21 benzene
TRANSCRIPT
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Chapter 21, Benzeneand and the Concept of
Aromaticity
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Benzene - Kekul
In 1872, August Kekul proposed the following
structure for benzene.
This structure, however, did not account for the
unusual chemical reactivity of benzene.
CH
CH
CH
CHC
H
C
HC
C
CC
C
C
H
HHH
HH
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Benzene - Resonance
We often represent benzene as a hybrid of two
equivalent Kekul structures. Each makes an equal contribution to the hybrid and
thus the C-C bonds are neither double nor single, butsomething in between.
Benzene as a hybrid of two equ ivalen tcontributing structures
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Benzene - Resonance Model
The concepts of hybridization of atomic orbitals
and the theory of resonance, developed in the1930s, provided the first adequate description ofbenzenes structure.
The carbon skeleton is a planar regular hexagon.
All C-C-C and H-C-C bond angles 120.
sp2-sp
2
sp2-1s109 pm
120
120
120
139 pm
C
C
C
C
C
C H
H H
H
H H
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The Pi System of Benzene
(a) The carbon framework with the six 2p orbitals.
(b) Overlap of the parallel 2p orbitals forms one torusabove the plane of the ring and another below it
this orbital represents the lowest-lying pi-bondingmolecular orbital.
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Benzene-Molecular Orbital Model
The molecular orbital representation of the pi
bonding in benzene.
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Orbitals of the pi System of BenzeneNumber ofnodal surfaces
0
1
2
3
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Benzene - Resonance
Resonance energy:The difference in energy
between a resonance hybrid in which theelectrons are delocalized
and
the most stable one of its hypotheticalcontributing structures in which electrons arelocalized on particular atoms and in particularbonds.
One way to estimate the resonance energy of benzeneis to compare the heats of hydrogenation of benzeneand cyclohexene.
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Benzene- Resonance Energy
Experimentaldata
Model
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Concept of Aromaticity
The underlying criteria for aromaticity were
recognized in the early 1930s by Erich Hckel,based on molecular orbital (MO) calculations.
To be aromatic, a compound must
Be cyclic. Have one p orbital on each atom of the ring.
Be planar or nearly planar so that there is continuousor nearly continuous overlap of all p orbitals of the
ring. Have a closed loop of (4n + 2) pi electrons in the cyclic
arrangement of p orbitals.
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Frost Circles
Frost circle: A graphic method for determining
the relative order of pi MOs in planar, fullyconjugated monocyclic compounds.
Inscribe in a circle a polygon of the same number ofsides as the ring to be examined such that one of the
vertices of the polygon is at the bottom of the circle.
The relative energies of the MOs in the ring are givenby where the vertices of the polygon touch the circle.
Those MOs
Below the horizontal line through the center of the ringare bonding MOs.
on the horizontal line are nonbonding MOs.
above the horizontal line are antibonding MOs.
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Frost Circles
Frost circles describing the MOs for monocyclic,
planar, fully conjugated four-, five-, and six-memberedrings.
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Relationship of hexa-1,3,5-triene to benzene
How does the linear triene relate
to benzene?
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Relationship of hexa-1,3,5-triene to benzene
?
R l i hi f h 3 i b
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Relationship of hexa-1,3,5-triene to benzene
?
Look at orbitals 2 and 3.
p2
p3
Curvearound
Antibonding,destabilizing
Bonding,stabilizing
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Aromatic Hydrocarbons
Annulene:A cyclic hydrocarbon with a
continuous alternation of single and doublebonds.
[14]Annulene is aromatic according to Hckels
criteria.
[14]Annulene
(aromatic)
HH
H H
H
H
H
H
HH
H
H
H H
n = 3
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Aromatic Hydrocarbons
[18]Annulene is also aromatic.
[18]Annulene
(aromatic)
H
H
H
H
H
H
H
H
HH
H
H
H
H
H
H
H
H
n = 4
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Aromatic Hydrocarbons
According to Hckels criteria, [10]annulene should be
aromatic; it has been found, however, that it is not. Nonbonded interactions between the two hydrogens
that point inward toward the center of the ring forcethe ring into a nonplanar conformation in which
overlap of the ten 2p orbitals is no longer continuous.
