The Photochemistry of 4‐Chlorophenol in Water Revisited: The Effect of Cyclodextrins on Cation and Carbene Reactions

The photochemistry of 4‐chlorophenol ( 1 ) in water and in the presence of cyclodextrins has been studied by means of steady‐state and time‐resolved experiments. These have shown that 1 undergoes photoheterolysis of the CCl bond in the triplet state to yield the 4‐hydroxyphenyl cation 3 2 in equilibrium with 4‐oxocyclohexa‐2,5‐dienylidene, 3 3 . These triplet intermediates scarcely react with a n nucleophile, such as water, nor abstract hydrogen from this solvent, thus they are long‐lived (≈1 μs). Specific trapping of both intermediates has been achieved. The cation adds to 2‐propenol, k add ∼1.3×10 8  m −1 s −1 , to form the long‐lived phenonium ion 11 (with λ max = 290 nm), which then converts to 3‐(4‐hydroxyphenyl)propane‐1,2‐diol ( 10 ). Carbene 3 3 is trapped by oxygen to give benzoquinone and is reduced by D ‐glucose ( k q = 8.5×10 6  m −1 s −1 ) to give the phenoxyl radical ( 8 ) and phenol ( 9 ). Cyclodextrins have been found to trap the intermediates much more efficiently ( k q = 9.4×10 8  m −1 s −1 with β ‐CD), which indicates that inclusion is involved. Ground state 1 forms inclusion complexes with 1:1 stoichiometry and association constants of 140 and 300  M −1 with α ‐ and β ‐CD, respectively. Complexation does not change the efficiency or the mode of photofragmentation of 1 ; however, it does influence the course of the reaction because the major portion of the intermediates are reduced to phenol within the cavity ( k ′ red ≥5×10 7 s −1 ) either via a radical 8 or via a radical cation 9 + . . Under these conditions, neither 2‐propenol nor oxygen trap the intermediates to a significant extent.