Page 273 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 257

9.4.1. Determination of chemical reaction paths

The use of radioisotopes in the study of the steps in a chemical reaction system is well
established. Let us consider a few examples to illustrate this technique.
If phenol is labeled with deuterium or tritium in the hydroxyl group and warmed to a
temperature slightly below decomposition, the labeled hydrogen migrates to other hydrogen
positions of the benzene ring either by intra-molecular rearrangement or by inter-molecular
reactions. However, using C6HsOT and C6H4TOH yields C6H4TOT, which can be formed
only through inter-molecular collisions, thus eliminating intra-molecular rearrangement as
the reaction mechanism.
The study of the reaction steps in the photosynthesis of carbohydrates from atmospheric
CO 2 in the presence of light and chlorophyll is an outstanding example of the value of the
tracer technique. The overall process (which involves many steps) can be written as

light
6 CO 2 + 12 H20 C6H1206 + 6 02 + 6 H20
chlorophyll

Using partition chromatographic technique and tracers of 14C, 32p, and T, Calvin and co-
workers were able to identify the intermediate steps involved. The experimentalprocedure
is usually as follows (Fig. 9.7). Plants are placed in atmospheres containing 14C-labeled
CO 2 and irradiated with light. After different irradiation times the plants are removed and
sections are digested to dissolve the material. A few drops of the solution containing the
substance to be separated (metal ion, organic molecule, etc.) are placed a few centimeters
from the end of a paper strip. The paper strip is hung vertically and dipped into a solution
so that the initial point of placement of the substance is near the bottom of the strip above
the solution level. Capillary forces draw the solution upwards and bring it into contact with
the adsorbed substances at the starting point. As this occurs the substance moves a certain
distance up the paper from the starting point with the distance traveled dependent on the
kind of paper, the solution used, and the chemical properties of the substance. In such
experiments, a certain Rf value for each substance can be defined as

distance the substance of interest has traveled
Rf = (9.16)
distance the liquid front has traveled

A typical solvent system for metal ions may be a mixture of acetone, dilute HCI, etc.,
while for organic substances it is possible to use mixtures of phenol and water,
acetylacetonate and water, etc. Figure 9.8 shows a two-dimensional paper chromatogram;
in this case it has been run initially with a particular solvent mixture, then (after drying)
turned 90 ~ and run with a second solvent mixture, thus increasing the selectivity of the
separation. The separated substance can be quantitatively recovered by cutting out the spot
and leaching the compound from it. The substance is identified either from its Rf value or
further analyze~l by standard methods.