Lesson Explainer: Properties of Benzene Chemistry

In this explainer, we will learn how to describe the properties of benzene, explain its structure, and name its derivatives.

Benzene is a cyclic hydrocarbon that contains six carbon atoms and six hydrogen atoms. Each carbon atom is bonded to two carbon atoms and a single hydrogen atom. Benzene has the molecular formula CH66 and it can be classed as an aromatic hydrocarbon or arene.

It is widely thought that Michael Faraday discovered benzene in 1825 by isolating it from an oily film that had been deposited from gas lamps. The structure of benzene was then slowly determined as scientists suggested different ways that it might be bonded.

Definition: Arene or Aromatic Hydrocarbon

Arenes are hydrocarbons that contain at least one benzene ring structural unit.

The German chemist Friedrich August Kekulé originally proposed that benzene had a rather simple and static structure. He claimed that benzene was a six-membered hydrocarbon ring that contained a cyclic arrangement of alternating carbon–carbon single bonds (CC) and double bonds (CC). Kekulé subsequently revised his hypothesis; in 1872, he stated that the structure of benzene was not static but that the carbon–carbon double bonds and single bonds rapidly exchanged positions. He proposed that there was a dynamic equilibrium between two essentially equivalent cyclic hydrocarbon ring structures.

Example 1: Identifying Which Scientist Helped Clarify the Structure of Benzene

Which chemist is often credited with proposing and publishing the correct structure of benzene?

  1. Lavoisier
  2. Kekulé
  3. Faraday
  4. Dalton
  5. Hückel

Answer

Kekulé was the first scientist to predict that benzene has an unusual resonance structure, and although his proposition was somewhat inaccurate, he is nonetheless still credited with proposing and publishing the correct structure of benzene. This statement can be used to determine that option B is the correct answer for this question.

This model proved to be too simplistic and it was later revised again by other scientists. Scientists now explain the unique physical and chemical properties of benzene in terms of delocalized electron density interactions. They state that benzene molecules have delocalized rings of electron density that form when the unhybridized p orbitals of carbon atoms interact with one another. Each carbon atom has four valence electrons and three of these electrons are used to create sigma (𝜎) bonds. The last remaining valence electrons interact with one another and create 𝜋 bonds that spread out over the whole hydrocarbon ring structure.

Definition: Delocalized Electron

Delocalized electrons are electrons that are spread out over entire organic molecules, ions, or solid metal structures.

The delocalized electron model satisfactorily explains the chemical and physical properties of benzene and it is now a universally accepted scientific hypothesis. Most chemists do, in fact, use nothing more than a drawing of a circle inside a hexagon to represent benzene molecules. This skeletal formula is supposed to convey the idea that benzene molecules contain a ring of delocalized electron density. The circle shows that benzene molecules have 𝜋 bonds that spread out over the whole hydrocarbon ring structure. It is imperative to state here though that this is just one way of expressing the structure of benzene molecules. Some chemists prefer to use the Kekulé structure. The following diagram shows how benzene molecules can be represented as a circle inside a hexagon or with the original Kekulé structure.

This explainer will sometimes use a circle inside a hexagon to represent benzene and benzene derivatives. It will also occasionally use the original Kekulé structure to represent benzene and benzene derivatives. The use of both structures will make students more comfortable with the different benzene representation styles that are used by different chemists and chemistry textbooks.

It is worth pointing out here that benzene and benzene derivatives are not the only molecules that have a delocalized ring of electron density. Benzene is actually just one member of a whole series of molecules that have delocalized rings of electron density. These molecules are known collectively as the aromatic compounds and they include molecules like naphthalene and anthracene which have the CH108 and CH1410 formulas. The structure of the naphthalene molecule is shown in the following figure.

The delocalized electron model is able to explain the unique properties of benzene. Benzene is known to have a surprising lack of chemical reactivity and it does not tend to decolorize bromine water at room temperature and atmospheric pressure. This inability to decolorize bromine water is difficult to explain if we assume that benzene has three CC double bonds, but it is less challenging to explain if we assume that the 𝜋-bond electrons are spread out over the whole benzene ring structure. Bromine molecules would readily react with benzene molecules if they contained electron-rich CC double bonds. Benzene molecules must not have any electron-rich CC double bonds whatsoever. They must contain much less reactive delocalized rings of electron density instead.

