Secondary a-deuterium isotope effects in the reaction of
2-phenylethyltrimethylammonium iodide with hydroxide ion in
aqueous solution at 97.0° and with ethoxide ion in ethanol at 40.0°
have been found to be very small. The results .suggest that the
hydrogen compound reacts slightly faster than the deuterium
compound. The secondary a-deuterium isotope effect in the reaction
of 2-phenylethyl bromide with sodium ethoxide in ethanol is
appreciably larger and ammounts to 9 per cent per deuterium
atom at 59.8°. The mechanistic implications of these results are
discussed

Ašperger, S.

Klasinc, L.

Pavlović, D.

Repozitorij Farmaceutsko-biokemijskog fakulteta

Secondary a-deuterium isotope effects in the reaction of
2-phenylethyltrimethylammonium ion by hydroxide ion in
aqueous solution and by ethoxide ion in ethanol

S. Ašperger

L. Klasinc

D. Pavlović

Hrčak - Portal of scientific journals of Croatia

CR 0 AT IC A CHEMIC A ACT A 36 (1964) 159 CCA-340 541.124:546.11.02 Original, Scientific Paper Secondary a-Deuterium Isotope Effects in the Reaction of 2-Phenylethyltrimethylammonium Ion by Hydroxide Ion in Aqueous Solution and by Ethoxide Ion in Ethanol S. Asperger, L. Klasinc, and D. Pavlovic Department of Physical Chemistry, Institute »Ruder Boskovic«, Zagreb, and Faculty of Pharmacy and Biochemistry, University of Zagreb, Zagreb, Croatia, Yugoslavia Received September 8, 1964 Secondary a-deuterium isotope effects in the reaction of 2-phenylethyltrimethylammonium iodide with hydroxide ion in aqueous solution at 97.0° and with ethoxide ion in ethanol at 40.0° have been found to be very small. The results .suggest that the hydrogen compound reacts slightly faster than the deuterium compound. The secondary a-deuterium isotope effect in the reaction of 2-phenylethyl bromide with sodium ethoxide in ethanol is appreciably larger and ammounts to 9 per cent per deuterium atom at 59.8°. The mechanistic implications of these results are discussed. It was previously shown1•2 that the secondary ~-deuterium isotope effect in the SNl solvolysis (aquatiian) of 2,2-dideutero-1 ,1-di-(trideuteromethyl) pro-pyl-dimethyl-sulphonium iodide amounts to 250/o (3.30/o per deuterium atom), while the isotope effect run the solvolysiis of the corresponding chloride [2,2--diideutero-1,1-di-(tddeuteromethyl) propyl chloride] in 800/o aqueous ethanol was found by Shiner to be 135% (170/o per deuterium atom). It was also of iJnterest to determine the magnitude of the secondary a-deu-terium isotope effect in the reaction of an »onrum« 1salt. In react1ons :iJilvolving carbonium ion transition states when deuterium is 11iniked to a-carbon the retardation usually amounts to 10-20 per cent per deuterium atom1-6• The effect is explai:ned in terms of the reduction :iJil the force constant for the bending 1of the C-H bood .in the sp2-hybridized activated complex5. It has also been shown2 that in the concerted E 2 reaction of 2-phenylethyl--1,l-d2 bromide with sodium ethoxide in ethanol there is a secondary a-deu-teurium isotope effect of 9 per cent per deutePi!um atom at 59.8°. On the other hand a very small secOiildary a-deuterium isoto:pe effect was found m the reaction of 2-phenylethyldimethylsulphonium-1,l-d2 bromide w,ith sodium hydro~ide in aqueous soluti:cm (kn/kD = 1.015)2• The importance of this obser-vation is lessMed very much because of the extensive a-hydrogen exchange in the sulphonium .. sal t2• 1,s. We have now determined the secondary a-deuter.ium isotope effects for the reaction of 2-phenylethylt;rimethylammonrum-l,l-d2 •i.odide with sodium hydro-xide in aqueous solution and with sodium etho:J.Qide in ethanol <md obtained