1. 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

2. 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

3. 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)

4. 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

5. Cyclooctatriene Dianion is aromatic
[16]annulene cation and anion are aromatic

6. 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

7. 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

8. 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 = kcal/mol
F2 reaction is explosive
Cl2 reaction exothermic; use AlCl3 or FeCl3 as catalyst
I2 endothermic

9. 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

10. 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

11. 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

12. 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)

13. 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

14. 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

15. 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