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Acyl Chloride Reactivity
MCQ questions and answers.
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MCQ Questions
1. In a Friedel–Crafts acylation, why is ethanoyl chloride activated by anhydrous AlCl3 before attack by benzene?
A.
AlCl3 converts ethanoyl chloride into a more reactive acylium ion, CH3CO+
B.
AlCl3 reduces ethanoyl chloride to ethanal
C.
AlCl3 replaces the chlorine atom with a hydride ion
D.
AlCl3 converts benzene into a powerful nucleophile by hydrogenation
Correct Answer:
AlCl3 converts ethanoyl chloride into a more reactive acylium ion, CH3CO+
Explanation:
The carbonyl oxygen of ethanoyl chloride coordinates with AlCl3, strongly polarizing the C–Cl bond and promoting formation of the resonance-stabilized acylium ion, CH3CO+. This electrophile reacts with benzene in the acylation step.
2. Which feature of an acyl chloride most directly determines its high reactivity toward Friedel–Crafts acylation in the presence of AlCl3?
A.
The strong electron-withdrawing effect of the carbonyl group increases the electrophilicity of the acyl carbon
B.
The chlorine atom donates a lone pair strongly into the acyl carbon orbital
C.
The acyl chloride contains an aromatic ring that stabilizes the intermediate
D.
The acyl chloride readily forms a carbanion at the carbonyl carbon
Correct Answer:
The strong electron-withdrawing effect of the carbonyl group increases the electrophilicity of the acyl carbon
Explanation:
The carbonyl group polarizes the acyl chloride, leaving the carbonyl carbon electron deficient. Coordination with AlCl3 further enhances this electrophilicity and facilitates departure of chloride as the acylium species forms.
3. When propanoyl chloride reacts with AlCl3 during Friedel–Crafts acylation, which electrophilic species is principally generated?
A.
CH3CH2CO+
B.
CH3CH2CH2+
C.
CH3CH2CO−
D.
CH3CH2Cl2+
Correct Answer:
CH3CH2CO+
Explanation:
Propanoyl chloride, CH3CH2COCl, loses chloride after coordination with AlCl3 to generate the propanoyl acylium ion, CH3CH2CO+. The positive charge is delocalized between the acyl carbon and oxygen.
4. Why does benzoyl chloride generally produce the benzoyl electrophile without the rearrangement commonly associated with some carbocations?
A.
The acylium ion is resonance-stabilized and its structure does not require formation of a freely rearranging alkyl carbocation
B.
Benzoyl chloride cannot coordinate with AlCl3
C.
The chlorine atom prevents any polarization of the carbonyl group
D.
Benzoyl chloride reacts through a radical chain mechanism
Correct Answer:
The acylium ion is resonance-stabilized and its structure does not require formation of a freely rearranging alkyl carbocation
Explanation:
Acyl chlorides form acylium ions, represented by RCO+ and its resonance form R–C≡O+. These ions are stabilized by resonance and do not rearrange like ordinary alkyl carbocations, so the carbon skeleton of the acyl group is retained.
5. Which reaction mixture would be least suitable for generating an acylium ion from an acyl chloride in Friedel–Crafts acylation?
A.
Acetyl chloride with anhydrous AlCl3
B.
Benzoyl chloride with anhydrous AlCl3
C.
Propionyl chloride with anhydrous AlCl3
D.
Acetyl chloride with aqueous AlCl3
Correct Answer:
Acetyl chloride with aqueous AlCl3
Explanation:
Anhydrous Lewis acid conditions are required because water hydrolyzes or deactivates acyl chloride and destroys the Lewis acidity of AlCl3. Consequently, aqueous AlCl3 is ineffective for generating the acylium electrophile.
6. What is the principal role of chloride ion released when an acyl chloride interacts with AlCl3?
A.
It combines with AlCl3 to form a Lewis-acid complex such as AlCl4−
B.
It attacks benzene to form chlorobenzene before acylation
C.
It reduces the acylium ion to an aldehyde
D.
It abstracts a proton from the acyl chloride to form an alkene
Correct Answer:
It combines with AlCl3 to form a Lewis-acid complex such as AlCl4−
Explanation:
As chloride leaves the acyl chloride–AlCl3 complex, it is strongly stabilized by coordination to AlCl3, giving an AlCl4−-type species. This chloride capture helps drive formation of the acylium ion required for acylation.
7. An acyl chloride RCOCl is replaced by the corresponding ester RCOOR' in a proposed Friedel–Crafts acylation. Why is this substitution generally less effective under standard AlCl3 conditions?
A.
The alkoxy group is a poorer leaving group in the activated ester complex than chloride is in the acyl chloride complex
B.
Esters contain no carbonyl group and cannot act as electrophiles
C.
The alkyl group of the ester immediately rearranges into an acylium ion
D.
AlCl3 reacts only with hydrocarbons and never with oxygen compounds
Correct Answer:
The alkoxy group is a poorer leaving group in the activated ester complex than chloride is in the acyl chloride complex
Explanation:
Acyl chlorides are especially reactive because chloride can depart readily after coordination with AlCl3. In esters, the alkoxy-derived leaving group is less favorable under these conditions, so acylium-ion generation is less efficient.
8. Why must an acyl chloride be used in the presence of a Lewis acid rather than simply mixed with benzene for efficient Friedel–Crafts acylation?
A.
The Lewis acid increases acyl chloride electrophilicity by coordinating to the carbonyl oxygen and facilitating chloride departure
B.
The Lewis acid converts benzene into cyclohexane, which then attacks the acyl chloride
C.
The Lewis acid supplies the hydrogen atoms needed to reduce the acyl chloride
D.
The Lewis acid prevents the carbonyl group from participating in the reaction
Correct Answer:
The Lewis acid increases acyl chloride electrophilicity by coordinating to the carbonyl oxygen and facilitating chloride departure
Explanation:
Neutral acyl chlorides are not sufficiently strong electrophiles for rapid reaction with benzene. Coordination of AlCl3 to the carbonyl oxygen makes the acyl carbon more electron deficient and promotes acylium-ion formation.
9. Which acyl chloride would generate the most resonance-stabilized acylium ion upon treatment with AlCl3?
A.
Benzoyl chloride, C6H5COCl
B.
Acetyl chloride, CH3COCl
C.
Propionyl chloride, CH3CH2COCl
D.
Butanoyl chloride, CH3CH2CH2COCl
Correct Answer:
Benzoyl chloride, C6H5COCl
Explanation:
Benzoyl chloride forms the benzoyl acylium ion, C6H5CO+, whose positive charge is stabilized by resonance involving the carbonyl group and conjugation with the phenyl ring. The other listed acylium ions lack this additional aryl conjugation.
10. After benzene attacks the acylium ion generated from CH3CH2COCl and AlCl3, what ultimately happens to the acyl chloride-derived chlorine-containing species during work-up?
A.
Hydrolysis releases the ketone and converts the aluminum chloride complex into inorganic chloride-containing products
B.
It becomes part of the ketone carbonyl oxygen
C.
It remains covalently bonded to the acyl carbon in the final ketone
D.
It is oxidized into an aldehyde substituent on the aromatic ring
Correct Answer:
Hydrolysis releases the ketone and converts the aluminum chloride complex into inorganic chloride-containing products
Explanation:
The acyl group becomes attached to benzene, while the product ketone remains coordinated to AlCl3. Aqueous work-up hydrolyzes this complex, releasing the ketone and leaving chloride in inorganic aluminum-containing species.
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