonds
Partial charges ( $\delta^+$ and $\delta^-$ ) on atoms
The tetrahedral intermediate
Both steps clearly labeled

Summary

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Key Points to Remember

Carboxylic acid derivatives all contain the acyl group ( $\text{R-C=O}$ ) and can be hydrolyzed back to carboxylic acids: esters, acyl chlorides, acid anhydrides, and amides

Esterification requires heating an alcohol with a carboxylic acid in the presence of concentrated $\text{H}_2\text{SO}_4$ catalyst - it's a reversible reaction that produces sweet-smelling esters

Hydrolysis of esters can be carried out under acidic conditions (reversible, produces carboxylic acid and alcohol) or alkaline conditions/saponification (irreversible, produces carboxylate salt and alcohol)

Acyl chlorides are highly reactive and useful synthetic reagents - they react readily with alcohols, phenols, water, ammonia, and amines to form esters, carboxylic acids, and amides without needing catalysts or harsh conditions

Nucleophilic addition-elimination mechanism involves two steps: nucleophile adds to the carbonyl carbon (forming a tetrahedral intermediate), then elimination of the chloride ion regenerates the $\text{C=O}$ double bond - remember the shape changes from trigonal planar → tetrahedral → trigonal planar

Carboxylic Acid Derivatives (OCR A-Level Chemistry A): Revision Notes