MCQ Chapter 2 1 Which of the following cycloalkanes has the lowest heat of combustion per CH2 group? A Cyclopropane B Cyclobutane C Cyclopentane D Cyclohexane Solution D 2 The general formula of a cycloalkane is A CnH2n+2 B CnH2n C CnH2n-2 D CnHn Solution B 3 In the presence of light, alkanes react with molecular chlorine to give substitution products. So methane can be converted to chloromethane etc. How many dichlorinated structural isomers can be formed by halogenation of butane with chlorine in the presence of light? A 2 B 3 C 5 D 6 Solution D 4 What is the IUPAC name for the following compound? A Trimethylpentane B 2,4,4-trimethylpentane C 2,2,4-trimethylpentane D 2,2,4,4-tetramethylbutane Solution C 5 How many constitutional isomers are there with formula C4H9Cl? A 3 B 4 C 5 D 6 Solution B 6 Which of the following C6H12 isomers has the highest heat of combustion? A Cyclohexane B Methylcyclopentane C cis-1,3-dimethylcyclobutane D Trimethylcyclopropane Solution D 7 Two hydrocarbons found in crude oil are decane, C10H22 and hexadecane, C16H34. Which of the following statements about the properties of hexadecane compared with the equivalent property of decane is correct? A Hexadecane has a lower boiling point B Hexadecane is less viscous C Hexadecane is more volatile D Hexadecane catches fire less easily Solution D 8 Which two of the following products are produced by the complete combustion of fuels? I CH4; II CO; III CO2; IV H2O; V NH3 A I and III B I and II C III and IV D IV and V Solution C 9 Water has a higher boiling point than compounds of similar molecular weight. What is the best explanation for this? A Extensive hydrogen bonding B Water is a polar covalent compound C Water is largely dissociated leading to large electrostatic forces D London dispersion forces exist between the molecules Solution A 10 Which of the following molecules does NOT have a permanent dipole moment? CHCl3 CH2Cl2 CH3Cl CCl4 A B C D Solution D 11 Which of the following molecules HAS a permanent dipole moment? A CH4 B CO2 C Cyclohexane D NH3 Solution D 12 What is the IUPAC name for Cl A chlorotrimethylbutane B 2-chloro-2,4-dimethylpentane C 4-chloro-2,4-dimethylpentane D 1-chloro-1,1-3trimethylbutane Solution B 13 What is the IUPAC name for A cis-1,3-dimethylcyclopentane B trans-1,3-dimethylcyclopentane C 1,3-dimethylcyclopentane D 1,3-dimethylcyclopentene Solution A 14 What are the most important intermolecular forces in H2? A Hydrogen bonding B Dispersion forces C Dipole-dipole interactions D Electrostatic interactions Solution B 15 What is the IUPAC name for Cl A chloroethylcycloexane B 2-chloro-1-ethylcyclohexane C 1-chloro-1-ethylcyclohexane D 1-ethyl-1-chlorocyclohexane Solution C 16 Cyclohexane boils at a higher temperature than n-hexane. What is the best explanation for this observation? A Cyclohexane stacks better in the liquid phase B Cyclohexane has a lower molecular weight than hexane C Cyclohexane can hydrogen bond D Dipolar forces are stronger in cyclohexane Solution A 17 Which of the following molecules does NOT show sp3 hybridization at the central atom A CH4 B CH3F C C2H4 D C2H6 Solution C 18 The angles between sp3 orbitals are approximately A 90 o B 109 o C 120 o D 180 o Solution B 19 The stick drawing of CH3CH2CH(CH3)CH(CH3)CH2CH3 is A B C D Solution A 20 The stick diagram for 2,5-dichloro-2,5-dimethylhexane is A Cl Cl B Cl Cl C Cl Cl D Cl Cl Solution D 21 Which of the following cycloalkanes would have the LOWEST heat of combustion per CH2 group? A Cyclopropane B Cyclopentane C Cyclohexane D Cyclodecane Solution C 22 Which of the following hydrocarbons would be expected to have the LOWEST boiling point? A n-hexane B 2-methylpentane C 2,2-dimethylbutane D They would all have the same boiling point Solution C 23 For which of the following molecules are dispersion forces NOT the main source of intermolecular attraction? A He B CH4 C CF4 D CH3F Solution D Chapter 2 – Structure, bonding and nomenclature of alkanes Test questions - solutions • 1 What are the most important intermolecular forces in • (a) He (b) N2 (c) O2 (d) Br2 (e) HBr (f) H2O (g) Chloroethane (h) hexane? • (a) He dispersion forces • (b) N2 dispersion forces • (c) O2 dispersion forces • (d) Br2 dispersion forces • (e) HBr dipole-dipole interactions • (f) H2O hydrogen bonding • (g) Chloroethane dipole-dipole interactions • (h) Hexane dispersion forces • 2 Give an example of each of the following • (a) A substituted methane with a permanent dipole moment • Chloromethane • (b) A linear triatomic molecule without a permanent dipole moment • CO2 • (c) A molecule that forms strong hydrogen bonds •H2O • (d) A cis-disubstituted cycloalkane • 3 What