The deprotonation of internally hydrogen‐bonded acids. Mechanistic conclusions from the influence of general bases

The reactions between alizarin yellow G and six different bases B (including OH − ) and between tropaeolin 0 and eight different bases have been investigated at 25°C and an ionic strength of 0.5 M , using the temperature‐jump method. From the form of the log k B versus Δp K curves it is concluded that for alizarin yellow G the observed relaxation time is due chiefly to a diffusion‐controlled reaction between the base and that fraction which is present in the “open” non‐hydrogen‐bonded form, whereas for tropaeolin 0 the base attacks the hydrogen bridge. The dissociation constants for the internally bound hydrogen have been measured under the same conditions of temperature and ionic strength, using a spectrophotometric method.