chemistry : class11 Ortho Para Directing Effects : Friedel-Crafts acylation : Hydrocarbons MCQ Questions and Answers
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Ortho Para Directing Effects
MCQ questions and answers.
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MCQ Questions
1. When anisole undergoes Friedel–Crafts acylation with acetyl chloride in the presence of anhydrous AlCl3, which product is expected to predominate?
A.
2-methoxyacetophenone
B.
3-methoxyacetophenone
C.
4-methoxyacetophenone
D.
A mixture containing equal amounts of all three isomers
Correct Answer:
4-methoxyacetophenone
Explanation:
The methoxy group is a strong ortho–para directing group because of resonance donation. The para position is favored over the ortho position because it experiences less steric hindrance during acylation.
2. The major product formed when toluene undergoes Friedel–Crafts acylation with propanoyl chloride and AlCl3 is best represented as:
A.
2-methylpropiophenone
B.
3-methylpropiophenone
C.
4-methylpropiophenone
D.
Phenyl propyl ketone without a methyl substituent
Correct Answer:
4-methylpropiophenone
Explanation:
The methyl group activates the ring and directs electrophilic substitution to its ortho and para positions. The para product predominates because acylation at that position is less sterically hindered than ortho acylation.
3. In Friedel–Crafts acylation of p-xylene, which statement correctly describes the positions available for substitution?
A.
Only one pair of equivalent positions is available, and both positions are equivalent by symmetry
B.
Four nonequivalent positions are available, with the position between the methyl groups being uniquely favored
C.
Substitution occurs only at positions meta to both methyl groups
D.
Substitution gives equal amounts of four constitutionally different products
Correct Answer:
Only one pair of equivalent positions is available, and both positions are equivalent by symmetry
Explanation:
In p-xylene, positions 2 and 6 are equivalent, as are positions 3 and 5. However, both methyl groups direct substitution toward positions 2 and 6, which are ortho to one methyl group and para to the other. These two positions therefore give the same product.
4. Acetanilide is subjected to Friedel–Crafts acylation under conditions that permit reaction. Which regioisomer is expected to be the major product?
A.
The product formed by acylation ortho to the acetamido group
B.
The product formed by acylation meta to the acetamido group
C.
The product formed by acylation para to the acetamido group
D.
A product formed by replacement of the acetamido group
Correct Answer:
The product formed by acylation para to the acetamido group
Explanation:
The acetamido group, –NHCOCH3, donates electron density to the ring through resonance and is ortho–para directing. The para position is generally favored because it is less crowded than the ortho position.
5. When p-cresol undergoes Friedel–Crafts acylation, which ring positions are most strongly favored by the combined directing effects of its substituents?
A.
Positions 2 and 6, equivalent and ortho to the hydroxyl group
B.
Positions 3 and 5, equivalent and meta to the hydroxyl group
C.
Position 4, because it is para to both substituents
D.
Positions 2 and 5, because each is ortho to only one substituent
Correct Answer:
Positions 2 and 6, equivalent and ortho to the hydroxyl group
Explanation:
The hydroxyl group is a much stronger ortho–para director than the methyl group. In p-cresol, positions 2 and 6 are activated by the hydroxyl group and are equivalent by symmetry, so acylation occurs preferentially there.
6. For Friedel–Crafts acylation of m-cresol, which position is expected to be favored when steric effects are considered along with directing effects?
A.
Position 2, ortho to both the hydroxyl and methyl groups
B.
Position 4, para to the hydroxyl group and ortho to the methyl group
C.
Position 5, meta to both the hydroxyl and methyl groups
D.
Position 6, ortho to the hydroxyl group and para to the methyl group
Correct Answer:
Position 4, para to the hydroxyl group and ortho to the methyl group
Explanation:
The hydroxyl group strongly directs substitution to positions 2, 4, and 6, while the methyl group also favors positions 2, 4, and 6. Position 4 is para to the strongly directing hydroxyl group and is less sterically hindered than position 2; it is therefore generally favored.
7. If the para position of anisole is blocked by a substituent that does not strongly alter the directing pattern, Friedel–Crafts acylation is expected to occur predominantly at which position relative to the methoxy group?
A.
The meta position
B.
The ortho position
C.
The ipso position with displacement of the blocking group
D.
No position because the methoxy group becomes meta directing
Correct Answer:
The ortho position
Explanation:
The methoxy group remains an ortho–para director. When the para position is unavailable, electrophilic acylation occurs at the less hindered available ortho position rather than at a meta position.
8. In the Friedel–Crafts acylation of 1-ethyl-4-methoxybenzene, which positions are expected to be favored when the directing strengths of the substituents are compared?
A.
Positions 2 and 6, ortho to the methoxy group
B.
Positions 3 and 5, ortho to the ethyl group
C.
The position para to both substituents
D.
Positions meta to both substituents
Correct Answer:
Positions 2 and 6, ortho to the methoxy group
Explanation:
Both ethyl and methoxy groups are ortho–para directors, but methoxy is the stronger activating director because of resonance donation. Positions 2 and 6 are ortho to methoxy and equivalent by symmetry, so they are preferentially acylated.
9. Which directing prediction is most appropriate for further Friedel–Crafts acylation of acetophenone, assuming the reaction can be carried out?
A.
Acylation occurs mainly ortho to the existing acetyl group
B.
Acylation occurs mainly para to the existing acetyl group
C.
Acylation occurs mainly meta to the existing acetyl group
D.
Acylation occurs equally at all available positions
Correct Answer:
Acylation occurs mainly meta to the existing acetyl group
Explanation:
The acetyl group, –COCH3, withdraws electron density by resonance and induction. It is therefore a meta director, so a second electrophilic substitution is oriented predominantly to the meta position rather than to the ortho or para positions.
10. A benzene ring contains both a methoxy group and a nitro group in the 1,3-positions. In predicting the site of Friedel–Crafts acylation, which principle is most important?
A.
The nitro group dominates and directs acylation to positions ortho and para to itself
B.
The methoxy group dominates and directs acylation to positions ortho and para to itself, provided those positions are not strongly deactivated by the nitro group
C.
Both groups direct only to the position meta to both substituents
D.
The methoxy group becomes meta directing because it is attached to a ring containing a nitro group
Correct Answer:
The methoxy group dominates and directs acylation to positions ortho and para to itself, provided those positions are not strongly deactivated by the nitro group
Explanation:
Methoxy is an ortho–para directing electron-donating group, whereas nitro is a strongly deactivating meta director. In a disubstituted ring with competing effects, the strongly activating methoxy group generally has the greater influence on orientation, although the nitro group can substantially reduce the overall reactivity of the ring.
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