[10]Annulene
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Aromatic Hydrocarbons
What is remarkable relative to [10]annulene is that if
the two hydrogens facing inward toward the center ofthe ring are replaced by a methylene (CH2) group, thering is able to assume a conformation close enough toplanar that it becomes aromatic.
CH2
Bridged [10]annulene
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Antiaromatic Hydrocarbons
Antiaromatic hydrocarbon:A monocyclic, planar,
fully conjugated hydrocarbon with 4n pielectrons (4, 8, 12, 16, 20...).
An antiaromatic hydrocarbon is especially unstablerelative to an open-chain fully conjugated hydrocarbon
of the same number of carbon atoms.
Cyclobutadiene is antiaromatic.
In the ground-state electron configuration of thismolecule, two electrons fill the p
1
bonding MO.
The remaining two electrons lie in the p2 and p3nonbonding MOs.
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Cyclobutadiene
The ground state of planar cyclobutadiene has two
unpaired electrons, which make it highly unstable andreactive.
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Cyclooctatetraene
Cyclooctatetraene, with 8 pi electrons is not aromatic;
it shows reactions typical of alkenes. X-ray studies show that the most stable conformation
is a nonplanar tub conformation.
Although overlap of 2p orbitals occurs to form pi
bonds, there is only minimal overlap between sets of2p orbitals because they are not parallel.
viewed from above viewed through an edge
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Cyclooctatetraene
MO energy diagram for a planar conformation of
cyclooctatetraene.
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Heterocyclic Aromatics
Heterocyclic compound: A compound that
contains more than one kind of atom in a ring. In organic chemistry, the term refers to a ring with one
or more atoms that differ from carbon.
Pyridine and pyrimidine are heterocyclic analogsof benzene; each is aromatic.
Pyridine
N
N
N
Pyrimidine
1
2
3
4
5
6
12
3 4
5
6
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Pyridine
The nitrogen atom of
pyridine is sp2
hybridized.
The unshared pair ofelectrons lies in an sp2
hybrid orbital and is nota part of the six pielectrons of thearomatic system (thearomatic sextet).
Resonance energy ofpyridine is134 kJ (32kcal)/mol.
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Furan and Pyrrole
The oxygen atom of furan is sp2 hybridized.
one unshared pairs of electrons on oxygen lies in anunhybridized 2p orbital and is a part of the aromaticsextet.
The other unshared pair lies in an sp2 hybrid orbital
and is not a part of the aromatic system. The resonance energy of furan is 67 kJ (16 kcal)/mol.
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Other Heterocyclics
Purine
Indole
N
N
NN
N
H
H
N
H
CH2 CH2 NH 2
Serotonin(a neurotransmitter)
HO
Caffeine
N
NN
N
O
O
H3 C
CH3
CH3
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Aromatic Hydrocarbon Ions
Any neutral, monocyclic, unsaturated
hydrocarbon with an odd number of carbonsmust have at least one CH2 group and, therefore,cannot be aromatic.
Cyclopropene, for example, has the correct number of
pi electrons to be aromatic, 4(0) + 2 = 2, but does nothave a closed loop of 2p orbitals.
Cyclopropene Cyclopentadiene Cycloheptatriene
CH2 CH2CH2
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Cyclopropenyl Cation
If, however, the CH2 group of cyclopropene is
transformed into a CH+
group in which carbon is sp2
hybridized and has a vacant 2p orbital, the overlap oforbitals is continuous and the cation is aromatic.
Cyclopropenyl cation represented as a hybridof three equ ivalen t contributin g structures
+
H
H
H
H
H
H
H
H
H+
+
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Cyclopropenyl Cation
When 3-chlorocyclopropene is treated with SbCl5, it
forms a stable salt.
This chemical behavior is to be contrasted with that of5-chloro-1,3-cyclopentadiene, which cannot be made
to form a stable salt.
