VJC 2024 Alkanes Tutorial

1 Victoria Junior College 2024 JC1 H2 Organic Chemistry Tutorial: Alkanes Summary of Reactions Complete the following table. Starting compound Reaction type Reagents & Conditions Products formed (CH3)2C=CH2 Reduction OR (CH3)2CHCH3 CH4 CCl4 + HCl CHCH3 CH3 CH3 Limited Cl2, UV light (OR high temperature OR heat) CHCH3 CH3 CH2Cl CClCH3 CH3 CH3 + HCl Introduction 1 Write down the displayed formula and skeletal formula for each of the following. (a) 2,2–dimethylbutane (b) 3–ethyl–2,4–dimethylpentane 2 Name the following compounds. (a) CH3(CH2)2CH(CH3)CH(CH3)2 (b) CH3(CH2)2CH(CH3)CH(C2H5)CH(CH3)(C2H5) Reaction mechanism 3 Chloroalkanes and bromoalkanes can be made by the reaction of the corresponding halogen with alkanes, for example: C2H6 + Cl2 → C2H5Cl + HCl (a) State the conditions needed for this reaction. (b) Name and describe the mechanism of this reaction. (c) Why is this reaction described as given in (b)? (d) Suggest why it is not possible to make iodoalkanes by this method. [N10/III/1(c) & (e)]

2 4 Propane undergoes free–radical substitution when mixed with chlorine and exposed to ultra–violet light. Which compounds are possible products from this reaction? 1 CH3CH2CH2Cl 2 CH2ClCH2CH2Cl 3 CH3CH2CH2CH2CH2CH3 4 CH3CH2CH2CH2CH3 A 1, 2 and 3 B 1, 3 and 4 C 1 and 2 only D 2 and 3 only [N19/I/18] 5 Thiocyanogen, (SCN) 2 is a symmetrical pseudohalogen which shows similar chemical reactivity to halogens. When pentane is mixed with thiocyanogen in the presence of UV light, it reacts via the free radical mechanism and compound A was isolated. CH3CH(CH2)2CH3 | S–CΞN Compound A (a) (i) Identify the radical formed in the initiation step for the formation of compound A. (ii) Write an equation to show how this radical is formed from (SCN) 2. You are to draw the full structural formula of (SCN)2 and use ar rows to indicate the movement of electrons so that it is clear which bonds are broken. (b) (i) Propose the 2 steps for the propagation stage of the reaction mechanism in the formation of compound A from pentane. (ii) Draw the two isomers of compound A which show different behaviours towards plane–polarised light. [Modified 2014 VJC Promo P2S2] Substitution at primary, secondary and tertiary carbon atoms 6 When an alkane F, with the formula C 6H14, reacted with bromine under ultraviolet light, it produced only two isomeric monobromo compounds G and H, with the formula C6H13Br. Compound H was chiral. (a) Suggest the structures of F, G and H, explaining your reasoning. (b) Hence, state the expected ratio of the two isomeric monobromo compounds G and H formed based on the probability of substitution of the different classes of hydrogens by halogens. (c) From the above reactions, ratio of products obtained from probability consideration differs from experimental yield. (i) Suggest another factor that will affect the ratio of products obtained. (ii) Explain the factor which you have stated in (c)(i). [Modified N08/III/2(g)]

3 Environmental Pollution and use of Catalytic Converters 7 A catalytic converter is part of the exhaust system of modern cars. Which reactions occur in a catalytic converter? 1 2CxHy + (4x + y)NO → 2xCO2 + yH2O + (2x + y/2)N2 2 2CO + 2NO → 2CO2 + N2 3 CO2 + NO → CO + NO2 A 1, 2 and 3. B 1 and 2 only C 2 and 3 only D 1 only [N14/I/38] Identification of reaction pattern 8 In an industrial process, heptane vapour is passed over a heated catalyst to make methylbenzene. CH3CH2CH2CH2CH2CH2CH3 CH3 + 4H2 Under similar conditions, which of the C8H18 isomers could give 1,4–dimethylbenzene? CH3 CH3 1,4–dimethylbenzene 1 CHCH2CH2CH2CH3 CH3CH2 CH3 2 CHCH2CH2CH CH3 CH3 CH3 CH3 3 CHCH2CH2CH2CH2CH3 CH3 CH3 A 1, 2 and 3 B 1 and 3 only C 1 and 2 only D 2 and 3 only [Modified N12/I/40]

