It’s tough to be a graduate student. If you are a Professor, you can sit in your ivory tower and think about all manner of nutty ideas and, as long as none of them violate any laws of thermodynamics, they will be eventually reduced to practice (and improved!) by our capable graduate students and postdocs. But the devil is in the detail and all those brave souls are left figuring out how to lower the kinetic barriers of reactions we contemplate. There are reactions (unfortunately, a lot), in which there are just too many energetically similar pathways, which is why we get in trouble with by-products… Apart from this insignificant detail (I am being sarcastic), chemistry is deceptively simple: any idea about an isolable endpoint of synthesis that is not uphill in energy, is worth the risk. Needless to say, you can design special conditions and isolate uphill intermediates (e.g. carbocations), but this would amount to imposing a kinetic barrier of some sort. Now, are there ridiculous (but seemingly plausible) ideas out there that can throw us for a loop unless we sit down and think about them for a second? Here’s one of the problems I like to discuss with my colleague, Professor Jik Chin. Consider the following generalized process:
Imagine that you want to develop a catalyst that would run this reaction. Can such catalyst exist? No, it can’t. The way this reaction is written is sheer nonsense. For this conversion to have a chance to work in the forward direction, the Gibbs free energy change must be less than zero. In the example above we clearly have no entropy change and enthalpy does not change either. In addition to the violation of the Second Law of thermodynamics, there is a problem with the principle of microscopic reversibility here as any catalyst that works in the forward direction should be capable of catalyzing the opposite process. Of course, stoichiometric reactions can be designed and there are many solutions for this “R into S” type of problem. Enzymes can do this too (and catalytically!), but those reactions are coupled processes, which means that there is something else that goes on with either your product or your starting material. Hence, the energy of the product is not the same as the energy of the starting material. You can break microscopic reversibility with photochemistry, but if you are interested in thermal activation, any catalyst that you think might promote the aforementioned process, will necessarily have to violate microscopic reversibility and the Second Law. Back to my starting point: unless we propose thermodynamically ludicrous ideas, being a Professor is the best job out there. The way George Olah would say, “Hey – I am doing my hobby and the University even pays me for it!”