18O labelling

Figure 2. Rates of scrambling of carbonyl-18O label in acetyl peroxide at 80°C. in solvents of varying viscosity...

Figure 3. Rates of over-all disappearance of acetyl peroxide calculated from the rates of scrambling of carbonyl-18O label, assuming the only solvent effect to be the cage effect...

Few examples have been reported (5, 8, 9, 10, 12, 24) of cage recombination of simple alkoxy or acyloxy radicals to form O—O bonds in isolable molecules. This paper explores further the implications of the observed (17, 22, 23) scrambling of label seen in acetyl peroxide carbonyl-18O recovered after partial decomposition. 

Isotope Scrambling During the Decomposition of Acetyl Peroxide-Carbanyl-18O. Determination of Carbonyl Label. Isotope scrambling... 

Having worked this out. one further labelling experiment showed that a tetrahedral intermediate must be formed an ester labelled with 18o in its carbonyl oxygen atom passes some of its l80 label to the water. We discussed why this shows that a tetrahedral intermediate must be formed on p. 282. 

In the achiral species 19, 20 and 21 the (R)- and (S)-1,2-propanediol binding modes are equivalent and no preferred retention or loss of the heavy oxygen isotope should be expected, even upon stereospedfic migration. This has been confirmed experimentally, i.e., about 50% of the 18O was retained in all three cases. In the chiral specimens 22, 23, 24 and 25, however, stereospecific migration of the unlabelled hydroxyl group should lead to chirally labelled geminal diols. Such a process should be preferred by the substrates 23 and 25 (Fig. 18). 

The 18O-equilibration technique was applied to several systems which undergo 1,2 alkyl or aryl shifts. The first study with a labeled sulfonate was carried out by Denney and Goldstein86 who rearranged (68) to (69) (25% scrambling of ether-180) and... 

The top mechanism is an SN1 on the alcohol the bottom mechanism is the standard addition-elimination on the carboxyl group. Labeling the alcohol with 18O will distinguish them. The 18O will be lost in the top mechanism, but it will be retained in the bottom one. 

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