Radical Recombination Kinetics
Miles Pickerlng Princeton University Princeton. NJ 08544
An experiment in physical organic chemistry
T h e experiment t o b e described in this paper is a truly inteerated one in t h a t a wide varietv of techniaues from both ph;sical a n d organic chemistry a i e brought t o hear on the problem. Furthermore, it is one of t h e few student kinetics experiments involving second order kinetics, and i t also displays the rare phenomenon of photochromism. The substance involved is the remarkable free radical ( 1 3 ) shown below:
Triphenylimidazyl Radical
This radical will be formed upon t h e dissolution of the dimeric precursor (Dimer I) in organic solvents and is bright purple in color. Recomhination of the radicals occurs over a period of minutes a t room temperature, h u t t h e resulting dimeric compound can he broken u p into the free radicals by the action of light. Sunlight will produce aremarkable and rapid color change in the solution, and the photolysis-recombination cycle can be run almost indefinitely. The dimeric precursor can he prepared easily from lophine, a commercially available, though expensive, starting material.
Then the free radical can be formed by dissolution in organic solvents.
purple b e radieal
Dimu
T h e recombination will occur in the dark t o produce another dimer:
Dimer I1
We have our student d o a multipart experiment involving the synthesis of the radical precursor, the measurement of the kinetics, a proof with ESR t h a t the purple species is a free radical, and the measurement of the extinction coefficient by radical trapping with hydroquinone.
Experiment Preparation of Dimer I
This compound, the "piezochromic dimerS'ofWhite and Sonnenberg, is the precursor of the free radieal. The material can be prepared
in a one-step reaction from lophine, which is available commercially, or can br synthesized by the procedure in the appendix. A solution otaleuholic KOH 110-12 g of KOH dissolved in 1lN ml of ethanulr is added a 6W-ml kokrr. 'l'hrre eroms oi luohine must be completely dissolved, by warming if necessary. A 606-ml beaker ,containine this solution and a stirrine bar is olaced in an ice hath. A mixture of30 ml uf 5.2J'bstdium hypwhlunte soluuon t(?hlumx~ or similar) and 42U ml of HKI is prepared. This sulutiun i* added 18) the r
