AQA A-Level Chemistry: Organic Chemistry Foundations

Propene reacts with hydrogen bromide at room temperature. The major product is 2-bromopropane (CH₃CHBrCH₃) rather than 1-bromopropane (CH₃CH₂CH₂Br).

Describe the mechanism for the reaction of propene with HBr, and explain why 2-bromopropane is the major product.

In your answer you should refer to the electrophile, the role of the C=C double bond, the intermediate formed, and the factor that determines which product predominates.

(6 marks)

A student prepared cyclohexene by the acid-catalysed dehydration of cyclohexanol, then purified the product by distillation. The equation for the reaction is:

C₆H₁₁OH → C₆H₁₀ + H₂O

The student started with 20.0 g of cyclohexanol and obtained 12.3 g of pure cyclohexene.

Relative molecular masses: cyclohexanol C₆H₁₁OH = 100.0; cyclohexene C₆H₁₀ = 82.0; water H₂O = 18.0.

(a) Calculate the percentage yield of cyclohexene. Show your working. (3 marks)

(b) Calculate the percentage atom economy for the formation of cyclohexene in this reaction. (2 marks)

(c) State one reason, other than experimental losses on transfer, why the percentage yield is below 100 %. (1 mark)

There are three alkene structural isomers with the molecular formula C₄H₈ (ignore cyclic structures).

(a) Draw or write the structural formula of, and name, each of the three alkene structural isomers of C₄H₈. (3 marks)

(b) One of these isomers exhibits E/Z (cis–trans) stereoisomerism. Identify it, and explain the two conditions that a molecule must satisfy in order to show E/Z isomerism. (2 marks)

1-iodopentane, CH₃CH₂CH₂CH₂CH₂I, is warmed under reflux with potassium cyanide dissolved in aqueous ethanol.

(a) Give the structural formula and name of the organic product, and name the mechanism taking place. (3 marks)

(b) Under the same conditions, 1-chloropentane reacts more slowly than 1-iodopentane. Explain this difference in rate. (2 marks)

Two alcohols are each warmed with acidified potassium dichromate(VI):

• Alcohol P: 2-methylbutan-1-ol, CH₃CH₂CH(CH₃)CH₂OH
• Alcohol Q: 2-methylbutan-2-ol, CH₃CH₂C(CH₃)₂OH

(a) For alcohol P, name the organic product formed when the reaction is carried out (i) under distillation and (ii) under reflux, and state the colour change seen in each case. (3 marks)

(b) Alcohol Q is not oxidised by acidified potassium dichromate(VI). Explain why. (1 mark)

(a) Give the IUPAC name of the compound with the structural formula CH₃CH₂CH(CH₃)CH₂Br. (1 mark)

(b) Ethanol is manufactured industrially by the direct hydration of ethene. State the reagent and the conditions (catalyst, temperature and pressure) required for this reaction. (2 marks)