Relative rates of electrophilic aromatic substitution - ACS Publications


of the electrophilic species, employing an inexpensive ... This graph is taken from the notebook of a ... since it constitutes one of the best interna...
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Relative Rates of

Joseph Casanova, Jr.

10s Angeles State College

Lor Angeles, California

Electrophilic Aromatic Substitution

The kinetic method has assumed a role of such importance in the study of organic reaction mechanisms that it is desirable to introduce some of the concepts and terminology of the method into the undergraduate laboratory a t an early time. Although a chemistry major will customarily perform one or more precise Emetic determinations in the physical chemistry laboratory, he may fail to appreciate the widespread applicability of kinetics to modern organic mechanism studies. Furthermore, nou-majors who usually do not study physical chemistry may not be exposed to the kinetic method a t all.

I

4

3

I t h

(secmas

I

5

la')

Figure 1. Normalized absorbance versus time for the bromination of some aromatic compounds in 9Oyoacetic acid a t 35'C. Bromine 10.05 MI and subrtrate (0.10 MI. Key: aspirin, 0; diphenyl ether, 0; ocetmilide, 0. Determined 3pectrophobmetrically (RJ.N.1.

nate method of analysis-visual color comparison-was examined and found to be satisfactory for some substrates. It is described in detail as Part 3-Alternate in the Experimental section. A typical result for the quantitative part of this experiment is illustrated by Figure 1. This graph is taken from the notebook of a student (R. J. N.). Data for quantitative visual estimation of relative rates for three aromatic systems are plotted in Figure 2 (data of J. E. S.). For a detailed discussion of the theoretical background to this experiment, the student may be directed to a recent paper by DeWolfe (1) or to a standard kinetics text (2). Most organic chemistry textbooks deal thoroughly with the subject of electrophilic aromatic substitution since it constitutes one of the best internally consistent bodies of information in organic chemistry. While the subject of orientation and sequence of reactivity is carefully discussed in most textbooks, little attention is directed to an appreciation of the actual magnitudes of rates and rate differences among various derivatives of benzene. The data in Tables 1 and 2 illustrate this difference. They are taken from recent literature (3,4). Table 1. Estimated Values of Partial Rate Factors for para-bromination of Some Aromatic Substrates b y Bromine in 8 5 % Acetic Acid (3)

R in CJIa

N(CH8)%

Partial rate factor (P,) 3 X

lo'*

OH

NHCOCHa

5 X 10"

1.2 X

OC&

lo9 7

X 10'

OCJIs

8 X lo'

Table 2. Calculated Values of Partial Rate Factors for para-bramination of Some Aromatic Substrates by Bromine in Aqueous Acetic Acid a t 25'C (4)

R in

t u brrna. r

caa

>on,

Figure 2. Bromine concentration versus time for the bromination of some aromatic compounds in 90% acetic osid at 24'C. Key: diphenyl ether, 0; p-nitmphenol, 1-1; acetanilide, 0. Determined b y visual mlor camporiron (J.E.S.).

We have designed a simple experiment which covers several aspects of kinetic phenomena. The experiment may be carried out in one three-hour laboratory session. We have chosen the bromination of various aromatic substrates in 90% acetic acid as the reaction to be studied. The rate of reaction is followed instrumentally by measurement of the rate of disappearance of the electrophilic species, employing an inexpensive visible/ultraviolet spe~trophotometer.~ An alter-

'The Bauach and Lomb "Speotronic 20," available at about $255 was employed for the experiment described here.

OH Partial rate factor 3 . 7 X (P,)

OCH,

OCJI,

1.1 X 10'0

1.0 X 108

C,H, 2.9 X 10"O1

ca, 1.8 X

ceca),

8.1 X 10'

Beyond general initial agreement, there is surprisingly little uniformity in details of the mechanism when the various reactions of this type are compared. D8erences occur mostly in the manner and timimg for generation and capture of the electrophilic species. Part of the complication also lies in the role of the catalyst in these reactions (6). They may vary from first order to third or fractional higher orders, wherein catalyst, solvent, water, electrophile, or aromatic substrate may appear in the rate expression (6). Trace amounts of water sometime exhibit an accelerating effect on Volume 41, Number 6, June 1964

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aromatic electrophilic substitution reactions. To eliminate this effect, in the present experiment an aqueous solution has been chosen as a solvent. The concentration of the aromatic substrate is set high enough so that it will not seriously change during the reaction, and it can be included in the rate constant for the reaction. Even then, complications by mixed k s t and second order dependence on bromine concentration and by the possible inhibitory effect of hydrogen bromide on the rate (7) lead to a complex and variable rate expression. In Part 1, decolorization rates are observed for a series of substrates immersed in a large vessel of water, so that the temperature remains nearly constant, and so that the samples are all exposed to the same temperature during their reaction period. The effect of temperature changes on the reaction rate is made apparent in Part 2. The Lambert-Beer law relationship between concentration and optical density may be experimentally verified by the student for bromine in 90% acetic acid a t a wavelength of 5200 A. The Experiment

