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Arenes - Organic Chemistry Tutor

The aromatic hydrocarbons also have the name arenes. They contain in their molecule one or more cycles made up from 6 carbon atoms. When the molecule is formed out of a single cycle, the hydrocarbons are mono nucleuses; when the molecule contains more than one cycle, the hydrocarbons are poly nucleuses. The simplest aromatic hydrocarbon, benzene, is compound out of just one such cycle; its formula is C6H6. The representation of benzene through a cycle of 6 carbon atoms with 3 double bounds was proposed by Kekule in 1865:

In some special conditions, benzene can be hydrogenated, the result being a cyclic hexane:




Under the influence of light, chlorine or bromine addition at the benzene’s molecule giving hexachlorocyclohexane: C6H6 + 3Cl2 C6H6Cl6. With ozone, the benzene gives a trizonide, which by decomposing (with water), passes in glycoxal. These addition reactions prove the following: benzene has a cycle of 6 carbon atoms; there are three double bounds in the cycle. Yet, the addition reactions at benzene take place only in special conditions, benzene usually giving substitution reactions, like:

With halogens: C6H6 + Cl2 C6H5Cl + HCl
With sulfuric acid: C6H6 + HO-SO3H C6H5-SO3H + H2O
With nitric acid: C6H6 + HO-NO2 C6H5-NO2 + H2O

The easy formation of the substitution products is a proof that benzene’s character is less unsaturated than the hydrocarbons with conjugated double bounds. Furthermore, benzene has a pronounced saturated character. Yet, this behavior does not correspond to Kekule’s structural formula, which says that three conjugated double bound should exist. Another critic brought to this formula is that she predicts the existence of for isomers than in reality.

If two hydrogen atoms in the benzene’s molecule are substituted with two bromine atoms, then, according tot Kekule’s rule, there should be two isomers containing two bromine atoms connected to two nearby carbon atoms (positions 1,2 and 1,6) at one the two carbon atoms being separated through a double bound, and at the other, through a simple bound. In reality, such isomers due exclusively to the position of the double bound are unknown. The symmetry of the benzenic cycle illustrates the existence of only one substituted product in the position 1,2 contrary to the representation in which benzene would have the structure 1,3,5-cyclohexatryene.

There are three isomers of the dibrominobenzene due to the substitution of the bromine at different positions of the cycle. They have very different boiling points. At one of this isomers (with boiling point = 1.8 Celsius degrees), the two bromine atoms are connected to the nearby carbon atoms; at the second isomer (with boiling point = -7 Celsius degrees), the bromine atoms are connected to two carbon atoms which are separated by a CH group (in which the hydrogen atom is not substituted); at the third isomer (with boiling point = 87 Celsius degrees), the bromine atoms are connected to two carbon atoms separated by two CH groups (in which the hydrogen atoms are not substituted). Therefore the positions 1,2 and 1,6 are equivalent, same as the positions 1,3 and 1,5:



Kekule tried to explain the unfit between the number of derivate substitute isomers of benzene in his formula and the existent ones, by evolving the hypothesis that in the molecules, the double bounds do not occupy steady position, but that they change their location with the simple bound, meaning they move, they “oscillate”.

Since the formula of Kekule does not express the equivalence between the C-C bounds in the benzene’s molecule and neither does it express the important property of benzene of giving privileged substitution reactions (and not addition reactions, as the double bounds in the formula indicate), some researchers started looking for explanations for the benzene’s structure which will also reverberate the characteristic properties of benzene.


Orto - Xylen
Ethyl benzene








The head of the series, at aromatic hydrocarbons is benzene (C6H6). The superior homologues of benzene, are: methylbenzene (also known as toluene), ethylbenzene, n-propylbenzene and isopropyl benzene. There can only be one monosubstituted derivate deferring to the fact that all the CH groups in the benzene must be equivalent.

The denomination of the arenes is made by adding at the substitutes name the ending “-benzene”. The disubstituted derivates of benzene are usually named by adding the prefixes “orto-” (“o-”), “para-” (“p-”) and “meta-” (“m-”). If benzene is substituted by two different radical, the numbering at the denomination of these is made in the alphabetical order.

