The Menschutkin Reaction A group experiment in a kinetic study P. W. C. Barnard a n d B. V. Smith Chelsea College, London, England SW3 6LX T h e application of kinetir methods to nucleophilic aliphatic substitution is a familiar topic in introdurtorv courses un reaction mechanism. A suitable experiment was required to demonstrate to a group of thirty or more students the use of conductivity measurements a s a method of following the rate of the reaction
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RX + R'rN R ~ ~ R + ' BXIn the course in auestion (which is of 12-wk duration) a maximum of 5 h r pLactical work per week could be undkrtaken. Any reactions studied have to he fast enoueh to enable a n infinity reading t o he taken in one day; otherwise, if they are left until the following week uncertainties may arise from evaporation of solvent and reversibility. With the Menschutkin reaction, pseudo-first order kinetics could he secured by using a n excess of amine and Guggenheim's method applied to the experimental results. T h e need for a n infinity reading is thereby removed. T h e reaction chosen
u Figure 1. The cell.
At time t , A ao - x l = a ~ e - ~ ( ~ l + ~ J By subtraction XI- x = aoe-btl (I - ec")
+
was studied in methanol (20-45%) and fulfills the conditions imposed by available time. T h e advantages of studying this system are a s follows:
This equation is of the form ln(x1 - x )
(1) It is a clean reaction to handle and can he considered irreversible
in the time needed for measurements. (21 The kinetic behavior is reuroducible. ($1 I t illustrates the applirntmn t r f the Guggenhrim method for obtnminl: a rate cmilant. ( 4 1 It ..huws the difference hetwern nppnrent kinetic urder and mulecularity. (5) By allotting experiments on a group basis, Arrhenius parameters are obtainable from different investigators'experimentsat several temperatures (6) Variation of the pyridine moiety leads to an understanding of a linear free energy relationship for this reaction. (7) The effect of solvent on reaction velocity can be checked readily. (8) The resultsof different experiments and the mechanism of substitution a n be discussed in the light of published data on related systems. This readion has been found valuable in all the aspects listed and the great interest of the group in the collation of their results was a justification of this study. T h e method of treatment of the results together with the necessary description of experimental detail will now he given. The Guggenhelm Method T h e usual graphical analysis of a first-order reaction means t h a t considerable weight is given t o the infinity value. A method of avoiding this was described by Guggenheim (I), which also has the virtue of shortening the period of time needed for following the reaction. Using the usual nomenclature,
282
Journal of Chemical Education
(2) (3)
= -kt +constant
Hence a plot of log(x1- x ) against t will furnish a straight line of slope 4 1 2 . 3 0 3 . Since it is assumed t h a t the observed chanee in a ohvsical .. parameter (e.g. conductance, absorbance e'tc.) is directly proportional t o the change in concentration of reactant (or product) the above equation takes the form
It was verified by a n independent series of measurements that, within the cuncentrathn range studied, the condurtanw uf N-phenacyl pyridinium b n m i d e (the productj inrreased smoothly with increasing concentration The Experlmenl The CeN
Individual runs were followed by measuring the change in conductance of the reacting solution contained within a closed cell connected by screened leads to a Wayne-Kerr Bridge, Model 221. The cell is shown in Figure 1 and required 100-120 ml of solution to cover the electrodes. The electrodes are circular discs of platinum held rigidly in place by the glass tube supports. Copper wires dipped into mercury-filled,sealed side arms as shown and platinum wires sealed in place complete the contact with the electrodes. Suitable concentrations of phenacyl bromide and pyridine are 0.005-0.015 M and 0.05-0.15 M, respectively. At 35'C this gives a convenient reaction rate (Table 11, and the measurements are complete within 2.5 hr. Materials
Phenacyl bromide (B.D.H.) was recrystallized from methanol or petroleum ether and stored in a vacuum desiccator over silica gel. It
has m.p. 49.5-50.0°C. Methanol (Burroughs, absolute) was fractionated from fresh quicklime using a 2-ft Vigreur column; the middle fraction (b.p. 6M5"C) was used in our initial experiments. It was then shown that the kinetic behavior of the rigorously purified and drled material was little different from that of the ordinary grade; consequently, this was used in the student experiments with a considerable saving in time and only a small induced error. Pyridine (A. R. Grade) was dried (solid potassium hydroxide) and distilled. The picolines and quinoline were good commercial grades which were treated similarly.
