Reactions of Aromatic Compounds Flashcards
How is aromaticity regained?
By the loss of a proton.
Name 5 reactions of electrophilic aromatic substitution.
Halogenation
Nitration
Sulfonation
Friedel-Crafts alkylation
Friedel-Crafts acylation
Explain halogenation.
Replacement of H (on benzene) by Br or Cl.
Explain nitration.
Replacement of H (on benzene) by NO2.
Explain sulfonation.
Replacement of H (on benzene) by SO3H.
Explain Friedel-Crafts alkylation.
Replacement of H (on benzene) by R.
Explain Friedel-Crafts acylation.
Replacement of H (on benzene) by RCO.
Give the 2-step mechanism of all electrophile aromatic substitution reactions.
- Addition of the electrophile to form resonance stabilized carbocation
- Followed by deprotonation with base
Is bromination of benzene exothermic or endothermic?
Exothermic
What compound is used most often as a catalyst in chlorination?
AlCl3 (FeCl3 will also work).
What catalyst is used in nitration? And what does it react with?
Sulfuric acid (H2SO4); HNO3
Sulfuric acid is used as a catalyst in nitration for…
the reaction to be faster and at lower temperatures.
Toluene reacts ___ times faster than benzene.
25
Why are ortho and para attacks preferred in nitration reactions?
Because their resonance structures include one tertiary carbocation.
Are nitro groups activators or deactivators?
Deactivators.
NO2 is also called a…
meta director.
Electrophilic substitution reactions for nitrobenzene are how many times faster/slower than what?
100,000 times slower than benzene.
Most electron-withdrawing groups are __________ (activators/deactivators) and ____-director.
deactivators; meta-
What are the worst activators?
Alkyls and phenyls
Para and ortho-directors are what? (deactivators/activators)
Activators.
Is CH3 an activator or deactivator?
Activator…para/ortho-director.
Is nitro an activator or deactivator?
Deactivator… meta-director.
Are halogens activators or deactivators? And what kind of director?
Deactivators; Ortho/para-director
Is methoxy an activator or deactivator?
Activator…para/ortho-director.
Why are halogens deactivators?
Because they react slower than benzene.
Why are halogens ortho-/para-directors?
Because the halogens can stabilize the sigma complex.
What can the lone pairs on halogens be used as?
To stabilize the sigma complex by resonance.
List the order of potential energy used for halogens from least to greatest. (Benzene, ortho, meta, para)
Benzene, ortho/para, meta
Compounds with lone pairs are most likely to be _________ (activators/deactivators).
activators
What is a sigma complex?
A resonance-stabilized carbocation.
The loss of a p+ gives us the sigma complex.
The synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl3, is what reaction?
Friedel Crafts Alkylation
Reactions of alkyl halide with Lewis acid in Friedel Crafts Alkylation produces what?
A carbocation, which is an electrophile.
Why is HF, a weak acid, preferred in the protonation of alkenes?
Because the fluoride ion is a weak nucleophile and will not attack the carbocation.
What can alcohols be treated with to form a carbocation?
BF3
What are 3 limitations of Friedel Crafts?
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.
Do Friedel Crafts reactions occur with NH2 groups?
No, because NH2 is a strong deactivator.
Give the general reactants, catalyst and products for Friedel Crafts Alkylation.
Benzene + alkyl halide; lewis acid (AlCl3); aryl halide + acid (HX)
3 differences of Friedel Crafts acylation from Alkylation.
Acyl chloride used instead of alkyl chloride.
Phenyl ketone is the product (less reactive than benzene).
No poly-acylations.
What is nucleophilic aromatic substitution?
The substitution of a halogen on a benzene ring by a nucleophile.
What are the 2 different nucleophilic aromatic substitutions?
Addition-elimination and elimination-addition.
Explain the addition-elimination mechanism.
Nucleophile adds to halogen substituted carbon and forms resonance stabilized carbanion.
Halogen is removed from the resonance of the electrons.
Reactivity Trend of Nu Aromatic Substitution: Electron-withdrawing groups _________ the intermediate
carbanion.
stabilize
Reactivity Trend of Nu Aromatic Substitution: Increasing the electronegativity of the halogen does what?
Increases the reactivity of the aryl halide.
Reactivity Trend of Nu Aromatic Substitution: A more electronegative halogen does what?
Stabilizes the intermediate carbanion by an inductive effect.
