Benzene
Benzene is the first aromatic
compounds and its molecule consists of six carbon atoms linked together in the
form of a regular hexagonal ring and each carbon atom is connected to a
hydrogen atom and double bonds and single carbon atoms alternate between carbon
atoms (composition I). Or each angle in the hexagon symbolizes a carbon atom
with a sphere combines with hydrogen and rotates the double and single bonds
(II), and can compensate for the rotation of the double and a single by a
circle inside the loop installation(III).
Resonance in benzene
In 1865, the scientific Kekule
proposed the two formulas (IV) or (V) for benzene as follows:
A diagram of how
carbon atoms bonded in a benzene molecule.
It is clear that the carbon atoms
in the formulas (IV and V) occupy similar places but differ in the places
assigned to the double bonds. The exchange of these links sites is called
resonance (Ringing).
That benzene is actually not a
reciprocal state between the two states of resonance IV and V are not part of
the molecules in the form of resonance and another part of the molecules in
the form of the other resonance.
But the real form of benzene is
modified for the two shapes is called the resonance hybrid. Where we observe
it’s in place there is a double bond in one of the shapes, it will be single in
other shape. When the average is taken, we get a hybrid form of resonance
containing of six carbon -carbon bonds, which are identical and of equal length
as the average case between the length of the single bond and the length of the
double bond.
Therefore, the shape of benzene is
usually painted in the form of a hexagonal ring, it has an inner loop (VI and
VII composition) rather than in the form of successive double and double aces
(IV and V composition) because it de- scribes the situation is more accurate.
However, drawing in the form of double bonds and individual successive is
better in the case of monitoring the movement of electrons as in reaction
mechanism state.
In brief, benzene is a molecular
formula C6H6 containing six identical carbon atoms and six hydrogen atoms. The
C-C bonds have equal lengths and it is in a median case between the length of
the single bond and the length of the double bond.
Special characteristics of benzene
1. Stability
It is a stable compound as indicated by its reactive proper- ties and relative resistance to chemical changes. That most unsaturated compounds tend to engage in addition reactions in which binary or triple bonds are saturated into individual bonds.
Note that
cyclohexane reacts easily with the bromine dissolved in carbon tetrachloride,
forming 1,2-Dibromocyclohexane, whereas benzene does not fully react under the
same conditions.
In order for benzene to react with
bromine, a catalyst (FeBr3) must be used to enter a substitution reaction (sub-
stitution) and not an addition interaction where the loop remains preserved in
its shape and this is evidence on the stability of benzene.
2. Resonance Energy
To understand the energy of the
resonance, we explain the following:
When a chemical reaction occurs, a
change in thermal energy. For example, cyclohexane hydrogenation is a heatemitting
reaction, releasing 120 kJ energy per mole of cyclohexane.
At the hydrogenation of 1,3 cyclohexane (cyclohexane containing two π-bonds) we expect the amount of energy to be released 240 kJ per mole, twice the liberated energy at the hydrogenation of cyclohexene because there is twice the number of double bonds in the cyclohexene.The real energy measured for this reaction was 232 kJ Per mole, it is very close to expected.
According to the same logic, we
expect that the energy released by hydrogenation of gasoline is three times the
liberated energy at hydrogenation of cyclohexane, ie 360 kJ per mole. The
actual energy of this has been found. The reaction is 208 kJ per mole and is
very different to our expectation.
The difference between the
expected energy value and the real energy value is 152 kJ per mole and this
amount of energy is called a resonance energy. This means that benzene contains
152 kJ per mole of energy is less than expected. So that gasoline is more
stable than expected by 152 kJ per mole.
3. Carbon-Carbon Bonds Length
Physical measurements showed that the lengths of car- bon-carbon bonds in benzene are equal and are of average length between the length of a single C-C and double C = C.
Benzene Derivatives Names
There are some systems in the
label as follows:
1. Monocompensation Components:
These compounds are called as
derivatives of benzene such as:
2. Dual-compensation Compounds:
When there are two groups on the
benzene ring, it is not enough to mention the names of the two groups only, but
they must be located on the benzene ring. Where the carbon atoms are numbered
for the benzene ring and the two groups take the smallest figures, for example,
1,2 Di-bromobenzene is:
The terms ortho may be used to
denote the site 2 and (meta) to denote site 3 or (para) to denote on site 4 for
the main compensator
If the two groups are different,
they are mentioned in the nomenclature according to the alphabets one after the
other and followed by the word benzene with its location at the beginning of
the name or called the compound as a derivative
Where we notice in the second
label we considered that the molecule of chlorobenzene is a benzene monocompensation
by chlorine. In the third label it was considered that the benzene
mono-compensation in nitro group, so it is the basis in the designation.
Dual-compensation benzene compounds are named in this way and as in the
following examples:
3. Multiple Compensation Compounds:
When there are more than two
groups on the benzene ring in which case it should be numbered the locations of
these groups, where the name Ortho, Barra and Meta are unacceptable. These
groups are named as a derivative of benzene or as a derivative of common names
if any. If the associated group are large, the compound is named in the general
designation of hydrocarbons. It takes the longest hydrocarbon chain as the base
of the name and benzene is named as a compensating group, where it is called
Phenyl. If benzene is replaced by a group, it is called Aryl.
