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J. CHEM. SOC. PERKIN TRANS. II 1989 Trends in Selectivity. Evidence from Rates and Products for Simultaneous Reaction Channels in Solvolyses of Benzoyl Chloride and Substituted Derivatives T. William Bentley" and In Sun Koot Department of Chemistry University College of Swansea Singleton Park Swansea SA2 8PP Rates of solvolyses of para-Z-substituted benzoyl chlorides (Z = OMe Me or CI) are reported for highly aqueous binary mixtures (with acetone ethanol and methanol cosolvents). Product data (ester and acid) are reported for a wide range of compositions of the aqueous binary alcohol mixtures (for Z = Me H or Cl). As water is added to alcohol the selectivity S ([ester] [water]/[acid] [alcohol] initially increases but then reaches a plateau and/or decreases. This reversal of the trend in S occurs at different solvent compositions for each substrate (Z = Me H or Cl) and is very close to the region of mechanistic change predicted previously from rate-rate profiles; these predictions have now been refined by replacing solvolyses of 1-adamantyl chloride by p-methoxybenzoyl chloride as the model S,1 process.The results provide new and sharply defined evidence for mechanistic change in nucleophilic substitutions at acyl carbon supporting the co-existence of two different mechanisms or reaction channels each of which could show variable structures of transition states. A quantitative dissection of observed rates into contributions from two reaction channels is described and an attempt is made-with partial success-to calculate variations in S values occurring as the mechanism changes. Trends in S values are calculated successfully.Nucleophilic substitutions at acyl carbon may proceed either by addition-elimination by sN1,or by direct sN2 displacement. *' From analyses of rate-rate profiles it was proposed that sol-volyses of benzoyl chloride in aqueous alcohol (ethanol or methanol) occurred by two mechanisms-a carbonyl addition- elimination mechanism in less aqueous alcohol and by a more polar mechanism having weak sN2 character in more aqueous media.3 The 'weak sN2' mechanism differs from the carbonyl addition mechanism in that it involves the development of positive charge on the carbonyl and hence is favoured by electron-donating substituents in the benzene Each reaction channel) which should fit one linear free energy of these two mechanisms could show variable transition state relationship (LFER).4.5 Deviations from an expected LFER for e.g.sN2tends towards sN1 as nucleophilic assistance solvolytic rate data for benzoyl chloride versus Y values led to decrease^.^ It may be considered that variations in transition proposals 2a,3a,7 that there were two competing mechanisms. If states within only one mechanism may be sufficient to account it is accepted that kinetic data for the sN2-sN1 spectrum of for the results. We now report further evidence from rate-rate mechanisms can be correlated by one appropriate LFER,' then profiles and independent evidence from product selectivities the competing mechanism must be different-presumably a Thesupporting the simultaneous operation of two distinct mechan- carbonyl addition proces~.~",~",~point of mechanistic isms or reaction channels.A straightforward mechanistic pat- change from predominantly carbonyl addition to predominantly occurred at tern will be revealed. These results are also of wider significance sN2-sN1 as predicted from rate-rate because in current discussions of reaction mechanisms it is increasingly aqueous solvent compositions for more electron- generally assumed that there is only one reaction channel. donating substituents in the benzene ring;3b a more precise Early proposals for mechanistic changes from sN2 to sN1 in LFER was recently reported for solvolyses of benzoyl chlor- solvolyses of acid chlorides require modifications.Previous ide,3' and details of the improved method of analysis of these arguments for an sN1 mechanism for solvolysis of benzoyl rate-rate profiles are reported here. chloride in 50% w/w aqueous acetone,6 have not been sup- A point of mechanistic change was not observed for sol-= NO2) in aqueous ported by recent attempts to trap a cationic i~~termediate.