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Benzene: Preparation - Properties - Resonance - Derivatives

 

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).

 

Benzene: Preparation - Properties - Resonance - Derivatives


 

Resonance in benzene

In 1865, the scientific Kekule proposed the two formulas (IV) or (V) for benzene as follows:

Resonance in benzene


A diagram of how carbon atoms bonded in a benzene molecule.


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.

A diagram of how carbon atoms bonded in a benzene molecule.


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.



Special characteristics of benzene 1. Stability


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.


Special characteristics of benzene 1. Stability


 

 

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.


2. Resonance Energy


 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.


2. Resonance Energy

 

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.


2. Resonance Energy

 

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.

 

3. Carbon-Carbon Bonds Length

 

 

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:


1. Monocompensation Components:

 

 

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:


2. Dual-compensation Compounds:

 

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


2. Dual-compensation Compounds:

 

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


2. Dual-compensation Compounds:

 

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:


2. Dual-compensation Compounds:


 

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.


Benzene is prepared in laboratory by heating sodium benzoate

 

 

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.


Benzene  From phenol:

 

 

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:

 

2. From benzene sulfonic acid:

 

 

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.


Benzene is burned with a bright

 

 

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:


Chlorine is added to benzene in the presence of light

 

 

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:


Chlorine is added to benzene in the presence of light

 

 

 

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 :


halogenation of benzene

 

 

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.

 

Sulfonation of benzene


 

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.


Nitration of benzene


 

 

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


 

Friedel-crafts Al-kylation: of 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.


Friedel-Kraft acetylation of 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:


Mechanism of electrophilic substitution reactions

 

 

These include three steps:

A-In the first step, the catalyst converts the reactant into an electron reagent (E+) according to the equation:


Mechanism of electrophilic substitution reactions


 B- In the second step, one of the double bonds of the re- agent benzene ring attacks the electron, forming a positive carbonium ion.


Mechanism of electrophilic substitution reactions

 

 

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.


Mechanism of electrophilic substitution reactions


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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