Structure of Benzene – History, Resonating Structures, Properties, Applications, Practice Problems and FAQ

Meet Jack Russel, my cute canine companion. I have to say that he is such a good sport and is always ready for games and fun. Additionally, he is really kind and amiable. He hops while he's playing, and when he's hungry, he signals you by standing up straight and sticking his tongue out. He is a devoted defender as well, so he sniffs and double-checks every dumb thing that comes his way. His day comes to a close with a restful nap.

The picture shows Jack as a unique person in all of his manifestations. He genuinely sums up the adoring little puppy that he is in all of his uniquely organised characteristics! Therefore, his different characteristics serve to illustrate who he is.

Benzene is an organic compound that is expressed through certain resonating structures. They are all a way of representing the true form of benzene and explain various characteristics and mannerisms that benzene exhibits. 

Without further delay, let's dive into understanding benzene and its structures in detail, right here!

TABLE OF CONTENTS

• Benzene – Definition
• Benzene – History and Discovery
• Benzene – Chemical Structure
• Resonance Structure of Benzene – Kekule Structure
• Dewar Benzene Structure
• Benzene – Characteristics
• Benzene – Physical Properties
• Benzene – Chemical Properties
• Benzene – Applications
• Benzene – Health Consequences
• Practice Problems
• Frequently Asked Questions – FAQ

Benzene – Definition

Benzene is the simplest aromatic hydrocarbon. Although benzene is a material that is formed naturally by volcanoes and forest fires and is found in many plants and animals, it is also a significant industrial chemical that is manufactured from coal and oil.

Benzene is a colourless liquid with a distinctive odour. Its main use is to make polystyrene. As a carcinogen and a very poisonous substance, exposure to it has been linked to leukaemia.

One of the fundamental petrochemicals, benzene is a component of crude oil that occurs naturally. Benzene is categorised as an aromatic hydrocarbon because of the continuous cyclic pi bonds that exist between the carbon atoms. is a common abbreviation. The stench near gas stations is partly caused by benzene, a colourless, extremely combustible chemical that has a pleasant scent.

It is largely utilised as a precursor in the production of compounds with more complicated structures, such as cumene and ethylbenzene, which are manufactured in yearly amounts of billions of kilos. Benzene is a significant industrial chemical, but its toxicity prevents it from being used extensively in consumer products.

Benzene – History and Discovery

The term "benzene" is derived from "gum benzoin" which is an aromatic resin produced in Southeast Asia. Since the early 16th century, it is known to European perfumers and pharmacists. By sublimating benzoin, an acidic substance known as "flowers of benzoin" or benzoic acid was produced. Thus, the hydrocarbon produced by benzoic acid came to be known as benzene, benzol, or benzin. 

Michael Faraday discovered benzene in compressed illuminating lighting gas in the year 1825. Eilhardt Mitscherlich, a German scientist, heated benzoic acid with lime to generate benzene in the year 1834. Later, benzene was extracted from coal tar in the year 1845 by the German scientist A.W. von Hofmann.

Friedrich August Kekulé, a German chemist who was teaching at the time in Francophone, Belgium, suggested that the structure was made up of a ring consisting of six carbon atoms with alternate single and double bonds in a paper he published in French in 1865.

Kekule’s symmetrical ring could explain the ratio of atoms in benzene. Using X-ray diffraction techniques, Kathleen Lonsdale, a crystallographer, eventually verified the cyclic nature of benzene in 1929.

Only a minor portion of benzene is generated nowadays from coal; the majority comes from the petrochemical sector. On Mars, benzene molecules have been found.

Benzene – Chemical Structure

Benzene has a molar mass of roughly . Benzene has the molecular formula and is an organic chemical. It comprises six carbon atoms and six hydrogen atoms. The benzene molecule is made up of six carbon atoms united in a planar ring, with one hydrogen atom bound to each carbon atom. Benzene is categorised as a hydrocarbon since it solely has carbon and hydrogen atoms.

Three carbon-carbon double bonds () and a six carbon-carbon single bonded ring (), symbolised by a hexagon, are present in the structure. The carbon atoms are represented by a corner that is bonded to other atoms. One carbon atom is bonded to one hydrogen atom () each.

The structure of benzene is conjugated as double bonds are separated by single bonds. Inside the hexagon that represents six electrons, a circle is used as an alternate sign.

As opposed to a bond, a bond is shorter. bonds in benzene have lengths that are longer than double bonds () but less than single bonds (). All six of the carbon-carbon bonds in benzene have the same length, which is in the middle of a single bond and a double bond's ranges. It is a regular hexagon and is planar.

