RI 2022 Alkanes v2.0

• Content • Alkanes (exemplified by ethane) RAFFLES INSTITUTION YEAR 5 H2 CHEMISTRY 2022 Lecture Notes 10: Alkanes (i) free-radical substitution reactions • Hydrocarbons as fuels 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 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 Lecture Outline 1 2 3 4 5 6 7 8 Introduction Isomerism in alkanes Physical properties Preparation of alkanes Reactions of alkanes Free-radical substitution Alkanes as fuels Summary References • Chemistry in Context (by Graham Hill & John Holman) • Cambridge International AS and A Level Chemistry (by Peter Cann & Peter Hughes) • Website: www.chemguide.co.uk aliphatic I organic compounds I hydrocarbons halogen derivatives hydroxy compounds carbonyl compounds aromatic carboxylic acids & derivatives nitrogen compounds saturated unsaturated .----I.__, .--__._I -.,.-__, I I I open-chain alicyclic alkenes alkynes arenes (acyclic or (ring) non-?clic) \ alkanes cycloalkanes CH CnH2n n 2n+2 -1-

_ 1---Ll_ln_tr_o_d_uc_t_:lo_n _____ _______________ ___ • The alkanes are the simplest of the homologous series found in organic chemistry. • Alkanes are hydrocarbons since they contain only carbon and hydrogen. Other hydrocarbons include alkenes, alkynes and arenes. • Alkanes are saturated hydrocarbons because they contain only carbon-carbon and carbon-hydrogen single bonds and do not contain any double or triple bonds. Unsaturated hydrocarbons such as alkenes and alkynes contain multiple bonds (either double or triple bonds) between carbon atoms. • The open-chain alkanes form a homologous series of saturated hydrocarbons with. the general molecular formula, CnH2n+2. Successive members differ in composition by one CH2 group (methylene group). • Each C atom in an alkane molecule is sp3 hybridised and has a tetrahedral molecular geometry around it. The bond angle around each C atom (in an open-chain alkane) is approximately 109.5°. • Cycloalkanes are alkanes that contain one or more rings of carbon atoms. They are saturated hydrocarbons in which the carbon atoms are arranged to form rings. • Cycloalkanes with only one ring have the same general molecular formula (i.e. CnH2n) as the alkenes with only one C=C bond. Tip: For an unknown compound with molecular formula CnH2n, it could likely be a cycloalkane or an alkene. The first three members of the open-chain alkanes are methane, ethane and propane. condensed structural formula displayed formula D-14 methane CH3CH2CH3 propane H I H-C-H I H H H I I H-C-C- H I I H H H H H I I I H-C-C-C-H I I I H H H 3-D structure H I _........c .... ,,,H H \ A H..-'109.5° ball-and-stick model The first four members of cycloalkanes are shown below. Cyclopropane C3H6 -2- H H I I H- C'-C-H I I H-C - C-H I I H H Cyclobutane C4H8 Cyclopentane C5H10 0 H H H \/ H \c,.,...c'-c1 H.- -..H I I H-..c c---H I 'c,.,... \ H / \ H H H ,,,.cH2 CH,, 'cH 2 I - I C~2 ,,,.cH2 CH2 Cyclohexane C6H12 0

