Explaining reaction mechanisms

Students need to have a fluency across a range of concepts and skills to gain a secure grasp of organic chemistry. In terms of reaction mechanisms, research has shown that learners frequently have trouble identifying electrophiles and nucleophiles in reactions. It’s particularly difficult for students to recognise the relationship between the pictorial depiction of reaction mechanisms and the underlying understanding of electron-deficient and electron-rich species.

Much research in this area has involved interviews with relatively small sample sizes. In a recent study, researchers evaluated nearly 20,000 written explanations of what occurs, and why, in reaction mechanisms to shed further light on students’ understanding of the processes involved.

The importance of pictorial depictions

Electrophiles, which are electron-deficient, are electron-seeking species. Nucleophiles, which are electron-rich, are nucleus-seeking species. The properties of a given species may be explicit, being depicted pictorially in the form of formal charges, labelled ‘dipoles’ and ‘lone pairs’. 

Students perform better in identifying electrophiles and nucleophiles where such explicit features are depicted, but they struggle where properties are implicit (not depicted pictorially). Species with negative charges and/or lone pairs may be easily identified as nucleophiles, while positively-charged species may be identified as electrophiles. Pi-bonds (explicitly depicted) and even some sigma bonds (eg, Al–H bonds in LiAlH₄), which can be thought of as shared pairs of electrons, have nucleophilic character and can therefore react with electrophiles.

In a chemical reaction, electrophiles and nucleophiles are complementary. An electrophile interacts with a nucleophile during a reaction and vice versa. In previous studies, students were more likely to correctly identify a nucleophile than an electrophile in a reaction. Since chemists need to be able to explain how and why two species interact in a reaction mechanism, it’s vital they can rationalise the roles of both reacting species. This then facilitates the prediction of the products for a specified reaction.

Students are often asked to reproduce a pictorial reaction mechanism in assessments, which can potentially be rote-learned, resulting in poor understanding of the underpinning theory. Instead, the researchers present a strong case for assessing the explanation of the mechanism in terms of the interaction between nucleophile and electrophile.

Being able to explain what’s happening at a molecular level is the key to understanding the processes involved in a reaction mechanism

The researchers categorised the sophistication of student explanations as: absent, descriptive, foundational or complex. Across both electrophiles and nucleophiles, over 54% of responses were classified as descriptive, while nearly 20% were classified as absent, showing room for improvement. Around 54.5% of explanations were at the same level of sophistication for both electrophiles and nucleophiles, and where there was a difference, there was a clear pattern that the electrophile level was lower than the nucleophile level, in line with previous studies.

Being able to explain what’s happening at a molecular level is the key to understanding the processes involved in a reaction mechanism. Providing a narrative to accompany the mechanism will help students correctly draw it from first principles.