The single bonds, also known as bonds, are produced by the overlap of hybridised atomic orbitals in the plane between the carbon nuclei. A bond and a bond make up the double bonds. The atomic above and below the plane of the ring overlap laterally to generate the bonds.

Within the ring, the circular electron density is evenly distributed through a above and below the ring. This is explained as resonance. 

Resonance Structure of Benzene – Kekule Structure

The electrons required for bonding are uniformly distributed among the six carbon atoms, which results in the intermediate distance between a () single bond and a () double bond. This is because of the delocalisation of electrons on all the six carbon atoms. The molecule of benzene is planar. While the valence bond description entails a superposition of resonance structures, the molecular orbital description entails the development of three delocalized orbitals covering all six carbon atoms.

Around the ring structure, the electrons are delocalized. This stabilises the benzene molecule by resonance. Each carbon-carbon link that results from this delocalization has a bond order of 1.5, indicating that they are stronger than typical bonds. A circle inside the hexagonal ring is used to describe the delocalization of electrons in the resonance hybrid of benzene.

Kekule first proposed two cyclohexatriene structures for benzene. When these structures are combined, they form the overall structure as contributing structures. In the hybrid structure given below, the hexagon stands for three double bonds and symbolises six electrons in a cluster of three molecular orbitals with a nodal plane in the molecule plane.

In addition to being characterised as a compound consisting of hydrogen and carbon atoms with alternate double bonds in the shape of the ring, the formulae and structure of benzene show it to be a pure aromatic hydrocarbon. As per Huckel’s rule of aromaticity, benzene is cyclic, conjugated, planar and has electrons (where ).

In the cyclic molecule of benzene (cyclohexatriene), which has three alternating double bonds, the single bond would have a length of and the double bond would have a length of . However, the length of each bond in a molecule of benzene is , showing that it is an average of the lengths of the single and the double bonds. This is owing to uniform electron distribution across the ring through the phenomenon of resonance. 

Dewar Benzene Structure

, often known as Dewar benzene or dewar benzene, is a bicyclic isomer of benzene having the chemical formula . Dewar benzene is termed so, in honour of James Dewar, who listed this structure in a list of potential structures in 1867.

It is also said falsely that Dewar claimed that his postulated structure is the actual structure of benzene. Dewar really only listed the structure as one of seven potential isomers of benzene and thought that his research on benzene only supported Kekulé's (correct) structure.

Dewar benzene structure has conjoined cyclobutene rings that create an acute angle. Keep in mind that Dewar structures are not benzene resonating structures. It is as depicted below.

Dewar benzene is not flat like benzene because the rings' joining carbons are connected to four atoms rather than three. The two cyclobutene rings form an angle where they are cis-fused to one another, and these carbons have a tendency toward tetrahedral geometry. The substance has a chemical half-life of two days and reverts to benzene despite having a significant amount of strain energy. Due to symmetry considerations based on orbital symmetry, this thermal conversion is relatively sluggish.

Benzene – Characteristics

• Petroleum is now used to extract huge amounts of benzene.
• Benzene has a planar regular hexagonal structure.
• The chemical compound benzene has the formula and is colourless with a pungent smell.
• Six carbon atoms make up the circular ring of benzene, which is joined by alternating single and double bonds.
• Sigma bonds hold one hydrogen atom per carbon atom.
• Benzene and its derivatives belong to the significant chemical class of aromatics.
• Benzene is used as a precursor for medicines, synthetic rubber, oil, plastics, and colours.
• Resonance causes the hydrogenation of benzene to happen much more slowly than it does for other organic compounds containing carbon-carbon double bonds. As a result, benzene is more challenging to oxidise than alkenes.
• The majority of reactions involving benzene fall under the category of electrophilic aromatic substitution, which preserves the ring while substituting one of the hydrogens attached to it.
• Because of how adaptable these reactions are, they are commonly used to produce benzene derivatives.