• 1.1 Nomenclature for Straight-Chain Alkanes • Straight-chain alkanes have their IUPAC names as shown in the table below. • Each member has the suffix 'ane'. Alkane Structural Skeletal formula formula IUPAC Names of Straight-Chain Alkanes propane CH3CH2CH3 A Number Number of carbon Name of carbon Name butane CH3CH2CH2CH3 /'./ atoms atoms octane CH3(CH2)sCH3 1 methane 11 undecane 2 ethane 12 dodecane 3 propane 13 tridecane 4 butane 14 tetradecane 5 pentane 15 pentadecane 6 hexane 16 hexadecane 7 heptane 17 heptadecane 8 octane 18 octadecane 9 nonane 19 nonadecane 10 decane 20 icosane* • Spelled "eicosane " prior to 1979 version of IUPAC rules 1.2 Nomenclature for Branched-chain Alkanes • The rules for naming branched-chain alkanes are summarised in the table below. prefix + parent + suffix J, ,j, ,j, What and where are the substituents? What is the longest carbon chain? What is the functional group? -ane 1. Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name. 2. Number each carbon atom in the longest chain, beginning with the end nearest a substituent. 3. Name the substituents attached to the longest chain (as alkyl groups). Give the location of each substituent by the number of the main-chain carbon atom to which it is attached. • An alkyl group is formed by removing a hydrogen atom from an alkane . • General formula of an alkyl group: CnH2n+1 • To name an alkyl group, change the -ane ending of the alkane to -yl. Alkyl group Structure methyl CHJ- an alkane: CnH2n+2 or R-H ethyl CHJCH2- propyl CH3CH2CHi- an alkyl group: CnH2n+1 or R- butyl CH3CH2CH2CH2- 4. When two or more substituents are present, list them in alphabetical order. When two or more of the same alkyl substituents are present, use the prefixes di-, tri-, tetra-, etc. to avoid having to name the alkyl group repeatedly. 5. Note: Use comma (,) to separate 2 numbers. Use hyphen (-) to separate a number and a word. Tip: Numbering your carbons is a useful way to draw structures. Practice it! ' -3-

-Worked Example 1 - Give the systematic (IUPAC) name for the following alkanes. (a) 1.3 Classification of hydrogen and carbon atoms • Carbon atoms are classified as primary (1°), secondary (2°), tertiary (3°) or quaternary (4°) depending on the number of carbon atoms to which they are bonded. two 1 ° carbons a 3° carbon a 4° carbon / I ~IH3 CH3 I CH3 -C~-CH 3 CH3- 1 H-CH 3 CH3 -C-CH 9-CH-CH 3 I - . a 2° carbon CH3 CH3 propane 2-methylpropane 2,2,4-trimethylpentane • A 1° C atom is bonded to one other C atom. • A 2° C atom is bonded to two other C atoms. • A 3° C atom is bonded to three other C atoms. • A 4° C atom is bonded to four other C atoms. • Hydrogen atoms are classified as primary (1°), secondary (2°) or tertiary (3°) depending on the~ of carbon atom to which they are bonded. H C C I I I Example: H-C-C H-C-C H-C-C t I t I 1 I 1° H H H C i H .,_3o H 1° H atom 2° H atom 3° H atom I A 1° H atom is bonded A 2° H atom is bonded A 3° H atom is bonded CH3CH2-C-CH 3 1 tH3 to a C atom that is to a C atom that is to a C atom that is bonded to one other C bonded to two other C bonded to three other C 2° H atom. atoms. atoms. -4-

__:::2:.......1.j _1s_o_m_e_ri_sm_ l_n_A_lk_a_n_es _____________________ 2.1 Constitutional (or structural) isomerism • Constitutional (or structural) isomerism arises when there exists compounds with the same molecular formula but different structural formulae. • From butane onwards, alkanes have constitutional isomers due to the branching of the hydrocarbon chains. Example 1 Example 2: Constitutional isomers with molecular formula CsH14 CH3CH2CH2CH3 CH3CH2CH2CH2CH2CH3 CH:3CH:zCl-l;z - CH- CH:3 CH:3CH;z-CH-CH:zCH:3 butane hexane I I CH:3 CH3 2-methytpentane 3-methylpentane CH-CH-CH 3 I 3 CH3 CH3 c~ CH3 I I I 2-methylpropane C~-CH-CH-CH3 c~c~-c-c~ I 2,3-di methyl butane 2,2-dimethylbutane c~ 2.2 Stereoisomerism • Stereoisomerism arises when there exists compounds with the same molecular formula and structural formula but differ in the way in which their atoms are arranged in space relative to one another. (a) Enantiomerism • Some branched-chain alka