Mr Cole Chemistry

Introduction to Acyl Chlorides and Acid Anhydrides Questions

Acyl Chlorides & Anhydrides

Carboxylic Acids Worksheet

Nomenclature & Structures

1. Draw the skeletal formula of following molecules.

a) Ethanoyl chloride
Ethanoyl chloride structure
b) 2-hydroxypropanoyl chloride
2-hydroxypropanoyl chloride structure
c) Ethanedioyl chloride
Ethanedioyl chloride structure

2. State the IUPAC name for the following molecules:

a) Structure A

Butanoyl chloride

b) Structure B

Cyanobutanoyl chloride

c) Structure C

3-oxo-2-methylbutanoyl chloride

3. Draw the following acid anhydrides:

a) Ethanoic anhydride
Ethanoic anhydride
b) Methanoic propanoic anhydride
Methanoic propanoic anhydride

4. State the IUPAC name of the following molecules:

a) Structure A

Ethanoic methanoic anhydride

b) Structure B

Methylpropanoic anhydride

Synthesis & Reactions

5. Ethanoyl chlorides can be made using a variety of techniques. In each case, ethanoic acid is reacted with a reactive non-metal chloride.

a) The first method is a reaction between ethanoic acid with phosphorous (V) chloride to form ethanoyl chloride, phosphorous oxychloride and hydrogen chloride.

i. What is the formula of phosphorous (V) chloride?

PCl5

ii. The reaction that occurs is not a redox reaction. What does this tell you about the oxidation state of the phosphorous in phosphorous oxychloride? Use this to determine the formula for phosphorous oxychloride.

It tells you the oxidation state does not change; it remains +5.

Since Oxygen is -2 and Chlorine is -1, for the Phosphorus to be +5 in the oxychloride, the formula must be POCl3.

iii. Given that you know the formula of each of the compounds involved, suggest the chemical equation for the formation of ethanoyl chloride using this method.
CH3COOH + PCl5
→
CH3COCl + POCl3 + HCl
b) The second method is to use PCl3 instead of phosphorous (V) chloride. The inorganic product is phosphoric (III) acid. Write the equation for this reaction.
3CH3COOH + PCl3
→
3CH3COCl + H3PO3

6. Acid anhydrides can be formed when two carboxylic acid groups react.

a) Write an equation for the reaction between two molecules of ethanoic acid to produce an acid anhydride.
2CH3COOH
⇌
(CH3CO)2O + H2O

b) Butenedioic acid can react to form a cyclic acid anhydride.

i. Butenedioic acid exists as two stereoisomers. Draw the skeletal formula of the two butenedioic acid isomers.
Butenedioic acid isomers
ii. Suggest which of them can form an acid anhydride, and draw the structure of the anhydride formed.

The Z-isomer (cis) can form the cyclic anhydride.

Maleic anhydride structure
7. Acyl chlorides are useful because they are more reactive than carboxylic acids. Explain why ethanoyl chloride is more reactive than ethanoic acid.

Both contain a carbonyl carbon susceptible to nucleophilic attack. Ethanoyl chloride is more reactive because the Chloride ion (Cl–) is a better leaving group than Hydroxide (OH–), which is a strong base.

8. Ethanoyl chloride and Ethanoic anhydride are not considered to be soluble, despite being able to make hydrogen bonds. Explain why a solution of these two chemicals cannot exist.

When ethanoyl chloride and ethanoic anhydride come into contact with water, they react (hydrolysis) instead of just dissolving.

  • Ethanoyl chloride reacts to form ethanoic acid and HCl.
  • Ethanoic anhydride reacts to form two molecules of ethanoic acid.
9. State what you would observe if you mixed silver nitrate solution with separate volumes of ethanoyl chloride and chloroethane. Explain the result.

Ethanoyl Chloride: Immediate white precipitate of AgCl due to rapid hydrolysis releasing Cl–.

