Aromaticity hydrocarbons aliphatic aromatic alkanes alkenes alkynes
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Aromaticity
hydrocarbons
aliphatic aromatic
alkanes alkenes alkynes
Aliphatic compounds: open-chain compounds and ring compounds that are chemically similar to open-chain compounds. Alkanes, alkenes, alkynes, dienes, alicyclics, etc.
Aromatic compounds: unsaturated ring compounds that are far more stable than they should be and resist the addition reactions typical of unsaturated aliphatic compounds. Benzene and related compounds.
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 known
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.
a) 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
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=CHCCH
CH2
H2C CH2
=C=C=CH2
=C=CH2
CH2 CH2
CH2
=CH2HC2=
=CH2
Which of these structures is benzene?
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
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 +
CH2
H2C CH2
=C=C=CH2
=C=CH2
CH2 CH2
CH2
=CH2HC2=
one possible
two
three +
two +
two
=CH2
three one
two
CH3CC-CCCH3
There are three disubstituted benzenes of any type: three dibromobenzenes C6H4Br2, etc.
CH2
CH2 CH2two possible
four
four
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:
Br
BrBr
Br
Br
Br
Br
Br
Br
Br
Br
Br
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.
But benzene doesn’t undergo the reactions typical of unsaturated hydrocarbons!
KMnO4 oxidation no reaction
Br2/CCl4 addition no reaction
HI addition no reaction
H2/Ni reduction no reaction
Reagent Cyclohexene Benzene
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.
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
substitutions
d) 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)
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!
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!
Resonance!We can draw more than one classic structure that differ only in where the electrons are. The two structures are of the same energy, so resonance is important. The molecule cannot be adequately represented by classic structures but must be thought of as a hybrid of the contributing structures. Additionally, the hybrid is more stable than any of the contributing structures (resonance stabilization energy).
Facts about benzene:
a) Formula = C6H6
b) Isomer number:
one monosubstituted isomer C6H5Y known
three disubstituted isomers C6H4Y2 known
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.
Aliphatic 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).
Aromatic 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.
Can we predict which compounds will be aliphatic and which ones will be aromatic like benzene? Yes.
In order to be aromatic, the compound must be:
1) cyclic with p-atomic orbitals on all members of the ring.
and
2) have 4i + 2 electrons in the p-orbitals of the ring (where i = 0, 1, 2, 3, …).
[ eg. = 2 or 6 or 10 or 14 or 18 or 22 or 26… ]
pi-electrons sp2 hybridized carbonsp atomic orbitals
.
0 pi-electrons 1 pi-electron 2 pi-electrons
2 pi-electrons
annulenes: monocyclic compounds with the formula:
-[-CH=CH-]n-
HC
HC CH
CH
4 pi electrons 6 pi electrons 8 pi electrons
10 pi electrons 12 pi electrons
aromatic
aromatic
aromatic
aromatic
•
•::
4 e-
4 e- 5 e-
does not have p-orbitals on all atoms
8 e- 7 e-
6 e-
6 e-
Nomenclature for benzene:
monosubstituted benzenes:
Special names:
CH3 NH2 OH
CO2H SO3H
toluene aniline phenol
benzoic acid benzenesulfonic acid
Br NO2 Cl
others as substituted benzenes
bromobenzene nitrobenzene chlorobenzene
Mercedes Benzene
Disubsituted benzenes:
ortho- meta- para-
1,2- 1,3- 1,4-
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-tribromoaniline
MD
MD
MD
MD
MD
MD
DDS
DDS
DDS
DDS
orthodoxparadox metaphysicians
orthodontists periodontists
napthalene anthracene phenanthrene
Some other aromatic hydrocarbons:
NOTE:
H2C
H2CCH2
CH2
CH2
H2C
C6H12
cyclohexane
HC
HCCH
CH
CH
HC
C6H6
benzene
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