VJC 2024 Alkenes Lecture Notes

VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT Mr. Rafiqi [muhammad.rafiqi.leman@vjc.edu.sg] H2 ORGANIC CHEMISTRY: ALKENES Lecture Outline 1 Introduction 2 Physical Properties of Alkenes 3 Preparation of Alkenes 3.1 Dehydration of Alcohols 3.2 Dehydrohalogenation of Halogenoalkanes 4 Reactions of Alkenes 4.1 Electrophilic Addition 4.11 Addition of hydrogen halides 4.12 Addition of halogens in CCl4 4.13 Addition of halogens in water 4.14 Addition of water 4.2 Reduction 4.3 Oxidation 4.31 Combustion 4.32 Formation of diol (mild oxidation) 4.33 Formation of carboxylic acids, ketones and/or CO2 (strong oxidation) 5 Tests for Alkenes 6 Summary Assessment Objectives 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 water/steam, hydrogen halides and halogens (ii) reduction via catalytic hydrogenation (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 additio n in alkenes, using bromine 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 References 1 A–level Chemistry by EN Ramsden 2 Chemistry – Longman A–level Guides by JGR Briggs 3 Understanding Advanced Organic and Analytical Chemistry by Tan J. and Chan K. S. 4 Chemistry – The Molecular Nature of Matter and Change by Silberberg Video Resources Electrophilic addition mechanism of alkenes Test for alkenes by bromine water Lecture 1 2 Pages 1–9 10-16 Complete by T3W8 T3W9 Tutorial Qns 1–4 5–7

2 1 Introduction • Alkenes belong to the homologous series of hydrocarbons containing a C=C bond. • General formula: CnH2n • The names of all alkenes end with ‘–ene’. The simplest alkene is ethene, CH2=CH2. • Isomerism occurs in alkenes containing more than three carbon atoms and occurs in three ways. (i) Constitutional / Chain isomers due to different arrangement of carbon atoms. (ii) Constitutional / Position isomers due to different position of C=C bond. (iii) Cis–trans isomers due to restricted rotation / lack of free rotation about C=C bond. E.g. C4H8 has four isomers (including both constitutional and cis–trans isomers). CH3CH2CH=CH2 CH2=C(CH3)2 but–1–ene 2–methylpropene cis–but–2–ene trans–but–2–ene • Nomenclature Step 1: Identify the parent chain (longest continuous carbon chain contain in