Table 1. Observed Conduclances from a Typical Run wlth Phenacyl Bromide (0.0123 M ) and Pyridine (0.124 M ) . Time (mitt)
k (m mha)
Time
k (m mho)
(min)
logAk
Ak
Safety Precautions Phenacylbromide is a powerful lachrymator; consequently, safety glasses should he worn when using it and care should he taken to keeo it off the skin. Hands should he washed as soon as final transference has been made. Care should also he taken when handling t h e pyridines. First Kinetic Run The requisite quantity of pyridine to make a solution in absolute methanol within the desired molarity range was weighed in a stappered weighing bottle and carefully washed into a volumetric flask. This solution was then transferred to the conductivity cell ensuring that the electrodes were covered completely by the solution. The cell and contents were allowed t o attain bath temperature. After attachment of the leads and any necessary adjustments of sensitivity and trim, the conductance of the solution was measured. The appropriate quantity of the organic bromide was weighed into a stoppered specimen tuhe and the reaction was started by addition of this solid to the solution in the cell. A clock was started a t the moment of addition. The cell stopper was replaced and the contents shaken thoroughly before the cell was reconnected to the bridge. This operation took 3 4 mi". After about 5 mi", readings of conductance were taken at l-min intervals (where temperatures were 35' or above) or 2-min intervals (209-350). It was convenient to allow a final interval of ninety min (at 35') before starting the second series of measurements. Twenty-five to thirty readings were taken in each set. At other temperatures, time intervals were adjusted accordingly. At some convenient time after the addition of the phenacyl bromide to the cell the specimen tuhe was reweighed.
Second and Subsequent Runs Within a group of students it is recommended that the following permutations of experiments are carried out: (a) Change of temperature; in this respect the range 20'-45'C
.mv.reA .....- - pasilv --....
is
:
ihl Chanyrs g l f nmcenrration < f reactant, a l t h t u ~ hthe ratiuof 1,a.r: hrumide is ~deallyset at ]*I, ~n practice it may be adjwted tt15:l withuur t,itinrton of the tirqt order kmetir turm Another useful set of results is gained by keeping one reactant a t constant concentration and investigating the value of k , and k* as the other concentration varies. (c) Changes in reactant; by using substituted pyridines and/or alkyl hromides of different structure the effect uf structure on reactivity may be explored fully. (d) Change of solvent; alcohols other than methanol can be used. The variations in rate of reaction with change of dielectric constant can be evaluated It will be appreciated that the pattern is determined by the number of students available and judicious choice of variables will mean that small or large groups can be kept fully occupied. Results (1) Good agreement has been found between this work and the literature values (21. It has been reoorted (31 that reaction in
conditions. The reliability of conductance change as a measure of velocity of reaction was checked in two ways: (a) The measurement of conductance of solutions of N-phenacyl pyridinium bromide in methanol (in the concentration range farmed in the runs) enabled an estimate of K.. to be made for nnv n ~ nA . value of the first order rate constant derived from
Table 2. Second-Order Rate Constants (I mol-' min-') for the Reactions Between Phenacyl Bromide and Pyrldlne in Methanol Temperature
Tempelahlre (" Cl
102k2
(" Cl
10Zk2
20.2 23.1 24.8 25.0 25.5 26.0 26.2 26.7 27.5 28.0
1.36 2.03 2.42. 2.45 2.47 2.70 2.73 2.74 2.75 2.75 2.79
29.4 32.5 32.9 34.0 34.3 34.8 35.3 36.0 37.0 42.8
3.05 3.59. 3.72 4.07 4.23 4.58 4.75 4.72. 4.73 5.02. 5.19 5.27 7.73
Table 3. Second-Order Rate Constants (I mol-' min-') for the Reactlon between Phenacyl Bromide and Selected Tertiary Amines Group NO. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Base (0.1-0.14 M)
CsHd CrHsN CsHsN CsHrN CsHrN 26H&H,N 3-CHaCsH,N 3-CH3CsHaN 3-CH&H,N 4-CH*CsH,N 4-CHaCsH4N 4-CH3CsH.N 4-CHzCsH4N 4-CH3CsHaN 3.5-(CHdtCsHsN Quinoline