Part 1 . Qualitative Rate Comparison. Fill a large beaker with warm tap water (35 2°C). Two milliliters of a solution of (1) acetanilide; (2) pnitrophenol; (3) benzene; (4) anisole; (5) chlorobenzene; (6) phenol; (7) diphenyl ether; (8) aspirin are placed in small test tubes. The solutions are 0.2 M in a r e matic substrate in 90% acetic acid. Under no circumstances should a metal spatula be used to mix any of the solutions. Trace amounts of certain metal ions appear to exhibit an inhibitory effect upon the reactions. A bromine stock solution (0.05 M in 90% acetic acid) is thermally equilibrated with the substrates. Add 2.0 ml of the bromine solution to one of the substrates. The solution should be quickly mixed with a glass rod until it is homogeneous. Note the time (in sec) required for each solution to decolorke. A faint yellow or colorless solution is considered to be the end point. In some cases the reaction is very slow. Part ?2. Temperature Effect on Rates. In each of four test tubes place 2.0 ml of a 0.2 M stock solution of acetanilide. Allow these test tubes to reach thermal equilibrium in different water baths which are close to 0, 30, 60, and 90°C. The 0' bath is an ice bath. The 90" bath is one which will be obtained by continuous heating of the water on a steam bath. Place 2.0 ml of the bromine stock solution in each of the four tubes containmg acetanilide. Record the time required for each solution to decolorize (faint yellow color). Plot l/time (sec, ordimate) versus temperature (absolute, abscissa). Part 8. Quantitative Rate Comparisons. The spectrophotometer is set a t a wavelength of 5200 A. Allow 2.0 ml of the stock solution of aspirin to equilibrate a t 35°C in a cuvette which is topped by a short cork. Rapidly, and in this order, carry out the following operations: open the cuvette; add 2.0 ml of the equilibrated bromine solution; stir quickly once only; replace the cork tightly; remove the cuvette from the water bath; dry it once very quickly with a paper towel, and place it in the spectrophotometer. This entire group of operations, with a little practice, can be performed in less than 10 seconds. Readings are best carried out by pairs of students. Record absorbance

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342 / Journal o f Chemical Education

a t 30, 60, 90, 120, 150, and 180 see. Take readings of the absorbance of this sample a t 15-min intervals to a t least to 4000 sec. Repeat the precedmg experiment with two other substrates--diphenyl ether and acetanilide. In these cases absorbance readmgs should be made a t 10-sec intervals for 1 min and thereafter a t 30-sec intervals until the change in absorbance between readings is only slight or imperceptible. This time will be about 5 min. First plot absorbance (ordinate) versus time (sec, abscissa) for each substrate. Extrapolate each graph to zero time to obtain Ao, the absorbance a t time zero. Divide the value of the absorbance a t each data point by the A. value appropriate to that run to normalize the data and obtain the values A/Ao for each point of each of the three substrates. F i l l y , plot on a single graph A / A Oversus time (sec) from 0-500 sec. Smce all data are now recorded on one graph and on the same adjusted scale, the relative rates are directly comparable. Part 8. Alternate. The rate of bromination of several aromatic substrates in 90% ,- acetic acid mav be visually estimated as outlined below. Stock solutions of bromine (0.5 M), pnitrophenol (0.05 M), acetanilide (0.05 M), and diphenyl ether (0.05 M), all in 90% acetic acid-water, are prepared. Seven standard color comparison solutions are prepared by dilution of 3.0 ml portions of the bromine stock solution to 4.0, 8.0, 16.0, 32.0, 64.0, 128, and 256 ml, respectively, with 90% acetic acid. A 4.0-ml portion of each of these solutions is placed in each of seven 13- X 100-mm test tubes. The test tubes are fastened in order of decreasing concentration of bromine in front of a white background. Mix a 2.0-ml portion of each of the substrates listed above with 2.0 ml of the bromine solution, stir the mixture quickly with a glass rod. Note the exact second of mixing. Record the time (in sec) a t which the color of the reaction mixture most closely matches each successive standard sample. Plot bromine concentration (molar, ordinate) against time (sec, abscissa) for each substrate. Acknowledgment

The author is grateful to Rebecca J. Nisson, James E. Scott, and Frederick Martm for assistance. Literature Cited (1) DEWOLFE, R. H., J. C ~ MEDUC., . 40,95 (1963). R. G., "Kinetics and Meeh* (2) FROST,A. A,, AND PEILRSON, nisms," 2nd ed., John Wileyand Sons, Ine., NewYork, 1953, E. S., "Mechanism and Structure in Orchap. 2; GOULD, ganic Chemistry," Holt, Rinehart and Winston, New York, 1959, pp. 159-75. (3) DE LA MARE,P. B. D., AND RIDD,J. H., "Aromatic Substitution-Nitration, Halogenation," Butterworths Scientific Publications, London, 1959, p. 140. (4) STOCK, L. M., AND BROWN, H. C., "Advances in Physical Oreanic Chernistrv." Vol. 1. V. GOLD. ' editor. Academic Press. k e w York, 1963; pp. G1,67-72. GOULD, E. S., pp. 440-445. HINE,J., "Physical Organic Chemistry," 2nd ed., McGrawHill, Inc., New York, 1962, chap. 16; INQOW,C. K., "Structuras and Mechanism in Organic Chemistry," Cornell University Press, Ithaca, New York, 1953, pp. .. 288295 and references cited therein. R. M., OTTENBERS, A,, AND ANDREWS, L. J., J. Am. KEEPER, Chem. Soc. 78,255 (1956); ANDREWS, L. J., AND KEEFER, R. M., J. Am. Chem. Soe., 78,4549 (1956).