It is also easier to understand the enthalpy change of hydrogenation for benzene if we use the delocalized electron density model. The enthalpy change of hydrogenation for cyclohexene is 120 kJ⋅mol−1 and we therefore expect that the enthalpy change of hydrogenation for a six-membered ring with three CC double bonds to be 360 kJ⋅mol−1. The enthalpy change of hydrogenation for benzene is only 208 kJ⋅mol−1 and this suggests that benzene molecules cannot contain three CC double bonds. The lower enthalpy change of hydrogenation value suggests the 𝜋-bond electrons are delocalized over the entire benzene ring.

Kathleen Lonsdale produced some of the most striking experimental evidence that helped to validate the delocalized model of benzene. Kathleen Lonsdale used X-ray diffraction analysis methods to show that all of the carbon–carbon bond lengths in benzene were identical. The distance between adjoining carbon atoms was smaller than the length of most single CC carbon bonds and longer than the length of most CC double bonds. The structure of benzene is the same from the perspective of each carbon atom and this suggests that the three 𝜋 bonds in benzene must be spread out equally over the whole benzene ring.

Example 2: Identifying the Most Reasonable Carbon–Carbon Bond Length for Benzene

The table shows the typical bond lengths of carbon–carbon bonds. Which of the following values seems most reasonable for the carbon–carbon bonds’ length in benzene?

Bond Type CCCCCC
Bond Length (pm) 154134120

  1. 130 pm
  2. 160 pm
  3. 120 pm
  4. 140 pm
  5. 110 pm

Answer

Kathleen Lonsdale showed that all of the carbon–carbon bond lengths in benzene were the same. The carbon–carbon bond lengths in benzene were longer than most CC double bonds and shorter than most CC single bonds. The value of 140 pm is smaller than the CC single bond length (154 pm) and longer than the CC double bond lengths (120 and 134 pm). These statements can be used to determine that D is the correct answer for this question.

Benzene can react with different compounds to form monosubstituted or multisubstituted arenes. The IUPAC developed a system for identifying and classifying arenes, and this nomenclature is used by most organic chemists. It is simple to classify most monosubstituted arenes because the correct name can be determined by merely identifying the parent chain and its single substituent.

The benzene ring is classed as the parent chain unless it is attached to an alkyl chain with other functional groups or to an unusually long alkyl chain that contains at least seven carbon atoms. The single substituent can be made up of different types of atoms. It can be made up of nothing but carbon and hydrogen atoms and it can be made up of other atoms like halogen atoms and nitrogen or oxygen atoms. The following figure shows the preferred IUPAC name for one type of organic compound that has the CHBr65 chemical formula. The bromo- prefix is red and the benzene stem is black.

The ethyl- and propyl- prefixes are used to describe single CH25 and CH37 substituents and the chloro- and fluoro- prefixes are used to describe single Cl and F substituents. The nitro- prefix is used to describe a single NO2 substituent.

However, some common molecules are given names like toluene and phenol when they should rightly be named methylbenzene and hydroxybenzene. The following image shows the structure of the toluene and phenol compounds. It also shows the structure of the benzoic acid and aniline molecules which should rightly be given the names carboxybenzene and aminobenzene.

Chemists also have special names for certain radical compounds that are formed from benzene and benzene derivative compounds. The radical compounds are collectively known as aryl radicals. They are formed when a hydrogen atom is removed from one of the carbon atoms in an arene. The following figure shows the structure of the phenyl radical. The phenyl radical is formed when a hydrogen atom is removed from one of the hydrogen atoms in a benzene molecule. The phenyl radical has the CH65. formula.

Example 3: Identifying the Correct IUPAC Classification for a Monosubstituted Arene Molecule

According to the IUPAC nomenclature, what name does the following benzene derivative have?

Cl

  1. Chlorophenol
  2. Benzene chloride
  3. Chlorophenyl
  4. Benzyl chloride
  5. Chlorobenzene

Answer

The benzene derivative only includes one chlorine atom substituent on the parent benzene ring structure. The correct prefix must be chloro- and the correct suffix must be -benzene. We can use these statements to determine that this compound should be called chlorobenzene. Thus, option E must be the correct answer for this question.

