Boiling Points and Densities of Acetates of Normal Aliphatic Alcohols

The boiling points, rates of change of boiling point with pressure, and densities of methyl acetate, ethyl acetate, n-propyl acetate, and n-butyl acetate were measured by precise comparative methods using water as the reference standard . As in previous work on the normal saturated hydrocarbons and normal aliphatic alcohols, it was found that the introduction of each CH2 group into the chain of a molecule containing a normal alkyl group of more than some small number n of carbon atoms causes dt/dp to increase by a constant value which is independent of the chemical nature of the molecule. CONTENTS Pag • • 1. Methods of measurement___________________________________ ____ 499 II. Method of extrapolation of the data_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ 500 III. Purification of materials________________________________________ 501 1. Methylacetate____________________ ______ ______ __ _____ __ 501 2. Ethylacetate__________ ____ ____ __ _______________ ___ ____ 501 3. n-Propyl acetate___ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ __ _ _ __ _ _ _ _ 501 4. n-Butyl acetate____________________________ _____________ 501 IV. Experimental results and conclusions_________________ ___________ 501 V. References_____ __ _ _ _ _ _ _ ____ _ _ _ _ __ _ _ __ _ __ _ _ _ __ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ 504 1. METHODS OF MEASUREMENT In previous papers it was shown that in the homologous series of normal saturated hydrocarbons [11] 2 and normal aliphatic alcohols [12], containing more than some small number n of carbon atoms, the addition of a CH2 group to the normal alkyl group to form the next higher normal alkyl group results in an Increase of 0.0029° C per millimeter of mercury in the coefficient dt/dp at the normal boiling point. Since this value was found to be independent of the chemical nature of the compound, the additivity of the effect of the CH2 group on dt/dp was postulated. In this paper a similar investigation of the relation between the dt/dp coefficient and the molecular weight of the compound in the series of acetates of normal aliphatic alcohols is described. In addition, there are reported the boiling points and densities of the following esters: methyl acetate, ethyl acetate, nQropyl aceta.te, and n-butyl acetate. For determining boiling points Swi~toslawski's ebulliometric technic [7] and comparative method with water as a primary standard was applied. Details of the experimental procedure for determining the boiling point and the coefficient I Guest Worker from the Polytechnic Institute, Warsaw, Poland. I The figures in brackets refer to reference, listed at tbe end of the paper. 126219-37--8 499 500 Journal oj Research oj the National Bureau oj Standards [Vol. 18 dt/dp have been described briefly in the paper on hydrocarbons [11]. Temperatures were measured with a platinum-resistance thermometer having a coiled filament [4], calibrated at this Bureau and furnished by C. H. Meyers. Normal boiling points were calculated by the use of the formula previously reported [11], t.=t,' + ~~: (100-t • .'), where t. represents the normal boiling point of the substance under investigation, t'. the boiling point actually measured, and t' '" the boiling point of water actually measured under the same pressure as that of the substance. The ratio I:::.t./I:::.t", at 1 atmosphere was <computed from boiling points of the substance and of water under the same pressure, slightly above and slightly below 1 atmosphere. For the density measurements, the differential method of twin picnometers [5], with water as a reference liquid, was used. To control the purity of substances investigated, Swi~toslawski's ebulliometric test of purity and his scale for classification of the preparations were used. II. METHOD OF EXTRAPOLATION OF THE DATA To compute the physico-chemical constants of the pure compounds a method of extrapolation which has been reported previously [13] was used. The method is similar to that of Swi~toslawski [7] and is based on the correlation of the data for a given property with the purity of the preparation expressed by I:::.t, the difference between its boiling point and condensation temperature. This difference is a function of the nature and concentration of the impurities in a substance and is zero if the substance is pure. Having made the measurements of I:::.t and of a given physico-chemical constant for a series of preparations of the same compound having different purities, as, for example, a few successive fractions of the distillate from an efficient column, one may plot I:::.t, the difference between the boiling point and temperature of condensation of each of the preparations, in a differential ebulliometer, against the data obtained for the given constant. Direct extrapolation of the curve to the point where I:::.t equals zero gives the constant corresponding to the pure substance or azeotropic mixture. If only one impurity is present, I:::.t is a very nearly linear function of its concentration, and so is the value of any property measured. Hence the relation between I:::.t and the given property is also very close to linearity. When there are several impurities, which generally is the case, and their concentration is different in each fraction of the distillate, I:::.t, as well as the property measured, is expressed by a curve whose slope and curvature depend on the nature and concentrations of the impurities. In general, the relation between I:::.t and the property measured may be expressed by a. curve. However, as the degree of purity of the substance investigated becomes higher, the curvature decreases, and for preparations whose purity is such that I:::.t is only a few thousandths of 1 degree, it becomes nearly linear, thus permitting a reliable extrapolation to I:::.t=O. Wojciechows ki] Smith Physical Constants oj Esters III. PURIFICATION OF MATERIALS 501 The best grades of substances that could be purchased were purified by distillation through an efficient 40-bulb column of the Swif;'toslawski type. Esters are difficult to maintain in a state of high purity, because they are very hygroscopic and readily hydrolyze on boiling . Preparations for the determination of boiling point and related properties were distilled directly into the ebulliometer to avoid contact with moist air. Only fractions distilling at constant temperature, as an evidence of their purity, were taken for measurement. A minute amount of water in the ebulliometer, mostly as water adsorbed on glass, may have slightly contaminated the preparations, but its effect was not differentiated from the effects of traces of other impurities.