Chapter 17- Reactions of Aromatic Compounds Flashcards
Why does the halogenation of aromatic rings require a catalyst?
Because alkenes are more reactive than benzene. The difference is resonance energy
What is the catalyst in Bromination? In chlorination? In Iodination?
FeBr3 in Bromination
AlCl3 in Chlorination
Iodination requires an oxidation catalyst: HNO3, HIO3, SO3, H2O2
What are the two methods for sulfonating aromatic rings?
- SO3/ H2SO4- “fuming sulfuric acid” or “oleum.” Used to sulfonate benzene or aromatic rings deactivated by EWGs
- H2SO4- Sulfuric Acid (96%). Used to sulfonate benzene rings activated by EDGs
The sulfonation reaction in oleum is reversible or irreversible?
Irreversible.
Sulfonation is (reversible or irreversible) in hot, dilute aqueous acid?
How is this reaction useful?
Reversible, it is useful b/c it can be used to install ortho/ para directing groups meta to one another, and then the sulfonic acid can be removed.
What is the catalyst required in the F-C alkylation? What are the akylating reagents?
Catalyst- AlCl3 (aluminum trichloride)
Alkylating reagents- alkyl halides
In the F-C alkylation, the catalyst complexes with the alkyl chloride to form what?
An incipient carbocation, which is not a true carbocation, but it acts like one.
What are the limitations of the F-C alkylation?
- Only works with alkyl halides (not vinyl or aryl halides)
- Carbocation rearrangements occur (hydride and methyl shifts)
- Strong EWGs on benzene ring inhibit reaction
- Multiple substitutions possible
What are the similarities and differences b/w F-C Alkylation and F-C Acylation?
Similarities:
- AlCl3 is catalyst
- Neither Acylation nor Alkylation of strongly deactivated rings not possible
Differences:
- Intermediate electrophile does not rearrange in Acylation
- Multiple substitutions not possible in Acylation
- Acylation uses acyl halides and anhydrides and alkylation uses alkyl halides
What are the two acylating agents in F-C Acylation?
- Acid Anhydride
2. Acid Halide
What is a very powerful tool in organic chemistry?
Synthesis of monosubstituted benzenes
Inductive electron withdrawal/ donation occurs through what framework? What about resonance electron withdrawal/ donation?
Inductive- sigma bond framework
Resonance- pi system
How do halides deactivate the ring? How do they direct ortho/ para?
Deactivation of ring- inductive withdrawal of electrons
Ortho/ para- directed via resonance donation
Which kind of reaction is the nucleophilic aromatic substitution?
Addition- elimination
Which substituents activate the ring for nucleophilic substitution?
Electron- withdrawing ortho and/ or para
What kind of the reactant is the one for elmination- addition Benzyne rxn? What reagent do you use for this rxn?
A halobenzene with no ewgs on the ring.
Use a strong base like NaNH2 for reagent
Why are Benzynes very reactive?
Because of the high strain on the triple bond
What method adds alkyl groups to the benzene without the multiple limitations of the F-C reactions?
Aromatic substitutions using organometallic reagents
How do you prepar lithium dialkylcuprate reagents (Gilman reagents)?
Reacting two eq of organolithium (2R- Li) with cuprious iodide (CuI).
2 R-Li + CuI —> R2CuI + LiI
Why can’t coupling using organocuprate reagents react via Sn2?
B/c vinyl and aryl halides work well in this reaction
What does the Heck Reaction entail? What does it produce? What is added to neutralize the HX produced?
Palladium- Catalyzed coupling of an aryl or vinyl halide with and alkene.
Produces C-C bond at the less subst end of alkene
Triethylamine or sodium acetate is added
What does the Suzuki Coupling Reaction entail?
Palladium- catalyzed substitution that couples an aryl or vinyl halide with an alkyl, alkenyl, or aryl borinic acid or boronate ester
What are the two ways to synthesize boronate esters?
- Alkene + alkyl, vinyl, or arylboronic acids
2. Hydroborating alkyne or alkene
What are required for catalytic hydrogenation? What are the possible catalysts?
Elevated heat and pressure required.
Possible catalysts: Pt, Pd, Ni, Ru, Rh
Reduction cannot be stopped at an intermediate stage
