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Friedel-Crafts Alkylation Comparison
MCQ questions and answers.
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MCQ Questions
1. Which reagent is commonly used in Friedel–Crafts alkylation of benzene?
A.
An alkyl halide with anhydrous AlCl3
B.
An acyl chloride with aqueous NaOH
C.
An alcohol with concentrated H2SO4 only
D.
An alkene with bromine water
Correct Answer:
An alkyl halide with anhydrous AlCl3
Explanation:
Friedel–Crafts alkylation commonly uses an alkyl halide in the presence of anhydrous aluminium chloride, whereas acylation uses an acyl halide or acid anhydride.
2. What is the main organic product of Friedel–Crafts alkylation of benzene with methyl chloride and AlCl3?
A.
Toluene
B.
Acetophenone
C.
Chlorobenzene
D.
Phenol
Correct Answer:
Toluene
Explanation:
Alkylation adds a methyl group to the benzene ring, forming toluene. Acylation with acetyl chloride would instead form acetophenone.
3. Which statement correctly compares Friedel–Crafts alkylation and acylation?
A.
Alkylation introduces an alkyl group, while acylation introduces an acyl group
B.
Both reactions introduce only halogen atoms
C.
Alkylation introduces an acyl group, while acylation introduces an alkyl group
D.
Neither reaction changes the benzene ring
Correct Answer:
Alkylation introduces an alkyl group, while acylation introduces an acyl group
Explanation:
In Friedel–Crafts alkylation, an alkyl group is attached to the aromatic ring. In acylation, an acyl group is attached.
4. Which reaction is more likely to give polyalkylation of benzene?
A.
Friedel–Crafts alkylation
B.
Friedel–Crafts acylation
C.
Neutralization
D.
Hydrogenation
Correct Answer:
Friedel–Crafts alkylation
Explanation:
The alkyl group introduced by alkylation activates the benzene ring toward further substitution, so polyalkylation can occur. The acyl group generally deactivates the ring.
5. Why can rearrangement occur during Friedel–Crafts alkylation?
A.
The reaction may involve a carbocation-like intermediate
B.
The benzene ring is completely reduced
C.
The acyl group always changes into a halogen
D.
Aluminium chloride prevents all intermediate formation
Correct Answer:
The reaction may involve a carbocation-like intermediate
Explanation:
Carbocation-like intermediates formed during alkylation can rearrange to form more stable carbocations. Friedel–Crafts acylation generally avoids this type of rearrangement.
6. Which reaction generally avoids carbocation rearrangement during substitution of benzene?
A.
Friedel–Crafts acylation
B.
Friedel–Crafts alkylation with a primary alkyl halide
C.
Alkylation using an unstable carbocation
D.
Repeated alkylation of toluene
Correct Answer:
Friedel–Crafts acylation
Explanation:
Acylation forms an acylium ion, which is resonance-stabilized and does not normally rearrange like a carbocation in alkylation.
7. What effect does an alkyl group usually have on the benzene ring after Friedel–Crafts alkylation?
A.
It activates the ring toward further electrophilic substitution
B.
It completely deactivates the ring
C.
It removes the aromatic character permanently
D.
It changes the ring into a cycloalkane
Correct Answer:
It activates the ring toward further electrophilic substitution
Explanation:
Alkyl groups are electron-releasing groups, so the alkylbenzene product is more reactive toward further electrophilic substitution than benzene.
8. What effect does an acyl group usually have on the benzene ring in comparison with an alkyl group?
A.
The acyl group deactivates the ring, while the alkyl group activates it
B.
Both groups activate the ring equally
C.
The acyl group activates the ring, while the alkyl group deactivates it
D.
Neither group affects the reactivity of the ring
Correct Answer:
The acyl group deactivates the ring, while the alkyl group activates it
Explanation:
An acyl group withdraws electron density from the ring and deactivates it, whereas an alkyl group releases electron density and activates it.
9. Which product is formed when benzene undergoes Friedel–Crafts acylation with acetyl chloride followed by reduction of the carbonyl group?
A.
Ethylbenzene
B.
Toluene
C.
Chlorobenzene
D.
Phenol
Correct Answer:
Ethylbenzene
Explanation:
Acylation first gives acetophenone, and reduction of its carbonyl group converts the acetyl group into an ethyl group. This is a useful alternative to direct alkylation because rearrangement is avoided.
10. Which limitation is more characteristic of Friedel–Crafts alkylation than of Friedel–Crafts acylation?
A.
Formation of rearranged products
B.
Introduction of an acyl group
C.
Use of an acylium ion
D.
Formation of a ketone as the initial product
Correct Answer:
Formation of rearranged products
Explanation:
Friedel–Crafts alkylation can produce rearranged alkyl groups because of carbocation-like intermediates. Acylation generally forms an unrearranged aryl ketone.
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