Define addition-elimination reaction .

An addition-elimination reaction is a two-step mechanism in which a nucleophile first adds across the C=O bond of an acyl chloride, then a small molecule (such as HCl) is eliminated.

What are the three reagents that can prepare an acyl chloride from a carboxylic acid?

Solid phosphorus(V) chloride (PCl 5 ), liquid phosphorus(III) chloride (PCl 3 ) with heat, and liquid sulfur dichloride oxide (SOCl 2 ).

True or False? When an acyl chloride reacts with ammonia, the HCl produced remains as a separate product in the reaction mixture.

False. The HCl formed is immediately neutralised by a second molecule of ammonia to form an ammonium salt (NH 4 Cl), so HCl is not observed as a separate product.

Hydrolysis of an acyl chloride produces a .......... acid and .......... gas.

Hydrolysis of an acyl chloride produces a carboxylic acid and HCl gas.

What products form when propanoyl chloride reacts with methylamine?

Methylpropanamide (a substituted amide) and methylammonium chloride are formed. The HCl produced is neutralised by the excess methylamine.

True or False? Acyl chlorides undergo esterification with phenols without requiring any additional conditions.

False. The reaction of acyl chlorides with phenols requires a base (e.g. NaOH) to deprotonate phenol and form the phenoxide ion , which is a better nucleophile. The reaction proceeds at room temperature — heat is not required.

Why are two molecules of ammonia (or amine) needed when an acyl chloride reacts with ammonia?

The first molecule of ammonia attacks the acyl chloride to form the amide . The second molecule neutralises the HCl that is produced, forming ammonium chloride .

Acyl chlorides can be prepared from carboxylic acids because they are .......... reactive. Their reactions with nucleophiles go to .......... unlike those of carboxylic acids.

Acyl chlorides can be prepared from carboxylic acids because they are more reactive. Their reactions with nucleophiles go to completion unlike those of carboxylic acids.

Hydrolysis is the breakdown of a compound using water. Acyl chlorides are hydrolysed readily at room temperature to form a carboxylic acid and HCl.

A nucleophile is a species that donates a lone pair of electrons to an electron-deficient atom. In acyl chloride reactions, nucleophiles such as water, alcohols, ammonia and amines attack the carbonyl carbon.

Describe the two steps of the addition-elimination mechanism for the hydrolysis of an acyl chloride.

Step 1: the lone pair of the oxygen atom from water attacks the carbonyl carbon ( addition across C=O). Step 2: an HCl molecule is eliminated to form the carboxylic acid .

True or False? In the esterification mechanism of an acyl chloride with a phenol, NaCl is eliminated rather than HCl.

True. When phenol is deprotonated by a base to form the phenoxide ion, the phenoxide attacks the carbonyl carbon and NaCl is eliminated as the small molecule.

In the addition-elimination mechanism, step 1 involves .......... of a nucleophile across the C=O bond, and step 2 involves .......... of a small molecule.

In the addition-elimination mechanism, step 1 involves addition of a nucleophile across the C=O bond, and step 2 involves elimination of a small molecule.

Why must phenol be converted to the phenoxide ion before it can react with an acyl chloride in the esterification mechanism?

The phenoxide ion carries a negative charge making its oxygen lone pair more available for nucleophilic attack on the carbonyl carbon. Phenol itself is a weaker nucleophile so it requires this activation.

True or False? The reaction of an acyl chloride with a secondary amine produces a substituted amide.

True. Secondary amines (with two alkyl groups on nitrogen) react with acyl chlorides via addition-elimination to form substituted (N,N-disubstituted) amides and the HCl is neutralised by excess amine.

What is the difference between the product formed when an acyl chloride reacts with ammonia versus a primary amine?

With ammonia, a non-substituted ( primary amide ) is formed. With a primary amine, a substituted ( secondary amide ) is formed, where the nitrogen carries one alkyl group.

In the addition-elimination mechanism of acyl chlorides with amines, the lone pair of the .......... atom attacks the carbonyl carbon, and .......... is eliminated.

In the addition-elimination mechanism of acyl chlorides with amines, the lone pair of the nitrogen atom attacks the carbonyl carbon, and HCl is eliminated.

An amide is an organic compound containing a -CONH 2 (or substituted variant) functional group, formed by the addition-elimination reaction of an acyl chloride with ammonia or an amine.

Hydrolysis is the breakdown of a compound by reaction with water. The ease of hydrolysis depends on the strength of the C-Cl bond and the electrophilicity of the carbon bonded to chlorine.

Place acyl chlorides, alkyl chlorides and aryl chlorides in order of decreasing ease of hydrolysis.

