The thermochemistry of bromination of phenols and anilines

• Main Author: Allott, Philip Hugh
• Published: Royal Holloway, University of London 1986
• Subjects: 547; Organic Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.704717

A method has been developed for bromination of aromatic compounds in aqueous solution in an isoperibol calorimeter. The enthalpies of bromination of phenol to 2,4,6-tribromophenol and of aniline, 2-bromo-aniline, 4-bromoaniline and 2,4-dibromoaniline to 2,4,6-tribromo-aniline were measured using this method. These values were used to derive the standard enthalpies of formation of 2,4,6-tribromophenol, 2-bromoaniline, 4-bromoaniline, 2,4-dibromoaniline and 2,4,6-tribromo-aniline. The enthalpies of combustion of 2,4,6-tribromophenol and 2,4,6-tribromoaniline were measured using a rotating-bomb calorimeter. These values were used to derive the standard enthalpies of formation of 2,4,6-tribromophenol and 2,4,6-tribromoaniline. The discrepancy between these latter standard enthalpies of formation and those found using the solution calorimetric method is discussed, and possible sources of systematic error indicated. The solution calorimetric method was adapted for the thermometric titration of phenols in aqueous solution with an aqueous solution of bromine. The sequential nature of bromination of five compounds -phenol, 2-bromophenol, 2-methylphenol, 3-methylphenol and 2-hydroxy-benzoic acid - was investigated using this technique. The reasons for this sequentiality are discussed. abstract continued overleaf A complete data-processing system has been designed for the solution calorimeter. A microcomputer is used for data acquisition from the AC bridge of the calorimeter through an analogue-to-digital interface. The bridge voltage readings, once uploaded to a minicomputer, are processed to yield the corrected temperature change. Enthalpies of reaction can then be calculated and statistical analyses performed. The problems encountered in developing the system are discussed, especially with regard to the calculation of the corrected temperature change from the noisy temperature data. A least-squares cubic spline is used for curve-fitting and calculation of the first derivative of temperature versus time.