Every first-year organic chemistry student learns early on about the inductive effect, which describes how certain atoms’ electronegativity can affect other atoms in the same molecule. This subtle electronic push and pull, they are told, extends through several nearby bonds to influence qualities such as acidity, susceptibility to certain reactions, and nuclear magnetic resonance readouts.
But that explanation isn’t quite accurate, says Mark Elliott, an organic chemistry lecturer at Cardiff University.
Elliott and three other chemists have published a study in the Journal of Chemical Education using computations to show that inductive effects don’t reach very far in uncharged molecules (2026, DOI: 10.1021/acs.jchemed.6c00141). This means that trends that often get chalked up to induction are usually better described by other qualities, such as polarizability.
“Some people may reach the conclusion that they’re not going to change anything” about how they teach these effects, Elliott says. But he thinks that doing away with discussing induction as a far-reaching effect is ultimately simpler for students. “This is a golden opportunity for us to have a look at what gets taught [and] how it gets taught.”
Many students’ first exposure to inductive effects comes from comparing the strengths of organic acids. Experimental observations say that halogens near a carboxylic acid group make it more acidic. The closer the halogens are to the acid group, the more of them there are, and the more electronegative they are, the more acidic the molecule tends to be in solution. So students learn that the halogens are inductively drawing electrons away from the acid group, making it easier for the molecule to part with H+.
“You have this nice, convenient explanation that is technically incorrect, but it leads to the observable,” Elliott says. And induction-based explanations falter in the gas phase. Away from the influence of solvents, acidity increases with the size of the halogen instead of its electronegativity.
The acidity of halogenated acids in water increases with the electronegativity of the halogen, but the trend reverses in the gas phase. These types of observations call for a more nuanced view of inductive effects in organic molecules, says Cardiff University’s Mark Elliott.
The researchers used a technique called natural population analysis to determine the partial charge distribution among atoms in substituted alkanes. They consistently found that a highly electronegative atom directly influences the partial charge only on the atoms directly attached to it.
Halogens can shift the pKa of a carboxylic acid from up to 9 atoms away, but larger and more polarizable atoms like chlorine have a greater effect than smaller, more electronegative fluorine. The researchers concluded that polarizability is a better explanation than pure induction. Smaller anions are more easily surrounded by water molecules, which explains the solution-phase trends.
Framing acidity trends in terms of polarizability and solvent effects isn’t significantly harder to explain to students, so if it’s more accurate, “I actually think that this is a good thing to incorporate,” says Danielle Jacobs, an organic chemist at Rider University. The important thing is having a consistent framework for students to use to predict molecular behavior.
“I don’t think there’s any significant discrepancies between how I’ve been teaching it and the findings in this paper,” says Justin Houseknecht, a physical organic chemist at Wittenberg University. He already instructs his students to focus on the charged species in acid-base equilibria and teaches them that some trends work differently in the gas phase compared with the solution phase.
Given that these topics first come up in the second week of class, there’s only so much nuance he can introduce to start off with, but Houseknecht says he never wants to lie to students. “It’s OK to give a watered-down or vague explanation, but to be straight up wrong sets our students up for problems.”
Inductive effects were first proposed nearly a century ago, Elliott says, and the chemists of yore drew reasonable conclusions based on the data they had available. But as tools have improved for teasing apart the complex blend of factors influencing molecules, it’s important to reevaluate old assumptions. “By showing the limits of the purely inductive effect, we force the use of a more rigorous observation,” he says.
Given that many intro-level organic students are aiming for med school, perhaps the real question may be whether inductive effects will come up on the Medical College Admissions Test.
