Lesson Video: Properties of Benzene Chemistry • Third Year of Secondary School

Video Transcript

In this video, we will learn how to describe the properties of benzene, explain its structure, and name its derivatives. Benzene is a hexagonal ring of six-carbon atoms, each bonded to a single hydrogen atom. The structure of benzene was first proposed by German chemist August Kekulé in 1865. Legend has it that he developed the cyclical structure after dreaming of a snake eating its own tail. As we will see in a moment, this initial structure is not entirely accurate. Kekulé later revised his proposal. While he maintained that alternating single and double bonds were the basis of the structure, he now claimed that the structure would continuously switch back and forth between two different forms. During this switch, the double bonds would all become single bonds, and vice versa.

Modern chemistry has revised this structure even further. We can’t accurately model the chemical properties of benzene with single and double bonds alone. We need to look at the orbitals that contain electrons to build a full understanding. The atomic orbitals of benzene’s carbon chain form balloon-shaped regions above and below the hexagonal plane of the molecule. However, when we consider where electrons are located in the molecule as a whole, we need to consider the molecular orbitals. In the case of benzene, the balloon-shaped atomic orbitals combine to form two doughnut-shaped molecular orbitals, one above the carbon ring and one below it.

While the precise details of these orbitals are beyond the scope of this video, we can summarize their effect by comparing Kekulé’s old model to this new model. In the old model of benzene, electrons are rigidly held in single and double bonds. In this new model of benzene, some of its electrons are delocalized, meaning they are not bound to a particular atom or bond. Instead, they can move freely around the molecule in those doughnut-shaped molecular orbitals.

Another word we might hear in this context is resonance, which refers to the movement of delocalized electrons. While delocalized electrons and resonance aren’t the exact same thing, in this situation, we can use them as a pair. For our purposes, if a molecule has delocalized electrons, it also has resonance. Sometimes, in chemistry, we draw skeletal formulas, which use straight lines to represent bonds between carbon atoms. Hydrogen atoms are assumed to be present. We can draw the skeletal formula of benzene two different ways. One way, inspired by benzene’s circular molecular orbitals, shows the delocalization of its electrons. Another way shows benzene as a structure with three double bonds. As we’ve mentioned before, this second structure is a less accurate portrayal. Let’s take a closer look at why this is the case.

When we talk about benzene, we also talk about a related yet distinct group of molecules called cycloalkenes. The “alk” part of cycloalkenes means we’re dealing with carbon atoms, cyclo- means it’s a ring of carbon atoms, and the suffix -ene means that there’s at least one double bond. In the example drawn here, we can see that cyclohexene is indeed a ring of carbon atoms with a double bond. At first glance, benzene would seem to belong to this group. But since some of its electrons are delocalized instead of being held in single and double bonds, benzene is not considered a cycloalkene. Benzene and cycloalkenes differ in their reactivity, energy, and structure.

If we take a closer look at these differences, we can see how the resonance in benzene affects its chemical properties. Let’s start by comparing their reactivity. Cycloalkenes are known for being highly reactive. Many molecules, such as water molecules or halogen molecules, can react with the double bond of the cycloalkene. As a result, they attach themselves to the carbon chain. In the reaction we’ve written here, bromine starts as a brown liquid. The product of this reaction is 1,2-dibromocyclohexane, or in other words two bromine atoms attached side by side on a ring of six carbons. This product is colorless. So if we carried out this reaction in the laboratory, we would see the liquid in the test tube change from brown to colorless.

The color change of the substance is an indication that a reaction is indeed occurring, supporting the notion that cycloalkenes are highly reactive. Most halogen molecules can react with most cycloalkenes in a similar way. On the other hand, if we expose benzene to bromine, no reaction will take place without a catalyst present. However, if we introduce the catalyst iron(III) bromide, the combination of benzene and bromine can produce bromobenzene, alongside hydrogen bromide. When comparing these two reactions, we can say that cyclohexene is more reactive than benzene. The electrons in its double bond readily react with other substances to form new compounds.

