Radical Addition Rates to Alkenes by Time-Resolved CIDNP:  2-Hydroxy-2-propyl Radicals

A novel time-resolved CIDNP technique is used to measure absolute rate constants for the addition of 2-hydroxy-2-propyl radicals to 10 1,1- and two 1,2- disubstituted alkenes in propan-2-ol- d 8 and toluene- d 8 solvents and in part in acetonitrile- d 3 and cyclohexane- d 12, at room temperature. 2-Hydroxy-2-propyl radicals are produced by photolysis of 2,4-dihydroxy-2,4-dimethylpentan-3-one and lead to a multiplet effect CIDNP of the self-termination product, propan-2-ol, which follows a second-order termination kinetics. Addition of alkene partially quenches this polarization through the addition reaction. Quantitative measurement of the time-dependence and its simulation by kinetic equations yields the addition rate constant k a . The technique is applicable to systems with rate constants between 10 5 and 10 9 M -1 s -1, a range covered by the alkenes used. The data correlate with the alkene electron affinity and confirm the 2-hydroxy-2-propyl radical as very nucleophilic. A solvent dependence of k a up to 1 order magnitude is observed and is related to the hydrogen-bonding ability of the solvent and radical and to the polarity of the solvent. A β-steric effect of the alkene substituents is also noted.