J. H. Cooley, J. D. McCown, ond R. M. Shill' University of I ~ O
Alcohols to Alkyl Halides ~ O
Moscow, 83843
A kinetics experiment
I
for elementary chemistry courses
W e wish to report a kinetics experiment which could bc used in high school, college introductory, or organic chemistry courses. The rate measurement is accomplished by direct observation of the change of length or volume of the insoluble layer of an alkyl bromide that is formed from a mixture of alcohol, hydrobromic acid, and sulfuric acid. ROH
+ HBr
-
RBr
+ H30
Straight lines are obtained from plots of log (length, length3 against t,ime,%and first-order rate constants are calculated from the slope. The simplicity of equipment (Fig. I) and measurement makes the experiment a convenient and inexpensive introduction to kinetics.
Figure 1.
Moking experimental obrervofionr.
The following procedure has been employed by undergraduate organic chemistry students a t the University of Idaho. To a mixture of 10 ml of n-butyl alcohol and 20 ml of 48% hydrobromic acid is added 10 ml of concentrated sulfuric acid in small portions. 280 / Journal of Chemical Education
These quantitites could he scaled down, but we also have students isolate the alkyl halide. The mixture is transferred to an 5-in. test tube, fitted with a condenser, and plunged into a beaker of gently boiling water, which serves as a constant temperature bath. A ruler with a millimeter scale can be used to measure the length of the layer of alkyl halide, or if a graduated cylinder is used as a reaction vessel, the change in volume with time can be obtained. Readings were made so as to avoid errors due to parallax, and the measurement was made to the bottom of the meniscus of the alkyl halide. Measurements were also made with the layer of alkyl halide being formed in a narrow neck of a tube used for Victor-Meyer determinations and by using a cathetometer to measure the length to the nearest hundreth of a millimeter. These refinements did not seem necessary to improve the experiment. During the first few minutes, measurements are difficult to make due to erratic boiling in the solution. Also, the readings made during the latter part of the reaction when changes in length with time are small were erratic when plotted. The time required to obtain 1, varied with the structure of the alcohol and with the acidity and temperature of the reaction. For a secondary alcohol 1, was obtained in about fifteen minutes, while for a primary alcohol I , was obtained after 5&50 min. Results obtained by the authors are plotted in the figures, and our undergraduate students have obtained similar data. The alcohols that have been studied include n-butyl, n-amyl, iso-amyl, see-butyl, and 2-pentyl, all of which give measurable rates. Tertiary alcohols react too rapidly. See-butyl alcohol is not easy to work with as the bromide has a boiling point below that of water, and the propyl alcohols were not studied. Solubility problems were encountered with n-hexyl alcohol, and the higher alcohols were not studied as they are even less soluble. It is surprising that the points come close to a straight lme throughout the reaction in view of the change in medium that must occur. We have no explanation to put forth for this happy circumstance. Rate constants are calculated from the slopes for each of the runs shown in the figures. Figures 2 and 3 show results with n-amyl alcohol, while Figure 4 shows results with sec'Participant from Mountain Home High School, Mountain Home, Idsho, in a NSF Summer Institute in Science and Mntha matics for high school teachers. 2 FROST, A. A,, AND PEARSON, R. G., "Kinetics and Mechanisms," John Wiley and Sons, Inc., New York, 1961, Ch. 3.
Concentrations and Data for the Various Components in the Reactions Graphed in Figure 4.
k, min.?
Temper* ture 'C
sec-Butyl Alcohol
HBr
HlSOl
Run by R. W m l s in undergraduate organic laboratory.
in the volume of alkyl halide formed, and reported second order velocity constants for a series of alcohols. Bennett and Reynolds6 have reported studies of the homogeneous system of alcohols and hydrogen bromide in phenol. Based on the results of Bennett and Reynolds, Ingold6 suggested a mechanism change from SN2to SN1between ethyl and isopropyl. The SN1process would be expected to show first-order kinetics, and the SN2process would show pseudo-first-order kinetics,
minuter Figure 2. Three rompler with 1 0 ml of n-omyl alcohol, 1 0 ml of sulfuric ocid,md 2 0 ml of hydrobromic acid. k = 0.085,0.089, ond 0.092 mirr'.
hutyl alcohol. Slower rates are found with primary alcohols, lower temperatures, and when sulfuric acid concentration is decreased (Fig. 3). No significant change in rate is observed for n-amyl alcohol when varying amounts of hydrobromic acid are used. The results were anticipated from the mechanisms suggested by Ingold3 as shown in the following scheme. ROH
+ He
ROE@ R@
+ BrQ
m
-+
ROHl
slow
--+
Rm
+RBr aiow
BrQ R--OH&
I
+ H1O
fast
--t B-R
Sx1 process
+ Ha0
S Nprocess ~
Few kinetic studies of the conversion of alcohols to allryl bromides have been made. Norris4 ran the reaction in sealed tubes, measured the rate by changes
minutes Figure 4. Four romples xi* wrying omovnh of aec-butyl alcohol, hydrobromic acid, ond sulfuris acid. (See the toble.1
under the conditions of our experiment. Either reaction process would show the observed rate enhancement with added sulfuric acid. The lack of dependence of the rate on hydrobromic acid concentration would suggest that the SN1 process is operating, but a mechanistic interpretation based on these few results is premature. The simplicity of equipment and visual observation of a product being produced make this experiment appealing for an introduction to rate measurements. We wish to acknowledge support for this work from the University of Idaho special research fund under project G 61.
5
10
I5
minutes Figure 3. All sampler contain 1 0 ml of n-amyl alcohd and 2 0 ml of hydrobromic odd. The dope increased wilh more sulfuric ocid: A, 8 ml; B, 1 0 ml; C, 1 3 ml; D, 2 0 ml.
IINGOLD,C. K., "Structure and Mechanism in Organic Chemistry," Cornell University Press, Ithacs, New York, 1953, p. 340. ' NORRIS, J. F., Z. physikal. Chem., 130, 662 (1927). NORRIS, J. F., Rec. trav. Chim., 48,885 (1929). F. M., J . Chem. Soe., 131 "BENETT,G. M., AND REYNOLDS, (1935). ' See footnote 4. Volume 44, Number 5, May 1967
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