Long Live the Intermediate!
What’s in between in a reaction matters just as much as what sets it off
One reason intermediates don’t get much press is that they are, well, intermediate. They decompose easily. They are not around in large concentrations, or for very long. They are fleeting. You have to be quick and handy to catch a glimpse of them, say, by observing a characteristic spectrum. Your method has to be exquisitely sensitive, for in principle the better the catalyst—the greater its turnover number—the less chance you will have to observe the intermediates. The title of this column may evoke a small smile from the catalytic chemist, who knows that if you have a long-lived intermediate, you are unlikely to have a good catalyst.
Enough talk; we need an illustration of the detection of an intermediate. In this context, I cannot resist showing one of the greatest scientific images of all time: the smoking gun in our understanding of how chlorofluorocarbons cause the ozone hole. It begins when chlorofluorocarbons, inert and harmless at sea level, are photolyzed in the stratosphere to give not one but a series of chlorine reservoirs: Cl2, HOCl, ClNO2. These compounds are adsorbed on ice crystals in polar clouds and then photolyzed by spring sunlight, easily releasing Cl atoms. These atoms then initiate a catalytic chain that decomposes ozone. The simplest possible mechanism for that decomposition, first proposed in the 1970s, is as follows:
But nothing is simple in this world (except for our minds when we listen to those political ads), and this mechanism is no exception. As musician Joe South wrote, “I never promised you a rose garden.” The sequence above requires oxygen atoms, which, as it turns out, are scarce in the Antarctic stratosphere. The actual course of events is more complicated, and includes the four essential steps below.
The overall reaction for the simpler mechanism, then, is:
for the second sequence, it is:
where hν represents energy from sunlight. In both mechanisms, Cl atoms are the catalyst and ClO is an intermediate. Like the Stille coupling, the second mechanism has multiple intermediates, also including ClOOCl and OOCl.
The very reactive intermediate ClO is not something you can pull off the shelf. In the 1980s a spectroscopic probe was created to detect the tiny amounts of ClO in the polar atmosphere. The probe was put on board a plane that took off in 1987 from Punta Arenas, at the southern tip of Chile, and flew at high altitude south toward the pole and into the ozone hole. The plane also carried a probe for ozone. The figure above shows the resulting measurements. They demonstrate, ever so clearly, that the ozone concentration goes down (that’s the hole!) just where the ClO concentration goes up. I hope you see the units of the ClO measurement (parts per trillion volume), to appreciate the experimental achievement here.
Characterizing a reaction intermediate is hard work—harder, I think, than finding a catalyst. Mind you, my catalytic chemist friends disagree. Their students struggle to discover new catalysts. But as an outsider, a theoretical chemist, when I look at the chemical literature, here is what I see: Reaction mechanisms, where intermediates are lurking, are easy to write down but often devilishly difficult to pin down. Into the establishment of mechanisms goes great experimental ingenuity in weighing the evidence from rates of reactions, isotope effects and the direct detection of intermediates. This is as true of reactions that are not catalytic as ones that are; few reactions proceed in one fell swoop, and most go through intricate sequences with fleeting intermediates galore.
As Lewis Carroll wrote in The Hunting of the Snark,
You may seek it with thimbles—and seek it with care;
You may hunt it with forks and hope;
You may threaten its life with a railway-share;
You may charm it with smiles and soap—
And you may hope that, even if you do not vanish, neither will your chemical reputation, as has that of more than one unfortunate seeker. By comparison, finding the overall reaction and the catalyst seems simpler. In favoring the catalyst, I would say that we take the easy road in what we admire.