Electrophilic addition
20.1 Types of organic reactions (7 hours)
2. Electrophilic addition reactions (Estimated 2 hours)
Pause for thought
How many chemists from the former Soviet era are known in the West?
Vladimir Markovnikov (1838-1904) was a Russian chemist who in 1869, on the basis of little evidence, formulated the rule that when hydrogen halides add to alkenes the hydrogen atom bonds to the carbon atom that already contains the most hydrogen atoms. This is useful for predicting the product but it is not an explanation.
The explanation in terms of the stability of the intermediate carbocation was put forward by Morris Kharasch (1895-1957) in 1933. Morris Kharasch was in fact born in Ukraine in the former Soviet Union but when he was 13 he emigrated to the US and studied at the University of Chicago.
Apart from Mendeleyev, Markovnikov is probably the only other Russian chemist whose name is generally known by western scientists although he worked as a postgraduate student under Alexander Mikhaĭlovich Butlerov (1828-1886) at Kazan University. (For a possible explanation as to why Soviet scientists have been 'airbrushed' out of Western science see my blog on How international is IB Chemistry?)
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 tipsAlthough the addition of bromine water to symmetrical alkenes is in the core (sub-topic 10.2), it makes sense for the mechanism of electrophilic addition to only be on the AHL part of the programme. This is because students really need to understand about sp2 hybridization and the formation of π bonds in order to be able to explain it. From their knowledge of electronegativity students should be able to see that a hydrogen halide molecule is polar so that the halogen will be δ- and the hydrogen δ+. It is this that makes the hydrogen atom of the hydrogen halide act as the electrophile. You will have to explain about induced dipoles to explain why halogens can also act as electrophiles. From sub-topic 14.2 students know that the carbon atoms are sp2 hybridized and that the π bond lies above and below the planar molecule so it is easy to see why alkenes attract an electrophile. I usually build on the addition reactions to symmetrical alkenes that they have already covered under Topic 10.2 and then stress how to use the curly arrows to show the movement of electron pairs and the formation of the intermediate carbocation. Sometimes to give evidence for the mechanism I tell them that with bromine and ethene the product is 1,2-dibromoethane in a non-polar solvent but if bromine water is used the product is actually 2-bromoethanol as the OH− ion preferentially reacts with the intermediate carbocation in place of Br−. Once they get the simple mechanism it is then quite easy to move on to the case of asymmetrical alkenes such as but-1-ene. Here the inductive effect needs to be brought in to explain the relative stability of the possible carbocations formed. As evidence that a CH3- group can have a marked effect I often give them the pKa values of methanoic acid and ethanoic acid. Just exchanging CH3- for H- makes the acid ten times weaker. Stress that Markovnikov’s rule only allows for predictions and is not an explanation. To test whether they really understand the mechanism give students the addition of an interhalogen, such as I-Cl, as an example. | Study guide
Pages 90-91 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 substitution 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 addition questions. Vocabulary listMarkovnikov IM, TOK, Utilization etc.See separate page which covers all of Topics 10 & 20. |
Teaching slides
Teachers may wish to share these slides with students for learning or for reviewing key concepts.
Electrophilic addition reactions:
Electrophilic addition reactions:
Electrophilic addition reactions:
Electrophilic addition reactions:
Electrophilic addition reactions:
Electrophilic addition reactions:
Electrophilic addition reactions:
Other resources
1. A good description of the straightforward addition of hydrogen bromide to ethene explaining why the mechanism occurs in the way that it does. The video is by James Mungall.
Mechanism of electrophilic addition
2. In this second video James Mungall explain the mechanism of Markovnikov addition.
3. For those who prefer music and a bit of polar bear 'humour' another video on three straightforward electrophilic addition reactions which might appeal to students.
Electrophilic addition reactions
4. From a TOK point of view it is interesting to look at how Morris Kharash came up with his carbocation explanation. Since Markovnikov formulated his rule there was considerable discrepancy in the literature about the products formed. Many people reported that they obtained the other isomer as the major product. Kharash showed that if the alkene was not purified beforehand it contains traces of peroxides. These formed free-radicals and the radical mechanism resulted in the formation of the anti-Markovnikov addition product. If only pure freshly distilled alkenes are used then the ionic mechanism prevails and the addition occurs as predicted by Markovnikov. (See my blog on Fish oil.)