A New Twist to an Old Favorite The Hydrolysis of tert-Butyl Chloride J Turner University of Houston, Houston, TX 77204 Manv oreanic laboratow texts include several e x ~ e r i ments"inv&ing reactionkinetics. One of the reactions often usedis some variant of the hydrolysis oft-butyl chloride (1).This system is popular as a kinetics experiment for good reason: All the chemicals needed are inexpensive and ;eadily available, and the reaction conditions'can be chosen to provide a reasonable reaction rate. However, monitoring is almost invariably done by titration (2).Students at this stage have already done many titrations, maybe even an inorganic kinetics reaction. The only new concept is the use of anorganic substrate. Hence, the collective sigh of resignation from most classes upon being faced with "another tedious titration". We have developed a method for monitoring the reaction using NMR.' NMR spectroscopyis most frequently used for compound identification, and thus is presented in lectures this way. This experiment demonstrates that NMR is also useful in studies of reaction kinetics. It provides a valuable addition to the very few NMR kinetics experiments currently available (3-8). However. some modifications to the svstem are necessary becauke the deuterated solvents normally used for tBuCl hydrolysis are expensive, and residual protonated solvent may make spectral interpretation difficult. The variation described below uses aqueous trifluoroacetic acid (TFA)as the reaction medium, and gives a half-life of about 7 min. Ahonus of this experiment is that it also permits the simultaneous determination of the approximate and relative leaving moup abilities (LGA's) of the species chloride. water, and-trifludroacetate. There are several ways in which this experiment may be conducted by a class. Of course, each student may collect their own set of data, if the logistics of NMR availability allow. Otherwise, the instructor may wish to divide the class into groups of three or four students. Then responsibilities for data collection, tabulation, graphing, and interoretation are decided bv the eroun members. A third alternative is to simply give students photocopies of previously run sets of raw spectra. Similarly, if time available on the instrument is short, the experiment may be ended aRer half an hour, and the determination of relative LGA's omitted. or the ex~eriment may he assumed to be very close to equilibrium. Alternativelv. data from a hv~otheticalor ~reviouslvrun s~ectrum maybe provided by %e instructor'to allow chis cal&lation.
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Experimental Phase Calibration Place TFA in a clean, dry NMR tube to a depth of 11.2 in. Then add a small drop of tetramethylsilane (TMS). 'This experiment has been used in the second semester of an organic chemistry lab course for several years. Naturally, it could be used instead in a physical chemistry lab course. It is a modificationof an experiment used for some time with CD,COOD as the solvent, which gave an inconveniently long half-life.
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Journal of Chemical Education
Observe the TMS signal, correct the phase, and record the position of the phase knob. Now. tiltine the tuhe about 75~.carefullv add 0.5 in. of Hz0 the upper portion of the N'MR tubewith a Pasteur pipet. The surface tension of the water will keep it in place, allowing some to be added or taken out as necessary. This is important when the kinetic run Droper is studied (see below) because it allows the reactionto begin with a known amount, a t a predetermined time. Tap the water down into the TFA, and mix thoroughly by briefly vibrating the tube with the fingers, or by tilting the tube and rotating it. (It is important not to shake the tube, esoeciallv durine the kinetic run because this often leaves droplets out of tLe NMR field temporarily. When the droplets fall back into the field, confusing data may result). Make note of the new position of the phase knob needed to get the correct shape of the TMS signal for this aqueous TFA system. Kinetic Run Accurately weigh an oven-dried NMR tube, with cap, to i1mg or closer. Place 20-30 ing of t-BuC1 in the tube and
reweigh. Then add 1-1.2 in. of TFAand reweigh again. Add a small drop of TMS, weigh once more, and mix the tube contents without shaking. Observe the t-BuC1 signal with the phase in the initial (nonaaueous) Dosition noted above. and adiust the sienal strength to about 90% of the makmum chat can be recorded. Eiect the tube and s a m ~ l efrom the instrument. and caref;llY place 0.4-0.6 in. O
