Nucleophile: AQA A-level wants “electron pair donor”

The ruling

In June 2018, question 2 of AQA’s A-level Chemistry Paper 2 asked students to explain a nucleophilic substitution. Most could say that the carbon-bromine bond was polar and that the carbon was slightly positive. The third marking point was for the nucleophile, and the report records what went wrong with it.¹

Two refusals in one sentence. “Attracted to the carbon” is a description of where the nucleophile goes, not of what it is, and a definition mark is for the definition. “Electron donor” is closer, and it is the one worth understanding, because it is the answer of a student who knows the chemistry and has lost one word.

What the pair is doing

An electron donor, on its own, is a reducing agent: a species that hands over electrons to something else, one at a time if need be, and is oxidised in the process. Sodium is an electron donor. That is not what a nucleophile does, and the difference is the whole of the mechanism.

A nucleophile brings a pair of electrons, together, and uses them to form a new covalent bond. The hydroxide ion attacking a halogenoalkane does not give up an electron. It shares two of its own with the carbon, and those two electrons become the new carbon-oxygen bond. The pair is the bond. Nothing is oxidised, nothing is reduced, and a student who writes “electron donor” has described a redox process that is not happening.

That is why the word is not pedantry. “Electron pair donor” is a statement about bonding. “Electron donor” is a statement about redox. They are different claims, one of them is true of a nucleophile, and the mark scheme is written for the true one.

The arrow starts on the pair

The same idea is drawn as well as written, and the board is as strict about the drawing. Five years later the June 2023 Paper 3 report restated the rule for every mechanism on the specification: “A curly arrow in any mechanism must always start either from the line representing a covalent bond or from a lone pair drawn on an atom.”² Half the students scored full marks on that mechanism; the ones who did not had usually started an arrow from the wrong place.

A curly arrow shows the movement of a pair of electrons. So it has to begin where a pair is: on a bond, or on a lone pair drawn as two dots. An arrow that starts on an atom’s symbol, or on the minus sign of the ion, or from empty space near the oxygen, is drawing something that has no electrons in it. The written definition and the drawn mechanism are the same fact twice, and both marks turn on the same word.

The other names the paper refused

The June 2018 report is a short catalogue of the same habit: a word next to the right one, credited as the wrong one.¹

• The name of the mechanism. In question 7, on acyl chlorides, most students identified a correct reagent, “but ‘electrophilic addition’ and ‘reduction’ were frequent wrong answers for the name of the mechanism”. A mechanism’s name is a mark in its own right, and it is not interchangeable with the name of a different mechanism that also happens to involve a carbonyl.
• An observation, not a product. In the next part, “a common error was to give the name of the product, i.e. ‘HCl gas’, rather than an observation as required, or alternatively to describe this as a ‘white precipitate’.” You cannot see hydrogen chloride. You can see misty fumes. And a gas is not a precipitate, whatever colour it is.
• Hydrogenation, not hydration. In the benzene question, Level 3 answers were spoiled by “errors such as ‘hydration’ for hydrogenation”. Adding hydrogen and adding water are different reactions with names one syllable apart.
• Regenerated, not merely “not used up”. On the chlorine radical, “some candidates wrote about Cl· acting as a catalyst and not being used up, without stating that it was regenerated/reformed”. A catalyst is not one that is never consumed; it is one that is consumed and remade, and the second half is the mark.
• A complete reagent. For the nitrile-forming step, “some suggested the CN− ion, which is an incomplete reagent”. An ion is not something you can take from a bottle. Potassium cyanide is.

The report also says, near its top, that several of its comments repeat advice printed in the mark scheme’s own instructions to examiners, and that this advice “should be known and understood by students”.¹ That is the board telling you the rules are published and expecting you to have read them.

Credited and refused

All AQA, from the June 2018 Paper 2 and June 2023 Paper 3 reports.