VJC 2024 Alkanes Lecture Notes

VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT Mr. Rafiqi [muhammad.rafiqi.leman@vjc.edu.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 primary, secondary and tertiary 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 T3W7 T3W8 Tutorial Qns 1–2 3–8

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 7 C7H16 heptane CH3(CH2)5CH3 8 C8H18 octane CH3(CH2)6CH3 • Constitutional isomerism exists in alkanes containing more than three carbon atoms due to branching of chains. E.g. C4H10 (butane) has 2 constitutional isomers. straight chain branched chain C H H CH H H C H H C H H H

3 E.g. C5H12 (pentane) has 3 constitutional isomers. straight chain branched chains • Nomenclature of branched chain isomer Prefix – Parent – Suffix Substituents No. of carbon Functional in the longest chain group 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 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 nu mbered the longest chain from the right. The correct order is the one that gives the lower number at the first point of substituent.) 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 hydrocarbon s that contain a ring of carbon atoms are called cycloalkanes. • General formula is CnH2n. 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. 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, p osition 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 – Q2 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 increase s 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 • It has a lower boiling point than the straight chain isomer. Reason: Branched molecules are more spherical, hence less surface area of contact between molecules , resulting in a decrease in strength of instantaneous dipole–induced dipole interactions. • Effect of branching on packing and on melting point is irregular.

7 4 Preparation Reduction of Alkenes Reagent: H2 gas Condition: Ni catalyst, heat OR Pt or Pd catalyst alkene alkane where R’, R’’, R’’’, R’’’’: H or alkyl groups (can be the same or different