Recent research shows alkyl groups have an opposite effect to what we teach post-16
English exam board AQA issued a statement about the inductive effect and its role in explanations in A-level chemistry. I have been teaching A-level chemistry for nearly 20 years and I cannot remember ever reading a statement like this before. It’s unprecedented. So, what is the inductive effect and why is it suddenly making waves for teachers of advanced chemistry qualifications? What are the implications of this change for individual teachers and for wider policy and practice?
Why now?
Exam boards picked up a paper published in November 2024. In this work, researchers describe the historic evolution of the inductive effect explanation. But the results of their computational modelling show that alkyl groups would actually have the opposite inductive effect to the one we teach in most post-16 curriculums. Moreover, the contribution of the inductive effect to the observed reactivities of molecules is small compared to the other electronic effects.
What is the inductive effect?
Post-16 we tell students that alkyl groups have a positive inductive (+I) effect – they ‘push’ electrons away from themselves. IUPAC defines an inductive effect as ‘an experimentally observable effect (on rates of reaction, etc.) of the transmission of charge through a chain of atoms by electrostatic induction’. This is not a student (or even teacher) friendly definition.
An accessible definition of the inductive effect is elusive, different sources say different things. Crucially, the inductive effect is just one of a number of electronic effects within molecules including the mesomeric effect, hyperconjugation and polarisability. If students go on to study chemistry at university, they will find these other electronic effects are invoked in situations where they had previously used the inductive effect for explanations.
What are the implications?
There are no immediate implications for teachers. Curriculums and exam boards cannot respond dynamically as exam papers are created at least a year in advance of the exam sitting. So, you don’t need to change what you teach just now. Looking further into the future, it will be difficult for exam boards to change their explanations and keep them at a level appropriate for 16–18 year-olds, so this might lead to changes in content when they review their curriculums.
For Ireland’s Leaving certificate, the advice is to encourage teachers to use this research as an example of the progress of science. This meets the unifying strand’s first learning outcomes to ‘support students as they learn to work like scientists and engage with the process of science’.
Additionally, this research paper involves computational calculations under particular parameters (for example, gas phase molecules) which may not be directly applicable to the reactions we study at post-16. It’s not clear cut; not all explanations using the positive inductive effect of alkyl groups are incorrect.
Where is it used post-16?
The concepts where the inductive effect may be included in explanations vary in their coverage by the curriculums in different nations and by different exam boards but fall broadly into three categories. The researchers did not discuss all of these in the paper.
- Stabilisation of carbocation intermediates in the electrophilic addition mechanism of alkenes
Tertiary carbocations are more stable than secondary and primary carbocations as the positive inductive effect of the alkyl groups reduces the carbocation’s positive charge. The more alkyl groups attached to the carbon bearing the positive charge, the more stable the positive charge. This allows students to justify the observed regioselectivity in the addition of electrophiles to alkenes, the chemistry behind the Markovnikov rule.
- Contribution to the availability of nitrogen lone pairs acting as bases or nucleophiles
Amine bases become more basic as the number of alkyl groups attached to the nitrogen atom increases. For example, trimethylamine is more basic than methylamine. This is due to the positive inductive effect of the alkyl groups increasing the availability of the nitrogen lone pair, so it is more able to act as a nucleophile or base.
- Effect of substituents on the acidity of carboxylic acids
Electron-withdrawing substituents make carboxylic acids more acidic due to their negative inductive effect. Groups with a positive inductive effect make carboxylic acids less acidic as they destabilise the carboxylate anions.
What next?
This might be slightly worrying to chemistry teachers; you may feel we have been teaching the wrong chemistry for a long time. Or, you might be ambivalent about it and feel it doesn’t really matter. In my opinion, this represents a pivotal opportunity for the chemistry curriculum and its relationship with research. Curriculums are reviewed at various points over time but rarely does that review involve engagement with the academic chemistry literature or even the chemistry education literature. This announcement from AQA suggests a greater willingness to examine curriculums with a critical eye, presenting a unique opportunity for teachers, researchers and policymakers.
No comments yet