in both cases very small .isotope effects. In the case of the ammonium salt there is no observable a-hydrogen exchange8 as shown by mass spectrometric mea-160 S. ASPERGER, L. KLASINC, AND D. PAVLOVIC surements of trimethylamine collected from the reactions w.ith deuteroxide in deuterium oxide 1soluti-0n and with ethoxiide ion in ethanol-0-d. Therefore the difficulties caused by the a-hydrogen exchange 1n 1.he reaction of 2-phenylethyl-d~methylsulphonium salt a:re not present in the elimination reaction from the corresponding ammonium salt. RESULTS Secondary a-deuterium isotope effects in the reaction of 2-phenylethyl-trimethylammonium-1,1-d2 iodide with sodium hydroxide in aqueous solution. Trimethylamine formed in the reaction was removed from the aliquots taken at defonite re.action times by a stream of nitrogen and the rate wa·s fol-lowed potentiometrically by titraUon w.~th hydrochloric acid. The results are presented in Taible I. '!'ABLE I Rates of the E2 reaction of 0.1 M 2-phenylethyUrimethylammonium-1,l-d2 iodide with 0.2 M sodium hydroxide in aqueous solution at 97.0° C Compound PhCH2CH2N(CHa)aI PhCH2CD2N(CH3)al Atom D per Molecule 1.79 • Mean of : 1.40, 1.50, 1.55, 1.42, 1.47. • Mean of: 1.37, 1.40, 1.48, 1.48. 0 Standard error of the mean 104 k mole-1 sec-1 1 1.47' ± 0.07° I.43b ± o.o7• 1.023 The isotQPe effect amounted to 1.3 per cent per .deuterium atom at 97.0° C, taking into account the incomplete deuteration. Because .of the large standard error the results only suggest that the hydrogen compound reacts slightly faster than the deuterium compound. Secondary «-deuterium isotope effect in the reaction of 2-phenylethy!-trimethylammonium-1,1-d2 iodide with ethoxide ion in ethanol. The kinetics of the reaction was followed .spectrophotometrically by measurements of absorption of styrene ait 248 mµ. The :results are presented in Table II. TABLE II Rates of the E2 reaction of 0.0025 M 2-phenylethyitrimethylammonium-l,l-d2 iodide with 0.1437 M sodium ethoxide in absolute ethanol at 40.0° C Compound PhCH2CH2N(CHa)aI PhCH2CD2N(CHa)sI Atom D per Molecule 1.79 • Mean of: 52.7, 51.1, 52.5, 52.1, 51.6 • Mean of : 51.9, 49.3, 51.0, 49.2 0 Standard error of the mean 105 k mole-1 sec-1 1 52.0° ± 1.3° 50.4" ± 1.1 c 1.032 SECONDARY a - DEUTERIUM ISOTOPE EFFECTS 161 The isotope effect calculated for complete deuteration was 1.8 per cent per deuterium atom. Again, because of the la1rge standard error, the results suggest that also in ethanol the hydrogen compound .reacts slightly faster than the deuterium compound. DISCUSSION The mechanism of the elimination reaotio;n fr.om 2-phenylethyltrimethyl-ammo:nium i.on 1n alcoholic media has been studied by Ayrey et al.9 by measurements of 111itrogen-15 kinetic .isotope effect. They found a substantiial nitrogen isotope effect of 0.9 per cent .in •the reaction with ethoxide ion in ethanol at 60° (max~mum nitrogen isotope effect ait thts temperature would be about 3.2 per cent). We measured the nitrogen isotope effect ill aqeous ibase at 97° and obtained a small nitrogen isotope effect of 0.2 + 0.1 per cent (rejecting one negative value of the isotope effect)10• The difference between the nitrngen ,isotope effects found in alcoholic and aqueous solutions would be somewhat reduced, if the experiments .in alcoholic solutions were performed at 97° where ithe isotope effect would amount ,to about 0.7 per cent. However, 0.7 per cent is still a substantial .isotope effect and suggests that the .rupture of the C-N bond .in the transition state is ap-preciable. Bourns et al. 11 also measured the nitrogen ,isotope effect in aqueous media and found an isotope effect of