Phenyl benzene - styrene




Obtaining methods

1. The alkynes trymerization


2. The catalytic re-formation of the n-alkanes

CH3-CH2-CH2-CH2-CH2-CH34H2 +

3. The alkalization of the arenes (substitution reaction) – the Friedel Crafts alkalization

Ar-H + R-X HX + Ar-R - X=Cl,Br,I

4. The dry distillation of the carbon tar

5. From petroleum

Chemical reactions

1. The hydrogenation (catalyser: Ni/Pt/Pd), energetic conditions, temperature


2. Halogenation (Cl2, Br2 – at light)

+3Cl2 hexaclorciclohexan (HCH)

3. The substitution reaction at the nucleus

Ar-H + XY Ar-X + HY

4. The alkylation reaction

Ar-H + R-X HX + Ar-R

5. The halogenation reaction (FeX3 - catalyser, X=Cl,Br,I)

Ar-H + Cl2 HCl + Ar-Cl (chlorobenzene)
Ar-H + Br2 HBr + Ar-Br (brominobenzene)

6. The nitration reaction (highly concentrated HNO3 and H2SO4 solutions)

Ar-H + HNO3 H2O +

7. The brimstone reaction

Ar-H + H2SO4 Ar-SO3H + H20 (this reaction is reversible)

8. Reactions at the lateral chain, in the benzoic position (cu Cl2, Br2, at light)

+Cl2 HCl

9. The oxidation reaction

a) Complete (combustion)
C6H6 + 15/2O2 6CO2 + 3H2 + Q

b) Incomplete
1. At the nucleus
+ 9/2 O2 2CO2 + H2O

2. At the lateral chain in the benzoic position
Ar-CH3 + 3/2O2 Ar-COOH + 2H2O

Physical properties

The mononucleosis aromatic hydrocarbons are colorless, with a sweet and pervading smell. They are insoluble in water, but mixable in any proportion with the organic solvents (alcohols, ethers). Their boiling points are within 80 and 300 Celsius degrees. Generally, the benzene’s homologues have similar properties with benzene, yet, if the chain is longer, the physical properties are getting closer to the acyclic hydrocarbon’s properties.

The infrared specters can give certain indications over the presence of the phenyl radicals also over the position of some substitutes in the cycle. Hence, the two bands with a 1600cm (-1) and 1500cm (-1) frequency are correlated with the elongation of the vibrations from the C-C bounds in the aromatic cycle, in so much as the nearby bands of 3030cm (-1) frequency are characteristic C-H aromatic bounds.

The aromatic compounds can give absorption specters with several bands in the ultraviolet region. Therefore, benzene and alkylbenzenes can give two characteristic bands of 200 nm – 260 nm, the first of great intensity corresponding to the excitation of an electron pi from a conjugated system in an orbital pi*. This band is magnified and moved to bigger wave longueurs, when in the conjugated system, the hydrogens in the cycle are substitute by unsaturated groups. Hence, the styrene has a characteristic absorption band in the ultraviolet at 244 nm. Similar effects are caused when the substitute in the benzoic nucleus has pairs of non participating electrons (at the conjugation of the benzoic cycle). For example, phenol has210 nm band; aniline (C6H5NH2) has a 230 nm band. The second band in the benzene’s specter is of a lower intensity; also, she is influenced b the substitutes in the cycle.

It was ascertained that a present substitute in the benzoic nucleus influences not only the reactivity of the nucleus towards the reactant, but also determines the position where the attack is carries on. Therefore we can distinguish 2 types of substitutes:

First grade substitutes: attached to an aromatic nucleus, it orientates the substitution in the orto- and para- positions. This way 2 isomers are formed: orto and para. The first grade substitutes only have simple bounds from the atom connected directly to the nucleus.

Second grade substitutes: attached to an aromatic nucleus, it orientates the substitution in the meta position. The second grade substitutes have double and triple bounds from the atom connected directly to the nucleus.


» benzene is the aromatic hydrocarbon which is produced in the highest quantity on global plan
» benzene and some aromatic hydrocarbons are cancerous substances; the poly nucleuses aromatic hydrocarbons with condensed nucleuses applied in small quantities on the skin causes cancer in approximately a month. The majorities of these hydrocarbons are found in the engine combustion gases, in the cigar’s smoke, but also appear at the cremation of the wastages, in industry arsons, or at the preparation of fried meat.

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