For nearly a century, chemistry students have been teach electron density by being shown a picture of the electron being pulled through one bond after another. Now a new computational study argues that this familiar explanation of the inductive effect does not hold up consistently, especially when used to analyze neutral organic molecules. That finding is so important that it could prompt educators towards redrawing the lesson written on chemistry textbooks.
Researchers from Cardiff University, the University of Newcastle in Australia, and the University of New England worked together to find that the classic inductive effect might actually be limited to one bond in neutral molecules. We know this might sound like greek. So to clarify it their intention is not to overturn organic chemistry. What they want is to point out that one of the most common teaching shortcuts that have been used for more than a century doesn’t exactly work.
What the textbooks have taught for generations
The inductive effect is commonly used to explain how an electronegative atom (such as fluorine or chlorine) pulls the electron density through a molecule’s chain of single bonds. To get an idea of how it worked before, old textbook diagrams show a line of consecutive electrons. A pull is given to the first electron and then to each subsequent one. The initial pull is strongest at the first atom, but gradually weakens with each additional bond.
If we go back in time to the early 20th century, technology used for anything, in general, was not as advanced as it is today. In this sense, it is clear that the old models trying to explain how electrons work were extremely limited. In a recent interview, Dr. Edwin Johnson from Newcastle University talked about this difference, claiming that current computational models debunk the old model.
Computer models reveal a much shorter reach
To test their hypothesis the researchers used a theory called “density functional theory”. It is a new quantum theory that uses a quantum calculation method. They tested it on three established approaches for assigning charge to individual atoms.
In the pentane-based molecules model, adding fluorine changed the carbon directly attached to it, but no further effects were detected at others down the chain. The second carbon model did not behave like a simple midway point in a fading electron pull, either.
That’s because small changes were being affected by a phenomenon called hyperconjugation, an orbital interaction that can donate electron density in the opposite direction of the expected electron pull. If we translate this into simpler words, we can say that the conclusion shown by the tests is that the electronic message sent through the chain of electrons happens irregularly, contrary to what the old models proposed.
Chlorine exposes the flaw in the outdated shortcut
The outdated classroom rule says that because fluorine is more electronegative it is supposed to increase the acidity of a nearby carboxylic acid more than chlorine. Yet, opposed to what the rule says, the team’s calculations found the matching chloroacids were consistently more acidic than the fluoroacids.
We now know chlorine has a larger and more easily distorted electron cloud, which allows it to help stabilize negative charge through polarizability. Also, the surrounding environment matters too. Sometimes so much that it changes the apparent trend.
A whole-molecule view may replace the relay model
Long range electronic effects are not 100% discarded though. They can still appear in charged molecules but researchers argue that these should be understood through polarizability and the reorganization of molecular orbitals. It looks less like a pull traveling from bond to bond to more like a cushion changing shape when weight is placed on one of its sides.
The paper goes even further and questions some effects that were traditionally described as if they were “acting through empty space.” Currently, the hypothesis that there are “empty spaces” is no longer used, or in other words, it is known that even in what would be empty space, things are happening.
What changes for students and working chemists
No medicine, polymer, or agricultural chemical suddenly behaves differently because a textbook diagram is being challenged though. The question is: are students and researchers being given the clearest explanation for predicting that behavior? And if they are not, changing the explanation may help improve how that particular ramification is being studied.
For teachers the practical update may be to stop presenting the old outdated model. For scientists, clearer distinctions between electronegativity, polarizability, solvation, and orbital interactions could reduce confusion when interpreting acidity and reactivity. “If a foundational concept is taught inaccurately, misunderstandings can carry into more advanced science and research,” Johnson said.
Textbooks can change without overturning organic chemistry
This new study is a computational reassessment of old rules, not a claim that everything that has been done before was wrong. The authors noted that the official definition by IUPAC (the body that standardizes chemical terms worldwide) is so vague that it blends two different concepts together: the inductive effect and polarizability. Because of the vagueness the dividing line gets blurred, which causes the terms to end up overlapping in practice. And this is the reason why they want to update and clarify scientific nomenclature.
The long and short of it is that the old model can no longer handle the job, so the next logical step for experts in the field is to update how we learn and teach the workings of electrons. If we change the way we look at the principle of these ideas, everything else will change as well, and this shift will certainly change the future of chemistry.
