Lesson Video: Reactions of Benzene Chemistry

Video Transcript

In this video, we will learn about addition and substitution reactions of benzene. We will learn how to predict what products are formed in these reactions.

Benzene is an organic hydrocarbon molecule with molecular formula C6H6. It is colorless to slightly light yellow and is a liquid at room temperature. It is highly flammable, and it is a known carcinogen. The six carbons are organized into a ring structure, and each carbon is bonded to one hydrogen atom because of the alternating double and single bonds in the carbon ring. Bond angles are 120 degrees. We can represent this compound in a variety of ways. As it is drawn here, all with the double bonds move to these positions like this, in a skeletal form, or like this.

Benzene is aromatic. That is, it is planar, cyclic, with pi bonds in resonance, resonance referring to the delocalized pi bonding because of the ability of pi electrons to shift between carbon atoms. Resonance makes benzene a fairly stable compound resistant to certain reactions. Benzene does not react like conventional alkane molecules, even those that contain three carbon-carbon double bonds. However, benzene can undergo substitution and addition reactions under the right conditions. Let’s have a look at some of the substitution reactions of benzene.

A substitution reaction is a type of reaction where parts of a molecule are removed and replaced with other functional groups. This simple diagram shows a pink particle displacing the blue particle and replacing it. In benzene, one of the hydrogen atoms are replaced with another element or group. Let’s investigate four types of substitution reactions for benzene. The first is halogenation, where hydrogen is replaced with a halogen—fluorine, chlorine, bromine, or iodine. Here is the general equation for the reaction of benzene with a halogen. We have benzene with one hydrogen atom on each carbon. It reacts with a halogen molecule, and one of the hydrogen atoms in benzene is replaced with a halogen atom to give a halobenzene and a hydrogen halide.

A suitable catalyst is used to drive this reaction because remember, benzene is fairly stable. Typical examples of Lewis acid catalysts are aluminum chloride and iron three chloride. Now, let’s have a look at a specific halogenation reaction. If chlorine is the halogen, then the product is chlorobenzene as well as hydrogen chloride. This halogenation is specifically called a chlorination. If bromine is the halogen, our aromatic product is bromobenzene, and we also get hydrogen bromide. This type of halogenation is a bromination substitution reaction. Under these conditions, we get a monosubstituted product. We need different conditions to get more halogen atoms to substitute onto the benzene ring.

So far, we’ve only looked at chlorination and bromination of benzene. But under these conditions, fluorine is too reactive and iodine too unreactive to get a monosubstituted aromatic product. The second type of substitution reaction we’ll look at is called alkylation or Friedel–Crafts alkylation. This is where a hydrogen atom on benzene is replaced with an alkyl group. Two simple alkyl groups are the methyl and ethyl groups, which we designate with R in the general reaction equation. To alkylate a benzene molecule, we take benzene, an alkyl halide, a Lewis acid catalyst, and these conditions produce a monosubstituted alkylbenzene and a hydrogen halide. Let’s look at a specific example.

When the alkyl halide is methyl chloride, the aromatic compound product is methylbenzene. We can see that a methyl group has substituted for a hydrogen atom. Methylbenzene has the common name toluene. Toluene is an excellent solvent and is used in the production of other substances, for example, the explosive substance TNT, which is trinitrotoluene. Conditions can be varied to give multiple alkylations.

This next substitution reaction is called a nitration. This is where hydrogen is replaced with a nitro group, NO2. Here is the reaction equation and the conditions. Benzene is treated with a nitric acid–sulfuric acid mixture. This acid mixture produces NO2+ ions, or nitronium ions. It is these positive ions which are attracted to the relatively negative charge on the aromatic ring. These ions are referred to as electrophiles, meaning they are attracted to and potentially bond to atoms or molecules that contain electron pairs available for bonding. The substituted benzene product in this reaction is nitrobenzene. Again, this is a monosubstituted product. Polysubstituted benzenes are possible under the right conditions.

Now, let’s have a look at the last substitution reaction in this video, sulfonation. In sulfonation, a hydrogen atom on benzene is replaced with a sulfo group, SO3H. The reaction is benzene plus concentrated sulfuric acid, which together are then refluxed, producing benzenesulfonic acid and water. This compound contains one sulfo or sulfonic acid group. It is a commonly used cleaning agent in laundry detergent. SO3 from the sulfuric acid acts as the electrophile and is attracted to the delocalized electrons in the benzene molecule. We won’t go into the details of how this reaction occurs, but the end product contains a sulfo group, SO3H.