On the other hand, since benzene requires a catalyst to react with bromine, it is less reactive than cyclohexene. Specifically, it is less reactive because it has some delocalized electrons. Compared to the electrons in the double bond of cyclohexene, these delocalized electrons are less able to form new bonds with other substances. Benzene’s delocalized electrons make it more stable. In addition to being less reactive, it also lowers the amount of energy that’s stored in the bonds.

There is energy stored in the double bonds of compounds. So when we break a double bond, say, by turning cyclohexene into cyclohexane, energy is released. In this case, 120 kilojoules of energy are released for every mole of cyclohexene that we turn into cyclohexane. We call this value the enthalpy change of hydrogenation. It’s the change in energy when we hydrogenate, or add hydrogen atoms to, cyclohexene to turn it into cyclohexane.

A pattern begins to emerge when we look at the enthalpy change of hydrogenation for 1,3-cyclohexadiene. Hydrogenating two double bonds gives us an expected enthalpy change of negative 240 kilojoules per mole, or twice the enthalpy change of hydrogenating a single double bond. The experimental value here is not exactly negative 240 kilojoules per mole. But it only differs by a small amount, about eight kilojoules per mole.

If we extrapolated this pattern, we might expect the enthalpy change of hydrogenation for benzene to be negative 360 kilojoules per mole. However, if we measure the enthalpy change of hydrogenation for benzene, the actual value is negative 208 kilojoules per mole, much lower than expected. We can see here that benzene does not simply contain three double bonds, as it is more stable and less energetic than predicted. The effect of benzene’s delocalized electrons is on display once again. Their resonance adds stability to the compound, lowering its enthalpy change of hydrogenation.

Another observation that supports this model of benzene with delocalized electrons is the observed shape and structure of the benzene molecule. A technique called X-ray crystallography can reveal important information about the structure of molecules. In this situation, we can use it to measure the lengths of the bonds. A typical carbon-carbon single bond, like the ones found in cyclohexene, has a bond length of 154 picometers. A typical carbon-carbon double bond, like the one found in cyclohexene, has a bond length of 134 picometers.

However, the X-ray crystallography of the benzene molecule reveals something interesting. All six of its bonds have the same length, 140 picometers, although you might also see this number referenced as 139 picometers depending on the source. Cyclohexene has different bond lengths for its single and double bonds. So if we considered benzene to be a cycloalkene, we might expect it to have alternating short and long bonds in its structure. However, since some of benzene’s electrons are delocalized across the molecule instead of being localized to certain single and double bonds, all of its bonds have the same length.

Now that we’ve learned about the structure of benzene, let’s take a look at how to synthesize it. There are many ways to synthesize benzene. One such method uses ethyne molecules. Passing the ethyne molecules through a red hot iron tube or sometimes a red hot nickel tube at 873 kelvin produces benzene. Specifically, electrons from the triple bond of each ethyne molecule are used to make new bonds between the ethyne molecules. The formation of new bonds turns three ethyne molecules into a benzene molecule.

Other methods of synthesizing benzene involve removing functional groups from aromatic compounds. For example, we can pass hydroxybenzene over heated zinc dust to produce benzene and zinc oxide. We can also heat sodium benzoate, sodium hydroxide, and quicklime to produce benzene and sodium carbonate. Lastly, we can take benzene sulfonic acid and hydrate it with superheated steam to produce benzene and sulfuric acid.

The resonant ring structure of benzene shows up in many different compounds. We call these compounds benzene derivatives. As we mentioned earlier, we can also call them aromatic compounds. Some aromatic compounds have a halogen atom attached to the carbon ring. To name these compounds, we simply attach the appropriate prefix. Here we have chlorobenzene, fluorobenzene, and bromobenzene. Sometimes a functional group instead of just one atom is attached to the benzene ring. Again, we can select the appropriate prefix or suffix to come up with a name. With an attached ethyl group, it becomes ethylbenzene. With an attached aldehyde, it becomes benzaldehyde.