It is more challenging to name multisubstituted aromatic compounds because we have to specify the position of each substituent. It is customary to use the lowest possible combination of numbers to specify the position of the different substituents. The following figure shows the preferred IUPAC names of two different disubstituted arene molecules. The carbon atoms are ordered from one to six in a clockwise direction so that the substituents are designated by the lowest possible benzene ring position numbers. The benzene ring position numbers are red in this image.

You will notice here that the CHBrNO642 compound was given the 1-bromo-2-nitrobenzene name even though there are other alternatives such as the 1-nitro-2-bromobenzene classification. We are supposed to order the prefix terms alphabetically and give the first prefix term a carbon atom position number of one (1) if we can choose between different possible organic compound names.

It can be simpler to name multisubstituted arene compounds that are based on a special status molecule, like toluene, because its CH3 group is automatically given a carbon atom position number of one (1). The following figure shows the structure of an incredibly explosive molecule that has the CH(NO)CH62233 chemical formula. The preferred IUPAC name for this molecule is 2,4,6-trinitrotoluene because the methyl group is automatically given a carbon atom position number of one (1). The molecule is colloquially known as TNT.

Chemists sometimes use unofficial names for multisubstituted arenes. Some chemists use the ortho- (o-), meta- (m-), and para- (p-) prefixes to describe the different isomeric forms of disubstituted benzene derivatives. The following figure shows how some chemists use the ortho-dichlorobenzene or even o-dichlorobenzene name for what should rightly be called 1,2-dichlorobenzene. It also shows how the meta-dichlorobenzene and para-dichlorobenzene names can be used for what the IUPAC would call the 1,3-dichlorobenzene and 1,4-dichlorobenzene molecules.

The ortho- or o- prefix is used for 1,2-disubstituted isomers. The meta- or m- prefix is used for 1,3-disubstituted isomers and the para- or p- prefix is used for 1,4-disubstituted isomers. This naming convention is summarized in the following figure.

Monosubstituted benzene compounds can be used to make disubstituted benzene derivatives. The following figure shows how a monosubstituted benzene compound can be reacted with certain ions and molecules to produce an ortho-, meta-, or para-type product:

The composition of the original substituent (A) usually determines what type of disubstituted benzene isomer is formed during these substitution reactions. Ortho, para directing groups increase the production of ortho- and para-disubstituted benzene derivatives and meta directing groups increase the production of meta-disubstituted benzene derivatives.

Ortho, para directing groups are usually electron-donating groups and meta directing groups are usually electron-withdrawing groups. The halides are a notable exception because halide ions are both electron-withdrawing substituents and also ortho, para directing substituents.

The following table lists some common ortho, para directing groups and some common meta directing groups.

Common ortho, para directing groupsCommon meta directing groups
OR (RH, alkly, aryl, acyl)NO2
NR2 (RH, alkly, aryl)NO
R (Ralkly, aryl)NR3 (RH, alkly, aryl)
X (XF, Cl, Br, I)COR (RH, alkly, aryl, hydroxy)
COR (RH, alkly, phenoxy, NH2)
CX3 (XF, Cl, Br, I)
CN
SOH3

The table shows that alkyl groups can be classed as ortho, para directing groups. This suggests that toluene should usually produce ortho- and para-disubstituted benzene derivatives when it is reacted with an electrophile. The following figure shows how toluene tends to generate ortho- and para-chlorotoluene when it is reacted with chlorine molecules:

The table also suggests that nitrobenzene should usually produce meta-disubstituted benzene derivatives. The next figure shows how nitrobenzene tends to generate meta-chloronitrobenzene when it is reacted with chlorine molecules:

It is important to stress here that the ortho- (o-), meta- (m-), and para- (p-) prefixes are used to describe the different isomeric forms of disubstituted benzene derivatives. The ortho- (o-), meta- (m-), and para- (p-) prefixes cannot be applied to describe trisubstituted benzene derivatives or any other benzene derivatives that have more than three substituents.

Example 4: Identifying Which Molecular Group for X Will Result in Substitution Predominantly Occurring at the Meta Position

A singly substituted benzene derivative is shown. Which molecular group for X will result in further substitution occurring mostly at the meta position?

