1) Benzylic radicals are reactive intermediates in the α-halogenation of alkylbenzenes. Benzylic halogenation proceeds without use of a catalyst like FeBr3 and involves benzylic cation and SN2 reaction mechanisms. 2) Phenol synthesis can occur through three mechanisms - nucleophilic aromatic substitution when electron-withdrawing groups are present, elimination-addition via highly reactive benzyne intermediates under harsh conditions, or using arenediazonium salts with super-leaving nitrogen groups. 3) Phenols are weakly acidic due to resonance stabilization of the phenoxide anion. They undergo electrophilic aromatic substitution and other reactions like ether and ester formation. O

1. Chapter 22: ChemistryChapter 22: Chemistry
of Substitutedof Substituted
BenzenesBenzenes
Benzylic resonance forBenzylic resonance for ++//..
//--
All stabilized by the overlappingAll stabilized by the overlapping pp orbitalsorbitals

3. BenzylicBenzylic radicalsradicals areare reactive intermediatesreactive intermediates
in thein the αα-halogenation-halogenation of alkylbenzenesof alkylbenzenes
Therefore:Therefore: BenzylicBenzylic halogenationhalogenation
No EAS
(Which needs
FeBr3)

4. MechanismMechanism of benzylic halogenationof benzylic halogenation
The twoThe two
propagationpropagation
stepssteps

5. BenzylicBenzylic CationCation (S(SNN1 reaction)1 reaction)
Mechanism:Mechanism:

6. SSNN2 is accelerated2 is accelerated
Transition state is delocalizedTransition state is delocalized
SSNN

7. Benzylic Acidity: Resonance inBenzylic Acidity: Resonance in
Benzylic AnionsBenzylic Anions
Compare propene pCompare propene pKKaa ~ 40~ 40

8. Benzylic Oxidation-Benzylic Oxidation-
ReductionReduction
or Naor Na22CrCr22OO77
• C-C bond broken via:C-C bond broken via:
OH O OH
Can be stopped hereCan be stopped here

9. Neutral, mildNeutral, mild
Protecting groupProtecting group DiscardedDiscarded DeprotectedDeprotected
UntouchedUntouched
Benzylic hydrogenolysisBenzylic hydrogenolysis
CompareCompareterttert-Bu ethers as protecting groups, need acid for deprotection-Bu ethers as protecting groups, need acid for deprotection

10. Cis-1,2 = axial-equatorialCis-1,2 = axial-equatorial
Cis-1,4 = axial-equatorialCis-1,4 = axial-equatorial

11. Benzenol (Phenol)Benzenol (Phenol)
Keq = 1013
NomenclatureNomenclature
(Note: Phenol is an enol!)(Note: Phenol is an enol!)

12. • Phenol ethers:Phenol ethers: AlkoxybenzenesAlkoxybenzenes
PhenoxyPhenoxy
(from “phenyloxy”)(from “phenyloxy”)
e.g.e.g.
MethoxybenzeneMethoxybenzene
(Anisole)(Anisole)
OCH3
O
Natural productsNatural products

13. AcidityAcidity
(Enolate!)(Enolate!)
• Substituent effects:Substituent effects: Inductive + resonanceInductive + resonance

14. MultipleMultiple resonatingresonating e-withdrawinge-withdrawing substituentssubstituents
increase the acidity to that of mineral acidsincrease the acidity to that of mineral acids

15. Preparation of PhenolsPreparation of Phenols
Problem with EAS:Problem with EAS:
• Electrophilic aromatic substitution needs “Electrophilic aromatic substitution needs “++
OHOH””
equivalent. Difficult to come by.equivalent. Difficult to come by.
• When a leaving group is on the ring:When a leaving group is on the ring:
Nucleophilic aromatic substitutionNucleophilic aromatic substitution
OH
R R
+OH
OH
R
-OH
X (L)
R

16. ThreeThree mechanistic scenarios exist:mechanistic scenarios exist:
1.1. When electron withdrawing groups areWhen electron withdrawing groups are
present,present, nucleophilic aromatic addition–nucleophilic aromatic addition–
eliminationelimination takes placetakes place
2.2. Without e-withdrawing groups: harshWithout e-withdrawing groups: harsh
conditions enforce an elimination-conditions enforce an elimination-
addition sequence through reactiveaddition sequence through reactive
benzynebenzyne intermediateintermediate
3.3. With super L = NWith super L = N22,, phenyl cationphenyl cation
intermediatesintermediates

17. Case 1:Case 1:
We learned earlierWe learned earlier
that Sthat SNN2 is not2 is not
possible onpossible on
haloalkenes. Howhaloalkenes. How
does aromaticdoes aromatic
nucleophilicnucleophilic
substitution work?substitution work?