Acyl chlorides (most readily hydrolysed at room temperature) > alkyl chlorides (require strong alkali and reflux) > aryl chlorides (do not undergo hydrolysis).

True or False? Aryl chlorides do not undergo hydrolysis because the C-Cl bond has some double-bond character.

True. One of the lone pairs on the chlorine atom overlaps with the delocalised π system of the benzene ring, giving the C-Cl bond partial double-bond character and making it very strong and resistant to hydrolysis.

In acyl chlorides, two .......... atoms pull electrons away from the carbonyl carbon, leaving it very .......... and highly susceptible to nucleophilic attack.

In acyl chlorides, two electronegative atoms pull electrons away from the carbonyl carbon, leaving it very δ+ and highly susceptible to nucleophilic attack.

Why do alkyl chlorides require a strong alkali such as OH - for hydrolysis, rather than water alone?

The carbonyl carbon in alkyl chlorides is bonded to only one electronegative atom, so it is less δ+ than in acyl chlorides. OH - is a stronger nucleophile than H 2 O and is needed to break the stronger C-Cl bond.

True or False? Acyl chlorides can be hydrolysed at room temperature using water alone.

True. The carbonyl carbon in acyl chlorides is very δ+ due to two electronegative atoms, so the C-Cl bond is weakened enough for water to act as a nucleophile and hydrolyse it readily at room temperature.

Why is the C-Cl bond in aryl chlorides stronger than in alkyl chlorides?

In aryl chlorides, the chlorine lone pair overlaps with the delocalised π system of the benzene ring, giving the C-Cl bond some partial double-bond character and making it much stronger.

Alkyl chlorides are hydrolysed by refluxing with a .......... alkali such as .......... ions, because water alone is too weak a nucleophile.

Alkyl chlorides are hydrolysed by refluxing with a strong alkali such as OH - ions, because water alone is too weak a nucleophile.

Define aryl chloride .

An aryl chloride is a compound in which a chlorine atom is directly bonded to a benzene ring. The C-Cl bond has partial double-bond character due to overlap with the ring's π system, making aryl chlorides resistant to hydrolysis.

A LevelChemistryCambridge (CIE)Flashcards33. Carboxylic Acids & DerivativesAcyl Chlorides

Acyl Chlorides (Cambridge (CIE) A Level Chemistry): Flashcards

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Define addition-elimination reaction.

