JPJC 2026 Alkene Annotated Lecture Notes

2026 JPJC JC2 H2 Chemistry (9476) 1 Topic 10: Alkenes Jurong Pioneer Junior College 2026 JC1 H2 Chemistry (9476) Topic 10: Alkenes Learning Outcomes (LOs) Candidates should be able to: (a) explain the general reactivity of alkenes towards electrophilic reagents/electrophiles (b) describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: (i) electrophilic addition of steam (H2O(g)) with H 3PO4 catalyst), hydrogen halides (HX(g)) and halogens (X2(aq) or X2 (in CCl4)) (ii) reduction via catalytic hydrogenation , using H 2(g) and Ni catalyst (catalytic addition of hydrogen) (iii) oxidation by cold, alkaline solution of manganate(VII) ions to form the diol (iv) oxidation by hot, acidified solution of manganate( VII) ions leading to the rupture of the carbon-to-carbon double bond in order to determine the position of alkene linkages in larger molecules (c) describe the mechanism of electrophilic addition in alkenes, using bromine (Br2 (in CCl4)) with ethene as an example (d) apply Markovnikov’s rule to the addition of hydrogen halides to unsymmetrical alkenes, and explain the composition of products in terms of the stability of carbocation intermediates Section Content Los Page 1 Introduction 2 2 Structure of Alkenes Refer to Topic 8: Introduction to Organic Chemistry 2 3 Isomerism 3.1 Constitutional Isomerism 3 3.2 Cis-trans Isomerism 4 4 Physical Properties of Alkenes 4.1 Melting and boiling points Refer to Topic 3: Chemical Bonding 6 4.2 Solubility 8 5 Preparation of Alkenes 5.1 Elimination Reactions Refer to Topics: Halogenoalkanes and Hydroxy Compounds 9 5.2 Predicting Major Product formed: Zaitsev’s rule 9 6 Reactions of Alkenes 6.1 Electrophilic Addition Reactions (a), (b)(i), (c), (d) 11 6.2 Reduction with H2 to form Alkanes (Catalytic hydrogenation) (b)(ii) 22 6.3 Oxidation (b)(iii),(iv) 22 7 Identification Tests for Alkenes 7.1 Presence of C=C 27 7.2 Presence of Terminal alkene 27 8 Summary of Reactions 28 Recommended References : 1. Chemistry for advanced level – Peter Cann & Peter Hughes 2. Chemistry in Context – Graham C. Hill & John S. Holman 3. Organic Chemistry – John McMurry Lecture 1 page 1-17 Lecture 2 page 17-28

2026 JPJC JC2 H2 Chemistry (9476) 2 Topic 10: Alkenes 1 INTRODUCTION • Alkenes are unsaturated hydrocarbons that contain at least one C=C bond. • General molecular formula (where n is the number of C atoms) Aliphatic (i.e. non-cyclic) alkene containing one C=C bond: CnH2n Cyclic alkene containing one C=C bond: CnH2n−2 structural formula C H H C H H C H H C H CH3 C H H C H CH2CH3 molecular formula C2H4 C3H6 C4H8 There can be more than one C=C bond present in a given organic molecule. It is also possible to have C=C bond in cyclic structures. E.g. 2-methylbuta-1,3-diene cyclohexene cyclopentadiene Natural Occurrence of Alkenes: Ethylene (ethene): a plant hormone that regulates fruit ripening, flower wilting, and leaf abscission. β-Carotene: A pigment that gives carrots, sweet potatoes and other orange vegetables their orange colour. -carotene Limonene: Found in citrus fruit peels; responsible for that fresh, citrusy smell. Lycopene: The pigment that gives tomatoes their red colour. saturated  no multiple bond (i.e. all single bonds) unsaturated  has multiple bond(s) MF: C5H8 2 C=C  CnH2n-2 MF: C6H10 1 C=C + 1 ring  CnH2n-2 MF: C5H6 2 C=C + 1 ring  CnH2n-4 vs aliphatic alkane CnH2n+2 vs cycloalkane CnH2n − 2H − 2H 1 C=C  CnH2n − 2H 1 C=C  CnH2n − 4H 1 2 3 4