18. Does not work with
meta-nitro groups

19. Without e-withdrawing resonance stabilizers,Without e-withdrawing resonance stabilizers,
but with good L extreme conditions requiredbut with good L extreme conditions required
2,4,5-Trichlorophenol2,4,5-Trichlorophenol
(2,4,5-TCP)(2,4,5-TCP)
2,4,5-Trichlophenoxy-2,4,5-Trichlophenoxy-
acetic acid (2,4,5-T):acetic acid (2,4,5-T):
Butyl esters used asButyl esters used as
“Agent Orange” defoliant“Agent Orange” defoliant
in Vietnam War.in Vietnam War.
85%85%
Melt!Melt!
Cl Cl Cl
Cl Cl
OH Cl
Cl Cl
OCH2COOH1. NaOH, 150°C
2. H+, H2O
- NaCl
ClCH2COOH,
NaOH, H2O,
Cl Cl
Cl
Cl Cl
OH Cl
Cl
O
Cl
Cl
O
K2CO3, Cu, 240-250°C
DioxinDioxin

20. Case 2:Case 2: Elimination-Addition via BenzyneElimination-Addition via Benzyne
At highly elevated
temperatures
and pressures

21. Mechanism:Mechanism: Elucidated by usingElucidated by using 1414
CC labellabel
Indicates formation ofIndicates formation of benzynebenzyne (highly strained)(highly strained)
““Biradicaloid”Biradicaloid”

22. (not E2, but stepwise, via anion)(not E2, but stepwise, via anion)

23. Keto-Enol in Phenols: Current Stuff!Keto-Enol in Phenols: Current Stuff!
Angew. Chem. Int. Ed. 2006, 45, 98 –101

24. OH
R
-OH
X (L)
R
Recap: PhenolRecap: Phenol
SynthesisSynthesis
1.1.
2.2.
Addition-Addition-
EliminationElimination
BenzyneBenzyne
Elimination (via anion)-Addition (via anion)Elimination (via anion)-Addition (via anion)

25. Benzyne can be trapped in Diels-Alder reaction:Benzyne can be trapped in Diels-Alder reaction:
Case 3:Case 3: Super leaving group nitrogen;Super leaving group nitrogen;
Arenediazonium saltsArenediazonium salts
Primary aminePrimary amine Relatively stable,Relatively stable,
isolableisolable
Br
I
+
Mg
““Diazotization”Diazotization”
Makes the Grignard (a phenyl anion), which eliminates halideMakes the Grignard (a phenyl anion), which eliminates halide
NH2
NaNO2, HCl
N
N
+
:
N
N
-
:
:
+
+ ∆, H2O
OH: :
:

26. In general,In general,
R
HNO3
R
NO2
R
OH

27. Chemistry of PhenolsChemistry of Phenols
a.a. BasicityBasicity: Weak: Weak
• Less basic than alkanols, because e-pair is tiedLess basic than alkanols, because e-pair is tied
up byup by resonanceresonance with the benzene ringwith the benzene ring
b.b. Do not formDo not form
phenyl cationsphenyl cations::
O
H
H
+

28. c. But phenol is ac. But phenol is a leaving groupleaving group
d. Phenols formd. Phenols form ethersethers by Williamsonby Williamson
synthesissynthesis
O
H
R
+
X: :
::
-
OH
+ RX+

29. d. Phenols formd. Phenols form estersesters
10,000 Tons per year
But:But:
MoreMore
nucleophilicnucleophilic

30. e.e. Electrophilic Aromatic SubstitutionElectrophilic Aromatic Substitution
Para productPara product
predominatespredominates

31. Halogenation fast, does not need catalyst,Halogenation fast, does not need catalyst,
needs cooling to controlneeds cooling to control

32. Both reactive,Both reactive,
continue on....continue on....
f.f. BasicBasic conditions:conditions: PhenolatePhenolate, superactive EAS, superactive EAS
Like aldolLike aldol
First, dehydration:First, dehydration:
Both are hot Michael acceptors

33. Then, Michael additions:Then, Michael additions:
Plywood, insulation, molding compounds, granulated wood, laminates.

34. Even COEven CO22 reacts: Kolbe reactionreacts: Kolbe reaction
Salicylic acid (Salicylic acid (as saltas salt););
precursor to aspirin:precursor to aspirin:
Adolph W. H. KolbeAdolph W. H. Kolbe
1818-18841818-1884

