ASRJC 2024 H2 Chem Alkanes Notes

2024 JC1 H2 Hydrocarbons: Alkanes 2024/ASRJC/Chemistry 1 ANDERSON SERANGOON JUNIOR COLLEGE JC1 H2 CHEMISTRY HYDROCARBONS: ALKANES Lecture Outline Content 1 Introduction 2 Nomenclature 3 Physical properties 4 Isomerism in alkanes 5 Chemical reactivity of alkanes 6 Chemical reactions of alkanes 7 Chemical reactions of alkanes 8 Hydrocarbon as fuels 9 Car engines emissions and the environmental consequences 10 Conservation of finite resources Learning Outcomes 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 their catalytic removal (ii) gases that contribute to enhanced greenhouse effect (e) recognise that petroleum, a chemical feedstock, is a finite resource and the importance of recycling References 1. Chemistry for Advanced Level, Peter Cann, 2002 2. Chemistry, The Molecular nature of Matter and Change, Silberberg, 2003 3. Chemistry (2nd edition). Chris Conoley and Phil Hill 4. Chemistry in Context. Hill & Holman 5. Understanding Chemistry for Advanced Level. Ted Lister, Janet Renshaw

2024 JC1 H2 Hydrocarbons: Alkanes 2024/ASRJC/Chemistry 2 1. Introduction Hydrocarbons  Hydrocarbons are compounds containing only hydrogen and carbon, and are obtained from crude oil, a fossil fuel.  It is a major energy source.  Alkanes constitute the major part of crude oil. Alkanes are transformed into feedstock for the chemical industry and fuels in internal combustion engines, jet engines and power stations (See Section 8). Huge fractions of energy we use come from combustion of alkanes. Hydrocarbons Alkanes Alkenes Arenes Alkynes (for your information only)

2024 JC1 H2 Hydrocarbons: Alkanes 2024/ASRJC/Chemistry 3 Alkanes  Alkanes are saturated hydrocarbons. All the carbon atoms are joined by single covalent bonds only.  Each C atom is sp3 hybridised, tetrahedrally surrounded by H and other C atoms.  Alkanes can exist as straight chains, branched chains, and rings.  Saturated hydrocarbons with one or more rings of carbon are known as cycloalkanes.  General formula: • for open–chain (i.e. straight–chain and branched–chain) alkanes: CnH2n+2, • for cycloalkanes: CnH2n ; where n equals the number of carbon atoms.  If a hydrogen atom is removed from an alkane, the partial structure that remains is called an alkyl group (usually denoted as R, R’ etc.). Examples of straight–chain and branched–chain alkanes n Condensed structural formula Displayed formula 3–D structure Skeletal formula Alkyl (replace –ane with –yl) 1 CH4 methane C H H H H 109.5o –CH3 methyl 2 CH3CH3 ethane C C H H H H H H –CH2CH3 ethyl 3 CH3CH2CH3 propane C C C HH HH HH HH –CH2CH2CH3 propyl 4 CH3CH2CH2CH3 butane C C C H HH HH HH C H HH –CH2CH2CH2CH3 butyl CH3CH(CH3)2 2–methylpropane C C C H H H H H H H CH H H C C C H HH HH H CH3H CH3CH(CH3)CH2– 2–methylpropyl 5 CH3(CH2)3CH3 pentane C C C H HH HH HH C HH C H HH CH3(CH2)3CH2– pentyl H C H H H H C H H C H H H H C H H C C H H H H H

2024 JC1 H2 Hydrocarbons: Alkanes 2024/ASRJC/Chemistry 4 Examples of cycloalkanes (isomeric with alkenes i.e. same general formula) 2. Nomenclature using the IUPAC system 2.1 Nomenclature for Straight–Chain Alkanes The IUPAC system is the most common method used for naming most organic compounds. Straight– chain alkanes have IUPAC names as shown below. Each member has a suffix ‘ane’. n Molecular formula Displayed formula Skeletal formula Ball–and–stick model 3 C3H6 Cyclopropane 4 C4H8 Cyclobutane 5 C5H10 Cyclopentane 6 C6H12 Cyclohexane Alkane Structural formula Skeletal formula propane CH3CH2CH3 butane CH3CH2CH2CH3 hexane CH3(CH2)4CH3 octane CH3(CH2)6CH3 6