appr.oximaitely 1/3 of the maximum value, which was in agreement with their previous measurements in alcoholic media. Our results are being reinvestigated. The appreciable nitrngen isotope effects obtained by Bourns et al. .in alooholi.c and aqueous media suggest that in both solvents the rupture of the carbon-nitrogen bond is appreciably advanced .m the transition 1staite. In this paper we have shown that there !is :practically no secondary a-deuterium i!sotope effect in the eHmination 'in aqueous and ethanolic media. One could assume that the heteroly:sis of the C-N+ ibond in the ammonium salt requires a smaller electron transfer than the heterolysis of the corresponding halide, ,since for ain equivalent .bond extension the incipient carbonium ion of the ammonium salt must car.ry a larger net positive charge. In such a case when the hybridization changes from sp3 to sp2 w~thout the development of positive charge ·on carbon, it might be exipected that the out of plane bending frequency would go down which could lead to an 1sotoipe effect near zero. The salvation difference needed to heterolyze ithe initially non-ionic C--halide bond and the C-N+ must 1be appreciable. The salvation change is much smaller ~n the heterolysis ·of C-N, where the total ionic change remains the same. EXPERIMENTAL 2-Phenylethyltrimethylammonium Iodide Methyl iodide was slowly added (in 100 per cent excess) to 2-phenylethylamine in absolute ethanol at its boiling point and then the solution refluxed for two hours in the presence of sodium carbonate to assure neutral reaction. Excess of methyl iodide and alcohol were distilled off and the ammonium salt recrystallized from 162 S. ASPERGER, L. KLASINC, AND D. PAVLOVIC water. It decomposed at about 226°. The decomposition point is unreliable as a criterion of purity. Anal. C11H1sIN (291.16) calc'd: C 45.4; H 6.2; I 43.6.; N 4.80/o found: C 45.2; H 6.3 ; I 43.0; N 4.5% 2-Phenylethyltrimethylammonium-1,1-d2 Iodide 2-Phenylethylamine-1,1-d2 was prepared by reduction of benzyl cyanide with lithium aluminium deuteride (Metal Hydrides, Inc., 95.50/o deuterium) as described by Nystrom12 • The undeuterated amine was prepared in an analogous way .. 2-Phenyl-ethyltrimethylammonium-1,1-d2 iodide and its undeuternted analogue were then prepared from the amines as already mentioned. 2-Phenylethyltrimethylammoniu."Tl iodide prepared in this way decomposed at about 226°. Anal. C11H10D2IN (293.17) found: C 45.4; I 43.6 ; N 4.80/o The deuterated salt C11H 16D2IN was found to contain 1.79 atoms D per molecule. Deuterium analyses - wen~ performed by the »falling drop« method13. Duplicate analyses agreed to within 1.50/o. ! Kinetics Measurements in Aqueous Solution The reaction vessel made of alkali resistant boron free glass (Corning Glass Woriks, Corning, N. Y., U. S. A.) was fitted with a condenser and thermostated at 97 ± 0.010. Blanks showed that alkali did not react with the glass of the vessel during five reaction times. 2-Phenylethyltrimethylammonium iodide was 0.1 M and sodium hydroxide 0.2 M. Aliquots of 2 ml were withdrawn at definite times, diluted with water to 50 ml and a stream of nitrogen led through the solution during one hour to expel trimethylamine. The decrease of sodium hydroxide concentration was then determined potentiometrically. Kinetic Measurements in Alcoholic · Solution The reactions of 0.0025 M 2-phenylethyltrimethylammonium-1,1-d2 iodide and of the corresponding undeuterated salt, were carried out with 0.1437 M ethoxide ion in ethanol at 40 ± 0.01° C ,in the reaction vessel described above. The volume of the reaction mixture was 25 ml. At definite reaction times aliquots of 2 ml were with-drawn, made up to 50 ml with 96 percent ethanol and the light absorption was measured at 248 m~t using a Hilger H 700 spectrophotometer and quartz cells. The concentration of styrene was then determined using the molar extinction coefficient of freshly distilled styrene in ethanol at 248 mp. which amounts to 1.35 X 104 mole-1 cm-1 114• REFERENCES 1. S. Asper g er and N. I 1 a k ova c, Chi::m. and Ind. 1960, 1191. 