+
Cyclopropenyl
hexachloroantimonate
+
3-Chloro-
cyclopropene
H
H
ClSbCl5 SbCl6
-
Antimony(V)
chloride(a Lewis acid)
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Cyclopentadienyl Cation
If planar cyclopentadienyl cation were to exist, it wouldhave 4 pi electrons and be antiaromatic.
Note that we can draw five equivalent contributingstructures for the cyclopentadienyl cation. Yet this
cation is not aromatic because it has only 4 pielectrons.
Cyclope ntadienylte trafluoroborate
++
5-Chloro-1,3-cyclopentadiene
H
ClHAgBF4 BF4 - + AgCl
C C
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Cyclopentadienyl Anion, C5H5-
To convert cyclopentadiene to an aromatic ion, it
is necessary to convert the CH2 group to a CHgroup in which carbon becomes sp2 hybridizedand has 2 electrons in its unhybridized 2p orbital.
H
HH
HH
the origin of the 6 pi electronsin the cyclopentadienyl anion
Cyclopentadienyl anion(aromatic)
HH
HH
H:
H
H
H
HH
n = 1
C l di l A i C H
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Cyclopentadienyl Anion, C5H5-
As seen in the Frost circle, the six pi electrons of
cyclopentadienyl anion occupy the p1, p2, and p3molecular orbitals, all of which are bonding.
C l di l A i C H
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Cyclopentadienyl Anion, C5H5-
The pKa of cyclopentadiene is 16.
In aqueous NaOH, it is in equilibrium with its sodiumsalt.
It is converted completely to its anion by very strongbases such as NaNH2 , NaH, and LDA.
pKa15.7pKa16.0
Na+
+ H2OH
H
H
H
H
CH2 + NaOH :
C l h i l C i C H
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Cycloheptatrienyl Cation, C7H7+
Cycloheptatriene forms an aromatic cation by
conversion of its CH2 group to a CH+ group withits sp2 carbon having a vacant 2p orbital.
+
Cyclohep tatrienyl cation(Tropylium ion)
(aromatic)
H
HH
H
H
HH
H
HH
H
H
HH
+
N l t
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Nomenclature
Monosubstituted alkylbenzenes are named as
derivatives of benzene. Many common names are retained.
Toluene CumeneEthylbe nzene Styrene
Phenol Aniline Benzoic acid Anisole
COOHNH2 OCH3OH
Benzaldehyde
CHO
N l t
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Nomenclature
Benzyl and phenyl groups
(Z)-2-Phenyl-
2-butene
4-(3-Me thoxyphenyl)-
2-butanone
1-Phenyl-1-pentanone
O OH3CO
Ph
Benzene Phenyl group, Ph- Toluene Benzyl group, Bn-
CH3 CH2
Di b tit t d B
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Disubstituted Benzenes
Locate two groups by numbers or by the
locators ortho (1,2-), meta (1,3-), and para (1,4-). Where one group imparts a special name, name the
compound as a derivative of that molecule.
CH3
Br
COOH
NO2
Cl
NH2
CH3
CH3
2-Nitrobenzoic acid
(o-Nitrobenzoic acid )
3-Chloroaniline
(m-Chloroaniline)
4-Bromotoluene
(p-Bromotoluene)
m-Xylene
Di b tit t d B
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Disubstituted Benzenes
Where neither group imparts a special name, locate
the groups and list them in alphabetical order.CH2 CH3
Cl
NO2Br
1-Bromo-2-nitrobenzene(o-Bromonitrobenzene)
1-Chloro-4-ethylbenzene(p-Chloroethylbenzene)
1
2
3
4 2
1
P l b tit t d D i ti
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Polysubstituted Derivatives
If one group imparts a special name, name the
molecule as a derivative of that compound. If no group imparts a special name, list them in
alphabetical order, giving them the lowest set ofnumbers.
CH3
NO2
OH
Br
Br
NO2
CH2 CH3
Br
4
2
1
6
4
21
4
1 2
4-Chloro-2-nitro-
toluene
2,4,6-Tribromo-
phenol2-Bromo-1-ethyl-4-
nitrobenzene
Br
Cl
Ph l
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Phenols
The functional group of a phenol is an -OH group
bonded to a benzene ring.
