A Study of Hydrogen Exchange between Water and Lithium Borohydride James U. Piper Simmons College, 300 The Fenway, Boston, MA 021 15 This experiment was used in a senior undergraduate course in instrumental analysis and proved t o be an excellent measure of both laboratory technique and the students' abilitv to maninulate and i n t e r ~ r enumerical t data. Its objective is to determine the extent of tritium incorporation into lithium borohydride during partial decomposition by [3H]-HzO. The resulting labelled horohydride is used to reduce n-octvl aldehvde to l-13H1-l-octanol. which is counted as the 3,5-dinitrobenzoate derivative1. significant quenching- is observed for which correction must be made. Kreevoy and Hutchins (I) first observed the exchange reaction LiBH3CN+ D,O
LiBH,DCN
+ HOD
(1)
which accompanies the decomposition of lithium cyanoborohvdride bv water. The reaction is remarkable since hvdride is being incorporated into the molecule from n proton source. It is difficult to rationalize the orocess usior classical Lewis structures, and Kreevoy and itchi ins postilate the protonated cvanoborohvdride ion as an intermediate. Sodium borohydride does ndr undergo exchange with D?O, hut in 1970 Cornforth rewrted the r a ~ i dexchanre renction with lithium borohydride(2). Borohydrides are readily decomposed by water in a reaction which can be written as ~
~~~
~~
.
~~~~~~~
-
+ 2H,O
BH,-
-
BOz- + 4H,
(2)
Consider thecase in which the exchange reactionis infinitely faster than decomposition, and one mole each of borohydride and DzO are mixed. The decomposition would be preceded by the reaction BH,-
-
+ D20
complete deuterium exchange
(3)
Since one third of the hydrogens in the system are deuterium, after randomization hoth the borohydride and the water will be 33% deuterated. After exchange is complete, decomposition takes places according to eq 2 destroying 50% of the borohydride. Cornforth found that under these conditions the remaining borohydride contained 24.4% deuterium indicatine that exchanee is sienificantlv faster than decomoosition. s h e also did tlhe exccange with tritiated water, &ding 22% incorporation (67% of theory). I t is this method that is used here because of the ease of measurement of the label. The exchange reaction was run as described in the experimental section. The remaining, labelled lithium horohydride was used to reduce odanal to 1-octanol
4
0 11
CH,,(CH2)&H+ LiBH4
-
H '
(CHIICHI)&H~O)~BL~
Hz0
4
CH:,(CH2)&H20H (4)
which was converted to the 3,5-dinitrobenzoate (DNB) derivative CH,(CH,),CH,OH
+ 3b-(NO,),C,H,COCl3,5-(N0,)2C6H,COO(CH2)7CH3 (5)
442
Journal of Chemlcal Education
for counting (see Discussion). students first determined the approximate activity of the DNB by counting a sample which they estimated would contain 20,000 dpm. Based upon Cornforth's finding of 67%of the theoretical incorporation of tritium into the borohydride, one calculates that 25 pCi of 3H20 will result in a specific activity of 8.33 pCil11.12 mmole (0.67 X 0.33 X 25) or 3423 dpmlmg. The actual count gave about 2300 dpmlmg, uncorrected for quenching. Students then prepared a series of quenched standards from six identical samples of [3H]-toluene with about the same activity as the initial sample of DNB. They added cold octanol 3.5-dinitrohenzoate to each of five of the s a m ~ l e s over a r&ge from three times less to two times greater &an the weight of DNB in the initial sample. (In the absence of a microhalance, these samples can he measured volumetrically from a solution of DNB in scintillation cocktail.) Two different quenching curves were constructed from these samples, one based on a sample channels ratio (SCR) and the other on the "spectral index of the sample" (SIS), a parameter produced by the Packard Tri-Carh 300C system which we used (3). In the third part of the exercise, students prepared a series of five samples of the DNB over a range of weights, and used the calibration curves to correct the specific activities for ouenchine. Thev also used the internal standard (IS) methdd to corr&t fo; quenching so that they had tbred values to compare by simple statistical methods (4). The average values obtained by seven students were 2772 f 246,2597 f 175, and 2719 f 217 dpmlmg for the SCR, SIS, and IS methods, respectively. This corresponds to 51-54s incorporation of tritium into the lithium borohydride. Experimental Taa aol!nionof 121 mg (5.56 mmol) of LiBHI in 10 ml, ofTHF is added 100 # I . (100 mg. S 5 f i mmnl. 26 uCi) of 'H20 slowly with stirrma. l ' h r solution is refluxed for 20 min. cooled. and 1.5 r 111.7 mmolhf oetyl aldehyde added slowly with $timing: After adhition is complete, the solution is refluxed 45 min, coaled, acidified with HC1, diluted with water, and extracted with ether. The ether extracts are washed with water followed by saturated salt solution and are dried over anhydrous NazS04. The ether extracts are concentrated under reduced pressure at room temperature and to the residue is added 2.6 g (11.3 mmol) of 3,5-dinitrohenzoylchloride. The mixture is heated in asand hathat its meltine ooint for 10 min. cooled. and triturated with warm 2% sodium cn.idonatr. The solid(2.8g ) is t~ltercdand recrystallired to constant specific arrivity frum 95% ethanol. Snmples of 3-15 mg were counted lor 10 min each. ~~~~
~
Discussion The exchange reaction generates 15-20 uCi of hvdroeen . .. gas which i . ; s ~ & exhaustid l~ when therenction israrried out in an efficient hood. If one prefers to trap the hydrogen for disposal as liquid waste, tandem traps rontaining 1-octene and the highly arrive platinum catalyst generated in situ from NaBH, and rhloroplatinic acid (5) can be used. Howev-
'
Counting was performed wiih a Packard TrlCarb 300C equipped with the dprn option.
er, this procedure is not a substitute for a good hood. It should he noted that the 3,5-dinitrobenzoate derivative was chosen for pedagogical reasons in order to observe significant quenching during counting. Cornforth's use of henzophenone is place of octanal or the use of aurethane derivative would be technically preferable. Since the students prepared their own quenched standards, which in several cases contained a sample that was erroneously prepared, they had an opportunity to see how a microprocessor-controlled instrument responded to a poor standard. In our case, the instrument's plotting program essentially connects the points on the standard curve regardless of the smoothness of the curve. (Since the instrument
prints the plot, such errors are obvious.) This fostered some healthy skepticism of results which might be blindly generated by other instruments.
111 Krecvou. M.M.: Hutchina. J.E. C. J. Amer. Chrm. Soc.
izi co&fi;ih, R.H:,
1969.91.1329. . .
~etrohehron1970.~6.4635. (3) Ring. J. G.; Nguyen, D. c.;Everett. L. J. "Liquid Scintillation Counting from GCounts to Spectral Analysis'l Psckard lnstrvmentp Co.: Downers Grove. IL 60515. The s ~ ~ t rindex s l of the s a m ~ l eis calculated from the first moment of the linear pulse:height distribution of the sample. (4) Hendee. W. R.,"RadioactiveIsotopsinBiolagiealReaearehWiley: New York,1973. (5) Brown,H.C.;Brown. C. A. J. Amer. Chem. Soc. 1962.84,2827.See a1soFieser.L.F.; Fieser, M. '"Reagents far Organic Synthesis"; Wiley: New York,1967:Val 1.
Volume 63
Number 5
May 1986
443
