Edexcel A-Level Chemistry AS Paper 2, June 2018: Question 1

Mark scheme

Show the mark scheme

Question

Acceptable Answer Mark

Number

1 The only correct answer is A

B is incorrect because H2O is a nucleophile – via lone pairs

C is incorrect because NH3 is a nucleophile – via lone pair

D is incorrect because CN- is a nucleophile – via lone pairs

(1)

(Total for Question 1 = 1 mark)

How to answer it

Recognising Nucleophiles

Edexcel AS Level Chemistry • Multiple Choice

What this question tests

This question assesses your ability to define and identify a nucleophile based on its electronic structure. You need to recognise that nucleophiles are electron-pair donors (Lewis bases) that possess lone pairs of electrons or a negative charge, allowing them to attack electron-deficient areas (positive dipoles or carbon atoms).

Question 1: Multiple Choice Analysis

Which of the following does not act as a nucleophile?

✅ Correct Answer: A (HBr)

Hydrogen bromide acts as an electrophile (or an acid/proton donor), not a nucleophile. The H–Br bond is polarised due to the high electronegativity of bromine, making the hydrogen atom electron-deficient (partial positive charge, δ+). Therefore, it accepts a pair of electrons rather than donating one.

💡 Key Knowledge

Nucleophile definition: An electron-pair donor.
Requirements: Must have a lone pair of electrons or a negative charge.
H₂O: Oxygen has 2 lone pairs (acts as a nucleophile in hydrolysis reactions).
NH₃: Nitrogen has 1 lone pair (acts as a nucleophile in nucleophilic substitution with haloalkanes).
CN⁻: Carbon has a lone pair and a negative charge (a strong nucleophile).

🧠 Exam Technique

For multiple-choice questions asking what a species does not do, evaluate each option systematically. Cross out options that clearly fit the standard definition to isolate the odd one out. Remember to look closely at heteroatoms (O, N, halogens) and overall charges.

❌ Common Errors

Students sometimes confuse nucleophiles with electrophiles, especially when looking at polar molecules containing hydrogen halides like HBr or HCl . Always check whether the species has available lone pairs to donate to a positive center or if it is attacking unsaturated/electron-rich bonds using its own acidic proton.

Mark Allocation: 1 mark for selecting A. (Options B, C, and D are incorrect because they all possess lone pairs or negative charges that enable them to function as nucleophiles).

Topics

Organic Chemistry · Topic 6: Organic Chemistry I