Download Senior Science topics Programme

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.