VJC H2 Chem Chpt 12 Alkanes Lecture Notes

VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT Mr Chang Siang Wen [chang.siangwen@vjc.sg] H2 ORGANIC CHEMISTRY: ALKANES Lecture Outline 1 Introduction 2 Cycloalkanes 3 Physical Properties 4 Preparation 5 Reactions 5.1 Combustion 5.11 Limited supply of oxygen 5.12 Unlimited supply / excess of oxygen 5.2 Formation of Halogenoalkanes 5.21 Free radical substitution mechanism 5.22 Reactivity of halogens 5.23 Rate of substitution at 1o, 2o and 3o carbon atoms 6 Petroleum and its Environmental Consequences 6.1 Environmental Pollutants 6.2 Use of Catalytic Converter 6.3 Enhanced Greenhouse Effect Assessment Objectives Candidates should be able to: (a) explain the general unreactivity of alkanes, including towards polar reagents (b) describe the chemistry of alkanes as exemplified by the following reactions of ethane: (i) combustion (ii) free–radical substitution by chlorine and by bromine (c) describe the mechanism of free– radical substitution with particular reference to the initiation, propagation and termination reactions (d) recognise the environmental consequences of: (i) carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the internal combustion engine and of their catalytic removal (ii) gases that contribute to the enhanced greenhouse effect (e) recognise that petroleum, a chemical feedstock, is a finite resource and the importance of recycling References 1 Chemistry in Action by Michael Freemantle 2 ‘A’ Level Chemistry by E.N. Ramsden 3 Chemistry – Longman ‘A’ Level Guides by J.G.R. Briggs 4 Organic Chemistry by H.L. Heys 5 Organic Chemistry by B.J. Stokes Lecture 1 2 Pages 1–8 8–15 Complete by 18 Aug 23 Aug Tutorial Qns 1–3 4–7

2 Hydrocarbons (Compounds containing only carbon and hydrogen atoms) Aliphatic hydrocarbon Alicyclic hydrocarbon Aromatic hydrocarbon or Arenes (Benzene and its alkyl derivatives) 1. Alkanes 2. Alkenes 1. Cycloalkanes E.g. cyclohexane 2. Cycloalkenes E.g. cyclohexene 1 and 2 contain rings of carbon atoms. 1. Benzene 2. Benzene derivatives E.g. methylbenzene 3. Naphthalene and others ALKANES 1 Introduction • Homologous series of saturated aliphatic hydrocarbon of general formula CnH2n+2. • Names of alkanes end with ‘ane’. n molecular formula name structural formula 1 CH4 methane CH4 2 C2H6 ethane CH3CH3 3 C3H8 propane CH3CH2CH3 4 C4H10 butane CH3(CH2)2CH3 5 C5H12 pentane CH3(CH2)3CH3 6 C6H14 hexane CH3(CH2)4CH3 • Constitutional isomerism exists in alkanes containing of more than three carbon atoms due to branching of chains. E.g. C4H10 has 2 constitutional isomers. straight chain branched chain CC HH HH HH CC CC CC HH HH HH HH HH HH HH C H H CH H H C H H C H H H

3 E.g. C5H12 has 3 constitutional isomers. straight chain branched chain • Nomenclature of branched chain isomer Prefix – Parent – Suffix Substituents No. of carbon Functional group in the longest chain Step 1: Identify parent chain (longest continuous carbon chain) and name compound after it. Step 2: Identify substituent(s) of parent chain which are alkyl group( s) (general formula: CnH2n+1). E.g. alkane parent chain substituent(s) butane methyl hexane methyl, ethyl Step 3: Determine position(s) of alkyl group(s) by numbering the c arbon atoms consecutively from one end of parent chain to the other. (i) Number such that a smaller number is given to the carbon atom(s) attached to the substituent(s). E.g. alkane correct name wrong name 2–methylbutane 3–methylbutane CC HH HH HH CC HH CC HH HH CC HH HH HH CC HH HH HH CCHH33 CCHH CCHH22 CCHH33 CCHH33 CCHH33 CCHH CCHH22 CCHH CCHH22 CCHH33 CCHH33 CCHH22 CCHH33 CCHH33 CCHH CCHH22 CCHH33 CCHH33 CC HH HH HH CC CC CC HH HH CC HH HH HH HH HH HH HH CC HH HH HH CC CC CC CC HH HH HH HH HH HH HHHH HH 1 2 3 4

