ch 15 benzene reactivity l.
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Ch. 15 Benzene Reactivity. Huckel’s Rule: 4n + 2 p electrons = aromatic 4n p electrons = antiaromatic Nonconjugated = nonaromatic 1,3-cyclobutadiene 4n electrons (n = 1) Air sensitive and very reactive, going to 1,3-butadiene structures

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ch 15 benzene reactivity
Ch. 15 Benzene Reactivity
  • Huckel’s Rule: 4n + 2 p electrons = aromatic

4n p electrons = antiaromatic

Nonconjugated = nonaromatic

    • 1,3-cyclobutadiene
      • 4n electrons (n = 1)
      • Air sensitive and very reactive, going to 1,3-butadiene structures
      • Its p-overlap actually destabilized the molecule
slide2
The destabilization can be seen in the its rapid Diels-Alder reaction where it behaves as either the diene or dienophile
    • 1H NMR is very unlike benzene
  • 1,3,5,7-cyclooctatetraene
    • 4n electrons (n = 2) antiaromatic
    • Reactive like normal polyene
    • 1H NMR like alkene (5.68 ppm)
    • Structure is not planar or symmetric
slide3
Aromatic Cyclic Polyenes
    • [18]annulene has 18 p-electrons (n = 4 in 4n +2)
    • (CH)x gives the name of cyclic polyenes
      • Benzene = (CH)6 = [6]annulene
      • Cyclobutadiene = (CH)4 = [4]annulene
    • Planar molecule with equivalent C—C bonds
    • Stable with an benzene-like NMR
  • MO Explanation of Huckel’s Rule
    • Extended cyclic p systems have similar MO diagrams
      • Highest and lowest are not degenerate, all middle MO’s degenerate
      • 4n electrons does not fill the bonding MO’s (not stabilized)
      • 4n + 2 electrons does fill all bonding MO’s (stabilized)
slide4
Charged Molecules follow Huckel’s rule
    • 1,3-cyclopentadienyl anion is aromatic
      • Cyclopentadiene is not fully conjugated
      • The CH2 group has a pKa of 16 (very acidic for carbon)
      • The resulting anion is conjugated and aromatic (6 p electrons)
      • The cation is antiaromatic and very unstable
    • Cycloheptadienyl Cation is aromatic
      • Loss of a hydride in a reaction with bromine easily forms the cation
      • The cation has 6 p electrons and is aromatic
slide5
Cyclooctatriene Dianion is aromatic
  • [16]annulene cation and anion are aromatic
slide6
Electrophilic Aromatic Substitution
    • Reactivity of Benzene
      • Benzene is quite unreactive, but can be attacked by electrophiles
      • Electrophiles substitute themselves for one of the ring H’s
      • The double bonds (aromaticity) are not disturbed
      • Under the same conditions, a conjugated polyene would polymerize
    • Mechanism of Electrophilic Aromatic Substitution
      • E+ attacks benzene p-cloud forming a cationic intermediate
      • Intermediate loses H+ to regenerate the aromatic ring, now substituted
slide7
Step 1 is endothermic. The cation is less stable than the aromatic.
  • Addition of X- at this point would be a normal alkene addition, but would produce a nonaromatic product

5) Loss of H+ is favored (exothermic) because in reforms the aromatic ring

slide8
Halogenation of Benzene
    • Benzene is unreactive with X2 alone (not electrophilic enough)
    • A Lewis Acid catalyst activates X2 to become more electrophilic
    • Mechanism
    • For Br2, DH = -10.5 kcal/mol
      • F2 reaction is explosive
      • Cl2 reaction exothermic; use AlCl3 or FeCl3 as catalyst
      • I2 endothermic
slide9
Nitration
    • Benzene is unreactive towards HNO3 until H2SO4 activates it
    • Nitronium ion (NO2+) can react with benzene
  • Sulfonation
    • Benzene will not react directly with sulfuric acid
    • Fuming sulfuric acid contains 8% SO3
slide10
Mechanism
  • This reaction can be reversed.
    • The hydration of SO3 to H2SO4 is very exothermic, so SO3 reforms to undergo this reaction.
    • We can use sulfonation to block, then take it off to do another reaction
  • Benzenesulfonic acids have had many uses:
    • Detergents
    • Good Leaving groups for SN1 and SN2 reactions
    • Sulfonamide Antibiotics
slide11
Friedel-Crafts Alkylation
    • We need a reaction to form Aromatic C—C bonds
    • The Friedel-Crafts reaction gives us that power
    • Reactivity of alkyl halides: RI < RBr < RCl < RF
    • Lewis acids we can use: BF3, FeCl3, AlCl3, AlBr3
    • Mechanism
    • Intramolecular Friedel-Crafts Reactions are possible
slide12
Any carbocation is susceptible to Friedel-Crafts reaction
  • Limits of the Friedel-Crafts Reaction

a) Polyalkylation is difficult to stop (more than one R group adds)

slide13
Skeletal rearrangements of Alkyl groups frequently occur
      • We need to find a better way to make aromatic C—C bonds
  • Friedel-Crafts Acylation
    • Acyl Halides (and anhydrides) can add to benzene similarly to alkyl halides
    • Carboxylic Acids can be turned into acyl halides
slide14
Carboxylic acids plus acyl halides can form anhydrides
  • Both acyl halides and anhydrides give Acylium Ions in reaction with Lewis Acids
  • Acylium Ions can do electrophilic aromatic substitution
  • Advantages of Acylation:

a) There is no danger of rearrangement

Acyclium Ion

slide15
The electron withdrawing nature of the carbonyl group deactivates the benzene ring, so it won’t undergo multiple substitutions.
  • Lewis acid complexation increases the electron withdrawing ability of the carbonyl, further preventing a second reaction.
    • One full equivalent of AlCl3 is needed (not catalytic)
    • Must have aqueous workup to release product