As the end of summer rapidly approaches and the start of semester looms like an unassailable mountain on the horizon, I have the urge to sit down and answer the age-old back to school essay question ‘what did you do over the summer’. I shall refrain for now, and console myself by answering a different question, posted by CENews on their International Year of Chemistry Blog for a chemistry blog carnival: what is your favourite reaction?
This question is very difficult, because everyone’s favourite chemical reactions should probably be those involved in respiration, the chemistry that keeps us alive, but there are also a whole host of fascinating and beautiful reactions that serve less practical purposes. Should my favourite reaction be something frivolous like the reaction that makes a firework go? Should it be something sensible that most of my research hinges on? Should it be the chemical reaction I do most often in the lab, or the one that works best? And in that initial panic of simply not being able to chose (frankly, a similar panic to that experienced when picking dessert in a very nice restaurant), my eye falls to an undergraduate dissertation sitting on the desk next to me. On the front cover is a wonderful image, created by the student, representing the goal of what was then a brand shining new research project (and also one of the first project students I supervised). The dissertation covers those first tentative and, let’s face it, largely unsuccessful first steps that break fresh ground in a new area of research. The dissertation, however, represents hours of work by an incredibly committed student and despite things not always working, she persevered. I should pick one of those reactions.
I’m going to pick a reaction that is simple to execute, welcomingly high yielding and from which incredibly beautiful structures can be constructed with a little effort. I’ll pick the Michael addition reaction of methyl acrylate and ethylene diamine in methanol. I’m specifying the solvent because my understanding of the serendipitous discovery of poly(amidoamine) dendrimers hinges on the addition of methanol giving an unexpected reaction product (figure 1).
The reaction above, producing the branched product, seems very straightforward, and it is. Some of my past project students may be smiling at this point because it is, in many ways, deceptively straightforward. Measure out the chemicals, mix, stir, and remove the solvent. For this first step it is easy, but then things start getting bigger and bigger until we end up with something that looks like figure 2.
Out of a very simple reaction (no, I’m not getting into the method) and its opposite, the addition of ethylene diamine to the terminal methyl ester, a very beautiful and symmetrical molecule can be created. As dendrimers go, this one is small which indicates the limit of my molecule drawing patience. This chemistry tests the patience of many – despite the high yielding reactions, they are not high yielding enough and defects start to creep into the structures. The odd side reaction becomes a big deal when those reactions are carried out hundreds of time on one molecule. There aren’t many occasions in chemistry where that is the case. And those dendrimers can be fiendish to characterise, trapping reagents and solvents within their arms, trying to get a decent looking (but broadly lumpy) clean NMR spectrum is an art form. That’s part of the fun though.
We don’t make dendrimers, we make dendrons (a 1/4 of figure 2 with reactive roots), but many of the difficulties of the synthesis still occur and trip people up.
So my favourite reaction forms amide or peptide bonds, similar to Carmen Drahl over at The Haystack (as I post this, the link isn’t working, I’ll double check later, it’s probably my end).