1,2-Benzenediol(Catechol)
1,4-Benzenediol(Hydroquinone )
3-Methylphenol(m-Cresol)
Phenol
OH OHOHOH
OHCH3
OH
Acidit of Phenols
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Acidity of Phenols
Phenols are significantly more acidic than
alcohols.
OH H2 O
CH3CH2 OH H2 O
O-
CH3CH2 O-
H3 O+
H3 O+
pKa = 9.95+
+
+
+ pKa = 15.9
Acidity of Phenols
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Acidity of Phenols
Separation of water-
insoluble phenolsfrom water-insolublealcohols.
Acidity of Phenols (Resonance)
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Acidity of Phenols (Resonance)
The greater acidity of phenols compared with alcohols
is due to the greater stability of the phenoxide ionrelative to an alkoxide ion.
These 2 Kekulstructures are
equivalent
HH
OO O O
H
O
These three contrib uting s tructuresdelocalize the negative charge
onto carbon atoms of the rin g
H
OO O O
H
O
Phenol Subsitituents (Inductive Effect)
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Phenol Subsitituents (Inductive Effect)
Alkyl and halogen substituents effect acidities by
inductive effects: Alkyl groups are electron-releasing.
Halogens are electron-withdrawing.
p-ChororophenolpKa 9.18
m-ChlorophenolpKa 8.85
PhenolpKa 9.95m-
CresolpKa 10.01p-
CresolpKa 10.17
OH OH OH OH OH
CH3CH3
Cl
Cl
Phenol Subsitituents(Resonance Inductiion)
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Phenol Subsitituents(Resonance, Inductiion)
Nitro groups increase the acidity of phenols by both an
electron-withdrawing inductive effect and a resonanceeffect.OH
NO2
OH OH
NO2PhenolpKa 9.95
p-NitrophenolpKa 7.15
m-NitrophenolpKa 8.28
Acidity of Phenols
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Acidity of Phenols
Part of the acid-strengthening effect of -NO2 is due to
its electron-withdrawing inductive effect. In addition, -NO2 substituents in the ortho and para
positions help to delocalize the negative charge.
+ +
delocalization of negative
charge onto oxygen furtherincreases the resonancestabilization of phenoxide ion
O
O O
N
O
O
NO
Synthesis: Alkyl Aryl Ethers
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Synthesis: Alkyl-Aryl Ethers
Alkyl-aryl ethers can be prepared by the
Williamson ether synthesis: but only using phenoxide salts and haloalkanes.
haloarenes cannot be used because they areunreactive to SN2 reactions.
no reaction+X RO-Na
+
Synthesis: Alkyl Aryl Ethers
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Synthesis: Alkyl-Aryl Ethers
OH CH2 =CHCH2 ClNaOH, H2 O, CH2 Cl2
OCH2 CH=CH2
Phenyl 2-prope nyl ether
(Allyl phenyl ether)
+
Phenol 3-Chloropropene(Allyl chloride)
OH
O
O
CH3 OSOCH3NaOH, H2 O, CH2 Cl2
OCH3 Na2 SO4+
Methyl phenyl e ther(Anisole)
+
Phenol Dimethyl sulfate
Synthesis: Kolbe Carboxylation
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Synthesis: Kolbe Carboxylation
Phenoxide ions react with carbon dioxide to give
a carboxylate salt.
OH
NaOH
H2
O
O-Na
+
CO2
H2
O
OH
CO-Na
+O
HCl
H2 O
OH O
COH
Phenol Sodiumphenoxide
Sodium salicylate S alicylic acid
Mechanism: Kolbe Carboxylation
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Mechanism: Kolbe Carboxylation
The mechanism begins by nucleophilic addition of the
phenoxide ion to a carbonyl group of CO2.