Temp (O c )
102k2
26.7 28.0 32.9 ' 36.0 42.8 * 34.9 23.1 28.7 ' 35.5 25.1 29.2 31.8 ' 35.2 39.3 23.1 ' 36.1
2.75 2.79 4.07 5.02. 5.19 7.73 0.48 2.75 4.51 6.98 3.72 5.19 6.09 8.00 10.09 4.25 2.99
the bromide ion liberated. First order rate constants so measured were in very good agreement with those obtained from conductance measurements. This method of following a run is suggested far circumstances where conductivity measurements cannot be made. (2) The results in Table 1were obtained from a typical run with phenacyl bromide and pyridine a t 32.9% A plot of log Aa versus t gave a straight line of slope of -0.00219 min-' whence k l = 5.05 1mol-' min-I. X 10W min-' and k n = 4.07 X (3) In Table 2 are collected some rate constants for the reaction of pyridine with phenacyl bromide in methanol illustrating the variation with temperature. An Arrhenius plot gave an Activation energy of 11.7 kcal mol-' (49.1 kJ mol-'1. (4) In Table 3 are collected results obtained with different bases. Some of the values from experiments with pyridine are also inVolume 58
umber 3
March 1981
283
Table 4.
The Effect of Replacing a Protlc Solvent by a Dipolar Anrotic One
Reaction
Solvent
Temp ('C)
2-CH3 k/ko
+
Figure 2. Linear free energy relationship fmthe system RCshN CsH5COCH2Br. 1 = Quinoiine; 2 = pyridine: 3 = 3-methylpyridine; 4 = 4-methyipyridine; 5 =
eluded for comparison. Although the temperatures used do not permit an exact comparison, the runs marked with astar can be used to examine how approximately the results conform to a linear free energy relationship. Figure 2 has been drawn using these values. Discussion
In the following discussion we have tried to make comments on our own results and to include salient noints from the verv considerable literature. From many pertinent references we have selected the followine four maior areas to illustrate the value of this study as a teaching exercise, and insrrurturs may find it hrloful to refer u, the literaturr citcd when oraanizinc . . tutorial groups.
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Linear Free Energy Relationships The results from this ex~erimeotare sufficient to illustrate the operation of a linear free energy relationship. In Figure 2 are olotted the values of loe.. h?- aeainst oh,: . . -. the straieht line accommodating points fur pyridine and 3- and J-methylpyridines iiassociated witha relationshi~ufthetvDe lo2 hlh,) = p a . In a more extended study (3), it has been ihownthat for eight suhstituents in the pyridine ring, the reactivity toward phenacyl bromide, in nitrobenzene, obeys the relationship log h, - log h~ = pod', where the symbols have their usual meanings. The value of p0 (-3.92 to -4.11, depending upon temperature) indicates that the reaction is favored by electron releasing substituents in the pyridine ring. It may he noted that oO has been used to signify "corrected" values for p suhstituents (4) and rn suhstituents, originally designated om by Taft (5). Steric Effects The large deviation associated with the point for 2methylpyridine (Fig. 2) is consistent with the operation of the steric effect of a 2-methyl group. Data for related reactions with 2-, 3-, and 4-methylpyridine are collected in Tables 3 and 4 and show clearly the order h4.M. > k a . > ~ h~z . ~ ,(6-8). Reference to the literature shows that steric effects have been observed as a function of substitution in a ranae of amines andlor halides (9). According to Sisti and ~ e m e g e (101, r osubstitution in phenacvl chloride (CsHsCOCHzCI) - gives . rise t o nucleoph3e-sub&itution in phenacyl chloride (CsH5COCHzCI) gives rise to nucleophile-suhstituent interaction; differences in the rate of substitution by iodide ions and pyridine were explained by coulomhic forces in the transition state. Examination of Dreiding models of these systems is recommended as an aid to understanding interactions in the transition state. ~
~
(3) Solvent Effects and Transition State Structure The Menschutkin reaction is particularly suhject to solvent effects. The data in 'l'ahles 4 and 5 are illustrarive (7, b l and 284
Journal of Chemical Education
Table 5.
