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Transcript
Reaction Mechanisms of Selected Organic Reactions Teachers' Notes Level S4-S6 Duration: 15 minutes Learning objectives The programme is for students’ knowledge enrichment. It aims to help students better understand the following reactions by visualising their reaction mechanisms 1. addition of hydrogen to alkenes 2. 3. addition of hydrogen halides/halogens to alkenes substitution reaction of haloalkanes The programme consists of five parts: 1. Hydrogenation of alkenes This part introduces to students the characteristic reaction of alkenes, addition reaction, as exemplified by the hydrogenation of ethene. The process in which the reaction is speeded up by the presence of a catalyst is illustrated by 3-D animation. 2. Addition of hydrogen halides to asymmetrical alkenes This part introduces to students the Markovnikov’s rule which can be used to predict the major product in the addition of hydrogen halides to asymmetrical alkenes. The mechanism of the reaction between 2-methylpropene and hydrogen bromide is taken as an example for illustration. 3. Addition of halogens to alkenes In this part, the reaction mechanism of how bromine is added to ethene is discussed. 4. Substitution of haloalkanes with sodium hydroxide (SN2) The reaction mechanism of the SN2 substitution reaction of primary and secondary haloalkanes with sodium hydroxide is discussed. 5. Substitution reactions of haloalkanes (SN1) In this part, the reaction mechanism of the SN1 substitution reaction of tertiary haloalkanes is discussed. The hydrolysis of 2-bromo-2-methylpropane is used as an example for illustration. Suggested Learning Activities Teachers may introduce the relevant parts to students for extended learning according to teaching schedule. Suggestions on activities after viewing the programme are as follows: 1. Teachers may discuss with students how scientists combine basic chemical principles, understanding of the reactivity of different chemical species with experimental observations to deduce the most likely sequence of elementary steps and thus the mechanism of a particular reaction. Knowledge of reaction mechanisms facilitates scientists to plan for synthesising new compounds from some “building block” molecules. However, students should be reminded that a mechanism can never be established with absolute certainty but represents only scientists’ best understanding and interpretation of experimental observations. 2. Alcohols and haloalkanes are especially valuable “building block” molecules in pharmaceutical and agricultural industries to make targeted compounds. Teachers may encourage students to explore the importance of alcohols and haloalkanes in synthetic chemistry.