Preparation of benzene
A- In laboratory
Benzene is prepared in laboratory
by heating sodium benzoate with sodium hydroxide (NaOH) in a glass distillation
device where, benzene is obtained from the distillate at 80oC.
B- Industrially:
Benzene industrially is prepared
in several ways, the most important of which are:
1. From phenol:
Phenol is heated with Zn-dust in a
distillation apparatus and then benzene is obtained from an 80oC distillate.
2. From benzene sulfonic acid:
Benzene sulfonic acid is heated with dilute hydrochloric acid or dilute sulfuric acid to boiling point and under high pressure according to the following equation:
Physical properties of benzene
1. Colorless liquid, flammable, has a specific aromatic smell
and it is toxic.
2. Boiling point 80 °C and freezing point 5 °C.
3. Its density is less than the density of water and
didn’t mixed with it.
4. a good solvent for non-polar organic materials such as
grease, oils, resins and others.
Chemical properties of benzene
Benzene is a relatively stable
chemical compound com- pared to unsaturated compounds for the presence of resonance
phenomenon.
It is not affected by concentrated
bases or concentrated hydrochloric acid nor by strong oxidizing agents such as
potassium permanganate but suffers from a number of re- actions, such as
combustion and addition and substitution.
A- Combustion:
Benzene is burned with a bright
and flame due to the ratio of its high carbon content 92.3% and gives carbon
dioxide and water with heat liberation.
B- Additive reactions:
Chlorine is added to benzene in
the presence of light and there is a reaction accompanied by popping the
reactants leading to the formation Hexachlorocyclohexane as described in
equation A:
Benzene is also reduced to
hydrogen at high temperatures and under high pressure in the presence of a
catalyst such as platinum to cyclohexane, as in the following equation:
C- Substitution reactions
(compensation):
One of the most important
reactions of benzene and its derivatives is the possibility of replacing
(compensation) one of the hydro- gen atoms by atom or different group (e.g.
alkyl group R, nitro group NO2, sulfonic group
SO3H, halide X or acetyl group in the presence of an appropriate catalyst, which is help for reaction. Examples of substitution reactions is:
A- Halogenation
It is the process of replacing one
of the hydrogen atoms with a halogen atom (such as chlorine Cl2 or Br2) with a
catalyst such as ferric chloride FeCl3 or ferric bromide FeBr3 :
B- Sulfonation
It is the process
of replacing one of the hydrogen atoms with the sulfonic group SO3H, for
example benzene reacts with the dark concentrated sulfuric acid at room
tempera- ture to compose benzene sulfonic acid.
C- Nitration
Replace a hydrogen atom with a
nitro group NO2 where benzene reacts with a mixture of concentrated nitric and
sulfuric acid at a temperature of 45 °C to compose of nitrobenzene.
D- Friedle- Crafts reactions
1. Friedel-crafts Al-kylation:
It is the process of replacing one
hydrogen atom in the alkyl group (R) with an appropri- ate catalyst. When
benzene reacts with alkyl halide (R-X) in presence of catalyst like dry
aluminum chloride (AlCl3) lead to compose of alkyl benzene
2. Friedel-Kraft acetylation:
is the process of replacing an atom
hydrogen in the acetyl group. Benzene reacts with acetylcholine in the presence
of dry aluminum chloride to compose of acetyl benzene.
Mechanism of electrophilic substitution reactions
The substitution reactions above
are called electrophilic substitution reactions. Where there are electrons
detectors is also called electrophilic reagent (E). The electron elec-
trophilic reagent is a reagent that needs electrons and can be in the form of a
positive charge that can form covalent bonds with carbon atoms like X+, NO
+,R+, RC+O.
A double link is rich in electrons
therefore, it is a source of electrons for the reagent that electrophilic
reagents need it. Electron-rich reagent are called Neocluphilic reagents
(Nu)(see chemistry book for the fourth stage). By reviewing these reactions we
can symbolize all these reactions by the following general reaction:
These include three steps:
A-In the first step, the catalyst converts the reactant into an electron reagent (E+)
according to the equation:
B- In the second step, one of the double bonds of the re- agent benzene ring attacks the electron, forming a positive carbonium ion.
C- In the third step, a proton (positive hydrogen atom H+) is withdrawn by Nu-Catalyst. The
output from the first step forms the other output H-Nu while the benzene ring
re- turns to the aromatic state.
This mechanism, known as
electrophilic compensation mechanism is a complex subject which we believe is
above the level of student understanding at this stage. So we were limited to
describe general mechanical only to explain how the reaction occurs in a
briefly method, these mechanics are studied in detail in advanced stages.
References
1. K. J. Denniston c J. J.Topping c and R. L.Caretc
“General Organic and Biochemistry”c Mc-Graw- Hillc New York
(2004).
2. K.W. Whittenc R.E. Davis and L. M. Peckc “General Chemistry” 7th ed.
Holt Rinehart and Winstonc New York (2010).
3. Clayden, J.; Greeves, N. and Warren, S. (2012) Organic
Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.
4. Streitwieser, Andrew; Heathcock, Clayton H.; Kosower,
Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4:
Medtech (Scientific International, reprint of revised 4th edition, Macmillan,
1998). ISBN 978-93-85998-89-8.
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