ANALYTICAL EDITION
May 15, 1941
357
The head which was finally designed is shown in the accompanying diagram. Essentially, i t consists of a condenser, C, of sufficient capacity to condense all vapor reaching it; a thermometer well, A , which is offset so as to avoid any disturbance due to proximity of the condenser or to contact with the cooled reflux; and a large-bore stopcock, B, through which product, in vapor form, may be withdrawn through the thermometer well. All parts are so constructed and leveled that there are no indentations in which reflux can be retained; even the tube to the stopcock is short and slanted upward to permit condensate to return readily to the column. To allow vapor to pass freely throu h the thermometer well, a bridging tube, D,is sealed between t%e condenser tube and the thermometer well. The bridge, which is slanted downward in the direction of the well, catches a small fraction of the reflux and diverts it through the thermometer well to help prevent the accumulation of dead vapors in that space. I n practice, the head is surrounded by a celluloid cylinder to lessen rapid temperature fluctuations due to air currents in the room. Heat leakage through this, however, causes a small condensation in the upper parts of the thermometer well and prevents the upper parts of the well from acting m a vapor pocket. Tests made of the operating characteristics of this head have shown t h a t it can be handled even b y novices with a minimum of instruction. With the vaporization rate fixed, i t is possible to control the reflux ratio accurately merely b y controlling the product rate with the stopcock; and by checking the temperature with a thermometer having scale divisions of 0.1' C., i t has been simple to follow the approach of the column t o equilibrium under total reflux, to note with fair accuracy the time required for equilibrium to be attained, and to observe signs of flooding long before other signs than that given by the temperature change made their appearance. Although the head has thus far been used only with columns of 1.2-cm. diameter and from 100- to 170-cm. height, packed with glass or h'ichrome helices, its performance should b e equally excellent with other laboratory columns. U
Bromination of Phenols by Means of Bromide-Bromate Solution SIR: A recent article (4) with the above title presented bromine titrations for a variety of substituted phenols, many .of which show good agreement with theory. Others, however, including 0- and p-cresols, four xylenols, and other alkyl phenols, were reported to be overbrominated under the conditions used by Sprung. He ascribed this discrepancy with theory to side-chain substitution. These fractional titration values were considered reproducible, and if so, are satisfactory for some purposes. However, a method which gives accurate values agreeing with theory was described (8) several years ago and applied to phenols, aromatic amines, and nitro aromatics. We have used the method occasionally since then, and never had any trouble with overbromination of cresols. Typical results giving the number of moles of bromine per mole of cresol are as follows: 0-Cresol 1.996 1.985 1.998
m-Cresol 3.007 2.996 3 . no0 3.007
p-Creso 2 . on6 2.014 2.006
Also, 50 per cent o-cresol, 50 per cent rn-cresol, 2.502 (theory 2.500); one third of each cresol, 2.337 (theory 2.333). Similar results (1) were obtained for the four xylenols which Sprung found were overbrominated. The essential difference in procedure seems to be in the time of standing with excess bromine. The nuclear substitutions have
velocity constants (3) of the order of at least lo6, so that no delay is necessary after the appearance of free bromine, and the merest trace of excess is sufficient. Since the formation of bromine is a time reaction, however, an excess of bromidebromate can be used; but by employing a lower concentration of acid (sulfuric or hydrochloric) the production of bromine can be controlled to any desired rate. As soon as a yellow color a p pears, bromination can be stopped by addition of potassium iodide. A sharp distinction between nuclear and side-chain substitution is thus possible. The iodine can be titrated immediately with sodium thiosulfate. To accelerate attainment of the end point more acid can be added before or during this titration. Sprung used about eight moles of potassium bromide for one of potassium bromate. A much smaller proportion of bromide, a little over two moles, is sufficient, as explained in the original article ( 2 ) . I t may be that this lower concentration of salt facilitates accurate titration.
Literature Cited (1) Francis, A. W.,
J. Am. Chem. Soc., 48, 1635 (1926). (2) Francis, A. W., and Hill, A. J., I b i d . , 46, 2498 (1924). (3) Francis, A. W., Hill, A. J., and Johnston, J., I b i d . , 47, 2213, 2228 (1925).
(4)
Sprung, M. M., IND.ENG.CHEM.,Anal. Ed., 13, 35 (1941). ALFREDW. FRANCIS Socony-Vacuum Oil Co. Paulsboro, N. J.
A. J. HILL Yale University New Haven, Conn.
