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This week, the organic chemistry lab I’m TA’ing dehydrated 2-methylcyclohexanol with phosphoric acid to give a mixture of 1- and 3-methylcyclohexene. The product ratio was determined by GC analysis… At least it was supposed to be. The students for the most part did a good job of working through the lab quickly and efficiently, then it was upstairs to the GC room. about halfway through the GC runs… the power went out. That of course means the GCs shut down, the voltage sensor stops working, and the data no longer transfers to DataStudio. Oh well, it’s the thought that counts, right?
While I’ve had some trouble finding demonstrations to fit some of the labs this semester, I had no such problem this week. I knew exactly what I wanted to do – dehydrate sugar with con’c sulfuric acid(more). The sulfuric acid easily dehydrates the carbohydrate (i.e. the hydrate of carbon) to give essentially elemental carbon and water. Oh, and a lot of heat. The water escapes as steam and the expanding gas (as well as some sulfur dioxide) aerates the residual carbon. The black mess rises out of the beaker like it’s possessed. It pretty cool (although kinda gross) looking.
I used 60-mL of ordinary table sugar in a 100-mL beaker. 30 mL of concentrated sulfuric acid (without stirring) gave an approximately 12 inch column of carbon after about 2 minutes. The video below is a good representation (although, I think it might be in time-lapse mode. My experience has never formed the column that quickly). I appreciate that the person in the video is using gloves… but I would not recommend trying this one on your stove at home. The fumes are not good for you.
There was a J. Chem. Ed. article from Willamette University in Oregon out a few years ago about a variation of the dehydration without using concentrated sulfuric acid. Instead, they combine the dehydrating sugar demo and the flaming gummy bear demo and use potassium chlorate. the potassium chlorate/sugar mixture is wetted with ethanol and the mound is ignited. The same effect occurs, but without the need for concentrated sulfuric acid. It’s not clear that the fumes are any more safe, but the need for con’c acid is avoided. Here’s a good description with pictures from the University of Minnesota.
Finally, one of my students asked if this is the same principle behind the Black Snake fireworks. Eh… essentially yes, but not exactly. It seems that for the homemade versions at least, a mixture of sugar and sodium bicarbonate are ignited in ethanol. The combustion produces heat and gas and a carbonaceous black solid. The solid is again aerated with the expanding gasses and the snake is formed. So it’s the same, but different!
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