Chloroethane: No immediate change; the C-Cl bond is strong and resistant to hydrolysis.

Nomenclature & Structures

1. Draw the skeletal formula of following molecules.

a) Ethanoyl chloride
Structure
b) 2-hydroxypropanoyl chloride
Structure
c) Ethanedioyl chloride
Structure

2. State the IUPAC name for the following molecules:

a) Structure A

Butanoyl chloride

b) Structure B

Cyanobutanoyl chloride

c) Structure C

3-oxo-2-methylbutanoyl chloride

3. Draw the following acid anhydrides:

a) Ethanoic anhydride
Ethanoic anhydride
b) Methanoic propanoic anhydride
Methanoic propanoic anhydride

4. State the IUPAC name of the following molecules:

a) Structure A

Ethanoic methanoic anhydride

b) Structure B

Methylpropanoic anhydride

Synthesis & Reactions

5. Ethanoyl chlorides can be made using a variety of techniques. In each case, ethanoic acid is reacted with a reactive non-metal chloride.

a) The first method is a reaction between ethanoic acid with phosphorous (V) chloride.

i. What is the formula of phosphorous (V) chloride?

PCl5

ii. Write the chemical equation for the formation of ethanoyl chloride using this method.
CH3COOH + PCl5
→
CH3COCl + POCl3 + HCl
iii. Show, using oxidation numbers, whether the reaction is a redox reaction or not.

Not Redox.

Oxidation state of P in PCl5 is +5.

In POCl3, Oxygen is -2 and Chlorine is -1 (total -3). Therefore P is +5.

Since the oxidation number of Phosphorus does not change, it is not a redox reaction.

b) The second method is to use phosphorus (III) chloride instead of phosphorous (V) chloride, forming the acyl chloride and phosphoric acid.

i. Write the equation for this reaction.
3CH3COOH + PCl3
→
3CH3COCl + H3PO3
ii. Show, using oxidation numbers, whether the reaction is a redox reaction or not.

Not Redox.

In PCl3, P is +3.

In H3PO3, H is +1 (total +3) and O is -2 (total -6). P must be +3.

No change in oxidation number.

6. Acid anhydrides can be formed when two carboxylic acid groups react.

a) Write an equation for the reaction between two molecules of ethanoic acid to produce an acid anhydride.
2CH3COOH
⇌
(CH3CO)2O + H2O

b) Butenedioic acid can react to form a cyclic acid anhydride.

i. Butenedioic acid exists as two stereoisomers. Draw the skeletal formula of the two butenedioic acid isomers.
Butenedioic acid isomers
ii. Suggest which of them can form an acid anhydride, and draw the structure of the anhydride formed.

The Z-isomer (cis) can form the cyclic anhydride.

Maleic anhydride structure
7. Acyl chlorides are useful because they are more reactive than carboxylic acids. Explain why ethanoyl chloride is more reactive than ethanoic acid.

Both contain a carbonyl carbon susceptible to nucleophilic attack. Ethanoyl chloride is more reactive because the Chloride ion (Cl–) is a better leaving group than Hydroxide (OH–), which is a strong base.

8. Ethanoyl chloride and Ethanoic anhydride are not considered to be soluble, despite being able to make hydrogen bonds. Explain why a solution of these two chemicals cannot exist.

When ethanoyl chloride and ethanoic anhydride come into contact with water, they react (hydrolysis) instead of just dissolving.

  • Ethanoyl chloride reacts to form ethanoic acid and HCl.
  • Ethanoic anhydride reacts to form two molecules of ethanoic acid.
9. State what you would observe if you mixed silver nitrate solution with separate volumes of ethanoyl chloride and chloroethane. Explain the result.

Ethanoyl Chloride: Immediate white precipitate of AgCl due to rapid hydrolysis releasing Cl–.

Chloroethane: No immediate change; the C-Cl bond is strong and resistant to hydrolysis.

Return to Topic