4 E.g. alkane correct name wrong name 3–ethylhexane 4–ethylhexane 3–methyloctane 6–methyloctane 2–ethylheptane (ii) If some alkyl groups occur more than once as substituents, indicate prefix di–, tri–, tetra–, etc. E.g. alkane name 2,2–dimethylpentane 2,2,4–trimethylpentane (wrong name: 2,4,4–trimethylpentane) (iii) Arrange substituents in alphabetical order , ignoring the prefixes such as di– or tri–. E.g. 4–ethyl–2–methylhexane (wrong name: 2–methyl–4–ethylhexane) (Wrong name: 3– ethyl–5–methylhexane or 5– methyl–3–ethylhexane. This is obtained when we numbered from the right . The correct order is the one that gives the lower number at the first point of substituent.) CCHH33 CCHH22 CCHH22 CCHH CCHH22 CCHH33 CCHH22 CCHH33 CCHH33 CCHH CCHH22 CCHH22 CCHH22 CCHH22 CCHH33 CCHH22 CCHH33 CCHH33 CC CCHH22 CCHH22 CCHH33 CCHH33 CCHH33 CCHH33 CC CCHH22 CCHH CCHH33 CCHH33 CCHH33 CCHH33 CCHH33 CCHH CCHH22 CCHH CCHH22 CCHH33 CCHH33 CCHH22 CCHH33 3 3 2 1 1 2 1 2 3 4 5 6 2 4 2 8 7 6 5 4 4 5 6 1 1 3 3 4 5 5

5 Self–practice 1 (Check your answers on SLS Alkanes Lesson 1) Name the following organic compounds using the IUPAC system. (i) CH3 CH CH2 CH CH3 CH2CH3 CH3 (ii) CH3 C CH3 CH2CH3 CH2 C CH3 CH2CH2CH3 H 2 Cycloalkanes • Saturated hydrocarbons that contain a ring of carbon atoms are called cycloalkanes. • General formula is C nH2n. E.g. name structural formula skeletal formula Cyclopropane Cyclobutane Cyclopentane Methylcyclopentane Cyclohexane 1,2–dimethylcyclohexane • Chemical behaviour of cycloalkanes is similar to the open–chain alkanes. HH22CC HH22CC CCHH22 HH22CC HH22CC CCHH22 CCHH22 CCHH22 HH22 CC HH22CC HH22CC CCHH22 HHCC HH22 CC HH22CC HH22CC CCHH22 CCHH33 HH22CC HH22CC CCHH22 CCHH HHCC HH22 CC CCHH33 CCHH33 CH2 CH2 CH2 CH2 CH2 CH2

6 Making Thinking Visible Question: Why is it not necessary to name methylcyclopentane as 1–methylcyclopentane? Answer: Only one substituent on cyclopentane. Hence, position number is not required as there is no distinction between the carbon atoms. Question: Why is 1,2–dimethylcyclohexane not named as 3,4– dimethylcyclohexane or 5,6–dimethylcyclohexane? Answer: The position numbers are chosen to be as low as possible. You can now attempt Tutorial Questions: Q1 – Q3 3 Physical Properties I. Solubility, Density and Viscosity • Alkanes are non–polar. They are insoluble in polar solvents such as water but soluble in non–polar solvents, such as benzene, ether (e.g. dimethyl ether, CH3−O−CH3) and CCl4. Reason: Alkanes do not dissolve in water since the energy released from the weaker permanent dipole–induced dipole interactions between the polar water molecules and non–polar alkane molecules is insufficient to overcome the stronger hydrogen bonding between the water molecules. • Alkanes float on top of water and thus are less dense than water. • Viscosity (more viscous, less fluid) of liquid alkanes increases with increasing molecular mass as long molecules have stronger instantaneous dipole −induced dipole interactions (due to greater number of electrons). II. Boiling and Melting Points (i) Straight chain alkanes • At room temperature and atmospheric pressure, first four alkanes (C 1–C4) are gases; next thirteen alkanes (C5–C17) are liquids; C18 or more are solids. Reason: Boiling point increases with number of carbon atoms as more energy is required to overcome the increasing strength of instantaneous dipole– induced dipole interactions due to increasing number of electrons. • Melting point increases in a zig–zag pattern as intermolecular forces in a crystal depend on the size of the molecules and how well they are packed into the solid lattice. (ii) Branched chain isomers