An aromatic hydrocarbon, also known as arene (or aryl hydrocarbon), is a hydrocarbon that has sigma bonds and delocalized pi electrons between carbon atoms to form rings. Benzene, C6H6, and other aromatic hydrocarbons have such distinct features from ordinary open-chain conjugated polyenes like 1,3,5-hexatriene that it is convenient to classify them as a new class of compounds known as arenes.
What is Aromatic Hydrocarbon (Arene)?
Aromatic hydrocarbons are defined as “unsaturated hydrocarbons with one or more planar six-carbon rings known as benzene rings to which hydrogen atoms are attached.”
The name “aromatic” originally referred to their pleasant odors (for example, cinnamon bark, wintergreen leaves, vanilla beans, and anise seeds), but it now refers to a specific type of delocalized bonding. Aromatic hydrocarbons (also known as arenes or aryl hydrocarbons) are hydrocarbons that include sigma bonds and delocalized π electrons between carbon atoms to form rings.
Classification and Nomenclature of Arenes
A wide range of substituted benzenes exists, with one or more of the ring’s hydrogen atoms replaced by other atoms or groups. Almost all of these compounds retain the particular stability associated with the benzene nucleus. Following are a few examples of “benzenoid” hydrocarbons, with the hydrocarbon substituents including alkyl, alkenyl, alkynyl, and aryl groups.
They are divided into three groups based on the number of fused benzene rings. There are three types of benzene ring fusion: monocyclic, polycyclic, and linear.
Monocyclic Aromatic Hydrocarbon
- Monosubstituted benzenes are named in the same way as other hydrocarbons are, with -benzene as the parent name. C6H5Br denotes bromobenzene, C6H5NO2 denotes nitrobenzene, and C6H5CH2CH2CH3 denotes propylbenzene.
- Alkyl-substituted benzenes are termed differently depending on the size of the alkyl group. If the alkyl substituent has fewer carbons than the ring, the arene is referred to as an alkyl-substituted benzene. A compound is known as a phenyl-substituted alkane if the alkyl substituent is greater than the ring (seven or more carbons). When the benzene ring is regarded a substituent, the name phenyl, pronounced fen-nil and frequently abbreviated as Ph or ᶲ (Greek phi), is used for the]C6H5 unit. Furthermore, the C6H5CH2] group is referred to as benzyl.
- The prefixes ortho (o), meta (m), and para (p) are used to name disubstituted benzenes. The two substituents in an ortho-disubstituted benzene are in a 1,2 relationship on the ring, whereas those in a meta-disubstituted benzene are in a 1,3 relationship and those in a para-disubstituted benzene are in a 1,4 relationship.
Polycyclic Aromatic hydrocarbon
There is a large number of polycyclic aromatic compounds containing benzene rings in common ortho locations. Polynuclear aromatic hydrocarbons are the name given to the parent chemicals of this class.
Linear fusion of Aromatic Hydrocarbon
Naphthalene, anthracene, and phenanthrene are three notable examples. The rings in anthracene are joined linearly, whereas in phenanthrene they are connected angularly.
Physical Properties of Arenes [Aromatic Compounds]
- Many arenes derivatives have pleasant scents, which is why they are commonly referred to as aromatic hydrocarbons. However, arenes are often poisonous, and inhaling their vapors should be avoided. In contrast to alkanes and alkenes, which burn with a bluish flame and leave minimal carbon residue, volatile arenes are very combustible and burn with a brilliant, sooty flame.
- They are less dense than water and very insoluble. Boiling points are shown to increase fairly frequently with molecular weight, although there is little link between melting point and molecular weight.
- The melting point of a chemical is greatly dependent on its symmetry; benzene melts 100° higher than toluene, while the more symmetrical p-xylene has a higher melting point than either the o- or m-isomer.
- These chemicals have aromaticity (additional stability provided by resonance).
- The carbon-to-hydrogen atom ratio in these compounds is relatively high.
- Electrophilic substitutions and nucleophilic aromatic substitutions are common in these compounds.
Synthesis of Aromatic Hydrocarbons
With the use of petroleum or coal tar, the compounds are manufactured commercially. However, we can also produce similar compounds in the lab using a few specific chemical reactions. The parent compound of all other aromatic hydrocarbons is thought to be benzene. If we can produce benzene or some of its derivatives, we will obtain an aromatic hydrocarbon. Several methods for the manufacture of aromatic compounds are discussed below:
Cyclic Polymerization of Alkyne
In the case of alkynes, cyclic polymerization is one of the most important reactions. It was the original way to create benzene. To create benzene, acetylene is pushed through a tube that is 873K hot and red hot. It is ethyne’s cyclic polymerization. Similar polymerization of other alkynes can produce other alkylbenzenes.
Decarboxylation of Aromatic Acids
Aromatic hydrocarbons are produced when benzoic acid or a derivative is heated with soda lime or CaO.
Reduction of Phenol
Phenol vapours are reduced by passing through highly heated zinc dust. Benzene is created as a result of this reaction.
Reactions of Aromatic Hydrocarbons
Most benzene reactions keep the highly stable delocalised ring of bonding electrons intact. This is accomplished by exchanging an atom or group of atoms for one or more hydrogen atoms linked to the benzene ring. An electrophile is usually the first to attack, drawn to the high electron density surrounding the benzene ring.
Electrophilic Aromatic Substitution Reactions
An atom linked to an aromatic ring is replaced with an electrophile in electrophilic aromatic substitution processes. Aromatic nitrations, aromatic sulfonation, and Friedel-Crafts reactions are examples of such reactions.
