HYDROLYSIS AND NMRSPECTRA OF SOMETHIOL ESTERS
J . OTq. Chem., VOZ. 58, NO.25, 1973 4239
Study of the Alkaline Hydrolysis and Nuclear Magnetic Resonance Spectra of Some Thiol Esters’
JOHN P.IDOIJX* Department of Chemistry, Florida Technological University, Orlando, Florida
368816
PHILIP T . It. H W A N AXD G ~ C. TIINKEY HANCOCK Departmenl of Chemistry, Texas A & M University, College Stalwn, Texas 77843 Received June 69, 1973 The alkaline hydrolysis rate constants at, 3s” in 40% aqueous p-dioxane and the nmr chemical shifts have been measured for nine thiolacetates, CH&OSR’, nine methyl thiol esters, RCOSCH3, and seven disubstituted thiol esters, RCOSIt’. Alkaline hydrolysis rate constants are controlled almost exclusively by steric effects of the R and R’ groups. The nmr substituent chemical shifts (SCS) of thiol esters are Controlled largely by steric and sixnumber effects of the 11 and It’ groups. All of these correlations are discussed and compared with correlations previously reported for similar oxygen esters, RCOOR‘, on the basis of differences in various structural features of the two series. A contributing structure involving negative charge development a t the sulfur atom is apparently not of importance for simple thiol esters.
I n the alkaline hydrolysis of esters, RCOOR’, the reaction site in the rractant state is trigonal and unsaturated while thc reaction site in the intermediate is tetrahedral and saturatcd. Jlorrover, thc rate-determining step in the mechanism3 for the rcaction is the coordination of the hydroxyl ion with the carbonyl carbon. Consc.qucntly, in the absonce of a,p unsaturation in the I2 group, the alkaline hydrolysis rate constant for RCOOIt’ is affccted by polar and steric dfects of the lt’group. For example, (’(1 1 has been found4 for thc alkaline hydrolysis rat(. constants of nine mcthyl esters, RCOlog k = 1.25
+ 1.750* + 0.848 ESc- 0.383 (n - 3), (100.0)
(100.0)
R
=
(100.0) 0.998, s = 0.043 (1)
OCHB, in 40% aqueous p-dioxane at 35”. In eq 1, k is thr second-ordcr ratc constant, u* is Taft’s polar substituent c0nstant,5~h’,~ is Taft’s steric substituent constant5b corrected4 for hyperconjugative effvcts, n is the number of a hydrogens in thc R group, R is the multiple corrdation coeficimt,Gaand s is thc standard deviation from regression.‘ja The numbtm in parcntheses below the three coc4ficimts of cq 1 arc’ the percentage confidencc lcvels as dctcrmincd by “Student’s’’ t tc.sts.6b It is apparent from cq 1 that log k is an almost exact function of u*, Est, and (n - 3), with each of these thrw independent variables being highly significant. Further, Newman7 has shown that, for the csterification of carboxylic acids and th(1 hydrolysis of estws, the six numbcr of a substituent (ie., the number of atoms in thc six position from thti carbonyl oxygen atom as atom one) makes a largc contribution to the (1) Presented in part at the Florida Section .Imerican Chemical Society Meeting-in-Miniature, Key 13iscayne, Fla.. M a y 18-20, 1972. (2) Abstracted in part from t h e Ph.L). Dissertation of P. T . R . 11.. Texas A & hl University, AUK1965. 73, 1626 (1951). (3) M . L. Bender, J . A m e r . C h e n . SOC., (4) C. K . Hancock, E. A. Meyers, and B. J. Yager, J . A n e r . Chem. Soc.. 89, 4211 (1961). ( 5 ) R . 1%‘. T s f t , Jr., “Steric Effects in Organic Chemistry,” M. S. S e w m a n , E d . , Wiley, New York, K.Y., 1856: (a) p 619; (b) p 598. (6) G . W. Sneilecor, “Statistical Methods,” 6th e d , The Iowa State College Press, Ames, Iow., 1966: (a) Chapter 14; (b) ~ ~ 4 6 , 4 1and 8 , 441: ( 0 ) Chapter 6. ( 7 ) M. S.Newman, J . A n e r . Chem. SOC.,71, 4783 (1950).