X

  1. Cl
  2. CH3
  3. NO2
  4. NH2
  5. OH

Answer

The following table shows some common ortho, para directing groups next to some common meta directing groups.

Common ortho, para directing groupsCommon meta directing groups
OR (RH, alkly, aryl, acyl)NO2
NR2 (RH, alkly, aryl)NO
R (Ralkly, aryl)NR3 (RH, alkly, aryl)
X (XF, Cl, Br, I)COR (RH, alkly, aryl, hydroxy)
COR (RH, alkly, phenoxy, NH2)
CX3 (XF, Cl, Br, I)
CN
SOH3

The table can be used to determine that the chloride (Cl), methyl (CH3), amino (NH2), and hydroxy (OH) groups are all ortho, para directing groups. The table can similarly be used to determine that nitro (NO2) groups are meta directing groups. These statements suggest that substitution would predominantly occur at the meta position if group X was made to be a nitro (NO2) group. Thus, we can conclude that option C is the correct answer for this question.

Benzene molecules can be produced through different industrial and laboratory processes. Benzene can be prepared from ethyne through cyclic polymerization reactions. The ethyne molecules produce a benzene product when they are passed through a red-hot iron or nickel metal tube:

Benzene can also be prepared from aromatic acids through decarboxylation reactions. Sodium salts of sodium benzoate can be heated with soda lime (NaOH) to produce benzene and sodium carbonate (NaCO23):

+NaOH+Na2CO3ΔCOONaCaO

Example 5: Identifying What Products Are Formed When Sodium Benzoate Is Reacted with Soda Lime

Benzene can be prepared from its derivative, sodium benzoate, according to the following reaction scheme:

CaOΔNaOH+?+COONa

What other product is formed in this reaction?

  1. NaCO23
  2. CH4
  3. NaOCa
  4. Ca(OH)2
  5. CHOONa

Answer

Sodium salts of sodium benzoate can be heated with soda lime (NaOH) to produce benzene and sodium carbonate (NaCO23). The reaction is summarized in the following equation:

+NaOH+Na2CO3ΔCOONaCaO

We can use these statements to determine that the unknown product is sodium carbonate (NaCO23) and also that option A is the correct answer for this question.

Catalytic reforming is another chemical process that can be used to produce benzene from long-chain alkanes. Hexane molecules are passed over a suspended platinum catalyst at a temperature of roughly 500C and a pressure of approximately 20 atm. The reaction produces benzene molecules and hydrogen gas:

Benzene molecules can also be obtained from phenol molecules. The phenol reactants are transformed into a benzene product when they are passed over the surface of a hot zinc powder. The phenol reactants are reduced during this chemical reaction process:

Benzene molecules can also be retrieved from coal rocks. The coal rocks are initially heated to a high temperature in an airless oven to produce a liquid coal tar product. Chemists then use the fractional distillation process to extract benzene from the liquid coal tar mixture. Benzene is obtained at 8082C.

Benzene liquids have many interesting physical and chemical properties. The benzene liquid is colorless; it is a highly flammable liquid but it burns with a black smoky flame. The liquid is usually described as having a pleasant aromatic smell and it is known to undergo both addition and substitution chemical reactions. Benzene has a boiling point of 80.1C and it is not miscible with water at room temperature and atmospheric pressure.

Benzene undergoes both addition and substitution chemical reactions but many of these chemical processes are not facile because benzene molecules are not very reactive. Benzene molecules contain 𝜋 bonds that are spread out over the whole hydrocarbon ring structure and they tend to be less reactive than cyclic alkenes. Chemists frequently use catalysts or a high temperature and pressure to speed up the addition reactions of benzene because it would otherwise react very ineffectively.

Key Points

  • The 𝜋 bonds in benzene spread out over the entire hydrocarbon ring structure and form delocalized clouds of electron density.
  • The IUPAC nomenclature dictates how chemists should classify benzene derivatives.
  • Benzene can be produced through cyclic polymerization reactions and decarboxylation reactions and through catalytic reforming.
  • Meta directing substituents increase the yield of meta-benzene derivatives.
  • Ortho, para directing substituents increase the yield of ortho- and para-benzene derivatives.

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