shape would you predict for each of the following molecules? Consider geometry at nitrogen and oxygen as well as carbon. • (a) CHCl3 • Approx. tetrahedral, sp3 hybridization • (b) CF4 • Perfect tetrahedron, sp3 hybridization • (c) CH3CH2OH • CH3, CH2 - approx. tetrahedral. • Bent at O, with two lone pairs. • All atoms are sp3 hybridized • (d) CH3NHCH2CH3 • CH3, CH2 approx. tetrahedral. • Pyramidal at N, with one lone pair. • All atoms are sp3 hybridized • 4 Give a systematic name for each of the following molecules • (a) Cl Cl • This is a 1,4-disubstituted cyclohexane - no relative stereochemistry is indicated. • If the two carbons attached to the ring had been methyl groups, it would be 1,4-dimethylcyclohexane • But each of the methyl groups is substituted by chlorine, so • 1,4-bis(chloromethyl)cyclohexane • (b) • 3-ethyl-3-methylpentane • (c) 1 4 8 2 3 6 5 7 • This time we must be careful to find the longest chain in a less conventional drawing – this will be named as an octane • The substituents are at C-3 and C-6 • So 3,6-diethyloctane 2 • (d) 1 4 3 Br 5 • We need to check the longest chain carefully as this a less conventional drawing • 3-(bromomethyl)pentane • (e) • cis-1,3-diethylcyclohexane • (f) F • 1-fluoro-1,2,2-trimethylcyclobutane • (g) F • cis-1-fluoro-3-ethylcyclohexane • 1R,2S-1-fluoro-3-ethylcyclohexane • (h) 3 1 2 4 Br • We number the molecule to put the bromine on the lowest numbered carbon atom • 1-bromo-2,2-dimethylbutane • 5 Draw a stick structure for each of the following molecules • (a) 5-bromodecane Br • (b) 2,2-dimethylbutane • (c) 2-iodoethylcyclohexane I • (d) 4,4-difluoroheptane F F • (e) 3-ethyl-4-methylnonane • (f) 1-chloro-1-methylcyclopentane Cl • (g) cis-1,2-dichlorocyclooctane Cl Cl • (h) 1-dichloromethyl-1-methylcyclohexane Cl Cl • 6 For each of the following pairs of molecules, predict which would have the higher boiling point. Give a brief reason • (a) hexane and cyclohexane • hexane 69 oC, cyclohexane 81 oC. Cyclohexane stacks better in the liquid phase • (b) hexane and 2,2-dimethylbutane • Hexane 69 oC, 2,2-dimethylbutane 49 oC. • Substituents make packing more difficult in the liquid phase, and reduce intermolecular interactions • (c) hexane and 1-fluorohexane • Hexane 69 oC, 1-fluorohexane 93 oC. • In 1-fluorohexane there are dipole-dipole forces to overcome as well as the dispersion forces that exist between all molecules. Chapter 2 Alkanes Test questions 1 What are the most important intermolecular forces in (a) He (b) N2 (c) O2 (d) Br2 (e) HBr (f) H2O (g) Chloroethane (h) hexane 2 Give an example of each of the following (a) A substituted methane with a permanent dipole moment (b) A linear triatomic molecule without a permanent dipole moment (c) A molecule that forms strong hydrogen bonds (d) A cis-disubstituted cycloalkane 3 What shape would you predict for each of the following molecules? Consider geometry at nitrogen and oxygen as well as carbon. (a) CHCl3 (b) CF4 (c) CH3CH2OH (d) CH3NHCH2CH3 4 Give a systematic name for each of the following molecules (a) Cl Cl (b) (c) (d) Br (e) (f) F (g) F (h) Br 5 Draw a stick structure for each of the following molecules (a) 5-bromodecane (b) 2,2-dimethylbutane (c) 2-iodoethylcyclohexane (d) 4,4-difluoroheptane (e) 3-ethyl-4-methylnonane (f) 1-chloro-1-methylcyclopentane (g) cis-1,2-dichlorocyclooctane (h) 1-dichloromethyl-1-methylcyclohexane 6 For each of the following pairs of molecules, predict which would have the higher boiling point. Give a brief reason (a) hexane and cyclohexane (b) hexane and 2,2-dimethylbutane (c) hexane and 1-fluorohexane Chapter 2 Alkanes and friends – structure, bonding, properties, nomenclature • At the end of the last chapter, we had learned how to combine atomic orbitals into covalent bonds for simple systems – two s-orbitals in molecular hydrogen and an s and a p-orbital to make a C-H bond, and p-orbitals to make either σ- or π-bonds. • Unfortunately, these unsophisticated pictures do not allow us to understand even the simplest organic molecule, methane. • Carbon is four-valent, and the methane molecule has been long known, and convincingly shown, to be a perfect tetrahedron, but none of the orbitals we have seen so far are directed towards the corners of a tetrahedron. • In order to make the right orbitals, we will need to mix, or “hybridize” s- and p-orbitals. • Organic chemists need a model that reliably produces and predicts what we need to understand organic molecules and their reactions. • Physical chemistry involves more serious mathematics and needs other parameters; it may be more accurately descriptive but it is less usable for the complex molecules involved in organic chemistry.