K. L. L O ~ ~ W O Lebanon Valley College Annville, Pennsylvania
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Stoichiometrv of the Reaction Bromine witi Phenols
The purpose of this investigation is to establish the stoirhiometry for the reaction of a number of phenols with bromine, and to demonstrate the rapid and quantitative nature of the reaction of bromine with enoln. The experiment may also he used to present the idea of equivalent, mass by finding the mass of phenol that is equivalent to one mole of bromine or to illustrate t,he orienting and activating influences of the hydroxyl group in aromatic electrophilic substitutions. The problem presented to the students is to find the stoichiometry for the react,ion indicated by the following equation ArOH
+ n Br* HOAc
Ar(Br),OH
+ n HBr
by establishiug the value of n. The quantitative procedure used for this investigation is that published by Ingberman.' Other procedures can be used, such as that of Smith and Shriner.% The phenol is reacted wit,h bromine in acet,ic acid, the excess bromine reacted with iodide, and the liberated iodine titrated with standard sodium thiosulfate. Appropriate precautions are given the students for handling bromine, phenols, and t,heir solut,ions. Eastman Kodak phenols have
' INGEERMAN, A. K., Anal. C h m . , 30,1003(1958).
SMITH, W. T., JR. AND SHRINER, R. L. "The Examination of New Organic Compounds," John Wiley & Sons, Inc., New York, N. Y., 1956, p. 125. 2
482
/
Journal o f Chemical Education
been used without further purification except that beta-naphthol should he recrystallized from a cyclohexane-benzene solvent. The bromine-acetic acid solution is standardized by each student by the addition of iodide ion and titration of the liberated iodine with standard sodium thiosulfate. A quant,ity of the assigned phenol is placed in a glassstoppered Erlenmeyer flask; the ranges specified are 200 to 250 mg for mononuclear phenols and 400 to 500 mg for naphthols. Exactly 25.00 ml of an approximately 0.L5 M solution of bromine in acetic acid is pipetted into the flask. This is followed by 5 ml of a ,5yosolution of pyridine in acetic acid; the pyridine catalyzes the bromination. The mixture is allowed to stand in the dark (to prevent possible side-chain bromination) for ,510 minutes, after which 10 ml of au aqueous solution of 25y0 potassium iodide is added. Standard sodium thiosulfate (approximately 0.2 A t ) is used to titrate the liberated iodine. The use of starch as an indicator to enhance the end point is advantageous, but not necessary. The following student results for n on three phenols indicate the accuracy attainable. The number of trials and the standard deviation from the mean, s, are given. Resorcinol, pyrogallol, and hydroquinone gave erratic results. pnitrophenol p-phenylphenol beta-naphthol
1.96 2.00 1.15
12 trials 12 trials 15 trials
s 8
s
= = =
0.0352 0.0365 0.0718
