Teaching aromatic chemistry post-16

We live in a colour-filled world. A chromophore is a molecule which absorbs light of a particular wavelength and reflects colour as a result. We commonly refer to chromophores as coloured molecules. These molecules often have large sections of alternating single and double bonds. Where adjacent pi bonds are parallel to one another, they can merge or link together to form extended molecular orbitals that we call conjugated pi systems. The conjugated pi systems can interact with longer wavelengths of light, which is how some of them produce colour. Mauveine, also known as Perkin’s mauve, is a chromophore and was one of the first synthetic dyes. It has conjugated pi systems that interact with longer wavelengths of light and produce a mauve colour.

What students need to know

Aromatic compounds are conjugated cycloalkenes that have enhanced stability because of the delocalisation of the electrons in the pi system. Benzene, C₆H₆, is the archetypal aromatic compound. It has a conjugated pi system with six electrons delocalised around the ring in a six-centre pi orbital.

The conjugated pi system extends around the whole ring rather than three isolated pi bonds in three sides of the hexagon. We often draw benzene with a circular ring to represent the pi system.

Both alkenes and aromatic compounds react with electrophiles which are attracted to the pi electron density. A conjugated pi system has greater thermodynamic stability than isolated pi bonds. This affects the reactions of aromatic compounds which undergo electrophilic substitution rather than the electrophilic addition reactions of alkenes. Stronger electrophiles are needed to react with benzene than with alkenes.

The mechanism of electrophilic substitution at a benzene ring involves the formation of a carbocation intermediate with the positive charge spread around a five-carbon delocalised pi system with four electrons left in it. We get this electrophilic substitution as the delocalised pi ring is restored.

We do not get electrophilic addition as this would break up the pi ring and lose the extra stability it confers.

In these mechanisms E⁺ represents an electrophile, :Nuc^- represents a nucleophile.

Common misconceptions

Most teachers start this topic with a comparison of the structure that Kekulé proposed for benzene – cyclohexa-1,3,5-triene – and a structure with a delocalised pi ring. Ensure that students understand the extra thermodynamic stability that benzene, with its delocalised pi ring, has compared to the theoretical cyclohexa-1,3,5-triene. Enthalpy diagrams, such as the one here, help but can contribute to the misconception that when we draw Kekulé’s structure we mean the theoretical cyclohexa-1,3,5-triene rather than benzene.  Students can think that the resonance structures represent distinct entities rather than a single concept. Remind learners these structures are representations, not reality.

Another common misconception about the mechanism of electrophilic substitution is that the carbocation reactive intermediate is electron deficient at the carbon which has formed a new bond to the electrophile.

Ideas for your classroom

Before you show your students the Kekulé structure for benzene you have an ideal opportunity to introduce the concept of double bond equivalents, DBE, to help with structure determination. Both double bonds and rings reduce the number of hydrogens in a molecular formula by two. Hexane C₆H₁₄ has no double bonds. Cyclohexane and hexene both have the formula C₆H₁₂, each has one DBE. Benzene C₆H₆, has four DBE. Download the accompanying resource for an entertaining question on DBE. 

At the start of this topic, (re)teach how pi bonds are made from the overlap of parallel p orbitals. Point out to students that the resulting pi orbitals have electron density above and below the sigma bond between the two atoms.

Evidence for delocalised structure of benzene

Because of the hazards associated with benzene students will not be handling it, so introduce context. Tell the story of Kekulé proposing his structure for benzene after dreaming one night of a snake swallowing its own tail.

Point out that the bond lengths of the C–C bonds in benzene are all the same and intermediate between the longer C–C single bond and the shorter C=C double bond.

Electron density maps provide evidence of this bond length by enabling us to measure the distance between nuclei.

Physical models or good diagrams can help students understand that the pi molecular orbital in benzene goes around the whole ring.

Explain that with this six-electron six-centre pi system, students are moving beyond covalent bonds that are simply two shared electrons.

Use the thermodynamic evidence against cyclohexa-1,3,5-triene to explore students’ grasp of thermodynamic stability. The electrons could occupy the space of three separate pi bonds but the transformation into a delocalised pi ring is exothermic so that’s what the electrons do. Like a ball rolling down to the bottom of a hill. Link this to the relative stability of the carboxylate ions compared to the alkoxide ions, if you have covered this topic.

More ideas

When teaching about nitration, explain the outline method of producing TNT (trinitrotoluene) and how each successive nitration requires a higher temperature as each NO₂ group deactivates the ring.

Revisit the structure of graphene – a single layer from graphite. Show learners how they can now explain the delocalised electrons between layers they met earlier as electrons in a conjugated pi system that extends across the whole layer.

When teaching acylation or alkylation, emphasise the importance of these reactions as a means to change the carbon skeleton in synthetic routes.

Checking for understanding