JPJC 2026 Alkane Annotated Lecture Notes

2026 JPJC JC1 H2 Chemistry (9476) 1 Topic 9: Alkanes Jurong Pioneer Junior College 2026 JC1 H2 Chemistry (9476) Topic 9: Organic Chemistry Hydrocarbons - Alkanes 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 in the presence of ultraviolet light at room temperature (c) describe the mechanism of free-radical substitution with particular reference to the initiation, propagation and termination reactions , exemplified by reaction of ethane with chlorine Recommended References : 1. Understanding Chemistry – Free Radical Mechanism URL: http://www.chemguide.co.uk/mechanisms/frmenu.html#top 2. Understanding Chemistry for Advanced Level (3rd Edition) by T. Lister & J. Renshaw 3. Fundamentals of Organic Chemistry (7th Edition) by J. McMurry Contents: Section Content LOs Page 1 Introduction 1.1 Hydrocarbons 2 1.2 Alkanes 3 2 Nomenclature Refer to Topic 8: Introduction to Organic Chemistry 4 3 Isomerism in Alkanes Refer to Topic 8: Introduction to Organic Chemistry 5 3.1 Constitutional isomerism 3.2 Stereoisomerism 4 Physical Properties of Alkanes 4.1 Boiling point and melting point Refer to Topic 3: Chemical Bonding 6 4.2 Solubility 6 4.3 Density 7 5 Preparation of Alkanes 7 6 Chemical Reactions of Alkanes 6.1 Combustion (a), (b)(i), (ii), (c) 8 6.2 Free-radical substitution 8 7 Uses of Alkanes 7.1 Commercial fuels 14 7.2 Aerosol propellants / refrigerants 14 Appendix 15

2026 JPJC JC1 H2 Chemistry (9476) 2 Topic 9: Alkanes 1. INTRODUCTION 1.1 Hydrocarbons • Hydrocarbons are compounds containing carbon and hydrogen only. • There are 3 main types of hydrocarbons, namely, saturated, unsaturated and aromatic. 1.2 Alkanes • Alkanes are the simplest and least reactive homologous series found in organic chemistry. • Alkanes are saturated hydrocarbons (i.e. contains only C –H and C –C single bonds). • All the C atoms in alkanes are sp3 hybridised, therefore every C atom has a tetrahedral shape. • Alkanes can be classified as 2 types: (a) non–cyclic alkanes (straight chain and branched) (b) cyclic alkanes (ring) Recall: four sp3 hybridised orbitals (a) Non–cyclic Alkanes • General formula: CnH2n+2 • Contain zig–zag chain of C atoms (tetrahedral about each C). Examples of non–cyclic alkanes: Compound Molecular formula Structural formula Skeletal formula propane C3H8 CH3CH2CH3 2–methylbutane C5H12 CH3 CH3CHCH2CH3 Hydrocarbons Non-cyclic alkanes e.g. Aromatic Aliphatic Saturated Cyclic alkanes e.g. Unsaturated Arenes e.g. (all unsaturated) Alkynes e.g. Alkenes e.g. Aliphatic compounds: carbon atoms are joined in a straight, branched or non-aromatic ring. Do you know? Aliphatic: “Fatty” Aromatic: “Fragrant”

2026 JPJC JC1 H2 Chemistry (9476) 3 Topic 9: Alkanes (b) Cyclic Alkanes (Cycloalkanes) • General formula: CnH2n (same general formula as alkenes) Examples of cyclic alkanes: Compound name Molecular formula Structural formula of cycloalkane Skeletal formula of cycloalkane Cyclobutane C4H8 Cyclopentane C5H10 Cyclohexane C6H12 Note: cyclohexane  benzene!!! In cyclohexane, each C is bonded to 2 hydrogen atoms; But in benzene, each C is bonded to 1 hydrogen atom. Compound name Molecular formula Hybridisation state of C atom Represented as: Cyclohexane (saturated) C6H12 ______ cyclohexane C6H12 (___________) Benzene (unsaturated; aromatic) C6H6 ______ benzene C6H6 (___________) cyclohexane benzene sp3 sp2 non-planar planar

