Reactions of Aromatic Compounds

Question 1

How is aromaticity regained?

Answer 1

By the loss of a proton.

Question 2

Name 5 reactions of electrophilic aromatic substitution.

Answer 2

Halogenation
Nitration
Sulfonation
Friedel-Crafts alkylation
Friedel-Crafts acylation

Question 3

Explain halogenation.

Answer 3

Replacement of H (on benzene) by Br or Cl.

Question 4

Explain nitration.

Answer 4

Replacement of H (on benzene) by NO2.

Question 5

Explain sulfonation.

Answer 5

Replacement of H (on benzene) by SO3H.

Question 6

Explain Friedel-Crafts alkylation.

Answer 6

Replacement of H (on benzene) by R.

Question 7

Explain Friedel-Crafts acylation.

Answer 7

Replacement of H (on benzene) by RCO.

Question 8

Give the 2-step mechanism of all electrophile aromatic substitution reactions.

Answer 8

1. Addition of the electrophile to form resonance stabilized carbocation
2. Followed by deprotonation with base

Question 9

Is bromination of benzene exothermic or endothermic?

Answer 9

Exothermic

Question 10

What compound is used most often as a catalyst in chlorination?

Answer 10

AlCl3 (FeCl3 will also work).

Question 11

What catalyst is used in nitration? And what does it react with?

Answer 11

Sulfuric acid (H2SO4); HNO3

Question 12

Sulfuric acid is used as a catalyst in nitration for…

Answer 12

the reaction to be faster and at lower temperatures.

Question 13

Toluene reacts ___ times faster than benzene.

Answer 13

25

Question 14

Why are ortho and para attacks preferred in nitration reactions?

Answer 14

Because their resonance structures include one tertiary carbocation.

Question 15

Are nitro groups activators or deactivators?

Answer 15

Deactivators.

Question 16

NO2 is also called a…

Answer 16

meta director.

Question 17

Electrophilic substitution reactions for nitrobenzene are how many times faster/slower than what?

Answer 17

100,000 times slower than benzene.

Question 18

Most electron-withdrawing groups are __________ (activators/deactivators) and ____-director.

Answer 18

deactivators; meta-

Question 19

What are the worst activators?

Answer 19

Alkyls and phenyls

Question 20

Para and ortho-directors are what? (deactivators/activators)

Answer 20

Activators.

Question 21

Is CH3 an activator or deactivator?

Answer 21

Activator…para/ortho-director.

Question 22

Is nitro an activator or deactivator?

Answer 22

Deactivator… meta-director.

Question 23

Are halogens activators or deactivators? And what kind of director?

Answer 23

Deactivators; Ortho/para-director

Question 24

Is methoxy an activator or deactivator?

Answer 24

Activator…para/ortho-director.

Question 25

Why are halogens deactivators?

Answer 25

Because they react slower than benzene.

Question 26

Why are halogens ortho-/para-directors?

Answer 26

Because the halogens can stabilize the sigma complex.

Question 27

What can the lone pairs on halogens be used as?

Answer 27

To stabilize the sigma complex by resonance.

Question 28

List the order of potential energy used for halogens from least to greatest. (Benzene, ortho, meta, para)

Answer 28

Benzene, ortho/para, meta

Question 29

Compounds with lone pairs are most likely to be _________ (activators/deactivators).

Answer 29

activators

Question 30

What is a sigma complex?

Answer 30

A resonance-stabilized carbocation.
The loss of a p+ gives us the sigma complex.

Question 31

The synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl3, is what reaction?

Answer 31

Friedel Crafts Alkylation

Question 32

Reactions of alkyl halide with Lewis acid in Friedel Crafts Alkylation produces what?

Answer 32

A carbocation, which is an electrophile.

Question 33

Why is HF, a weak acid, preferred in the protonation of alkenes?

Answer 33

Because the fluoride ion is a weak nucleophile and will not attack the carbocation.

Question 34

What can alcohols be treated with to form a carbocation?

Answer 34

BF3

Question 35

What are 3 limitations of Friedel Crafts?

Answer 35

Reaction fails if benzene has a substituent that is more deactivating than halogens.
Rearrangements are possible.
The alkylbenzene product is more reactive than benzene, so polyalkylation occurs.

Question 36

Do Friedel Crafts reactions occur with NH2 groups?

Answer 36

No, because NH2 is a strong deactivator.

Question 37

Give the general reactants, catalyst and products for Friedel Crafts Alkylation.

Answer 37

Benzene + alkyl halide; lewis acid (AlCl3); aryl halide + acid (HX)

Question 38

3 differences of Friedel Crafts acylation from Alkylation.

Answer 38

Acyl chloride used instead of alkyl chloride.
Phenyl ketone is the product (less reactive than benzene).
No poly-acylations.

Question 39

What is nucleophilic aromatic substitution?

Answer 39

The substitution of a halogen on a benzene ring by a nucleophile.

Question 40

What are the 2 different nucleophilic aromatic substitutions?

Answer 40

Addition-elimination and elimination-addition.

Question 41

Explain the addition-elimination mechanism.

Answer 41

Nucleophile adds to halogen substituted carbon and forms resonance stabilized carbanion.
Halogen is removed from the resonance of the electrons.

Question 42

Reactivity Trend of Nu Aromatic Substitution: Electron-withdrawing groups _________ the intermediate
carbanion.

Answer 42

stabilize

Question 43

Reactivity Trend of Nu Aromatic Substitution: Increasing the electronegativity of the halogen does what?

Answer 43

Increases the reactivity of the aryl halide.

Question 44

Reactivity Trend of Nu Aromatic Substitution: A more electronegative halogen does what?

Answer 44

Stabilizes the intermediate carbanion by an inductive effect.