Practice important
Polyacylation Prevention
MCQ questions and answers.
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10
MCQ Questions
1. Why does Friedel–Crafts acylation usually give mainly a monoacylated product?
A.
The acyl group deactivates the benzene ring toward further electrophilic substitution
B.
The acyl group makes the ring highly electron-rich
C.
The acyl group leaves the ring immediately after substitution
D.
The acyl group changes into an alkyl group
Correct Answer:
The acyl group deactivates the benzene ring toward further electrophilic substitution
Explanation:
The first acyl group is electron-withdrawing, so it reduces the reactivity of the ring and helps prevent further acylation.
2. Which property of the acyl group helps prevent polyacylation in Friedel–Crafts acylation?
A.
It strongly activates the aromatic ring
B.
It withdraws electron density from the aromatic ring
C.
It increases the number of hydrogen atoms on the ring
D.
It removes the aromaticity of the ring permanently
Correct Answer:
It withdraws electron density from the aromatic ring
Explanation:
The acyl group withdraws electron density, making the monoacylated ring less reactive toward another acylation.
3. After benzene is converted into acetophenone by Friedel–Crafts acylation, why is further acylation difficult?
A.
The methyl group completely blocks all ring positions
B.
The carbonyl group deactivates the aromatic ring
C.
The benzene ring becomes saturated
D.
The carbonyl group changes into a halogen
Correct Answer:
The carbonyl group deactivates the aromatic ring
Explanation:
In acetophenone, the carbonyl-containing acyl group withdraws electron density from the ring, discouraging a second acylation.
4. Which reaction is generally preferred when the aim is to avoid polyacylation of benzene?
A.
Friedel–Crafts acylation
B.
Repeated nitration
C.
Repeated sulfonation
D.
Complete hydrogenation
Correct Answer:
Friedel–Crafts acylation
Explanation:
Friedel–Crafts acylation generally stops after one acyl group is introduced because the acyl group deactivates the ring.
5. What happens to the reactivity of an aromatic ring after one acyl group is introduced?
A.
It increases greatly toward another acylation
B.
It decreases toward another electrophilic acylation
C.
It remains exactly unchanged
D.
It becomes non-aromatic immediately
Correct Answer:
It decreases toward another electrophilic acylation
Explanation:
The newly attached acyl group withdraws electron density, so the ring becomes less reactive and polyacylation is reduced.
6. Which product is usually formed predominantly when benzene reacts with ethanoyl chloride in the presence of anhydrous aluminium chloride?
A.
A monoacylated product, acetophenone
B.
A benzene ring with three ethanoyl groups
C.
A cyclohexane derivative
D.
A phenol derivative
Correct Answer:
A monoacylated product, acetophenone
Explanation:
The first ethanoyl group forms acetophenone and deactivates the ring, helping prevent further acylation.
7. Which statement correctly describes the role of the first acyl group in preventing polyacylation?
A.
It activates the ring for a second acyl group
B.
It deactivates the ring for attack by another acylium ion
C.
It removes the aluminium chloride catalyst permanently
D.
It converts the ring into an alkene
Correct Answer:
It deactivates the ring for attack by another acylium ion
Explanation:
The first acyl group lowers the electron density of the ring, making attack by another acylium ion less favorable.
8. Why is polyacylation less common in Friedel–Crafts acylation than in Friedel–Crafts alkylation?
A.
An acyl group deactivates the ring after the first substitution
B.
An acyl group activates the ring after the first substitution
C.
Acylation does not involve an aromatic ring
D.
Alkyl groups cannot attach to benzene
Correct Answer:
An acyl group deactivates the ring after the first substitution
Explanation:
The electron-withdrawing carbonyl group in the acyl substituent reduces the ring's tendency to undergo another electrophilic substitution.
9. A benzene ring already bearing a carbonyl-containing acyl group is less likely to undergo which reaction?
A.
A second Friedel–Crafts acylation
B.
Formation of the first acyl group
C.
Loss of all carbon atoms
D.
Conversion into an inorganic salt without reagents
Correct Answer:
A second Friedel–Crafts acylation
Explanation:
The existing acyl group deactivates the ring, so introduction of a second acyl group is less favorable.
10. What is the main reason that Friedel–Crafts acylation is useful for preparing a single acylated aromatic product?
A.
The first acyl group reduces the ring's reactivity toward further acylation
B.
The first acyl group makes the ring more reactive than benzene
C.
The reaction destroys every remaining ring position
D.
The acyl group prevents formation of any aromatic product
Correct Answer:
The first acyl group reduces the ring's reactivity toward further acylation
Explanation:
Because the acyl group is electron-withdrawing, the monoacylated product is less reactive and polyacylation is naturally limited.
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Polyacylation Prevention
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