Studies of Reaction Mechanisms Using Isotopes

To determine the total volume of blood in the body, a known volume of blood with hemoglobin having radioactive iron (Fe59) is supplied to the subject. After sufficient time for complete circulation, a known volume of blood is withdrawn, and the radioactivity remeasured. The observed dilution in radioactivity then allows the calculation of the volume of diluting agent (in this case the subject s blood). 

At several points within the text we have referred to instances in which isotope studies have furnished information concerning the possible path of a reaction. For example, the oxidation of SOl with ClOj in water enriched with O18 (p. 366) yields SO having practically no enrichment with O18, indicating that the reaction has taken place by direct oxygen transfer from CIO to SQ. Similarly, the oxidation of Or(II) with Co(NH3)5Cl2+ must proceed through transfer of a chlorine atom from the cobalt ion to the chromium ion, for it has been shown that one Cr—Cl bond is formed for each chromium(II) ion oxidized. Representing the reaction schematically  

If the oxidizing agent, Co(NH3)5Cl2+, is furnished with radioactive chlorine, the product, Cr(H 0)5Cl2+, contains active chlorine. However, if the chlorine in Co(NH3)5Cl2+ is inactive but the reaction is carried out qu ekly in the presence of active Cl in solution, essentially none of the active chlorine is incorporated into the chromium complex. 

In connection with exchange studies on complexes (Chap. 23), it was pointed out that the complex Cr(C 04)J a was inert —that is, that radioactive 0 0)4 present in solution with this complex became incorporated into the complex at a very low rate. On the other hand, when the enantio-morphs of this ion are separated, then redissolved in water, racemization 

The hydrolysis of the carbonato complex I is roughly analogous to ester hydrolysis, for the complex may be split either at the Co—O bond or the inner C—O bond. When the reaction is carried out in water labeled with O18, the O18 is found to be present in the resulting aquo complex, II, but the label has been diluted by a factor of two. The resulting CO2 is not labeled. This indicates that both Co—0 and C—0 bond breakage occurs, but the Co—O bond is broken first  

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