O
C
O
O
OC
H
OO OH
CO
O
A cyclohexadienone
intermediate
+
Sodium
phenoxide
Salicylate anion
keto-enol
tautomerism
(1) (2)
Go back to aromatic
structure
Synthesis: Quinones
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Synthesis: Quinones
Because of the presence of the electron-donating
-OH group, phenols are susceptible to oxidationby a variety of strong oxidizing agents.
H2 CrO4
Phenol 1,4-Benzoquinone(p-Quinone)
O
O
OH
Quinones
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Quinones
OH
OH K2Cr2O7
OH
OH
H2SO
4
K2Cr2O7
H2SO
4
O
O
O
O
1,4-Benzoquinone(p-Quinone)
1,2-Benzenediol(Catechol)
1,2-Benzoquinone(o-Quinone)
1,4-Benzenediol(Hydroquinone)
Quinones
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Quinones
Readily reduced to hydroquinones.
1,4-Benzoquinone(p-Quinone)
(reduction)
1,4-Benzenediol(Hydroquinone)
O
O
OH
OH
Na2 S2O4 , H2 O
Coenzyme Q
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Coenzyme Q
Coenzyme Q is a carrier of electrons in the
respiratory chain.O
O
MeO
HMeO MeO
MeO
OH
OH
Hn n
Coenzyme Q(oxidized form)
Coenzyme Q(reduced form)
reduction
oxidation
Benzylic Oxidation
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Benzylic Oxidation
Benzene is unaffected by strong oxidizing agents
such as H2CrO4 and KMnO4 Halogen and nitro substituents are also unaffected by
these reagents.
An alkyl group with at least one hydrogen on its
benzylic carbon is oxidized to a carboxyl group.
2-Chloro-4-nitrotoluene 2-Chloro-4-nitrobenzoic acid
H2 CrO4
O2N Cl
CH3
O2N Cl
COOH
Benzylic Oxidation
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Benzylic Oxidation
If there is more than one alkyl group on the benzene
ring, each is oxidized to a -COOH group.
1,4-Dimethylbenzene(p-xylene)
1,4-Be nzenedicarboxylic acid(terephthalic acid)
CH3H2 SO4
K2 Cr2 O7H3 C COH
O
HOC
O
Benzylic Chlorination
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Benzylic Chlorination
Chlorination and bromination occur by a radical
chain mechanism.CH3
Cl2+
CH2Cl
HCl+
Toluene
heator light
Benzyl chloride
( PhCO2) 2 , CCl4
NBS
Br
Ethylbenzene 1-Bromo-1-phenylethane(racemic)
Mechanism: Benzylic Reactions
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Mechanism: Benzylic Reactions
Benzylic radicals (and cations also) are easily
formed because of the resonance stabilization ofthese intermediates.
The benzyl radical is a hybrid of five contributingstructures.
C
C
C
C C
Benzylic Halogenation
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Benzylic Halogenation
Benzylic bromination is highly regioselective.
Benzylic chlorination is less regioselective.
(PhCO2 )2 , CCl4
NBSBr
Ethylbenzene 1-Bromo-1-phenylethane(the only product formed)
Cl
Cl2 +
heator light
1-Chloro-2-phenylethane
(10%)
Ethylbenzene
+
1-Chloro-1-phenylethane
(90%)
Cl
Hydrogenolysis
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Hydrogenolysis
Hydrogenolysis:Cleavage of a single bond by H2
Benzylic ethers are unique in that they are cleavedunder conditions of catalytic hydrogenation.
O H2Pd/ C
OH
Me+
Benzyl butyl ether Toluene1-Butanol
+
this bondis cleaved
Synthesis, Protecting Group: Benzyl Ethers
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Synthesis, Protecting Group: Benzyl Ethers
The value of benzyl ethers is as protecting
groups for the OH groups of alcohols andphenols.
To carry out hydroboration/oxidation of this alkene,the phenolic -OH must first be protected; it is acidic
enough to react with BH3 and destroy the reagent.
OH
1. ClCH2 Ph
O Ph
2. BH3 THF
Et 3N 3. H2 O2/ NaOH
H2Pd/ C
O Ph
OH
OH
OH
2-(2-Propenyl)phenol
(2-Allylphenol)