OCR A-Level Chemistry: Basic Organic, Alkanes & Alkenes

Methylpropene, $(\text{CH}_3)_2\text{C}=\text{CH}_2$(CH3​)2​C=CH2​, reacts with hydrogen bromide gas at room temperature. The reaction gives 2-bromo-2-methylpropane as the major product and only a trace of 1-bromo-2-methylpropane.

Describe and explain the mechanism of this reaction. In your answer you should name the type of mechanism, describe the role of the C=C π electrons and the movement of electron pairs (curly arrows in words), name the intermediate, and use the relative stability of the carbocation intermediates to explain why 2-bromo-2-methylpropane is the major product (Markovnikov's rule).

(6 marks)

A gaseous hydrocarbon X is a single compound containing only carbon and hydrogen. When a $0.860\,\text{g}$0.860g sample of X is burned completely in excess oxygen, the only products are $2.64\,\text{g}$2.64g of carbon dioxide and $1.08\,\text{g}$1.08g of water.

Substance	Mass / g	$M_r$Mr​
Carbon dioxide, CO₂	2.64	44.0
Water, H₂O	1.08	18.0

(a) Use the combustion data to determine the empirical formula of X. (4 marks)

(b) The relative molecular mass of X is $86.0$86.0. Deduce the molecular formula of X, and state whether X could be an alkane or an alkene, justifying your choice using the general formula. (2 marks)

(Relative atomic masses: $A_r(\text{C}) = 12.0$Ar​(C)=12.0, $A_r(\text{H}) = 1.0$Ar​(H)=1.0.)

An alkene W has the molecular formula C₄H₇Cl and contains one C=C double bond. One of its structures can be written as $\text{CH}_3\text{-CCl}=\text{CH-CH}_3$CH3​-CCl=CH-CH3​ (2-chlorobut-2-ene).

(a) Explain why $\text{CH}_3\text{-CCl}=\text{CH-CH}_3$CH3​-CCl=CH-CH3​ shows E/Z stereoisomerism, with reference to the C=C bond. (2 marks)

(b) Using the Cahn–Ingold–Prelog (CIP) priority rules, deduce whether the stereoisomer drawn with the two methyl groups on opposite sides of the double bond is the E or the Z isomer. Justify the priority assignment on both carbons of the C=C. (3 marks)

(Atomic numbers: Cl = 17, C = 6, H = 1.)

In the upper atmosphere, chlorine radicals ($\text{Cl}\cdot$Cl⋅) generated from pollutants can attack ethane, $\text{C}_2\text{H}_6$C2​H6​, which is present in trace amounts. The first propagation step is:

$\text{Cl}\cdot + \text{C}_2\text{H}_6 \rightarrow \text{C}_2\text{H}_5\cdot + \text{HCl}$Cl⋅+C2​H6​→C2​H5​⋅+HCl

(a) State the type of bond fission occurring when the C–H bond breaks in this step, and explain why the species formed are described as radicals. (2 marks)

(b) Write an equation for the second propagation step, in which the ethyl radical reacts with a chlorine molecule, $\text{Cl}_2$Cl2​, and explain why these two steps make the overall process a chain reaction. (2 marks)

(c) Write one possible termination step for this system. (1 mark)

A branched alkene Y has the structural formula $\text{CH}_2=\text{C(CH}_3\text{)CH}_2\text{CH}_3$CH2​=C(CH3​)CH2​CH3​.

(a) Give the IUPAC name of Y. (2 marks)

(b) Y can be polymerised by addition. Write the repeat unit of the addition polymer formed from Y (as a structural formula in text), and state the empirical-style name of this polymer in the form poly(monomer). (2 marks)

This question is about alkanes and alkenes.

(a) Define the term structural isomers. (1 mark)

(b) In the free-radical substitution of methane by chlorine, write one equation for a termination step that produces ethane. (1 mark)

(c) State what you would observe when bromine water is shaken with an alkene such as ethene, and what this tells you about the alkene. (1 mark)