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Sources and Names of
Aromatic Hydrocarbons
 From high temperature distillation of coal tar
 Heating petroleum at high temperature and pressure over

a catalyst

1

Bonding in Benzene

Benzene’s Unusual Structure
 All its C-C bonds are the same length: 139 pm —





between single (154 pm) and double (134 pm) bonds
Electron density in all six C-C bonds is identical
Structure is planar, hexagonal
C–C–C bond angles 120°
Each C is sp2 and has a p orbital perpendicular to the

plane of the six-membered ring

3

Aromaticity and the 4n + 2 Rule
 Huckel’s rule, based on calculations – a compound is

aromatic if it is a






planar
cyclic molecule with
alternating double and single bonds and has
4n+ 2  electrons (n is 0,1,2,3,4)

 For n=1: 4n+2 = 6; benzene is stable and the electrons

are delocalized

4

Heats of Hydrogenation as Indicators
Benzene has 3 unsaturation sites but gives off
of Stability
only 206 kJ/mol on reacting with 3 H molecules
2

Therefore has ~ 150 kJ more “stability”
than an isolated set of three double bonds
H2 + C=C normally gives ~118 kJ/mol
So could expect 3 C=C’s to give off 356kJ/mol)

2 conjugated C=C’s in cyclohexadiene
add 2 H2 to give off 230 kJ/mol

Disubstituted Benzenes
 Relative positions on a

benzene ring
 ortho- (o) on
adjacent carbons
(1,2)
 meta- (m)
separated by one
carbon (1,3)
 para- (p)
separated by two
carbons (1,4)

6

• Benzene does react with bromine, but only in
the presence of FeBr3 (a Lewis acid), and the
reaction is a substitution, not an addition
...


7

C6H6 reacts with HBr to form
one isomer of C6H5Br
Br
Br2, cat
...


Ladenburg benzene
C6H6

Br2, cat
...


3 unique C6H4Br2
(+ HBr)

• A benzene substituent is called a phenyl group, and it can be
abbreviated in a structure as “Ph-”
...


9

• Use “phene” (the French name for benzene) as the
parent hydrocarbon name, not benzene
• Name substituents on aromatic ring by their position
from OH

Naming
An old name for benzene was phene, and its
hydroxyl derivative came to be called phenol
...

Likewise, o-, m-, and p-cresol are acceptable names
for the various ring-substituted hydroxyl
derivatives of toluene
...

Numbering of the ring begins at the hydroxylsubstituted carbon and proceeds in the direction
that gives the lower number to the next
substituted carbon
...


OH

OH

OH

OH
CH3

CH3

Phenol

CH3

o-Cresol

m-Cresol

p-Cresol
NO2

Cl

H2
C

4-Benzyl-2-chlorophenol

OH

HO

CH2CH3

4-Ethyl-3-nitrophenol

Phenols, syntheses:
Industrial process from readily available cumene
Forms cumene hydroperoxide with oxygen at

high temperature

Phenols, syntheses:
1
...
Alkali fusion of sulfonates
SO3 Na

NaOH,H2O
300o

ONa

H+

OH

Phenol Acidity
• Phenols (pKa ~10) are much more acidic than alcohols (pKa
~ 16) due to resonance stabilization of the phenoxide ion
• Phenols react with NaOH solutions (but alcohols do not),
forming salts that are soluble in dilute aqueous solution
• A phenolic component can be separated from an organic
solution by extraction into basic aqueous solution and is
isolated after acid is added to the solution

Substituted
• Can be more or less acidic than phenol itself
• An electron-withdrawing substituent makes a phenol
more acidic by delocalizing the negative charge
• Phenols with an electron-donating substituent are
less acidic because these substituents concentrate
the charge

Effect of electron-withdrawing groups is most
pronounced at ortho and para positions
...

OH

OH

OH
NO2

NO2

NO2

O2 N

NO2

NO2
19

Nitro-Phenols
• Phenols with nitro groups at the ortho and para
positions are much stronger acids

Reactions:
alcohols

phenols

1
...
PX3

NR

3
...
as acids

phenols are more acidic

5
...
ester formation (similar to alcohols)
OH
CH3

O
+ CH3CH2C

OH

H+

O
CH3CH2C

O

H3C
O
H3C C

OH
COOH

O
COOH

+ (CH3CO)2O

salicyclic acid

aspirin

+ H2O

3
...

OCH2CH3

OH
+ CH3CH2Br, NaOH
CH3

CH3

4) Coupling with diazonium salts
(EAS with the weak electrophile diazonium)
N2 Cl

OH

OH
CH3

CH3
+
benzenediazonium
chloride
an azo dye

N
N

5 Oxidation of Phenols: Quinones

OH

O
-2 e
+2 e

OH

, -2H+
, +2H+
O

quinone
hydroquinone
Reaction of a phenol with strong oxidizing agents yields a quinone
Fremy's salt [(KSO3)2NO] works under mild conditions through a radical mechanism

Reactions of Phenols

Quinones in
Nature
• Ubiquinones mediate electron-transfer processes involved
in energy production through their redox reactions



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