We’ve looked at four substitution reactions of benzene. Let’s now have a look at some addition reactions. An addition reaction is a type of reaction where two or more molecules combine to form one larger molecule, without any byproducts forming. When benzene undergoes an addition reaction, say, with this substance here, one of the pi bonds can open up, adding two new atoms onto the ring to form an addition product with two new substituents. Or if a second pi bond opens up, or even the third, the product would contain four or six new substituents, respectively. Let’s look at a specific example.

A hydrogenation is when the atoms of one or more hydrogen molecules H2 add onto the ring. If one molecule of hydrogen reacts for every one molecule of benzene in the presence of a catalyst, such as nickel, platinum, or palladium, delocalization on the benzene ring is lost as a double bond opens up, and a diene functionality is produced, called 1,3-cyclohexadiene. Remember, there was a hydrogen atom here and here, represented by these two hydrogen atoms in the product. The two new hydrogen atoms that have been added are here.

We can simplify the structure as follows. In this skeletal structure, we know that there are two hydrogen atoms here and two here because these carbons only have single bonds. Sometimes, this compound is referred to as cyclohexa-1,3-diene. Either way, cyclo- tells us that this is a cyclic structure and hexa- that there are six carbons in the ring. The one and the three tell us that the two double bonds, the diene, are on carbons number one and three.

Successive hydrogenations produce cyclohexene where a hydrogen atom has been added on here and here to fully saturate those two carbon atoms. And eventually, the fully saturated cycloalkane, cyclohexane, is produced, where two further atoms are added on here and here. Cyclohexane is an excellent solvent and a starting material in the production of many other substances. Often, this hydrogenation reaction is carried out at high temperature and pressure. We can simplify these successive steps by showing the overall reaction like this. Benzene plus three hydrogen molecules under high temperature, pressure, and the presence of a catalyst produces the fully saturated cycloalkane, cyclohexane.

The last addition reaction we’ll look at is very similar to hydrogenation. This is halogenation. We’ve heard about halogenation before in the substitution reactions. This halogenation is a bit different. Atoms from one or more halogen molecules add on rather than substitute for hydrogen atoms. The general equation for full halogenation addition is shown here. The addition of halogen atoms at double bonds is successive. However, when we have three halogen molecules per molecule of benzene, all three double bonds of benzene open up and six halogen atoms are added on. The end result of full halogenation is the opening up of all three carbon-carbon double bonds. We have six halogen atoms added on, one at each carbon atom.

The conditions necessary for benzene to undergo halogenation addition are heat and ultraviolet light. With three equivalents of chlorine, thermal energy, ultraviolet light, and enough time, complete chlorination addition of benzene occurs, giving us a hexachloro product. If we had bromine instead of chlorine, the reaction would be similar. Notice that no catalyst is necessary for this addition reaction.

Now, let’s summarize what we’ve learned about the reactions of benzene. We learned that because benzene is aromatic, it is stable, but it can undergo addition and substitution reactions under the right conditions. We saw that a substitution reaction is a type of reaction where parts of a molecule are removed and replaced with other functional groups and that an addition reaction is a type of reaction where two or more molecules combine to form one larger molecule without any byproducts forming.

We looked at four substitution reactions of benzene and two addition reactions. In an halogenation substitution reaction, benzene reacts with a halogen to form a halobenzene. The other product is a hydrogen halide HX. The conditions necessary for this reaction are a Lewis acid catalyst such as aluminum chloride or iron three chloride. In an alkylation substitution reaction, benzene reacts with an alkyl halide. Here too, a Lewis acid catalyst is needed and the products are an alkylbenzene and a hydrogen halide.

Sulfonation of benzene is performed by refluxing benzene in concentrated sulfuric acid. A sulfonic acid group is substituted onto the benzene ring, and water is a byproduct. To nitrate a benzene molecule, in other words, to replace a hydrogen atom with an NO2 group, nitric acid and sulfuric acid are added to benzene and the mixture produces nitrobenzene as well as water. Only monosubstituted products are shown here, but polysubstituted products may form under the right conditions.

The first addition reaction we looked at was the hydrogenation of benzene. Heat and a metal catalyst, such as nickel, platinum, or palladium, are necessary. And when there are three equivalents of hydrogen, a fully saturated cyclohexane product is formed. The last addition reaction we looked at is when hot benzene is reacted with a halogen under ultraviolet light. This is called a halogenation addition reaction. If there are three equivalents of the halogen molecule and enough time, then all three double bonds in benzene can open up to add on six halogen atoms.