2026 JPJC JC2 H2 Chemistry (9476) 3 Topic 10: Alkenes 2 STRUCTURE OF ALKENES Each C atom of the C=C bond is sp2 hybridised and the geometry/shape around each C atom is trigonal planar. Using ethene as an example: • Each C atom has three sp2 hybrid orbitals and one unhybridised 2p orbital. The sp2 hybrid orbitals lie on the same plane (refer to diagram (a)) and the unhybridised 2p orbital is oriented perpendicular to this plane (refer to diagram (b)). • The C=C bond comprises of one  bond and one  bond. Diagram (c) shows the orbitals that form the C=C bond in an alkene. C CH HH H  bond formed by sideways overlap between the unhybridised 2p orbital of a C atom and the unhybridised 2p orbital of the other C atom (b) (c) (a)  bond formed by head-on overlap between a sp2 hybrid orbital of a C atom and the sp2 hybrid orbital of the other C atom  bond formed by head-on overlap between a sp2 hybrid orbital of C atom and the 1s orbital of H atom  bond ` ` ` `  bond (3 bond pairs & no lone pair) mixing of one 2s & two 2p orbitals to form three sp2 hybrid orbitals Sigma () bond and pi ( ) bond are covered under Topic 3: Chemical Bonding. Hybridisation is covered under Topic 8: Introduction to Organic Chemistry. (trigonal planar shape) Lecture Notes from Introduction to Organic Chemistry

2026 JPJC JC2 H2 Chemistry (9476) 4 Topic 10: Alkenes 3 ISOMERISM 3.1 Constitutional Isomerism (Refer to Introduction to Organic Chemistry pg. 13) Constitutional isomerism occurs when compounds share the same molecular formula but have different structural formulae. There are three types of constitutional isomerism: positional, chain and functional group isomerism. Alkenes can exhibit two types of constitutional isomerism: • Chain isomerism, where compounds differ in the arrangement of their carbon skeleton (through different chain lengths or branching patterns) • Positional isomerism, where the position of the carbon -carbon double bond (C=C) varies within the molecule Notably, aliphatic alkenes containing one C=C bond are isomeric with cycloalkanes, demonstrating functional group isomerism. To illustrate these concepts, let's examine all constitutional isomers with the molecular formula C4H8: but-1-ene but-2-ene 2-methylpropene cyclobutane These compounds demonstrate various types of isomerism: • But-1-ene and but-2-ene are positional isomers, having the same carbon chain length and functional group but different C=C bond positions • But-1-ene/but-2-ene and 2-methylpropene are chain isomers, sharing the same functional group but differing in their carbon skeleton arrangement • But-1-ene/but-2-ene/2-methylpropene and cyclobutane are functional group isomers, containing different functional groups Do Tutorial Qn 1, 2 1 2 3 4 1 2 3 4 1 2 3 IUPAC Nomenclature Rules for Alkenes: 1. Select the longest continuous carbon chain containing the C=C double bond as the parent chain. 2. When numbering the chain, first, assign the lowest possible number to the C=C bond; then assign smaller numbers to prefixes (if any).

2026 JPJC JC2 H2 Chemistry (9476) 5 Topic 10: Alkenes 3.2 Cis-trans Isomerism in Alkenes (Refer to Introduction to Organic Chemistry pg. 14) (i) Criteria for Cis-Trans isomerism in Alkenes: • Restricted rotation about the C=C bond due to the presence of  bond • Two different groups of atoms attached to EACH C atom of C=C bond For example, but-2-ene has two different groups (CH 3 group and H atom) attached to each C atom of C=C bond. The two possible configurations of but-2-ene are shown below. The groups of atoms are still connected the same way − both molecules are but-2-ene. However, both have different spatial arrangements of their groups. The two CH3 groups can be either on the same side of C=C bond in cis-but-2-ene or on the opposite sides of C=C bond in trans-but-2-ene. However, these two molecules cannot be interconverted spontaneously without breaking the  bond. As such, but-2-ene exists as two different stereoisomers (i.e. cis-trans isomers). However, cis-trans isomerism is not possible if one C atom of C=C bond is attached to t