I love organic chemistry but am well aware that this view of things is not held by all teachers and students!
However, I feel I teach the subject well, but when it comes to lab work I do not do so well and this is something I need to work on. Today’s blog post aims to help me (and you) overcome this fear by giving you an idea for a simple lab to carry out (once you have the necessary reagents – that may be the hard part!)
The lab in question is to do with the nucleophilic substitution reaction of haloalkanes, but it also carries over into the bonding and structure topic and the energetics topic – so you could use it in a variety of different teaching situations.
If you recall, haloalkanes react via a SN1 or SN2 mechanism. The rate of reaction will be governed by the strength of the carbon halogen bond. The weaker the bond, the larger the size of the halogen. This is because the bonding pair of electrons are further away from the nuclei of both atoms, hence the electrostatic force of attraction is weaker so the bond is easier to break.
So we would predict iodoalkanes to react faster than bromoalkanes which will react faster than chloroalkanes (if you had it, a fluoroalkane would react even slower).
Set up the experiment as follows:
Take some ethanol (approx. 1ml) and to it add a couple of drops of haloalkane (obtaining the haloalkane could be the tricky thing here). Use a water bath to obtain a temperature of approx. 60oC. Then add approx. 5ml of 1.0M NaOH (to supply the OH- nucleophile) and 5ml of 0.05M silver nitrate and observe. To ensure a fair test, all the halogenalkanes should contain the same carbon backbone.
The role of the silver nitrate is to test for the substituted halide ion. Iodide ions form the yellow AgI almost immediately, the bromide forms AgBr after around 15 or so second, and finally the chloride forms AgCl – but this can take around 5 minutes or so. The silver halides are all formed as precipitates so are nice and easy to see.
It should be noted that you the teacher are responsible for risk assessing this lab and to carry it out before trying it with students.
Do you have any good, ‘low tech’ labs you carry out in the organic chemistry topic? If so, please feel free to post them below.