OCR A-Level Chemistry: Alcohols, Haloalkanes & Analysis

A student warms three separate haloalkanes — 1-chlorobutane, 1-bromobutane and 1-iodobutane — with aqueous sodium hydroxide. In each case the C–X bond is broken and butan-1-ol is produced.

Selected mean bond enthalpies are given below.

Bond	Mean bond enthalpy / kJ mol⁻¹
C–Cl	338
C–Br	276
C–I	238

Describe and explain the mechanism by which aqueous hydroxide ions convert 1-bromobutane into butan-1-ol, and explain, using the data, why the rate of hydrolysis follows the order 1-iodobutane > 1-bromobutane > 1-chlorobutane. Describe the mechanism in words and equations, naming the nucleophile, identifying the carbon attacked and stating how the bonding electrons move. Do not draw a diagram.

(6 marks)

A volatile organic liquid Z contains only carbon, hydrogen and oxygen. Combustion analysis shows that Z has the percentage composition by mass shown below.

Element	Percentage by mass / %
Carbon	64.84
Hydrogen	13.60
Oxygen	21.56

The mass spectrum of Z shows a molecular ion peak (M⁺) at m/z = 74.

($A_r$Ar​: H = 1.0, C = 12.0, O = 16.0)

(a) Use the percentage composition to determine the empirical formula of Z. (3 marks)

(b) Use the empirical formula together with the molecular-ion m/z value to determine the molecular formula of Z, showing your reasoning. (3 marks)

A chemist has a colourless liquid P with molecular formula C₃H₆O. The two structures consistent with this formula are propanal (an aldehyde) and propan-2-ol after partial dehydration is ruled out — in fact the two candidates being considered are propanal and prop-2-en-1-ol (CH₂=CH–CH₂OH).

The infrared spectrum of P shows the following key absorptions.

Absorption / cm⁻¹	Relative intensity
2810 and 2710	medium (two sharp bands)
1715	strong
2950	medium

A broad absorption in the range 3200–3550 cm⁻¹ is absent.

Use these data to deduce the identity of P. In your answer, assign the absorptions to the bonds responsible and explain how the data distinguish propanal from prop-2-en-1-ol.

(5 marks)

For decades the compound CCl₂FCClF₂ (a chlorofluorocarbon, CFC) was used as a refrigerant. In the upper atmosphere, ultraviolet light breaks one of its C–Cl bonds to release a chlorine radical, which then catalyses the breakdown of ozone (O₃).

The C–Cl bond enthalpy in this molecule is about 340 kJ mol⁻¹ and the C–F bond enthalpy is about 485 kJ mol⁻¹.

(a) Use the bond-enthalpy data to explain why ultraviolet light breaks a C–Cl bond rather than a C–F bond in this molecule. (2 marks)

(b) The chlorine radical (Cl•) reacts with ozone, and the product then reacts further to regenerate the chlorine radical. Write the two equations for these radical steps and explain why a small amount of chlorine can destroy a large amount of ozone. (3 marks)

A fragrance chemist is working with butan-2-ol and 2-methylpropan-2-ol, two isomeric alcohols of formula C₄H₁₀O. She treats each, separately, with acidified potassium dichromate(VI) and warms the mixture under reflux.

(a) State the type of alcohol that each molecule is, and predict whether each is oxidised by warm acidified potassium dichromate(VI). (2 marks)

(b) For the alcohol that is oxidised, name the organic product and state the colour change observed in the reaction mixture. (2 marks)

This question is about reactions and analysis of organic compounds.

(a) State what is meant by the term nucleophile. (1 mark)

(b) Ethanol can be converted into ethene by dehydration. State a suitable reagent or catalyst and the condition required for this dehydration. (1 mark)

(c) An infrared spectrum of an organic compound shows a strong absorption at 1700 cm⁻¹. State the bond responsible for this absorption. (1 mark)