2. S. Asp erg er, N. I 1 a k o v a c, and D. Pavlovic, Croat. Chem. Acta 3'1 (1962) 7,. 3. V. J . Shiner, Jr., J . Am. Chem. Soc. 75 (1953) 2925. 4. W. H. Saunders, Jr., S. Asper g er, and D. H. Edison, J. Am. Chem.. Soc. 80 (1958) 2421. K. Mis 1 ow, S. Bo r c i c, and V. Pre 1 o g, Helv. Chim. Acta 40 (1957) 2477. 5. A. Streitwie ,ser, Jr., R.H. Jagow, R. C. F a hey, and S. Suzuki, J. Am. Chem. Soc. 80 (1958) 2326. A. Streit w i es er, Jr., Annals New York Accid. Sci. 84 (1960) 576. 6. E. A. Ha 1 e vi, in Progress in Physical Organic Chemistry, Edited by Cohen, Streitwieser, Jr., and Taft, Interscience Publishers, New York 1963. 7. S. Asper g er, N. I 1 a k ova c, and D. Pavlovic, J. Am. Chem. Soc. 83 (1961) 5032. 8. S. A s·p erg er, D. Pavlovic, L. K 1 as inc, and I. Murat i, International Symposium on Orgcini c Reaction M echanism , Cork, Ireland, July 20-25, 1964, P31 ; a supplemented abstract in print by the Chemical Society, London. SECONDARY a - DEUTERIUM ISOTOPE EFFECTS 163 9. G. Ayre y, A. N. B our n s, and V. A. Vy a s, Canad. J . Chem. 41 (1963) 1759. Results earlier quoted by E. B u n c e 1 and A. N. B our n s, ibid. 38 (1960) 2457 .. 10. S. Asper g er, L . K 1 as inc, and M. P api c, XIX International Congress of Pure and Applied Chemistry, London, 10-17 July 1963, Al-17. 11. Private communication kindly communicated by Professor A. N. Bourn s. 12. R. F . Ny st r o m, J. Am. Chem. Soc. 77 (1955) 2544. 13. W. v on D o er in g and A. K. Hof f ma n n, J . Am. Chem. Soc. 77 (1955) 521. 14. W. H. Saunders, Jr. and S. 'Asper g er, J . Am. Chem. Soc. 79 (1957) 1612. IZVOD Sekundarni a-deuterijski izotopski efekti u reakciji 2-feniletiltrimetilamonijeva iona sa hidroksilnim ionom u vodenoj otopini i sa etoksi ionom u etanolu S. Asperger, L. Klasinc i D. Pavlovic Odredeni su sekundarni a-deuterijski izotopni efekti u reakciji 2- feniletiltri-metilamonijeva jodida sa hidroksilnim ionom u vodenoj otopini kod 97.0° i sa etoksi ionom u apsolutnom etanolu kod 40.0° !i nadeno je da su ti efekti vrlo mali. Dobiveni rezultati govore za to da vodikov spoj reagira nesto brZ:e od a-deuteriranog spoja. Sekundarni a-deuterijski izotopski efekti u reakciji 2-feniletil bromida sa natrije-vim etilatom u apsolutnom etanolu znatno je veci i iznosi oko 90/o po a tomu deuterija kod 59,.8°. Diskutirani su uzroci za mali izotopski efekt i znacenje tog efekta ' za reakcijski mehanizam. INSTITUT »RUDER BOSKOVIC« I ZA VOD ZA ANORGANSKU I FIZIKALNU KEMIJU FARMACEUTSKO-BIOKEMIJSKI FAKULTE'I ZAGREB Primljeno 8. rujna 1964. 

Secondary a-Deuterium Isotope Effects in the Reaction of
2-Phenylethyltrimethylammonium Ion by Hydroxide Ion in
Aqueous Solution and by Ethoxide Ion in Ethanol

Repository of Faculty of Pharmacy and Biochemistry University of Zgreb

University of Zagreb Repository

Secondary a-deuterium isotope effects in the reaction of 2-phenylethyltrimethylammonium ion by hydroxide ion in aqueous solution and by ethoxide ion in ethanol

Abstract

Similar works

Full text

Available Versions

Repozitorij Farmaceutsko-biokemijskog fakulteta

Hrčak - Portal of scientific journals of Croatia

Repository of Faculty of Pharmacy and Biochemistry University of Zgreb

University of Zagreb Repository