A reactive, esterified hydroxy group is particularly a hydroxy group esterified with a strong, inorganic or organic acid, preferably a hydrohalogen acid, and as hydrochloric acid, hydrobromic acid, or hydroiodic acid, further sulphuric acid or a strong organic sulphonic acid such as a strong aromatic sulphonic acid, e.g. benzenesulphonic acid, 4-bromobenzenesulphonic acid or 4-toluenesulphonic aci