Aromatic Hydrocarbons || What is an example of an aromatic hydrocarbon?

Benzene

Aromatic hydrocarbons are carbon and hydrogen compounds in the molecule of which there is a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologs – products of substitution of one or more hydrogen atoms in a benzene molecule for hydrocarbon residues.

The structure of the benzene molecule

The first aromatic compound – benzene – was discovered in 1825 by M. Faraday. Its molecular formula, C6H6, was established. If we compare its composition with the composition of the saturated hydrocarbon containing the same number of carbon atoms – hexane (C6H14).

We can see that benzene contains eight less hydrogen atoms. As is known, the appearance of multiple bonds and cycles leads to a decrease in the number of hydrogen atoms in a hydrocarbon molecule. In 1865, F. Kekule proposed its structural formula as cyclohexantriene-1,3,5.

Thus, the molecule corresponding to the Kekule formula contains double bonds, therefore, benzene must have an unsaturated character, i.e., it is easy to enter into the addition reactions: hydrogenation, bromination, hydration, etc.

However, the data of numerous experiments showed that benzene enters the addition reaction only under harsh conditions (at high temperatures and lighting), is resistant to oxidation. The most characteristic for him are substitution reactions, therefore, benzene is closer in nature to saturated hydrocarbons.

Trying to explain these discrepancies, many scientists have proposed various options for the structure of benzene. The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. In fact, carbon-carbon bonds in benzene are equivalent, and their properties are not similar to the properties of either single or double bonds.

Currently, benzene is denoted either by the Kekule formula or by the hexagon in which the circle is depicted.

What is the specific structure of Benzene?

Based on the data of studies and calculations, it was concluded that all six carbon atoms are in the state of sp2 hybridization and lie in the same plane. Non-hybridized p-orbitals of carbon atoms constituting double bonds (Kekule formula) are perpendicular to the plane of the ring and parallel to each other.

They overlap with each other, forming a single π-system. Thus, the system of alternating double bonds depicted in the Kekule formula is a cyclic system of conjugated, overlapping π-bonds. This system consists of two toroidal (donut-like) regions of electron density lying on both sides of the benzene ring. So, to represent benzene in the form of a regular hexagon with a circle in the center (π-system) is more logical than in the form of cyclohexantriene-1,3,5.

The American scientist L. Pauling proposed to represent benzene in the form of two boundary structures that differ in the distribution of electron density and are constantly passing into each other:

that is, to consider it as an intermediate compound, “averaging” of two structures.

Measurements of bond lengths confirm this assumption. It was found that all C – C bonds in benzene have the same length (0.139 nm). They are somewhat shorter than single C – C bonds (0.154 nm) and longer than double (0.132 nm).

There are also compounds whose molecules contain several cyclic structures, for example:

Isomerism and the nomenclature of aromatic hydrocarbons

The homology of benzene is characterized by the isomerism of the position of several substituents. The simplest homolog of benzene – toluene (methylbenzene) – does not have such isomers, the following homolog is presented in the form of four isomers:

The name of the aromatic hydrocarbon with small substituents is based on the word benzene. The atoms in the aromatic ring are numbered, starting from the senior substituent to the youngest:

If the substituents are the same, then the numbering is carried out along the shortest path: for example, the substance:

According to the old nomenclature, positions 2 and 6 are called ortho positions, 4 – para-, 3 and 5 – meta-positions.

Physical properties of aromatic hydrocarbons

Benzene and its simplest homologs under ordinary conditions are very toxic liquids with a characteristic unpleasant odor. They are poorly soluble in water, but good in organic solvents.

Substitution Reactions. Aromatic hydrocarbons enter into substitution reactions.

  1. Bromination: When reacting with bromine in the presence of a catalyst, iron (III) bromide, one of the hydrogen atoms in the benzene ring can be replaced by a bromine atom:
  2. Nitration of benzene and its homologs: When aromatic hydrocarbon interacts with nitric acid in the presence of sulfuric (a mixture of sulfuric and nitric acids is called a nitrating mixture), a hydrogen atom is replaced by a nitro group – NO2:

Reduction of nitrobenzene gives aniline, a substance that is used to produce aniline dyes:

This reaction is named after the Russian chemist Zinin.

Addition reactions. Aromatic compounds can also enter into reactions of addition to the benzene ring. In this case, cyclohexane and its derivatives are formed.

Hydrogenation: The catalytic hydrogenation of benzene proceeds at a higher temperature than the hydrogenation of alkenes:

Chlorination: The reaction takes place under ultraviolet light and is free radical:

Benzene Homologues

The composition of their molecules corresponds to the formula CnH2n-6. The nearest homologues of benzene:

All benzene homologues following toluene have isomers. Isomerism can be related both to the number and structure of the substituent (1, 2), and to the position of the substituent in the benzene ring (2, 3, 4). Compounds of the general formula C8H10.

According to the old nomenclature used to indicate the relative location of two identical or different substituents in the benzene ring, the prefixes are ortho- (abbreviated o-) – the substituents are located at adjacent carbon atoms, meta- (m-) – through one carbon atom and para- ( n-) – substitutes against each other.

The first members of the homologous series of benzene are liquids with a specific odor. They are lighter than water. They are good solvents. Homologs of benzene enter into substitution reactions:

Bromination:

Nitration:

Toluene is oxidized by permanganate when heated:

Sanjay Bhandari

Hello Friends, My name is Sanjay Bhandari. I am a chemistry Teacher.

Leave a Reply