IB Chemistry SL R3.4 Electron-pair sharing reactions questions

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Question 1

Multiple choiceMedium1 mark

The displayed equation shows a reaction between hydrogen chloride and hydroxide ions. Which species donates both electrons to the new bond, and which atom receives the donated pair?

Show answer and explanation

Answer: D. A nucleophile supplies both electrons in the new bond. The figure shows an O—H bond in water and the loss of the H—Cl bond; therefore O in OH⁻ supplies the pair to H in HCl.

A: Reverses the donor role: HCl provides the H atom, but does not supply the pair that forms the new bond.
B: Identifies OH⁻ as the donor but mistakes the Cl atom, which leaves as chloride, for the atom receiving the pair.
C: Treats the negatively charged chloride product as the nucleophile; it is formed after the O—H bond is made.

Remember: identify a nucleophile by the source of the bonding pair, not merely by its charge.

Question 2

Paper 2Medium6 marks

In aqueous solution, bromoethane reacts with hydroxide ions. Figure 1 shows the reactants; the products are not shown.

(a)

On Figure 1, draw one curly arrow to show the movement of the bonding pair in the C—Br bond during the reaction.

[1]

(b)

Write a balanced ionic equation for the reaction, using structural formulae for the organic compounds and including state symbols.

[2]

(c)

Using Figure 1 and your equation in (b), deduce the type of reaction and explain the electron-pair changes at outcome level. Do not give a detailed mechanism.

[3]

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