NJC Alkane 2023 Tutorial Answers

1 HYDROCARBONS - ALKANES AND CYCLOALKANES TUTORIAL Suggested answers Structure and naming 1 Give the IUPAC name for each compound. Identify any chiral center and draw its stereoisomers, if any. (a) CH3CH2CH2CH(CH3)CH2CH2CH3 4-methylheptane; no chiral C (b) CH3CH(CH3)CH(CH3)CH2CH3 2,3-dimethylpentane CH(CH3)2 C CH2CH3CH3 H enantiomers CH(CH3)2 C CH3 H CH3CH2 (c) 1,3-dimethylcyclohexane (2 chiral centres with internal mirror plane; actual no of stereoisomers = 3) non-superimposable mirror images C C CH3 HCH3 H C C CH3 H CH3 H ; superimposable mirror images (identical structures) C C H HCH3 CH3 C C H H CH3 CH3 2 Give the structural formula of the following alkanes. (a) 4-ethyl-3,4-dimethylheptane C CH H H C H H C CH3 H C CH2 CH3 C H H CH3 C H H H H H (b) a saturated compound with molecular formula C5H10 (cannot be alkene; must be cycloalkane - any of the structures below)

2 Physical Properties 3 Crude oil consists mainly of alkanes, the saturated hydrocarbons. The alkanes in crude oil can be separated because they have different boiling points. The table below shows the boiling points of some alkanes. Alkane Boiling point / oC Mr Butane 0 58 Pentane 36 72 Hexane 69 86 2-methylbutane 28 72 2,2-dimethylpropane 10 72 3-methylpentane ? 86 2,3-dimethylbutane 58 86 (a) State what is meant by the term saturated hydrocarbon. A compound containing: - only C and H atoms where all C atoms form 4 single covalent bonds each. - only single bonds, no multiple bonds (b) (i) Explain the trend in boiling points of the straight chain alkanes. boiling point increases from butane to hexane, due to: - increasing Mr; hence size of electron cloud increases - increasing electron cloud size leads to more easily induced dipole/ greater ease of distortion - strength of intermolecular id-id interactions increases, - bp increases as more E is required to break the increasingly stronger IMF (ii) Explain the difference in the boiling points of the three isomers with Mr = 72. - decreasing order of bp: pentane > 2-methylbutane > 2,2-dimethylpropane - increasing branching of isomers leads to smaller surface area of contact between molecules - hence weaker intermolecular id-id interactions - less E needed to break the IMF and hence decreasing bp in branched compounds (iii) With reference to your answer in b(ii), suggest a possible value for the boiling point of 3-methylpentane. Any value: 58 oC - 69 oC (higher than 2,3-dimethylbutane but lower than hexane)

3 Combustion of hydrocarbon 4 What is the volume of CO2 formed when y cm3 of a 50:50 mixture of methane and propane is completely burnt? A 2y cm3 B 5𝑦 2 cm3 C 4y cm3 D 5y cm3 CH4 + 2O2 → CO2 + 2H2O V / cm3 ½y ½y C3H8 + 5O2 → 3CO2 + 4H2O V / cm3 ½y 2 3y Total vol of CO2 produced = 2y cm3 Ans: A 5 When 15 cm3 of a gaseous hydrocar