Electrophilic substitution
20.1 Types of organic reactions (7 hours)
3. Electrophilic substitution reactions (Estimated 2 hours)
Pause for thought
1. The structure of benzene
The elucidation of the structure of benzene provides an interesting example of how science can progress. The structure of benzene had been puzzling chemists for years as none of the linear structures proposed could explain its properties. Friedrich Kekulé (1829-1896) was originally trained as an architect but switched to studying chemistry after hearing a lecture by Justus von Liebig. Kekulé was one of the originators of the theory of valency and the fact that carbon showed tetravalency. In 1865 he published a paper on the structure of benzene suggesting that it contained a hexagonal ring structure with alternate single and double carbon-carbon bonds. The evidence for this was mainly based on the fact that only one 1,2-disubstituted isomer of C6H4X2 existed as at that time bond lengths were not known.
Only when the 25th anniversary of the discovery was being celebrated in 1890 did Kekulé tell the story of how the discovery came to him through a dream. He dreamed that he was watching atoms twisting and entwining like snakes. One of the snakes caught hold of its own tail and it was this that gave him the inspiration to think of a closed ring structure. Whatever the truth of the dream story it was probably his training as an architect that gave him the ability to think of the structure in a different way. Later, in 1928, Linus Pauling introduced the idea of resonance in the structure. The model of benzene goes further than this for Higher Level as students need to know that each carbon atom in the benzene ring is sp2 hybridized and that the remaining electrons in the p orbitals form a delocalized π bond so that there is a volume of electron density above and below the planar ring which attracts electrophiles.
2. Why electrophilic substitution reactions of benzene are important.
The syllabus seems to limit the mechanism of electrophilic substitution reactions just to the nitration of benzene. This is a pity as once the idea of attack by an electrophile is understood benzene can be converted into many different classes of compounds by essentially the same reaction. In all of these reactions one of the hydrogen atoms in the benzene ring is substituted by an electrophile. The function of the H2SO4 in nitration and the halogen carrier, AlCl3, in chlorination, alkylation and acylation is to produce the electrophile. These catalysts do this by accepting a pair of electrons from the reacting species, HNO3, Cl2, RCl and ROCl, i.e. by functioning as Lewis acids.
Nature of Science
Organic reactions fall into a number of different categories. By understanding different types of organic reactions and their mechanisms, it is possible to synthesize new compounds with novel properties for use in diverse applications.
Collaboration between scientists on investigating the synthesis of organic compounds using new green chemistry pathways involves ethical and environmental implications.
Learning outcomesAfter studying this sub-topic students should be able to: Understand:
Apply their knowledge to:
| Clarification notesEmphasise the difference between the use of curly arrows and fish-hooks in reaction mechanisms. The use of partial charges (δ+ and δ-) and 3-D (wedge-dash) representations should be used where appropriate in explaining reaction mechanisms. Although typical conditions and reagents for all reactions should be known (e.g. catalysts, reducing agents, reflux etc.); more precise details, such as specific temperatures, do not need to be included. International-mindednessHow important in the global context is organic chemistry to green and sustainable chemistry? |
Teaching tipsThis is sub-topic contains a lot of good chemistry. If you approach it logically then there is no reason why students should find it difficult. They cannot just rely on memory though but need to understand fully the underlying mechanism. The key to understanding is the sp2 hybridization of each carbon atom. This results in a hexagonal planar molecule where each carbon atom is bonded by sigma bonds to two other carbon atoms and one hydrogen atom together with a delocalized pi bond over all six carbon atoms giving equal C to C bond lengths with a bond order of 1.5. It is easy to see that the volume of electron density above and below the ring will attract electrophiles. It is also easy to explain that substitution rather than addition will occur because the delocalization energy provides extra stability to the ring which does not need to be overcome during substitution reactions. When explaining the nitration of benzene you will need to explain the function of the sulfuric acid (to produce the nitronium ion electrophile, NO2+). Although probably not on the syllabus it is probably worth pointing out that it is exactly the same mechanism for other electrophilic substitution reactions except that now it is the halogen carrier which is needed to produce the electrophile. Stress the importance of the intermediate carbocation and look at the answers to the attached questions to see how the IB requires students to set out this mechanism in the exam to gain full marks. The syllabus now seems to be limited just to benzene so there is no need to go into the directing effects of substituents such as -CH3, -OH or -NO2. However, due to its carcinogenic properties, benzene is banned in school laboratories so the only way students can get 'hands on' experience of the required mechanism is to nitrate nitrobenzene. This is a traditional experiment and well worth doing for all the manipulative techniques it involves. | Study guide
Page 92 QuestionsFor ten 'quiz' multiple choice questions with the answers explained see MC test: Electrophilic addition & substitution reactions. Note that this quiz also includes questions on electrophilic addition as both form part of Topic 20.1. For short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers see Electrophilic substitution questions. Vocabulary listarene IM, TOK, Utilization etc.See separate page which covers all of Topics 10 & 20. Practical work |
Teaching slides
Teachers may wish to share these slides with students for learning or for reviewing key concepts.
Electrophilic substitution reactions:
Electrophilic substitution reactions:
Electrophilic substitution reactions:
Electrophilic substitution reactions:
Electrophilic substitution reactions:
Electrophilic substitution reactions:
Electrophilic substitution reactions:
Other resources
1. There are many videos available to explain electrophilic substitution - very few though limit themselves to just the nitration of benzene. The best way is to do it yourself! Below is one where everything that is said is also written on the slides but nevertheless it is a reasonable account of the electrophilic substitution mechanism for the nitration of benzene.
2. This is perhaps a much better video, not just because it contains Homer Simpson and doughnuts (!), but also because it covers all electrophilic substitution reactions and also considers the effects of all the activating and deactivating groups. This is actually a bit beyond the current IB syllabus but still worth showing (except that it sometimes uses non IUPAC names, e.g. toluene instead of methylbenzene).