Creating a Time Lapse Video for the Bromination of Hexane in the Presence of UV Light

My IB chemistry class is currently working its way through organic chemistry. One of the topics studied is the free-radical mechanism for the halogenation of an alkane.




For example:           CH4 (g)  +  Cl2 (g)  -->  CH3Cl (g)  +  HCl (g) (in the presence of UV light)


I am starting to think that I need to show my students more reactions (both in labs and as demonstrations) to help them expand their chemical understanding. I know I can't show the reaction above. But I found some hexane and bromine water in the chemical stockroom at my school so I set up a demo in class to show the bromination of hexane in the presence of UV light. (On a side-note, I showed the addition reaction of bromine water with hex-1-ene also.)

My lab tech told me today that this is likely the last round of bromine water she can make, as she's having a hard time finding a supplier that can actually ship to Thailand. So I decided to use a bit of technology and make a video of the reaction. It's quite slow, so I decided on a timelapse to show the reaction progress, along with snapshots every 15 minutes to see the decolorization a bit better.

Below are a few pictures from the reaction, along with a link to the video which I've uploaded onto YouTube. While watching the reaction and the video I wondered why the yellow-ish color of the bromine water stayed, even after mixing repeatedly and leaving it in the UV light. (I noticed the same thing with hex-1-ene. It would turn the bromine water clear when I swirled the test tube. This would last for a few minutes then the bromine water would eventually turn back to a faint yellow hue.) My students also noticed that the solution gets a bit cloudy after the reaction. This provides yet another question for us to explore. And of course rate of reaction, solubility and intermolecular forces are all at play here, so that could lead to additional discussions. I'm not sure we'll have time for this mini-exploration since the school year is almost finished, but maybe next year when I cycle back through organic chemistry we can revisit this reaction and try to learn more.


Are there any reactions you've recorded that you'd like to share?









Initial Experimental Setup






Reaction After 5 Minutes





Reaction After 10 Minutes





Reaction After 15 Minutes





Comparison of Hex-1-ene with Hexane with Bromine Water (without UV light)