In this explainer, we will learn how to describe addition and substitution reactions of benzene and predict what products are formed.
Benzene is a small aromatic hydrocarbon. It is volatile (boiling point: ) and carcinogenic and burns with a very sooty flame.
Benzene () is the most common example of an aromatic system. Benzene is substantially more resistant to hydrogenation than a simple alkene like cyclohexene as the delocalization of electrons in the ring confers extra stability.
The aromatic ring can be represented using alternating single and double carbon–carbon bonds, with dotted bonds of order 1.5, or with a circle in the middle of the ring. The former is sometimes more useful when drawing mechanisms, and the latter is a better representation of reality since all the carbon–carbon bonds in benzene are equivalent.
Here are a few different representations of benzene.
We will also look at both the addition and substitution reaction varieties of halogenation.
Benzene can undergo substitution reactions such as alkylation, nitration, and sulfonation, and addition reactions including hydrogenation. We will also look at both the addition and substitution reaction varieties of halogenation.
A substitution reaction of benzene will involve the replacement of one or more hydrogen atoms with other functional groups.
Definition: Substitution Reaction
A substitution reaction is a type of reaction where parts of a molecule are removed and replaced with other functional groups.
Definition: Functional Group
A functional group is a portion of a molecule with a particular composition and arrangement; functional groups usually behave in particular, predictable ways.
We will examine halogenation, alkylation, nitration, and sulfonation substitution reactions of benzene.
|Substitution Reaction||Benzene +||Catalysts||Products|
|Chlorination||or||Chlorinated benzenes +|
|Bromination||or||Brominated benzenes +|
|Alkylation (Friedel–Crafts)||or||Alkylated benzenes +|
|Nitration||()||Nitrated benzenes + water|
|Sulfonation||Concentrated ()||None||Sulfonated benzenes + water|
We will not be examining the mechanisms; nonetheless, these reactions are part of a class of reactions called electrophilic substitution reactions. During the reactions, reactants or portions of the reactants act as electrophiles (they are attracted to the relatively negative charge of the aromatic ring).
In halogen substitution reactions of benzene, hydrogen atoms on benzene are substituted with halogens, like fluorine, chlorine, bromine, and iodine. Many halobenzenes are useful as solvents and intermediates in the production of other products.
Definition: Halogen Substitution Reaction
A halogen substitution reaction is a type of reaction where parts of a molecule are removed and replaced with halogens.
Fluorine () is too reactive to use in practical applications, and iodine () is not reactive enough. There are, however, methods to fluorinate and iodinate benzene. Instead, we will focus solely on the chlorination and bromination of benzene using chlorine () and bromine () respectively.
When performing these reactions, we expect to isolate the monohalogenated products (i.e., chlorobenzene or bromobenzene).
The aluminum halide or iron(III) halide catalysts help weaken and polarize the halogen–halogen bonds, increasing the rate of the reaction.
The reaction for the monochlorination of benzene is shown below:
Further reactions with chlorine are possible to produce polychlorobenzenes like dichlorobenzenes and trichlorobenzenes.
The equivalent reaction equation for the monobromination of benzene is as follows:
Alkyl substitution reactions are a type of alkylation reaction. Here, benzene gains alkyl groups in place of hydrogen atoms. Toluene, also known as methylbenzene, is a common alkylbenzene that is used as a solvent and an intermediate in the production of other products, like trinitrotoluene (TNT).
Definition: Alkyl Substitution Reaction
An alkyl substitution reaction is a type of reaction where parts of a molecule are removed and replaced with alkyl groups.
The type of alkylation we will examine here is called a Friedel–Crafts alkylation.
There are many options for reactants and catalysts for Friedel–Crafts alkylation. One variant is the Friedel–Crafts alkylation of benzene by reaction with a chloroalkane, catalyzed by iron(III) chloride:
A chloroalkane () reacts with benzene, and this is catalyzed by iron(III) chloride (). The product is like benzene, but with an R group in place of one of the hydrogens. Under the right conditions, multiple alkylations could occur.