2024 JC1 H2 Hydrocarbons: Alkanes 2024/ASRJC/Chemistry 5 2.2 Nomenclature for Branched–Chain Alkanes In the IUPAC nomenclature, every name consists of: Prefix – Parent – Suffix Steps in naming alkanes: Step Example 1 Identify the parent chain [longest continuous carbon chain] and use it with the suffix. CH3 CH2 C CH2 CH3 CH2 CH2 CH3 H longest chain is hexane 2 Identify the substituent/side chain, if any (prefix). 1 ethyl group 3 Number the carbon chain. Assign lowest possible numbers to the group(s) cited as prefix. CH3 CH2 C CH2 CH3 CH2 CH2 CH3 H Ethyl group on C3 4 Use commas to separate numbers. Use hyphens to separate numbers and words. 3–ethylhexane [prefix]–parent–suffix Note: (For multiple, different types of substituents) 1. For compounds with different multiple substituents, we number the carbon chain to give the substituents the lowest number at the first point of difference. 2. If two or more alkyl groups are attached to the parent chain, they are named in alphabetical order. E.g. (1) butyl (2) ethyl (3) methyl (4) propyl Which is correct? [Circle it] ClBr 1 2 3 4 5 6 7 ClBr 1 2 3 4 5 6 7 ClBr 1 2 3 4 5 6 7 6–bromo–2–chloro–3–methylheptane 2–bromo–6–chloro–5–methylheptane 2–chloro–3–methyl–6–bromoheptane 1 2 3 4 5 6 substituents or side groups no. of C atoms in the longest continuous C chain functional group

2024 JC1 H2 Hydrocarbons: Alkanes 2024/ASRJC/Chemistry 6 3. If the same alkyl group occurs more than once, the prefixes di, tri, or tetra etc. are used to indicate the number of groups. E.g. CH3CH2CH2 C CH3 CH3 H CH3CH2CH2 C CH3 CH3 CH3 CH3 C CH2 C CH3 CH3 H CH3 CH3 2–methylpentane 2,2–dimethylpentane 2,2,4–trimethylpentane Note: (For naming of cycloalkanes) 1. Use the cycloalkane as the parent chain. 2. Number the carbons of the cycloalkane to give the substituents the lowest number at the first point of difference. Hence, the correct name for the following example is 1–ethyl–3–methylcyclohexane. 1 3 1 5 1–ethyl–3–methylcyclohexane 1–ethyl–5–methylcyclohexane Exercise 1 1. Write the IUPAC names of the following compounds. CH3 CH CH3CH2 C CH3 CH3 CH2CH3 2. Write the structural formula for the following compounds. 2,2–dimethylpentane 5–ethyl–2,2–dimethylheptane

2024 JC1 H2 Hydrocarbons: Alkanes 2024/ASRJC/Chemistry 7 3. Physical properties Alkanes contain C–C and C–H bonds. As the difference in electronegativity between C and H atoms is negligible (refer to the Pauling Electronegativity Values) , the C –H bond is essentially non –polar and alkanes are essentially non–polar with the following physical properties. 3.1 Boiling point / volatility  Alkanes have relatively low boiling points (or high volatility).  Table below shows some physical properties of straight–chain alkanes: name formula melting point / C boiling point / C density / g cm–3 (20 C) state at r.t.p. remarks methane CH4 −182 −162 gas gas The C 1 to C 4 straight– chain alkanes are colourless gases at room temperature. ethane CH3CH3 −183 −89 gas gas propane CH3CH2CH3 −188 −42 gas gas butane CH3(CH2)2CH3 −138 −0.5 gas gas pentane CH3(CH2)3CH3 −130 36 0.626 liquid The C 5 to C 17 alkanes are colourless liquids of increasing viscosity. The remaining alkanes containing 18 carbon atoms or more are solids at room temperature. hexane CH3(CH2)4CH3 −95 69 0.659 liquid heptane CH3(CH2)5CH3 −91 98 0.684 liquid octane CH3(CH2)6CH3 −57 126