Participation by neighbouring groups in addition reactions. Part III. Bromination in acetic acid and trifluoroacetic acid solvents

Abstract

Rate measurements and product analyses for bromination of CH₂CH[CH₂]_nOH (for n= 1–4) in acetic acid indicated substantial participation by the neighbouring hydroxy-group for n= 3 and a smaller but still significant participation for n= 4. The general pattern of behaviour was very similar to that observed previously for reactions in methanol and water solvents. However, in trifluoroacetic acid solvent the extent of such hydroxy-participation was drastically reduced (three being only 4% ring-closed product for n= 3). The effect is attributed to hydrogen-bonding between the solvent and the hydroxy-substituent, which reduces the nucleophilicity (and hence its effectiveness as a neighbouring group) of the hydroxy-substituent. As for the methanol reactions, there was no kinetic indication of any participation by neighbouring bromine in the bromination of CH₂CH[CH₂]_nBr in either acetic acid or trifluoroacetic acid solvent.