Solvent Variations in Menschutkin Reactions of Phenacyl Bromide Sol-
Base CsHsN CdW ~-CH&HIN ~-CH&HIN
vent
Temp I"c )
l@k2 [i moi-' mi"-')
CHsOH CsHsNOz CHsOH CsHsNOs
42.6 40.0 35.5 40.0
7.73 28.8 6.98 61.2
show the marked effect of reolacina a ~ r o t i solvent c hv dipolar aprotic one ( 1 1 ). Such an effbrt has hein interpreted as a;iiing from the i m p o m r e of better solvat~onof the transition state. rather than-any significant solvent effect on the reactants in a dipolar aprotic medium (12). These conclusions give a more quantitative measure of the predicted solvent effects as enunciated by Hughes and Ingold (13), based on a better understandine of the structure of the transition state and solvent effectithereon (14). From different studies the conclusion has been reached that the transition state is a .~.o r o x i mately one quarter along the reaction coordinate, i.e. it is reactant-like (14-1 7). Unhindered and hindered Menschutkin reactions both show a pressure-induced increase in reaction rate. This is greater for the hindered systems (la), and it was proposed that interpenetration of interfering groups was responsible (19). Such an explanation ignores solvent effects, and the agreement between calculations based on models and experiment is probably fortuitous. An alternative explanation has been offered (20) on the basis of the Hammond postulate (21) which states that when two similar reactions differ greatly in the amount of energy released, the more exothermic one will be faster and have an earlier transition state. From this it follows that the pressure-induced rate increase is associated with a more ~roduct-liketransition state for hindered svstems. For an interesting example of the contrasting solvent effects with intra- and inter-molecular Menschutkin reactions see reference (22).
(4Activation Parameters The Arrhenius activatiun energy (19.1 k.J mul-') and the pre-exponential factor (lay A = 5.21) are t ~ r t hlower than for many himolecular reactions. For related reactions the \,slues uf E A and log A fall within the ranges :%5X k.1 mol-' and The low value of E* 4 7 4 . 2 Im-' s-'. resnectivelv " 1. 3 ,. fi., 7.2'3). , would normall; be Bssociated with a fast reaction; but t h k ignores the entropy of activation which exerts a controlling influence. From the data given, A S = -34.7 cal. deg.-' mole-' is within the ranee reported for other Menschutkin reactions (-34 to -40 c 2 deg.-I mole-') (3,23). The large, neaative value of AS! sueeests that a more ordered. charged transition state is formedfrom the reactants.
Literalure Cited I l l Gu~genhcirn,E. A.,Phil. M a d . 1,538 11926). (2) Pearsun, R. G.. Langer, S. H., Williams. F. V., and MeGuim, W. J.. J. Amm Chem Sor..,
-. ~.-" ,*,OL,"
,."="> ,,m,',.
(8) Litvinenku,L. M.snd Perel'men,L.A.,Zhur. OPX.Khim..3.986 119671. 8 (11 Tait. R. W..and k w i . . LC.. J . A m r r C h o n S ~ ~ . , 8 1 , 5 3 4(19591. J A m r r . Chem.Sa.81,5352 (5) Taft,R. W.,Ehrenwn,S.,kwis,l.C.,andGliek,R.E..
\...,",. ,,OLO8
(61 Clarke. K. snd Ruthwel1,K. J. Chsm. Sor.. 1965l1960). (71 Fiicher.A.,Gailuway, W. and Vsughan, J.. J. Chem S o . , 8506 (19641 181 Br