Aluminum chloride () is also an appropriate catalyst for this reaction.
Example 1: Predicting the Product That Would Form from the Alkylation of Benzene given the Reaction Equation
Express the product that will be formed from the alkylation of benzene as shown in the reaction scheme.
In the reaction scheme, we can see benzene () reacting with 2-chloro-2-methylpropane. There is also a reagent above the reaction arrow: aluminum chloride, . This is a classic example of a Friedel–Crafts alkylation.
is a catalyst helping to weaken the carbon–chlorine single bond, accelerating the reaction with benzene.
In an alkyl substitution reaction like this, we expect hydrogens on the benzene to be replaced with alkyl groups. In this particular reaction, we have one equivalent of 2-chloro-2-methylpropane for each equivalent of benzene. Therefore, we expect a single hydrogen on the benzene to be replaced by the alkyl group from 2-chloro-2-methylpropane.
So the answer is
The nitration of benzene could also be called the nitro substitution of benzene, but that is uncommon. This is a nitro group: . Nitrobenzene is used in the production of many chemicals, such as polymer precursors, lubricants, dyes, and synthetic rubber. Polynitrated compounds are commonly used in explosives because as they heat up, they release oxygen gas that can participate in combustion reactions and nitrogen gas that creates extra pressure.
Definition: Nitro Substitution Reaction
A nitro substitution reaction is a type of reaction where parts of a molecule are removed and replaced with nitro groups ().
Nitration of benzene is achieved by treating benzene with a mixture of sulfuric acid () and nitric acid (). These two acids react to produce the nitronium ion (), which performs the role of the electrophile in this substitution reaction.
The mononitration of benzene is shown below:
Further reactions with nitric acid are possible, producing polynitrobenzenes, but the more nitro groups there are attached to benzene, the less reactive it is to further nitration.
Example 2: Identifying the Primary Use of Picric Acid in a Set of Uses
Picric acid, also known as 2,4,6-trinitrophenol, is a benzene derivative containing several nitro groups. Given the chemical groups it contains, what primary use might picric acid have?
- Fuel for cars
- Food preservation
Picric acid molecules contain three nitro groups. When picric acid heats up and combusts, lots of heat and gas are produced. It is a compact source of oxygen (which will react to form water and carbon oxides), and nitrogen atoms will combine to form nitrogen gas.
The heat and huge increase in gas pressure can create a devastating explosion. Dry, pure picric acid is extremely dangerous; if touched, it can explode.
From this, we would expect picric acid’s main application to be in explosives.
While compounds containing nitrogen and oxygen can be useful as fuel additives or fertilizers, the concentration of unstable nitro groups makes picric acid unsuitable for these applications.
So picric acid’s likely primary use is in explosives.
The sulfonation of benzene could also be called the sulfo substitution of benzene, but that is uncommon. This is a sulfo group: . Benzenesulfonic acid (a potential product of the sulfonation of benzene) is used as a cleaning agent in laundry detergents.
Definition: Sulfo Substitution Reaction
A sulfo substitution reaction is a type of reaction where parts of a molecule are removed and replaced with sulfo groups ().
Sulfonation of benzene is achieved by treating benzene with concentrated sulfuric acid () under reflux for a few hours.
The monosulfonation of benzene is shown below:
In the case of sulfonation, the electrophile is sulfur trioxide (), which forms in small amounts in concentrated sulfuric acid.
Alternatively, fuming sulfuric acid, also known as oleum (), can be used.
Sulfonated benzenes are not called sulfobenzenes; instead, they are called benzenesulfonic acids. This is similar to how carboxylating benzene produces benzoic acids.
An addition reaction of benzene will involve the addition of 2, 4, or 6 functional groups, without any by-products. Typically, these addition reactions involve two parts of a simple molecule being added to the carbons at either end of a carbon–carbon double bond.
