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BENZENBENZENEE
Benzene. This aromatic hydrocarbon was first discovered in
1825 but its structure was not generally agreed upon until 1946.
Facts about benzene:
a) Formula = C6H6
b) Isomer number:
one monosubstituted isomer C6H5Y known
three disubstituted isomers C6H4Y2 known04/21/23 Dr Seemal Jelani Chem-160 2
c) Benzene resists addition reaction, undergoes
substitution reactions.
d) Heats of hydrogenation and combustion are far lower than they should be.
e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
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•Formula = C6H6
Max. number of H’s for 6-carbons = 14
• Benzene only has 6 hydrogens. Given one degree of unsaturation (double bond or ring) for every two missing hydrogens less than the maximum, benzene has 4 degrees of unsaturation; that is four combinations of pi-bonds and rings.
CH3CC-CCCH3 HCC-CC-CH2CH3
HCCCH2CCCH3 HCCCH2CH2CCH
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CH3
HCCCHCCH CH2=CHCCCH=CH2
CH2=CHCH=CHCCH CH2=C=CHCH2CCH
CH3CH2=C=CHCCCH3 CH2=C=CCCH
CH=CH2
CH2=C=CHCH=C=CH2 CH2=C CCH
CH3CH=C=CHCCH04/21/23 Dr Seemal Jelani Chem-160 5
CH2
H2C CH2
=C=C=CH2
=C=CH2
CH2 CH2
CH2
=CH2HC2=
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=CH2
Which of these structures is benzene?
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b) Isomer number. There is only one monosubstituted benzene of any type: only one bromobenzene C6H5Br, only one nitrobenzene C6H5NO2, etc.
CH3CC-CCCH3 HCC-CC-CH2CH3
one possible three possibles
HCCCH2CCCH3 HCCCH2CH2CCH
three possible two possible
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CH3
HCCCHCCH CH2=CHCCCH=CH2
three two +
CH2=CHCH=CHCCH CH2=C=CHCH2CCH five + four + CH3CH2=C=CHCCCH3 CH2=C=CCCH three + three + CH=CH2
CH2=C=CHCH=C=CH2 CH2=C four + CCH four +
CH3CH=C=CHCCH four +04/21/23 Dr Seemal Jelani Chem-160 9
CH2
H2C CH2
=C=C=CH2
=C=CH2
CH2 CH2
CH2
=CH2HC2=
one possible
two
three +
two +
two
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=CH2
three one
two
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CH3CC-CCCH3
There are three disubstituted benzenes of any type: three dibromobenzenes C6H4Br2, etc.
CH2
CH2 CH2two possible
four
four
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No classical valence bond structure for C6H6 correctly explains the existence of only one monosubstituted benzene and three disubstituted benzenes. Kekulé (1890) proposed that the following were in rapid equilibrium: 1,3,5-cyclohexatriene
Br
BrBr
Br
Br
Br
Br
Br
Br
Br
Br
Br
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Interesting features of benzeneInteresting features of benzene
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•If benzene is 1,3,5-cyclohexatriene as Kekulé proposed, what should its chemistry be? Alkenes, dienes, cyclcoalkenes, etc. typically give addition reactions with electrophiles.
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KMnO4 oxidation no reaction
Br2/CCl4 addition no reaction
HI addition no reaction
H2/Ni reduction no reaction
Reagent Cyclohexene Benzene
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Benzene + 3 H2, Ni, room temp. NR
Benzene + 3 H2, Ni, 200oC, 1500 psi cyclohexane
Although highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead.
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Reactions of benzene:
1. Nitration
C6H6 + HNO3, H2SO4 C6H5NO2 + H2O
2. Sulfonation
C6H6 + H2SO4, SO3 C6H5SO3H + H2O
3. Halogenation
C6H6 + X2, Fe C6H5X + HX
4. Freidel-Crafts alkylation
C6H6 + RX, AlCl3 C6H5R + HX
substitutions04/21/23 Dr Seemal Jelani Chem-160 18
Heats of hydrogenation and combustion are far lower than they should be.
cyclohexene + H2, Ni cyclohexane + 28.6 Kcal/mole
1,3-cyclohexadiene + 2 H2, Ni cyclohexane + 55.4 Kcal/mole
(predicted value = 2 X 28.6 = 57.2 Kcal/mole)
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benzene + 3 H2, Ni, heat, pressure
cyclohexane + 49.8 Kcal/mole
(predicted value = 3 X 28.6 = 85.8 Kcal/mole)
Heat of hydrogenation for benzene is 36 Kcal/mole lower than predicted!
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e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
C—C single bonds 1.50 Å
C = C double bonds 1.34 Å
The bonds in benzene are all equal and 1.39 Å
but 1,3,5-cyclohexatriene has three double bonds and three single bonds!
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Facts about benzene:
a)Formula = C6H6
b)Isomer number:
one monosubstituted isomer C6H5Y known
three disubstituted isomers C6H4Y2 known
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c) Benzene resists addition reaction, undergoes
substitution reactions.
d) Heats of hydrogenation and combustion are far lower than they should be.
e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
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Hydrocarbons are open-chain and ring
compounds that react like open chain compounds:
Saturated: alkanes and cycloalkanes (typical reaction = substitution)
Unsaturated: alkenes, alkynes, dienes, cycloalkenes (typical reaction = addition).
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Aliphatic
Hydrocarbons are unsaturated ring
compounds that resist the typical addition reactions of aliphatic unsaturated compounds, instead undergoing substitution reactions
They are also much more stable than they should be.
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Aromatic
Nomenclature for benzene:Monosubstituted benzenes: Special names:
CH3 NH2 OH
CO2H SO3H
toluene aniline phenol
benzoic acid benzenesulfonic acid
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Br NO2 Cl
others as substituted benzenes
bromobenzene nitrobenzene chlorobenzene
Mercedes Benzene
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Disubsituted benzenes:
ortho- meta- para-
1,2- 1,3- 1,4-
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Br
Br
NO2
Cl
CH3
Br
o-dibromobenzene m-chloronitrobenzene p-bromotoluene
1,2-dibromobenzene 3-chloro-1-nitrobenzene 4-bromotoluene
Br
Br
If more than two groups on the ring, use numbers!
Br NH2
Br
Br
Br
1,2,4-tribromobenzene 2,4,6-tribromoaniline04/21/23 Dr Seemal Jelani Chem-160 29
napthalene anthracene phenanthrene
Some other aromatic hydrocarbons:
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NOTE:
H2C
H2CCH2
CH2
CH2
H2C
C6H12
cyclohexane
HC
HCCH
CH
CH
HC
C6H6
benzene04/21/23 Dr Seemal Jelani Chem-160 31