ASRJC H2 Chem 10. Alkenes

2024 JC1 H2 Alkenes 1 2024/ASRJC/CHEM ANDERSON SERANGOON JUNIOR COLLEGE JC1 H2 CHEMISTRY HYDROCARBONS: ALKENES Content/Outline Alkenes (exemplified by ethene) 1 Introduction 2 Nomenclature 3 Isomerism 4 Physical Properties 5 Preparation of alkene - Cracking - Elimination, including Saytzeff’s rule 6 Chemical Property 7 Reactions of alkene - Electrophilic addition, including Markovnikov’s rule - Oxidation - Reduction 8 Chemical Tests for alkene 9 Electronic Effect (Inductive Effect) Learning Outcomes 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 (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 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 the carbocation intermediates References 1. Chemistry for Advanced Level. Peter Cann & Peter Hughes 2. Chemistry (2nd edition). Chris Conoley and Phil Hill 3. Chemistry in Context. Hill & Holman 4. Understanding Advanced Organic and Analytical Chemistry, K.S Chan and Jeanne Tan

2024 JC1 H2 Alkenes 2 2024/ASRJC/CHEM 1 INTRODUCTION Alkenes are unsaturated hydrocarbons which contain one or more C=C double bonds in their structures. Alkenes have the general formula CnH2n. Cycloalkenes have the general formula CnH2n-2. (Note that cycloalkanes have the same general formula as alkenes e.g. C 3H6 represents propene as well as cyclopropane.) Each C atom of the C=C is __________________________. Hence the geometry (shape) about each of the double bonded C is trigonal planar. Two of the three sp2 hybrid orbitals of the carbon atom overlap head–on with the 1s orbitals of the two hydrogen atoms to form two C–H sigma () bonds. The remaining sp 2 hybrid orbital overlaps head –on with the sp 2 hybrid orbital of the neighbouring carbon atom to form a C–C sigma () bond. The unhybridised 2p orbital, which is perpendicular to the plane of the inter –nuclear axis , overlaps side–on with the 2p orbital of the neighbouring carbon atom to form a pi () bond. The  electron cloud lies above and below the plane of the inter–nuclear axis. bonding in ethene molecule x x H H x x H H Unhybridised 2p orbita