Ch12: Electrophilic aromatic substitution questions

Chapter 12 : Reactions of Arenes. Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution Answers

Qu 1

(a) First a Friedel-Crafts alkylation reaction using EtCl to add an Et group to the benzene followed by a radical bromination at the 2^o benzylic position (since it is the most stable radical), to give 1-bromo-1-phenylethane

(b) Freidel-Crafts acylation of benzene will give the ketone without rearrangement of the alkyl chain. The Wolff-Kishner reduction converts the C=O to a -CH₂- to give n-butylbenzene.

(c) FriedelCrafts alkylation of benzene with the 1^o system, isobutyl chloride will result in rearrangement via a 1,2-hydride shift to provide the more stable 3^o carbocation which alkylates the aromatic to yield t-butylbenzene.

Qu 2 We are looking at a bromination reaction (but the same would be true of any of the other electrophilic aromatic substitution reactions)

This relative order of reactivity depends on the substituent on the benzene. -Cl groups are weakly deactivating (induction), while -OH groups are strongly electron donating (resonance with the lone pairs), and -NO₂ groups are strongly electron withdrawing (due to resonance onto the electronegative O and induction due to the +ve N atom). Infact, phenols are reactive enough to undergo polybromination even in the absence of the Lewis acid catalyst.

Qu 3 Since alkyl groups are weak electron donors and therefore slightly activating, t-butylbenzene will direct to the ortho and para positions.

However, the large size of the t-butyl group will sterically hinder both of the ortho positions and thus favour para substitution over ortho. Thus, iii > i > ii.

Qu 4 (a) This is a Friedel-Crafts acylation. The curly arrow mechanisms and charges may be drawn as follows

(b) This is a Friedel-Crafts alkylation. The curly arrow mechanisms and charges may be drawn as follows:

Qu 5:

We are looking at a Friedel-Crafts alkylation reaction (but the same would be true of any of the other electrophilic aromatic substitution reactions). This relative order of reactivity depends on the substituent on the benzene. -CO₂Me groups are deactivating (resonance withdrawal by the C=O), while -OMe groups are strongly activating (resonance donation by the -O- lone pairs), and -CH₃ groups are weakly activating (inductive donation due to polarisability and hyperconjugation).

Qu 6: Both starting materials are ester but the aromatic ring is connected to opposite sides of the carboxylate.

For ethyl benzoate, the aromatic ring has a -CO₂CH₂CH₃ group attached which is an electron withdrawing group. Therefore the electrophilic aromatic substitution reaction will occur at the meta position giving 3-nitrophenyl ethanoate.
For phenyl ethanoate, the aromatic ring has a -OC(=O)CH₃ group attached. The -O- atom with it's lone pairs next to the ring makes this an electron donor, so ortho- and para- substitution occurs. Steric effects will favour the para- product.

Qu 7: The resonance energy of naphthalene (61 kcal/mol) compared to benzene (36 kcal/mol) means that one of the rings is less aromatic than the other and will be more reactive.

© Dr. Ian Hunt, Department of Chemistry

Qu 1
	(a) First a Friedel-Crafts alkylation reaction using EtCl to add an Et group to the benzene followed by a radical bromination at the 2^o benzylic position (since it is the most stable radical), to give 1-bromo-1-phenylethane

	(b) Freidel-Crafts acylation of benzene will give the ketone without rearrangement of the alkyl chain. The Wolff-Kishner reduction converts the C=O to a -CH₂- to give n-butylbenzene.

	(c) FriedelCrafts alkylation of benzene with the 1^o system, isobutyl chloride will result in rearrangement via a 1,2-hydride shift to provide the more stable 3^o carbocation which alkylates the aromatic to yield t-butylbenzene.

Qu 2	We are looking at a bromination reaction (but the same would be true of any of the other electrophilic aromatic substitution reactions)
	This relative order of reactivity depends on the substituent on the benzene. -Cl groups are weakly deactivating (induction), while -OH groups are strongly electron donating (resonance with the lone pairs), and -NO₂ groups are strongly electron withdrawing (due to resonance onto the electronegative O and induction due to the +ve N atom). Infact, phenols are reactive enough to undergo polybromination even in the absence of the Lewis acid catalyst.

Qu 3	Since alkyl groups are weak electron donors and therefore slightly activating, t-butylbenzene will direct to the ortho and para positions.
	However, the large size of the t-butyl group will sterically hinder both of the ortho positions and thus favour para substitution over ortho. Thus, iii > i > ii.

Qu 4	(a) This is a Friedel-Crafts acylation. The curly arrow mechanisms and charges may be drawn as follows


	(b) This is a Friedel-Crafts alkylation. The curly arrow mechanisms and charges may be drawn as follows:

Qu 5:
	We are looking at a Friedel-Crafts alkylation reaction (but the same would be true of any of the other electrophilic aromatic substitution reactions). This relative order of reactivity depends on the substituent on the benzene. -CO₂Me groups are deactivating (resonance withdrawal by the C=O), while -OMe groups are strongly activating (resonance donation by the -O- lone pairs), and -CH₃ groups are weakly activating (inductive donation due to polarisability and hyperconjugation).

Qu 6:	Both starting materials are ester but the aromatic ring is connected to opposite sides of the carboxylate.
	For ethyl benzoate, the aromatic ring has a -CO₂CH₂CH₃ group attached which is an electron withdrawing group. Therefore the electrophilic aromatic substitution reaction will occur at the meta position giving 3-nitrophenyl ethanoate. For phenyl ethanoate, the aromatic ring has a -OC(=O)CH₃ group attached. The -O- atom with it's lone pairs next to the ring makes this an electron donor, so ortho- and para- substitution occurs. Steric effects will favour the para- product.

Qu 7:	The resonance energy of naphthalene (61 kcal/mol) compared to benzene (36 kcal/mol) means that one of the rings is less aromatic than the other and will be more reactive.