total steric effect of that substituent. For example, eq 2 has been founds for the alkaline hydrolysis rate log k = 1.35
+ 0.688u* + 0 644 ESc+ 0.0477 ( A s ) , (95 3 )
(100 0 )
(99.4)
R = 0.997, s
=
0.070
(2)
constants of nine acc.tatc esters, CH?COOR’, in 40y0 aqueous p-dioxane a t 35”. I n c’q 2, A6 is the change in the six number, i.e., the differmce of the six number of a substitucnt in the It part of the. ester minus the six number of the same substituent in the R’ part of the ester. It is evident that eq 2 provides a quantitative relationship that is almost as exact as that of eq 1. For thc nine acetatr csters mentioned previously,* in addition to nino others, Iiosado and his coworkersg have found cq 3 to apply to the measured substituent chemical shifts (SCS) of these acetatcs relative to methyl acetato. In eq 3, u* has the same meaning as SCS (Hz)
=
0.953
+ 20 4u* + 1.11 ((2-6 no.), (100.0)
IZ
(100.0) = 0.943, s = 0.659
(3)
before and C-6 no. is the number of carbon atoms in the six p ~ s i t i o n . ~Equation 3 indicates, as concluded by ICan,l0 that thr SCS values of acetate esters are governed largely by polar arid six number effects of the R’ group. Kosado and his coworkers9 have also measured the SCS of a series of methyl esters rrlative to methyl acetate and have found that eq 4 applies for this series. SCS (Hx)= 0.446
+ 1 8 . 4 ~ *- 2.06 (n - 3), (100.0)
(100.0) R = 0.863, s
=
0.415 (4)
I n view of the diffcrences and of the multitude of substituent effccts c.xhibited by oxygen wters in alkaline hydrolysis and in substiturnt chemical shift correlations, JW h a w madc similar studies on the corresponding sulfur esters. The present papw is concerned with thc results of thew studim and with a (8) C. K. Hancock, 13. J. Yager, C. P. Falls, and J. 0. Schreck, J . Amer. C h e n . Soc., 8 5 , 1297 (1963). (9) 0 . Rosado-Lojo. C . Isters and oxygen e s t m which confirm this conclusion and morc! rcwntly CollingsZ1and ,his co\rorkcrs have questioned the importance of structure IV bawd on irifrarcd studies on thiol mtcrs and other acyl compounds. A. Thiolacetates, CHsCOSR'.-In view of the fairly good correlation provided by eq 3 for the acetates, the correlation of the SCS of the acyl mcthyl protons of eight of thc thiolacctates (benzyl thiolacetatc omitted) was also attcmptcd. u* and H-G no., whcrr H-6 no. (see Table IV) is the number of hydrogen atoms in thc
IDOUX, HWANC, A X D HANCOCK The relationship ulhich best fits the data is shown in eq 12. This equation accounts for 79% of the varianco SCS (Hz)= 1.30
(99.0)
Substituent
.4s R ' of CHsCOSR' C-6 no.u H-G no.b
A J R of RCOSCHa n0.O 1%-6no.*
C-6
CIIB 0 0 0 0 0 3 0 0 CzHs n-C3H7 1 2 0 3 0 6 0 0 i-C3H, 1 2 1 2 n-C4119 i-C4H9 2 1 0 6 1 5 0 3 s-C~H~ l-C,Hg 0 9 0 0 a Carbon six number, i.e., the number of carbon atoms in the six position from the carbonyl oxygen atom a2 atom number one. b Hydrogen six number, i.e., the number of hydrogen atoms in the six position from the carbonyl oxygen atom as atom number one.
six position from the carbonyl oxygen atom as atom number one, were found to bo insignificant variables. ( 1 7 ) A . h'. Raker and C;. H . Harris. J. Amer. Chem. S o c . . 81, 1923 (1960). (18) I . JVnllmark. .\I. 11. Krackov, S. C h u . and H . G . hlautner, J. Amcr. Chrm. Soc.. 92, 44.17 (1970). (19) U' 1'. Jrncks, L3. Scltaffharisen. K . Torniicim. a n d 11. White. J . Amcr. Chrm. S o r . . 9 3 , 3417 (1971). (20) I . Ivadso. Acta Chem. Scortd.. 16, 487 (1962). (21) A . J . Collinus, P. F . Jackson. and I