It is important to highlight that in electrophilic aromatic substitutions, the aromaticity of the aromatic component is conserved. As a result, aromatic rings and iodine, bromine, or chlorine can be employed in these processes to produce aryl halides.
Some electrophilic substitution reactions of arenes are given below :
Haloarenes are formed when arenes react with halogens in the presence of a Lewis acid, such as anhydrous FeCl3, FeBr3, or AlCl3. The catalysts in these reactions are known as ‘halogen carriers’.
We can think of the electrophile as a Br+ cation.
The Br+ cation and the ‘electron-rich’ benzene ring are attracted to each other, as the mechanism below shows.
Nitrobenzene is produced by heating benzene with a mixture of concentrated nitric acid and concentrated sulfuric acid. The introduction of the NO2 group into a molecule is referred to as nitration. The electrophile in this process is the NO2+ ion, also known as the nitronium ion (or nitryl cation). This is created by combining concentrated nitric acid with concentrated sulfuric acid:
This ‘nitrating mixture’ is refluxed with benzene at about 55 °C to make nitrobenzene
Step 1: The electrophile, NO2+, is attracted to the high electron density of the benzene bonding system. In NO2+, a pair of electrons from the benzene ring are transferred to the nitrogen atom, forming a new covalent bond. The delocalized ring of electrons in benzene is shattered at this location. There are now four bonding electrons and a positive charge distributed over five carbon atoms.
Step 2: However, when the C-H bond breaks heterolytically in step 2, the whole delocalised ring is recovered. Both electrons from the C-H covalent connection enter the bonding system of nitrobenzene, and hydrogen exist as an H+ ion. Because there are now six electrons dispersed throughout the six carbon atoms, the benzene ring’s chemical stability is preserved in this substitution reaction.
Nitrobenzene is further nitrated to provide 1,3-dinitrobenzene and 1,3,5-trinitrobenzene. The NO2 group is electron-withdrawing, in contrast to the electron-donating methyl group in methylbenzene (which activates the 2 and 4 positions in the benzene ring). This sort of group (which contains COOH) deactivates the benzene ring positions 2 and 4. As a result, when a nitro group is linked to the benzene ring, additional substitution occurs at the 3 and 5 positions.
Benzene sulphonic acid is formed when benzene is burned with fuming sulphuric acid.
Sometimes creating a new product requires scientists to alter the structure of an arene. Detergent production and the production of the chemicals required to create polymers, such as poly(phenylethene), sometimes known as polystyrene, are two examples. They can replace a hydrogen in the benzene ring with an alkyl group, such as a methyl (-CH3) or an ethyl (-C2H5) group, using a Friedel-Crafts reaction.
A side-chain is added to a benzene ring as a result of Friedel-Crafts processes.
Additionally, they are known as acylation or alkylation reactions.
Step 1: The reactions involve an electrophile carrying a positive charge on a carbon atom, or a carbocation, attacking the benzene ring. The electrophile is often produced by reacting a halogenoalkane with a catalyst made of aluminum chloride. The carbocation electrophile is produced as a result:
Step 2: The carbocation electrophile then attacks the benzene ring.
Step 3: The final stage involves regeneration of the aluminum chloride catalyst.
Addition Reaction of Aromatic Hydrocarbon
Synthesis of Cyclohexane
Cyclohexane forms when benzene is hydrogenated at high pressures and/or temperatures while being in the presence of nickel.
Benzene hexachloride (gammaxane) formation
Benzene hexachloride (C6H6Cl6) or gammaxane is formed when three chlorine molecules combine with benzene in the presence of ultraviolet light.
The coupling of two fragments with radical nature occurs in these kinds of reactions with the aid of a metal catalyst. The following types of bonds can form during coupling reactions involving aromatic hydrocarbons. The coupling processes of arenes can result in the formation of carbon-carbon bonds and other compounds like vinyl and alkyl arenes.
In these reactions, carbon-oxygen bonds can form, resulting in the synthesis of aryloxy molecules. In coupling reactions, carbon-nitrogen bonds can form, producing substances like aniline.
The arylation of perfluorobenzenes, as shown below, is an example of a coupling reaction using aromatic hydrocarbons.
Oxidation of the side-chain in arenes
The reactivity of an alkylarene’s alkyl side-chain can be affected by the presence of the benzene ring, such as in methylbenzene. Alkanes, for instance, often are not oxidized by a chemical oxidizing agent like potassium manganate(VII). In contrast, the alkane sidechain of alkylarenes is oxidized to create a carboxylic acid. For instance, when methylbenzene is refluxed with alkaline potassium manganate(VII), followed by acidification with diluted sulfuric acid or another potent oxidizer such acidified potassium dichromate(VI), benzoic acid is produced:
Uses of Aromatic Hydrocarbons
- Aromatic compounds are effective non-polar solvents due to their low reactivity and other properties. Thus, it is a component of paints, gasoline, and other products as additives. Model glues contain a solvent called methylbenzene.
- Aromatic hydrocarbons, which are essential for photosynthesis, are also contained in the chlorophyll, the green pigment found in plants.
- Phenanthrene is an aryl hydrocarbon used in the production of medicines, dyes, and explosives.
- Trinitrotoluene, also known as TNT, is a crucial aromatic hydrocarbon that is frequently utilized in explosives.
- Aromatic hydrocarbons are widely used in the petrochemical and plastics industries.
- These aromatic hydrocarbons are also present in the amino acids and nucleic acids that make up the human body.
- The manufacturing of mothballs requires the use of naphthalene.
Video on Naming Aromatic Hydrocarbon
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