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Define addition-elimination reaction.
An addition-elimination reaction is a two-step mechanism in which a nucleophile first adds across the C=O bond of an acyl chloride, then a small molecule (such as HCl) is eliminated.
What are the three reagents that can prepare an acyl chloride from a carboxylic acid?
Solid phosphorus(V) chloride (PCl₅), liquid phosphorus(III) chloride (PCl₃) with heat, and liquid sulfur dichloride oxide (SOCl₂).
True or False?
When an acyl chloride reacts with ammonia, the HCl produced remains as a separate product in the reaction mixture.
False.
The HCl formed is immediately neutralised by a second molecule of ammonia to form an ammonium salt (NH₄Cl), so HCl is not observed as a separate product.
Hydrolysis of an acyl chloride produces a .......... acid and .......... gas.
Hydrolysis of an acyl chloride produces a carboxylic acid and HCl gas.
What products form when propanoyl chloride reacts with methylamine?
Methylpropanamide (a substituted amide) and methylammonium chloride are formed. The HCl produced is neutralised by the excess methylamine.
True or False?
Acyl chlorides undergo esterification with phenols without requiring any additional conditions.
False.
The reaction of acyl chlorides with phenols requires a base (e.g. NaOH) to deprotonate phenol and form the phenoxide ion, which is a better nucleophile. The reaction proceeds at room temperature — heat is not required.
Why are two molecules of ammonia (or amine) needed when an acyl chloride reacts with ammonia?
The first molecule of ammonia attacks the acyl chloride to form the amide. The second molecule neutralises the HCl that is produced, forming ammonium chloride.
Acyl chlorides can be prepared from carboxylic acids because they are .......... reactive. Their reactions with nucleophiles go to .......... unlike those of carboxylic acids.
Acyl chlorides can be prepared from carboxylic acids because they are more reactive. Their reactions with nucleophiles go to completion unlike those of carboxylic acids.
Define hydrolysis.
Hydrolysis is the breakdown of a compound using water. Acyl chlorides are hydrolysed readily at room temperature to form a carboxylic acid and HCl.
Define nucleophile.
A nucleophile is a species that donates a lone pair of electrons to an electron-deficient atom. In acyl chloride reactions, nucleophiles such as water, alcohols, ammonia and amines attack the carbonyl carbon.
Describe the two steps of the addition-elimination mechanism for the hydrolysis of an acyl chloride.
Step 1: the lone pair of the oxygen atom from water attacks the carbonyl carbon (addition across C=O). Step 2: an HCl molecule is eliminated to form the carboxylic acid.
True or False?
In the esterification mechanism of an acyl chloride with a phenol, NaCl is eliminated rather than HCl.
True.
When phenol is deprotonated by a base to form the phenoxide ion, the phenoxide attacks the carbonyl carbon and NaCl is eliminated as the small molecule.
In the addition-elimination mechanism, step 1 involves .......... of a nucleophile across the C=O bond, and step 2 involves .......... of a small molecule.
In the addition-elimination mechanism, step 1 involves addition of a nucleophile across the C=O bond, and step 2 involves elimination of a small molecule.
Why must phenol be converted to the phenoxide ion before it can react with an acyl chloride in the esterification mechanism?
The phenoxide ion carries a negative charge making its oxygen lone pair more available for nucleophilic attack on the carbonyl carbon. Phenol itself is a weaker nucleophile so it requires this activation.
True or False?
The reaction of an acyl chloride with a secondary amine produces a substituted amide.
True.
Secondary amines (with two alkyl groups on nitrogen) react with acyl chlorides via addition-elimination to form substituted (N,N-disubstituted) amides and the HCl is neutralised by excess amine.
What is the difference between the product formed when an acyl chloride reacts with ammonia versus a primary amine?
With ammonia, a non-substituted (primary amide) is formed. With a primary amine, a substituted (secondary amide) is formed, where the nitrogen carries one alkyl group.
In the addition-elimination mechanism of acyl chlorides with amines, the lone pair of the .......... atom attacks the carbonyl carbon, and .......... is eliminated.
In the addition-elimination mechanism of acyl chlorides with amines, the lone pair of the nitrogen atom attacks the carbonyl carbon, and HCl is eliminated.
Define amide.
An amide is an organic compound containing a -CONH₂ (or substituted variant) functional group, formed by the addition-elimination reaction of an acyl chloride with ammonia or an amine.
Define hydrolysis.
Hydrolysis is the breakdown of a compound by reaction with water. The ease of hydrolysis depends on the strength of the C-Cl bond and the electrophilicity of the carbon bonded to chlorine.
Place acyl chlorides, alkyl chlorides and aryl chlorides in order of decreasing ease of hydrolysis.
Acyl chlorides (most readily hydrolysed at room temperature) > alkyl chlorides (require strong alkali and reflux) > aryl chlorides (do not undergo hydrolysis).
True or False?
Aryl chlorides do not undergo hydrolysis because the C-Cl bond has some double-bond character.
True.
One of the lone pairs on the chlorine atom overlaps with the delocalised π system of the benzene ring, giving the C-Cl bond partial double-bond character and making it very strong and resistant to hydrolysis.
In acyl chlorides, two .......... atoms pull electrons away from the carbonyl carbon, leaving it very .......... and highly susceptible to nucleophilic attack.
In acyl chlorides, two electronegative atoms pull electrons away from the carbonyl carbon, leaving it very δ+ and highly susceptible to nucleophilic attack.
Why do alkyl chlorides require a strong alkali such as OH^- for hydrolysis, rather than water alone?
The carbonyl carbon in alkyl chlorides is bonded to only one electronegative atom, so it is less δ+ than in acyl chlorides. OH^- is a stronger nucleophile than H₂O and is needed to break the stronger C-Cl bond.
True or False?
Acyl chlorides can be hydrolysed at room temperature using water alone.
True.
The carbonyl carbon in acyl chlorides is very δ+ due to two electronegative atoms, so the C-Cl bond is weakened enough for water to act as a nucleophile and hydrolyse it readily at room temperature.
Why is the C-Cl bond in aryl chlorides stronger than in alkyl chlorides?
In aryl chlorides, the chlorine lone pair overlaps with the delocalised π system of the benzene ring, giving the C-Cl bond some partial double-bond character and making it much stronger.
Alkyl chlorides are hydrolysed by refluxing with a .......... alkali such as .......... ions, because water alone is too weak a nucleophile.
Alkyl chlorides are hydrolysed by refluxing with a strong alkali such as OH^- ions, because water alone is too weak a nucleophile.
Define aryl chloride.
An aryl chloride is a compound in which a chlorine atom is directly bonded to a benzene ring. The C-Cl bond has partial double-bond character due to overlap with the ring's π system, making aryl chlorides resistant to hydrolysis.

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