Benzene – Physical Properties

• Benzene having the chemical formula , appears to be a colourless liquid with a pleasant fragrance.
• Benzene is insoluble in water but soluble in organic solvents.
• It burns with a smoky flame and it is flammable.
• The boiling point of benzene is
• The melting point of benzene is
• The molar mass of benzene is
• The density of benzene is comparatively lower than that of water and it is
• The resonance energy of benzene is

Benzene – Chemical Properties

• Benzene is a stable aromatic unsaturated hydrocarbon. Even though its molecular formula , indicates a high degree of unsaturation, it is not as reactive as alkenes or alkynes.
• It does not show the usual reactions of unsaturated hydrocarbons due to its extreme stability owing to its reasoning nature. 
• Benzene does not form any addition product with halogen acids and hypochlorous acid.
• It resists oxidation by alkaline (i.e., no decolourisation) and does not decolourise bromine solution. However, under special conditions, it shows some addition reactions. 
• In many respects, benzene behaves like a saturated hydrocarbon as it gives substitution reactions, i.e., hydrogen atoms are replaced by other atoms or groups (benzene is more reactive than alkanes).
• Benzene remains unaffected by alkalis like . 
• General reactions shown by benzene are addition reactions, electrophilic aromatic substitution reactions, and oxidation reactions. 

Benzene – Applications

Numerous industrial processes use benzene, including those that create lubricants, polymers, rubber, pigments, and synthetic fibres. However, due to its toxic and carcinogenic properties, benzene finds limited uses outside of the industry. The following is a list of the different uses for benzene.

• Metals were once degreased using benzene.
• Still today, phenol is produced using benzene.
• Dodecylbenzene, which is also used in detergents, and aniline, which is used to produce colours, are also made using benzene.
• It is used to make nylon fibres.
• Cumene, cyclohexane, nitrobenzene, ethylbenzene, and alkylbenzene are just a few of the numerous chemicals that may be produced using benzene as a starting material.

Benzene – Health Consequences

Solvents containing benzene cause the skin to become dry, itchy, cracked, and fissured when they come into contact with it.

Leukaemia and other diseases of the blood-forming organs, such as multiple myeloma, plasma cell cancer, as well as anaemia and low blood counts, have all been associated with benzene exposure.

It may also result in drowsiness, lung infections, or even bruising on the skin. The areas that create blood cells are particularly vulnerable to damage from benzene. The quantity and length of exposure determine the effects of benzene on human health.

Long-term exposure to high concentrations of benzene results in headaches, unconsciousness, nausea, convulsions, disorientation, and death.

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Practice Problems

Q 1. How many pi () electrons does benzene have?

a. 4
b. 2
c. 8
d. 6

Answer: D
Benzene has 3 and each bond has 2 electrons. Thus, the total number of electrons in benzene are 6. 

So, option D is the correct answer.

Q 2. Which is the simplest aromatic hydrocarbon among the following?

a. Ethyne
b. Acetylene
c. Benzene
d. Methane

Answer: C
Benzene is the simplest aromatic hydrocarbon. It is aromatic due to its cyclic, planar and conjugated nature along with the presence of 6-pi electrons (follows 4n+2 Huckle’s rule). 

So, option C is the correct answer.

Q 3. Find the number of sigma and pi bonds in benzene.

Answer: There are three single bonds in benzene. Three bonds and six bonds. All single bonds correspond to and all double bonds correspond to one and one bond. So, there are a total of 12 sigma () and 3 pi () bonds.

Q 4. What kind of reactions are undergone by benzene?

Answer: Benzene is an unsaturated and electron-rich compound. Hence, it undergoes electrophilic substitution reactions, whereby electrophilic species attach themselves to the benzene ring by substituting a hydrogen atom from any of the bonds. 

Example: Bromination of benzene.

Frequently Asked Questions – FAQ

Q 1. Do food items contain benzene?
Answer: Beverages that are citrus based, have greater chances of presence of benzene. Benzene was found in fruit juices, pickles, lime juices, mayonnaise, and salad dressing. Products that have been smoked or canned have more benzene. Benzene was present in food-grade aromas. One of the ways that food gets contaminated with benzene is through food packaging.

Q 2. Is benzene polar or nonpolar?
Answer: Due to the many nonpolar carbon-hydrogen bonds that are evenly spaced out around the molecular ring, benzene is a nonpolar chemical. The bond dipoles in this symmetrical molecule cancel out completely.

Q 3. Is benzene heavier than air?
Answer: At room temperature, the chemical benzene is a colourless or light yellow liquid. Benzene swiftly disappears into the atmosphere. Because its vapour is denser than air, it might settle in low-lying places.

Q 4. Mention some regular usable product made from benzene?
Answer: Plastics, resins, synthetic fibres, rubber lubricants, dyes, detergents, pharmaceuticals, and insecticides are all made with benzene. Volcanoes and forest fires naturally create benzene. Benzene may be discovered in houses in glues, adhesives, cleaning supplies, paint removers, cigarette smoke, and gasoline.

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