35. Allyloxyphenols undergo theAllyloxyphenols undergo the
Claisen rearrangementClaisen rearrangement
Mechanism:Mechanism:
An extension of electrocyclic reactions. The carbonAn extension of electrocyclic reactions. The carbon
analog is calledanalog is called Cope rearrangementCope rearrangement..
Ludwig ClaisenLudwig Claisen
1851-19301851-1930
6e aromatic TS6e aromatic TS

36. Cope RearrangementCope Rearrangement
Recall:Recall:
Dihydro analog of hexatriene ring closureDihydro analog of hexatriene ring closure
Arthur C. CopeArthur C. Cope
1909-19661909-1966

37. Release ofRelease of
ring strainring strain
D
D
D
D
D
D
D
D
DegenerateDegenerate
Cope rear-Cope rear-
rangementrangement
Exotic,Exotic,
degeneratedegenerate

38. Compare to normalCompare to normal
Claisen rearrangementClaisen rearrangement
Nature uses electrocyclic reactions:Nature uses electrocyclic reactions:
An extraordinary drug mechanismAn extraordinary drug mechanism

39. Benzenediols–Benzenediols–
Benzoquinones: RedoxBenzoquinones: Redox
CouplesCouples
(Unstable)(Unstable)
ReductionReduction
ReductionReduction
o-Benzoquinone
p-BenzoquinoneOxidation with Ag+
used in photography (→ Ag)

40. O
O
CN
CN
NC
NC
Quinones are oxidizing, i.e., they pick up electrons. Can be taylored withQuinones are oxidizing, i.e., they pick up electrons. Can be taylored with
substituents. Electron-withdrawing substituents increase oxidizingsubstituents. Electron-withdrawing substituents increase oxidizing
power:power:
TetracyanobenzoquinoneTetracyanobenzoquinone
(TCNQ)(TCNQ)
TCNQ
+
CN
CN
NC
NC
OH
OH
::

41. BenzoquinonesBenzoquinones as electron poor alkenesas electron poor alkenes
(i) 1,4-Addition(i) 1,4-Addition
(ii) Diels-Alder Reaction(ii) Diels-Alder Reaction
(Keto)(Keto) (Enol)(Enol)
Kinetically stableKinetically stable

42. ArenaminesArenamines
React like amines (e.g., acidic, basic,React like amines (e.g., acidic, basic,
make amides, and undergo alkylation)make amides, and undergo alkylation)
Less basic than RNHLess basic than RNH22
(p(pKKaa = 11-12), because of= 11-12), because of
resonanceresonance
ppKKaa = 4.63= 4.63
NH2:
H+
NH3
+

43. Arenediazonium SaltsArenediazonium Salts
Relatively stable because: a. ResonanceRelatively stable because: a. Resonance
Revisited:Revisited:

44. b. Extrusion of Nb. Extrusion of N22 forms bad phenyl (aryl) cationforms bad phenyl (aryl) cation
Phenyl cationPhenyl cation
However, NHowever, N22 elimination leads to useful substitutions.elimination leads to useful substitutions.
Recall phenol synthesis with water.Recall phenol synthesis with water. OtherOther nucleophilesnucleophiles
work, too:work, too:

45. Cu catalysisCu catalysis

46. Reductive removal of nitrogen:Reductive removal of nitrogen:
Strategic application: ArHStrategic application: ArH →→ ArNOArNO22 →→ ArNHArNH22
→→ Ar’NHAr’NH22 →→ Ar’HAr’H
Use of the amino substituent as an activator inUse of the amino substituent as an activator in
EAS, followed by removal.EAS, followed by removal.

47. Dyes by DiazocouplingDyes by Diazocoupling
This is an EAS withThis is an EAS with diazoniumdiazonium as electrophile.as electrophile.
Works only with veryWorks only with very activated arenesactivated arenes (i.e. phenol(i.e. phenol
or benzenamine derivatives). The resultingor benzenamine derivatives). The resulting
compounds are highlycompounds are highly coloredcolored ( n( n→→ππ*,*, ππ→→ππ**
electronic transitions):electronic transitions): Azo dyesAzo dyes

48. Industrial dyes contain –SOIndustrial dyes contain –SO33H groups: Water soluble.H groups: Water soluble.
Dye sticks by ionic bonding to charged sites in textile.Dye sticks by ionic bonding to charged sites in textile.
(and pH Indicators)(and pH Indicators)