Nucleophilic substitution

This question is about two metal hydrides, lithium aluminium hydride, LiAl H₄, and sodium borohydride, NaBH₄. Both hydrides are used as reducing agents in organic chemistry.

What is the systematic name of the alcohol C₆H₅CH₂OH?

Calculate the mass of LiAl H₄ that the student should use in Step 2 and the percentage yield of pure C₆H₅CH₂OH obtained in Step 4.

Give the mass of LiAl H₄ and the percentage yield of pure C₆H₅CH₂OH to 3 significant figures.

Percentage yield of pure C₆H₅CH₂OH =........ %

In the preparation, LiAl H₄ behaves as a nucleophile.

What is meant by the term nucleophile ?

The mechanism for Step 2 in the reduction of C₆H₅COOC₂H₅ is shown below. In the mechanism, LiAl H₄ can be represented as H^–.

Complete the mechanism by adding curly arrows to the three boxes.

Suggest why the student heated the mixture from Step 2 with dilute aqueous acid in Step 3.

NaBH₄ contains Na⁺ and BH₄^– ions.

Draw a ‘dot‑and‑cross’ diagram for NaBH₄.

Use a different symbol for the electrons from each element.

Show outer electrons only.

NaBH₄/ O₂ cells are being investigated as potential fuel cells.

• The positive electrode contains O₂.

• The negative electrode contains NaBH₄.

The reaction at the positive electrode and the overall cell reaction for a NaBH₄/ O₂ fuel cell are shown below.

Overall cell reaction: NaBH₄ + 2O₂ NaBO₂ + 2H₂O E = 1.64 V

What is the standard electrode potential of the negative electrode in this cell?

Construct the equation for the reaction at the negative electrode of this cell.

Which species could react as a nucleophile?

2 OH –

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

This question is about reaction mechanisms.

Chemists use curly arrows in reaction mechanisms.

What does a curly arrow show in a reaction mechanism?

Draw structures to show the products in the reaction mechanism below.

Use the mechanism in (ii) to explain what is meant by heterolytic fission.

An incomplete reaction mechanism is shown below.

What is the role of OH − in this mechanism?

Which compound does not react with nucleophiles?

A CH₃CH₂CHO

B CH₃CHCH₂

C CH₃CH₂COCH₃

D CH₃CH₂CH₂Cl

This question is about the hydrolysis of haloalkanes.

The rate of hydrolysis of a haloalkane depends on the halogen present.

State and explain how the halogen in the haloalkane affects the rate of hydrolysis.

A student hydrolyses a haloalkane, E, using the following method.

• 0.0100 mol of haloalkane E is refluxed with excess NaOH(aq) to form a reaction mixture containing an organic product F.

• The reaction mixture is neutralised with dilute nitric acid.

• Excess AgNO₃(aq) is added to the reaction mixture. 1.88 g of a precipitate G forms.

Organic product, F, has a molar mass of 74.0 g mol⁻¹ and has a chiral carbon atom.

Draw a labelled diagram to show how the student would carry out the hydrolysis of haloalkane E.

Analyse the information to identify E, F and G.

Show your working.