Definition: Addition Reaction
An addition reaction is a type of reaction where 2 or more molecules combine to form a larger molecule, without any by-products forming.
We will examine hydrogenation and halogenation addition reactions of benzene.
|Addition Reaction||Benzene +||Conditions||Catalysts|
|, , or catalyst|
|Chlorination||UV light |
|Bromination||UV light |
The hydrogenation of benzene, or the hydrogen addition reaction of benzene, involves the addition of 2, 4, or 6 hydrogens to the carbons of benzene. Cyclohexane is produced when benzene is completely hydrogenated. Cyclohexane is a useful solvent and a precursor in the production of other chemicals.
Definition: Hydrogen Addition Reaction
A hydrogen addition reaction is a type of reaction where a molecule combines with 1 or more molecules of hydrogen to form a larger molecule, without any by-products forming.
The reaction below shows the successive hydrogenation of benzene:
These reactions are performed using a nickel, palladium, or platinum catalyst, and moderate temperatures and pressures. A minimum of is necessary, but reactions are usually performed at –.
Here’s the complete hydrogenation of benzene:
Example 3: Predicting the Product of the Hydrogenation of Benzene in Excess Hydrogen
What product is produced from the hydrogenation of benzene using a nickel catalyst and excess hydrogen?
- Carbon dioxide
The hydrogenation of benzene involves the addition of 1 or more hydrogen molecules to each molecule of benzene in the reaction.
In excess hydrogen, we can expect that benzene will be completely hydrogenated.
Benzene () is an aromatic ring of carbon atoms, each bearing one hydrogen atom. It can be represented in a number of ways, but this is probably the most helpful at the moment:
Each carbon atom can potentially gain one more hydrogen atom (carbon atoms can form a maximum of four single bonds):
This molecule is called cyclohexane (it is a ring of 6 carbon atoms, saturated with hydrogen).
You might think that the answer is cyclohexene, or cyclohexa-1,3-diene, but these products only form if the amount of hydrogen is restricted. In excess hydrogen, the reaction would be
The halogenation of benzene, or halogen addition reaction of benzene, involves the addition of 2, 4, or 6 halogens to the carbons of benzene. Various hexachloroethane have been used to make pesticides that have since been banned.
Definition: Halogen Addition Reaction
A halogen addition reaction is a type of reaction where a molecule combines with 1 or more halogen molecules to form a larger molecule, without any by-products forming.
Theoretically, we could successively halogenate benzene with any halogen:
However, as with the substitution reaction, the reactions involving fluorine or iodine are impractical due to fluorine’s overly high reactivity and iodine’s extremely low reactivity (at least, this is the case if other reactants are not involved).
Therefore, we will only be examining the chlorine and bromine addition reactions of benzene.
Chlorine and bromine react in the same way. In the absence of a catalyst, UV light and heat are sufficient for complete halogenation.
The complete addition chlorination of benzene is shown below:
And here is the complete addition bromination of benzene:
- Benzene can participate in substitution reactions with halogens (like chlorine and bromine), chloroalkanes, nitric acid, and sulfuric acid, and in halogenations, alkylations, nitrations, and sulfonations.
- A substitution reaction is a type of reaction where parts of a molecule are removed and replaced with other functional groups.
Substitution Reaction Benzene + Catalysts Products Chlorination or Chlorinated benzenes + Bromination or Brominated benzenes + Alkylation (Friedel–Crafts) or Alkylated benzenes + Nitration () Nitrated benzenes + water Sulfonation Concentrated () None Sulfonated benzenes + water
- Benzene can participate in addition reactions with hydrogen and halogens (like chlorine and bromine), in hydrogenations and halogenations.
- An addition reaction is a type of reaction where 2 or more molecules combine to form a larger molecule, without any by-products forming.
Addition Reaction Benzene + Conditions Catalysts Hydrogenation –
, , or catalyst Chlorination UV light
None Bromination UV light