Three more aromatic compounds are hydroxybenzene, methylbenzene, and aminobenzene. While we can name these compounds with the prefix-plus-benzene method, they also have special names. We can call them phenol, toluene, and aniline, respectively. This is a good reminder that one compound in chemistry can often have multiple names. We just looked at chlorobenzene, a monosubstituted benzene derivative, monosubstituted meaning there is one substitution or one attached atom or functional group on the carbon ring. If we added another chlorine atom to the ring, we would get dichlorobenzene, a disubstituted derivative, meaning there are two substitutions on the carbon ring.

But what does this compound look like? Where do we attach the second chlorine atom? It could look like this, like this, or like this. How do we differentiate between these three versions of dichlorobenzene? First, let’s describe their differences. In the first version, the chlorine atoms are attached to adjacent carbon atoms. In the second version, the attachments are on carbon atoms that are themselves separated by one carbon atom. In the third version, chlorine atoms are attached to carbon atoms that are opposite from one another in the ring. It’s worth noting that these are the only three possible arrangements for dichlorobenzene. Any other arrangement we could draw would end up being a rotation or a reflection of one of these three original arrangements.

To name these three versions, we use a different prefix for each arrangement. With side-by-side attachments, it’s called ortho-dichlorobenzene. The second arrangement of slightly separated attachments is called meta-dichlorobenzene. The third arrangement with opposite attachments is called para-dichlorobenzene. We can also shorten these prefixes to single letters, referring to these compounds as o-dichlorobenzene, m-dichlorobenzene, and p-dichlorobenzene, respectively.

Another way of naming these compounds involves numbering the carbons one through six. By convention, we minimize the numbers of carbons with attachment. That usually means starting at a carbon with an attachment and moving around the molecule toward the next attachment. Since the chlorine atoms are attached to carbon number one and carbon number two, we can also call ortho-dichlorobenzene 1,2-dichlorobenzene. If we number the carbons in the diagram of meta-dichlorobenzene, we can see attachments on carbon number one and carbon number three, giving us the name 1,3-dichlorobenzene. Continuing this process for para-dichlorobenzene, we see attachments on carbon number one and carbon number four, giving us the name 1,4-dichlorobenzene.

But the question remains, how do we know which orientation of the disubstituted derivative will form? To answer this question, let’s look at an example reaction. If we brominate methylbenzene, we end up with hydrogen bromide and two different aromatic products. More specifically, this reaction produces 40 percent ortho-bromomethylbenzene and 60 percent para-bromomethylbenzene. The reason for this is that a methyl group is what’s called an ortho–para-directing group. It directs atoms and functional groups to attach in the ortho and para orientations. If we start with a different functional group attached to the benzene ring, we might get different results.

For example, if we nitrate nitrobenzene, the aromatic product is dinitrobenzene. Specifically, it is 93 percent meta-dinitrobenzene. The nitro group is a meta-directing group, meaning it directs atoms and functional groups to attach in the meta orientation. Other ortho–para-directing groups include hydroxy groups, amino groups, and the halogens. Other meta-directing groups include cyano groups, trifluoromethyl groups, sulfonyl groups, and carbonyl groups.

Lastly, if we work with benzene in the laboratory, it may be useful to know some of its physical properties. It is a colorless liquid with a sweet odor. It is less dense than water. As a nonpolar molecule, it is immiscible in water but soluble in nonpolar solvents. It has a moderate boiling point of 80.5 degrees Celsius and a melting point of 5.5 degrees Celsius. It’s flammable and burns with a sooty flame.

Now that we’ve learned about benzene, let’s review the key points of the lesson. A benzene molecule is a ring of six-carbon atoms each bonded to a single hydrogen atom. Benzene’s delocalized electrons provide stability. Benzene can be synthesized using a variety of methods. We can name benzene derivatives using prefixes or sometimes suffixes to represent the attached atoms and functional groups. And we can use numbered carbons or the prefixes ortho-, meta-, and para- to represent the arrangement. Lastly, benzene is a colorless liquid with a sweet odor.