Practice important
Acid Anhydride Reactivity
MCQ questions and answers.
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MCQ Questions
1. In a Friedel–Crafts acylation using ethanoic anhydride and anhydrous AlCl3, which electrophilic species is primarily generated from the acid anhydride?
A.
CH3CO+
B.
CH3CH2+
C.
CH3COO−
D.
CH3O+
Correct Answer:
CH3CO+
Explanation:
Ethanoic anhydride reacts with the Lewis acid AlCl3 to generate the resonance-stabilized acylium ion, CH3CO+, which attacks the aromatic ring.
2. Benzene is treated with propanoic anhydride in the presence of anhydrous AlCl3. Which ketone is formed after hydrolysis of the reaction mixture?
A.
Propiophenone, C6H5COCH2CH3
B.
Ethyl phenyl ether, C6H5OCH2CH3
C.
Acetophenone, C6H5COCH3
D.
Benzophenone, C6H5COC6H5
Correct Answer:
Propiophenone, C6H5COCH2CH3
Explanation:
Propanoic anhydride supplies the propanoyl group, CH3CH2CO−, producing the ketone C6H5COCH2CH3 after the acylation complex is hydrolyzed.
3. When benzene reacts with ethanoic anhydride under Friedel–Crafts acylation conditions, what is the principal role of the second acyl fragment of the anhydride?
A.
It becomes the alkyl group attached to benzene
B.
It forms an acetate-containing aluminium complex and is released during work-up
C.
It is reduced to methane by AlCl3
D.
It becomes the chlorine substituent on the aromatic ring
Correct Answer:
It forms an acetate-containing aluminium complex and is released during work-up
Explanation:
Cleavage of the acid anhydride by AlCl3 generates one acylium ion for electrophilic substitution, while the other acyl portion remains as an acetate-containing aluminium species.
4. Which acid anhydride is expected to give acetophenone when used to acylate benzene by the Friedel–Crafts method?
A.
Methanoic anhydride
B.
Ethanoic anhydride
C.
Propanoic anhydride
D.
Benzoic anhydride
Correct Answer:
Ethanoic anhydride
Explanation:
Ethanoic anhydride, (CH3CO)2O, generates the acetylium ion CH3CO+, which gives acetophenone, C6H5COCH3, upon reaction with benzene.
5. Benzene is acylated separately with ethanoic anhydride and benzoic anhydride, each in the presence of AlCl3. Which pair of products is expected?
A.
Acetophenone and benzophenone
B.
Ethylbenzene and toluene
C.
Benzaldehyde and acetophenone
D.
Phenol and benzoic acid
Correct Answer:
Acetophenone and benzophenone
Explanation:
Ethanoic anhydride generates CH3CO+ and forms acetophenone, whereas benzoic anhydride generates C6H5CO+ and forms benzophenone.
6. Why are acid anhydrides suitable acylating agents in Friedel–Crafts reactions with AlCl3?
A.
They readily provide acylium ions through Lewis-acid-assisted cleavage of the anhydride bond
B.
They directly provide free carbanions that attack benzene
C.
They reduce AlCl3 to aluminium metal
D.
They produce alkoxide ions that substitute hydrogen on benzene
Correct Answer:
They readily provide acylium ions through Lewis-acid-assisted cleavage of the anhydride bond
Explanation:
The carbonyl oxygens of the anhydride coordinate with AlCl3, facilitating cleavage and formation of the resonance-stabilized acylium ion required for acylation.
7. For the Friedel–Crafts acylation of benzene with propanoic anhydride, which structural feature of the acylium ion determines the carbon chain attached to the aromatic ring?
A.
The carbonyl carbon and the ethyl group of CH3CH2CO+
B.
Only the oxygen atom of the acetate ion
C.
The aluminium atom from AlCl3
D.
The hydrogen ion released from benzene
Correct Answer:
The carbonyl carbon and the ethyl group of CH3CH2CO+
Explanation:
The propanoyl electrophile CH3CH2CO+ transfers its entire acyl group to benzene, so the product contains the carbonyl carbon bonded to the ethyl group.
8. Which equation best represents the net transformation when benzene is acylated with ethanoic anhydride, followed by aqueous work-up?
Explanation:
One acetyl group is transferred to benzene to form acetophenone, while the other portion of the anhydride ultimately appears as acetic acid after hydrolysis.
9. A student replaces ethanoic anhydride with ethyl ethanoate in an intended Friedel–Crafts acylation of benzene using AlCl3. What is the key reactivity difference relevant to this substitution?
A.
Ethanoic anhydride generally generates the acylium ion more readily than an ester
B.
Ethyl ethanoate generates an alkyl radical instead of any electrophile
C.
Ethanoic anhydride cannot coordinate with AlCl3
D.
Ethyl ethanoate always gives benzophenone because it contains two oxygen atoms
Correct Answer:
Ethanoic anhydride generally generates the acylium ion more readily than an ester
Explanation:
Acid anhydrides undergo Lewis-acid-assisted cleavage to form acylium ions efficiently, making them effective acylating agents; esters are less readily cleaved in this way.
10. A mixed anhydride, CH3CO–O–COC6H5, is used as an acylating agent with benzene and AlCl3. Which pair of ketones represents the two possible acylation products, depending on which acyl group forms the electrophile?
A.
Acetophenone and benzophenone
B.
Toluene and ethylbenzene
C.
Benzaldehyde and phenol
D.
Acetic acid and benzoic acid only
Correct Answer:
Acetophenone and benzophenone
Explanation:
A mixed anhydride contains both acetyl and benzoyl groups. Formation of CH3CO+ gives acetophenone, while formation of C6H5CO+ gives benzophenone.
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Acid Anhydride Reactivity
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