What is Huckels rule?
An aromatic compound has the following criteria:
- CYCLIC: has to be a ring (or multiple)
- CONJUGATED: All atoms must have an unhybridized p orbital
- PLANAR: Aromatic rings are planar
- 4n + 2: The total count of pi electrons (from double bonds, lone pairs, or vacant orbitals) must follow the formula 4n+2
What are anti aromatic compounds
highly unstable, reactive, cyclic, planar, and fully conjugated molecules containing 4n pi electrons
What reagents/solvents are required for halogenation (bromination specifically) of benzene?
Br2 and FeBr3 (or Fe)
Describe the mechanism of halogenation of benzene
1) Br2 attacks FeBr3 to make FeBr5
2) Double bond of benzene attacks Br, kicking off FeBr4
3) Resonance stabilized carbocation
4) Bromide ion removes proton from carbocstion, regenerating FeBr3, while Creating products HBr and bromobenzene (substituted benzene)
How do we introduce nitro groups into aromatic rings
Nitration
What reagents/solvents are required for nitration of benzene
Nitric acid (HNO3) in the presence of H2SO4
Describe the steps of ntiration of benzene
1) Generation of ephile (NO2+):
Abstraction of proton from H2SO4 by nitric acid; then rearrangement to generate H20 + nitronium (NO2) cation
2) Benzene pi electron reacts with nitronium N+ to form resonance stabilized carbocation
3) Proton abstraction (by H2SO4) from carbocation to regenerate aromacity
What are the reagents/solvents for sulfonation of benzene
Sulfur trioxide (SO3) in the presence of H2SO4
Describe the mechanism of the sulfonation of benzene
1) Benzene pi electron reacts with S of sulfur trioxide (SO3(2-)) to form carbocation
2) H2SO4 abstracts proton, carbocation resolved
3) Protonation of SO3 benzene group by H2SO4
What are the reagents/solvents involved in Friedel crafts alkylation of benzene?
alkyl halide + lewis acid catalyst (usually alcl3)
Describe the mechanism o friedel crafts alkylation of benzene
1) Generation of electrophile: halogen of alkyl halide attacks alcl3 to form complex; partial dissociation onto R+ carbocation and AlCl4
2) Alkylation by complex and kicking of AlCl4 as LG OR* alkylation by carbocation (R+) directly
3) Regeneration of aromatic ring using AlCl4
Alkyl vs acyl group
Alkyl: R-
Acyl: RCO- (carbonyl + 2 R groups)
In the Friedel Crafts Acylation of benzene, where is the acyl group typically derived from?
An acid chloride
What are the reagents/solvents in Friedel Crafts acylation
Benzene + acid chloride in presence of lewis acid (usually AlCl3) and H2O
Describe the mechanism of Friedel Crafts acylation
1) Fomration of acylium ion: Acetyl chloride reacts with AlCl3 (O donates electrons to create triple bond, Cl abstracted by AlCl3) to form R-C=-O+
2) Benzene pi bond reacts with acylium ion, triple bond reduced to double bond
3) AlCL4 abstracts proton, carbocation resolved to form acylated benzene
Ortho vs. para vs. meta meaning in EAS
Ortho: 1,2
Para: 1,4
Meta: 1,3
What are the various electrophilic aromatic subsitution reactions?
Halogenation, Nitration, Sulfonation, Friedel Crafts Alkylation and Acylation
What are activating susbsituents?
Substituents that increase rate of EAS by donating e- density to ring
Activating substituents are usually _______ directors
ortho, para
Deactivating substituents are usually _______ directors
meta
What are some ortho/para directing groups?
NH2, NR2, OH, alkoxy; (RO), acylamino (NH-acyl), alkyl, acyloxy, phenyl, halogens
Which substituents are deactivating but ortho/para directors
Halogens
What are some meta directors (and deactivating)?
Carboxy, carboxyamido, carboxyalkoxy, acyl, sulfonic acid (SO3H), cyano (C triple bond N), Nitro (NO2)
These groups are which kinds of directors: Carboxy, carboxyamido, carboxyalkoxy, acyl, sulfonic acid (SO3H), cyano (C triple bond N), Nitro (NO2)
Meta
