Sept. 5, 1952
S O L V O L Y S I S OF 2 - B U T Y L - 1 - C l 4
[CONTRIBUTION FROM
THE
p-TOLUENESULFONATE
42S3
DEPARTMENT OF CHEMISTRY AND LABORATORY FOR NUCLEARSCIENCE AND ENGINEERING, MASSACHUSETTS INSTITUTE OF TECHNOLOGY]
Rearrangements in the Solvolysis of 2-Butyl-1-C14 p - T o l u e n e s u l f o n a t e 1 , 2 BY JOHN D. ROBERTS, WINIFREDBENNETT,ROBERT E. MCMAHON AND EDMOND W. HOLROYD, JR. RECEIVED FEBRUARY 8, 1952 Solvolysis of 2 - b ~ t y 1 - l - Cp-toluenesulfonate ~~ in acetic acid in the presence of acetate ion under conditions where the reaction rate is essentially independent of the acetate ion concentration leads to a mixture of 91 f 1% of 2-butyl-l-CI4 acetate and 9 =k 1% of the rearranged product, 2-butyl-4-C14 acetate. It has been shown that the rearrangement is not due to rearrangement of the starting material or to elimination t o yield butenes followed by addition of acetic acid. Much less rearrangement occurs in the hydrolysis of 2 - b ~ t y l - I - Cp-toluenesulfonate ~~ in 75% acetone-25% water (by volume). It is concluded that a non-classical cation of the type CH3qH-=CHCHa is not an important intermediate in the unimolecular @ hydrolysis or acetolysis of 2-butyl p-toluenesulfonate. 8
“’
In an earlier investigation,3 it was shown that isotope position rearrangements do not occur in the usual essentially irreversible, carbonium ion-type metathetical or elimination reactions of t-butyl and t-amyl derivatives. Such behavior was expected on the basis of the carbonium ion theory4 of molecular rearrangements since any isotope position rearrangements with t-butyl or t-amyl derivatives would involve interconversion of tertiary to less stable secondary (or primary) cations under unfavorable conditions. The situation with 2butyl derivatives is quite different in that the interconversion equilibrium of, for example, 2-butyll-CI4 (I) and 2-b~tyl-4-C’~ (11) cations could be established by simple 1,2-hydride shifts. The role of a more or less stable “ethyleneprotonium” ion5intermediate like TIP in such processes has not yet been established. It is possible that I11 is no more than
Information as to the best approximation to the actual state of affairs was sought in the present research by measurement of the extent of rearrangement in solvolyses of 2-butyl- l-C14p-toluenesulfonate (IV) under conditions where these processes are irreversible. It was expected that extensive rearrangement to yield 2 - b ~ t y l - 4 - Cderivatives ’~ would be observed if reasonably free cationic intermediates were involved and if 111 possesses comparable or greater stability than I or 11.
C H ~ C H Z C H C ’ ~ H ~ CH3CHCH3Cl4H3 @
e3
I
I1 H
,’e‘\ CH3CH---’CHC14H3 I11
a transition state through which rearrangement could occur more or less readily as shown schematically in the top curve of the energy diagram (Fig. I ) . Alternatively, 111and the isomeric cations I and I1 could have similar energies and be in more or less a dynamic equilibrium7 as indicated by the middle curve of Fig. 1. As a final clear-cut possibility, 111 could be substantially more stable than either of the cations I and I1 and could be formed directly and react without their intervention as indicated by the lowest curve of Fig. 1. (1) Supported in part by the joint program ot the Office of Naval Research and the United States Atomic Energy Commission. (2) Presented in part a t the Symposium on Reaction Mechanisms a t the 75th Anniversary Meeting of the American Chemical Society, September 7 , 1951. (3) J. D. Roberts, R. E. McMahon and J , S. Hine, THISJOURNAL, 71, 1896 (1949); 72, 4237 (1950). (4) Cf., L. P. Hammett, “Physical Organic Chemistry,” McGrawHill Book Co., Inc., New York, N . Y., 1940, pp. 317-325; G. W. Wheland, “Advanced Organic Chemistry,” John Wiley and Sons, Inc., 1949, pp. 451-534. New York, N . Y., ( 5 ) Cf.,D. J. Cram, THIYJOURNAL, 74, 2137 (1952). (6) For refs. to some other situations where intermediates similar to I11 have been proposed see S. Winstein and D. S.Trifan. ibid., 71, 2953 (1949). (7) M. J. S.Dewar, “The Electronic Theory of Organic Chemistry,” Oxford University Press, London, 1949, pp. 211-213.
R
3 ,.----., /----..,
c , RX I I11 I1 R’X Fig. 1.-Schematic energy diagram for extreme and intermediate formulations of reactions involving possible cations from 2-butyl derivatives labeled with CI4. RX and R‘X represent 2-butyl-l-C1‘ and 2 - b ~ t y l - 4 - Cderivatives, ’~ respectively. Curve A depicts a situation where cation I11 is less stable than I or 11. Curve B has I11 of comparable stability and readily interconvertible with I and 11. Curve C has 111 much more stable than I or I1 with solid lines representing the usual course of reaction, L e . , 111 being formed directly from R S or R’X without intervention of I or I1 being required
Synthetic and Degradative Methods The 2--butyl-l-CI4p-toluenesulfonate was prc-
1 TABLE I I < A ' i I I COS5l'AKTS FOR SOLVOLYSIS Olr 2-BUTYI. P-TOLUENESLTLFONATE :>olvcllt
'l'rinp.. 'C.