4 Supplementary Questions 1 What happens when one mole of ethane is mixed in the dark at room temperature with six moles of chlorine? A There is no reaction. B CH3CH2Cl and HCl are formed. C CH3CCl3 and HCl are formed. D CCl3 and HCl are formed. 2 Two structural isomers of molecular formula C6H14 are shown. P (CH3)2CHCH2CH2CH3 Q CH3CH2CH(CH3)CH2CH3 P and Q react with chlorine to form monochloro compounds C6H13Cl. How many possible structural isomers, each with formula C 6H13Cl, could be produced by P and by Q? isomers formed by P isomers formed by Q A 5 3 B 5 4 C 6 3 D 6 4 3 The use of Data Booklet may be relevant to this question. Which would be the easiest initiating step in a free –radical process (i.e. the one involving least energy)? A HCl → H• + Cl• B Cl2 → 2Cl• C CH4 → •CH3 + H• D (C6H5)CH3 → (C6H5)CH2• + H• 4 Which one of the following is a propagation step in the reaction between methane and chlorine when they are irradiated with light? A H• + Cl2 → HCl + Cl• B CH4 + Cl+ → CH3Cl + H+ C CH4 + Cl• → CH3• + HCl D CH4 + Cl• → CH3Cl + H• 5 When heated with chlorine, the hydrocarbon 2,2 –dimethylbutane undergoes free radical substitution. In a propagation step, the free radical X • is formed by the loss of one hydrogen atom. C CH3 CH3CH2 CH3 CH3 + Cl X + HCl How many different forms of X• are theoretically possible? A 1 B 2 C 3 D 4 6 The data below refers to straight chain alkanes. No. of C atoms Formula B.p. / oC Density of liquid alkane / g cm–3 1 CH4 –161 0.42 3 C3H8 –42 0.50 8 C8H18 126 0.70 15 C15H32 271 0.77 20 C20H42 344 0.78 Explain the trend in boiling point and in density as shown by these data. • •

5 7 Describe the reagents and conditions used to produce chlorocyclopentane from cyclopentane and the mechanism of the reaction. State the probability for chlorocyclopentane to undergo another substitution to form 1,2–dichlorocyclopentane. 8 Integrated question (a) Calculate the ∆Hr for each step as shown below with the given bond energy data. Bond Bond energy / kJ mol–1 Bond Bond energy / kJ mol–1 H–H 436 Cl–Cl 244 H–Cl 431 C–Cl 340 H–C 410 Free–Radical Substitution 1 Cl2 + CH4 + Cl2 Initiation  UV light ∆Hr = 2 Cl2 + CH4 + 2Cl• Propagation  ∆Hr = 3 Cl2 + •CH3 + HCl + Cl•  ∆Hr = 4 Cl• + CH3Cl + HCl + Cl• Termination  ∆Hr = 5 Cl2 + CH3Cl + HCl (b) Hence, draw the energy profile for the reaction between CH4 and Cl2. (c) Using the energy profile diagram, explain why the use of UV light is only required for a short period of time. (d) Calculate the ∆ Hr for the 2 equations and hence explain the feasibility of the two possible propagation steps. CH4 + Cl• → •CH3 + HCl ∆Hr = CH4 + Cl• → CH3Cl + H• ∆Hr = Numerical Answers to supplementary questions: 8(a) +244 kJ mol–1, –21 kJ mol–1, –96 kJ mol–1, –244 kJ mol–1 8(d) –21 kJ mol–1, +70 kJ mol–1