Alkyl inductive effects: Confusion in textbooks

Understanding the factors contributing to reactivity trends is a fundamental skill that underpins chemists’ ability to design and develop new reactions. One of the factors that students learn about early on is the inductive effect—the influence of the differing electronegativity between two atoms on the electron distribution within a bond. But Mark Elliott, an organic chemist at Cardiff University, asserts that chemists have for years been incorrectly teaching that concept for alkyl groups.

Alkyl groups have an electron-donating character, which is used to explain the stability of carbocations, trends in the acidity of carboxylic acids, and the basicity of amines. When alkyl groups’ electron-donating nature was observed in the 1920s and 1930s, chemistry pioneer Christopher Ingold concluded that it is the result of an inductive effect; that theory quickly entered—and has persisted in—textbooks.

But on closer inspection, this interpretation is counter-intuitive. Carbon is slightly more electronegative than hydrogen (2.52 versus 2.20 on the Pauling scale), meaning that an alkyl group should theoretically withdraw electrons from its adjacent atom. The observed electron-donating character must instead arise from other, more dominant factors.

According to Elliott, the subsequent discovery of hyperconjugation—the interactions between sigma orbitals on neighboring atoms—provides a more rational explanation. But modern resources for students still place an incorrect emphasis on the role of the inductive effect.

To solve the puzzle once and for all, Elliott and his colleagues used computations to demonstrate that alkyl groups do exert a subtle electron-withdrawing inductive effect (Org. Biomol. Chem. 2024, DOI: 10.1039/d4ob01572j). The team hope that its findings will encourage chemists to adopt greater rigor and accuracy in future explanations of reactivity trends, including the explications provided in student textbooks.

But rather than suggesting a theoretical misunderstanding by chemists, this inconsistency may reflect the challenge of effectively teaching complex theoretical concepts, particularly to secondary school age chemists, says Jonathan Clayden, an organic chemist at the University of Bristol and author of the undergraduate textbook Organic Chemistry. “We think of inductive effects as simpler to explain than molecular orbital theory. In secondary school teaching, [trends] may be explained with inductive effects where we would use conjugation to explain the same reactivity [at the undergraduate level].”

Regardless of the cause, Elliott thinks this inconsistency is extremely problematic educationally. The strength of his feeling is also evident in the new paper’s title: “Alkyl groups in organic molecules are NOT inductively electron-releasing.”

Though the forthright presentation of the team’s findings has surprised some researchers, they agree that the study has raised some important questions about chemistry education. “I think the authors have done well to point out that there are competing effects and their relative magnitude will determine the overall effect,” says Alastair J. J. Lennox, an organic chemist at the University of Bristol. “This is a really important concept that we should teach more.”

Clayden agrees. “I don’t think this is a big overturning of general opinion, but one of the important points they’ve raised is that aspect of balance between the simplicity of the explanation while making sure the explanation actually works,” he says.

And Elliott hopes his team’s evidence will gradually lead to changes in how chemistry is taught, especially during the tricky secondary school–undergraduate transition. “I’m very opposed to the situation that we often find ourselves in as university educators where we have to say to our students, ‘You learned this before but it’s not true,’” he says. “If I was rewriting one of my books, I would have a short statement that you do not need to worry about inductive effects for alkyl groups because, if the electronic effect is significant, it will not be an inductive effect!”