1niti:il [ R O T s ] , .\I
Iiiilial
[ K O A c ] . .If
Initial [KOT,].'L If
k . , hr.-1
(ROTS)
b: , OleAn,h (m*Jlr'l.J-~lir,-~ ?&
c
1 IOAcd (io, 0 0 , (i3 0 . (i I . . (1, OG 0.1i ... 0 ,W H O A ~ (5.0 .ti; 41; .11 ,32 .;t HOAC" (3. 0 5I . ,I6 0 . 4.j I6 .19 38 ,231 HOAcd ci5,(1 1:19:t . 100 .89 22 ( . 1ny 33 . 95 H,O-acetone' 65.0 . 1'18 ... .. .32 ... (23)Q 1.00 0 Potassium p-toluetiesulforiatc. Percentage of theoretical amount of olefin formed at practical completion of solvolysis; JOURNAL, 71, 1980 (1949). Fraction of reaction procletermincd by the hydrogenation procedure of J. D. Roberts, THIS ceeding by first-order processes at practical completion; calculated from kl and kp by the equation given in ref. 10. d Acetic acid containing lybacctic anhydridc. e Assumed to he the same as in the preceding run. The kinetics in this run were quite 25% water-75% acetone accurately first-order to ovcr 90% reaction after a small correction for the infinity titer was made. ( b y volurnc). 0 Calculated from the C'"-activity of thc carrier-diluted product from thc hydrolysis of IV. .
I
.
*
'
pared starting with methyl-Ci iodide* by the following scrics of reactions. The solvolysis reactions werc carried out in acetic acid and aqueous acetone C1.H I
hlg
--+
C"HlhIgI
1. CHjCH-CHO 2. H?O, NI-IIC~ --______)
CH~CII'CIIOIIC"I-IJ
1 . sdI-r 2 . CHJC6H4SO~C1 f
CI1,CH~CHC"~I~
OSO2CF,Nle€I3 IV
and yielded ?-butyl acetate or 2-butanol. In the fornIcr event, the ester was converted to %butanol by alkaline hydrolysis for degradation. The most satisfactory degradation procedure involved oxidation of labeled %butanol with excess sodium hypobromite to yield carbon tetrabromide, the C14activity of which gives the activity at C-1 of the 2-butanol. Purification of carbon tctrabromide W:LS found to bc sonic.vvhat siniplcr th:ui that of the iotloloriii obtaiiicd iii thc analogous osidatioii with sodiuni hy1)oiodit.c.solutioii. Experimental Results and Discussion I t was considered particularly desirable to study the uniniolecular solvolysis products of IV in acctic acid since much work has been done on reaction rates and rearrangements of benzenesulfonate esters in this s o l ~ e n t . Some ~ ~ ~ difficulty was encountered in selccting appropriate reaction conditions since it was considered imperative to use at least one eiliiivnlent of acetatc ion to each equivalent of IV to avoid foriiiatioii of t h t frec sulfonic acid. Howt ' cr, ~ coniplicatioiis w r e tlieii introduced by bit!iolecular sulxtitution (Sxyi)] and climination (E2) rvactions lietween acctutc ion arid IV. Presuniably, hiriiolecular substitution would proceed witho u t rcarrangeinent and suitable corrections could he inade if the appropriate rate constants were h o w r i . However, since both the unimolecular and bimolecular reactions may lead to substitution and elimination, complete analysis of the kinetics would bc difficult and i t was deenied preferable to find conditions where the biniolecular reaction is essentially negligible. The kinetics of the reaction of Si ' l h ? nielhyl-C1+ iedidP \ \ a s olrtaineil frciiri Tracerlab, Inc , on I Scatc. htt)niic Iiiier.#y Conimission Grunwald a n d 1%. lV. Jones, THISJ O U R S A L .
unlabeled I V with potassium acetate werc studiedLo under several different conditions (see Table I) and, as would be expected, the most satisfactory circumstances for practical climination of the bimolecular reaction involved low concentrations of IV and acetate ion with considerable neutral salt present. The extent of rearrangement in the acetolysis of IV was measured a t ionic strengths of 0.9 and 1.0 rll using acetate ion concentrations of 0.46 and 0.10 X . The experimental results along with those obtained in the hydrolysis of IV in aqueous acetone and some controls are given in Table 11. At 0.10 d l acetate, TABLE I1 REARRAKGEMEST 1'4 THE B LTYL- 1-Cl4 ~ - T ~ L u E S E S ~ L
1':XTENJ'
Solvent
OF
itial [ROT.;], .if
itial [KOXci, .If
itial [KO'ls],
0.46
0 . 13
IIOAc'
0.31
IIOAc' 1rr)Ac'
,111
.IO
18
.IO
1I(lLW
,,.
-''I
.If
2-
SOLVOLYSIb Olr I ~A ~r .65' ~ A T ~
--Radioactivitv"--2-butanol CHrd
. iil
,:IO05 zt l Z d !I13 3 1, !)SI1 j, 200 .S'I Iiili i (if1.j k I ,811 I ~ v ~ ? + Iu, j i~i ) d : t j , S!i - 1 .z 1'' , , , , , ,
..
..
*
Rearranpe.,h
"D
7
!I/
8 X I i' (.