2026 JPJC JC1 H2 Chemistry (9476) 4 Topic 9: Alkanes 2 NOMENCLATURE Recall: (From Topic 8: Introduction to Organic Chemistry) The naming of organic compounds follows a system developed by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC nomenclature is made up of 4 parts: No. of C atoms in the longest straight chain Type and position of principal functional group Prefix(es) Stem Stem−suffix Suffix Type and position of substituent(s) Type and position of C–C bond in the longest straight chain Using 2−methylbutane as an example: 4 C atoms in the longest straight chain 2−methyl but ane One –CH3 at C2 of longest straight chain All C–C bonds (single bond) in the longest straight chain Hence, the structural formula of 2−methylbutane is C C C C H H HH H H H H H CH3 14 3 2 Exercise 1: Draw the displayed and skeletal formulae of 2,2-dimethylpropane and propylcyclopentane. 2,2-dimethylpropane propylcyclopentane CCC C H C H H H H H H H H H H H 1 32 displayed formulae skeletal formulae displayed formula skeletal formula C C C C C H H H H H H H H C H C C H H H H H H H C

2026 JPJC JC1 H2 Chemistry (9476) 5 Topic 9: Alkanes 3 ISOMERISM Recall: (from Topic 8: Introduction to Organic Chemistry) 3.1 Constitutional (Structural) isomerism • Alkane can display two types of constitutional (structural) isomerism, ie. chain isomerism and positional isomerism • Both isomerism can only be exhibited by alkanes from C4H10 and above. e.g. Chain isomerism of C4H10 Positional isomerism of C6H14 CH3 CH3CH2CH2CH3 CH3CHCH3 CH3 CH3 CH3CHCH2CH2CH3 CH3CH2CHCH2CH3 3.2 Stereoisomerism 3.2.1 Cis-trans isomerism • Cis-trans isomerism occurs when there is restricted rotation due to presence of a structurally rigid bond. • In alkane, only cyclic alkanes are possible to exhibit cis-trans isomers, due to the presence of ring structure prevents free rotation of the C-C single bond. e.g. Br Br H H Br H H Br 3.2.2 Enantiomerism • Enantiomerism occurs when a molecule contains: (1) at least one chiral carbon and (2) no plane of symmetry. Since each C atom in alkane is sp3 hybridised, it can act as chiral centre when it is bonded to 4 different atoms or groups of atoms. e.g. 3.2.3 Total number of stereoisomers Maximum number of stereoisomers a compound possess = 2n Where n = number of stereocentres (i.e. chiral C and cis-trans centre) e.g. Testosterone There are a total of 6 chiral carbons and 0 cis-trans centre in the testosterone molecule. Thus, the total number of stereoisomers for this molecule is: 26 = 64 * * * * * * non-superimposable mirror images CH2CH3 C CH2CH2CH3 H CH3 CH2CH3 C CH2CH2CH3 H H3C * * butane 2-methylpropane 2-methylpentane 3-methylpentane

2026 JPJC JC1 H2 Chemistry (9476) 6 Topic 9: Alkanes 4 PHYSICAL PROPERTIES OF ALKANES (self study) Recall: (from Topic 3: Chemical Bonding) Since the electronegativity of C and H are similar ( i.e. negligible electronegativity difference), C–H bond is non –polar. Hence, alkanes molecules are non–polar and are held together by weak instantaneous dipole–induced dipole attractions. 4.1 Boiling point & melting point • Boiling point and melting point increase as number of carbon atoms in the alkane increases. • Straight–chain alkanes: C1 to C4 are gases, C5 to C17 are liquids, C18 and above are solids at r.t.p. Explanation: Increasing amount of energy is required to overcome the increasing strength of instantaneous dipole–induced dipole attractions between molecules since the number of electrons per mo