~" volyses of p-nitrobenzoyl chloride (1; Z Interpretation of the earlier trapping experiments is compli- ethanol or rnethan~l,~' and the rates and products can be and by correlated assuming third-order kinetics.'" There appear to be cated by the medium effect of the added n~cleophile,~" the possibility that an added nucleophile could induce a new at least four similar pathways differing only in the way two substitution pathway.alcohol or water molecules participate in the reaction (see later We now consider that the mechanistic change from sN2to SN1 discussion). All of these reactions are favoured by electron reflects variations within a single mechanistic spectrum (or withdrawal are probably general base catalysed,'" and may be broadly described as carbonyl additions. As changes in contri- butions from the four pathways are mechanistic changes we will On study leave from Department ofchemical Education Gyeongsang use terms like 'change in reaction channel' when more extensive National University Chinju Gyeongnam 620 Korea. changes in mechanisms are discussed. Solvolyses of (1; Z = Perkin Trans.2 1986 619 Table 2 [ref. NOz) model the carbonyl addition reaction channel and pro- J Corrigendum.J. Chem. SOC. 3(b)].Entries on the first four and final five rows are correct; the other ten vide useful data with which the other solvolyses will be entries are printed one row too low. The editor apologises for this error. compared. 1386 Table 1. Rate constants (k)for solvolyses ofp-methoxybenzoyl chloride (1; Z = OMe) in aqueous alcohols and aqueous acetone." AM kcal ASf/cal Solventb T/T kjs-' mol-' mol-' K-' 50E 25.0d 5.4 x lo-' 17.7 -0.3 40E -10.0 (3.50 f0.22) x 0.1 (1.41 f0.01) x lo-' 18.5 5.4 10.0 (4.57 f0.14) x lo-' 25.0' 2.6 30E -10.0 (2.06 & 0.09) x lo-' 0.0 (6.73 _+ 0.10) x lo-' 18.2 7.8 5.0 1.48 f0.01 25.0' 14 20E -9.9 (5.71 f0.15) x lo-' -5.0 1.02 & 0.04 17.7 7.8 0.0 2.02 & 0.02 25.0' 34 60M 25.0d 8.4 x lo-' 16.2 -4.5 50M 25.0'3' 2.4 16.1 -2.8 40M 25.0'3' 5.9 16.5 0.2 30M -20.0 f (6.53 _+ 0.30) x -10.0 (2.69 f0.09) x lo-' 18.2 8.4 25.0' 18 20M -9.9 (5.71 f0.27) x lo-' -4.9 1.13 & 0.02 18.7 11.5 0.0 2.16 & 0.07 25.0' 42 40A 25.0' 9.8 x lo-' 16.0 -4.9 30A -10.0 (8.25 f0.04) x 0.0 (2.8 0.02) x lo-' 17.2 2.0 25.0' 4.4 20A -10.0 (2.49 f0.01) x lo-' 0.0 (8.78 & 0.6) x lo-' 17.5 5.3 25.0' 14 1 cal = 4.184 J." Determined conductimetrically in duplicate except where noted otherwise; errors shown are average deviations. Solvent codes E = % vjv ethanolLwater; M = % vjv methanol-water; A = % vjv acetone-water.'Calculated from data at other temperatures. Reference 3(a). Reference 3(h). Triplicate measurement of rate constant. Table 2. Additional rate constants (k/s-') for solvolyses of p-methoxybenzoyl chloride (1; Z = OMe) in aqueous alcohols at 25 "C." vjv Alcohol Ethanol Methanol 98 (1.55 f0.02) x (1.42 f0.01) x 95 (2.66 0.05) x (2.09 k 0.01) x a Determined conductimetrically in duplicate; errors shown are average deviations. Electron-donating substituents and increases in solvent ion- izing power favour the S,1 mechanism e.g. hydrolysis of p-dimethylaminobenzoyl fluoride shows common-ion rate de- pression.' Solvolyses of p-methoxybenzoyl chloride (1; Z = OMe) in aqueous ethanol or methanol showed a slightly non- linear rate-rate profile us.Yc possibly indicating a point of mechanistic change.3b However more detailed studies and comparisons with solvolyses of 2,6-dimethylbenzoyl chloride showed that the slight non-linearity was due to a solvation effect." From substituent and solvent effects it appears that these solvolyses are close to sN1; 3b product formation may occur via solvent-separated ion pairs because selectivity for ethanol is less than that for water." Hence solvolyses of (1; Z = OMe) model the sN2-sN1 reaction channel and provide data for a second model compound for comparison with the other solvol yses. Because of possible complications in the interpretation of J. CHEM. SOC. PERKIN TRANS. II 1989 Table 3.Rate constants (kjs-') for solvolyses of para-Z-substituted benzoyl chlorides (1) in highly aqueous media" at 25 "C. Solvent z= c1 Z = H' Z = Me Z = OMed 60E 0.0078 ' 0.19/ 50E 0.0097 0.0 17 0.05 84 0.54 40E 0.0155 0.048 0.199 2.6 30E 0.0384 0.18 0.83 14 20E 0.0890 0.37 2.5 34 70M 0.0220 0.0197 0.04 15 0.32 60M 0.0254g 0.035 0.0844 0.84 50M 0.0295 0.063 0.242 2.4 40M 0.0428 0.134 0.554' 5.9 30M 0.0635 0.29 1.2' 18 20M 0.0994 0.54 3.1 ' 42 40A 0.0091 0.025 0.103 0.98 30A 0.0201 9 0.083 4.4 20A 0.0479 0.25 I .42 14 1OA 0.1 109 0.55 a New results determined conductimetrically in duplicate; errors for this new work are not shown but are typically < f2%. As for Table 1. Ref. 3(a). Data from Table 1. Ref. 4(b). Interpolated from a slightly curved plot of log(k/k,) for (1; Z = OMe) uerws Grunwald-Winstein Y values from ref.13. Ref. 3(b). Direct measurement at 25 "C supported by the following additional kinetic data k/s-' ("C):(1.63 & 0.02) x lo-' (0.0"C); (5.32 f0.07) x lo-' (1O.O"C); AH* = 17.6 kcal mol-' ASf = 2.6 cal K-' mol-'; extrapolation of these two results to 25.0 "C gives k = 2.7 s'. rate-rate profiles independent evidence for competing mechan- isms was obtained from product selectivities. Products of solvolyses of acid chlorides in aqueous alcohols are the parent acid and an ester. Selectivities (S)are defined by equation (1)" Results To complete the set of kinetic data required for this work additional results were obtained for solvolyses of (1; Z = OMe) (Tables 1 and 2) and (1; Z = C1 Me) (Table 3).Solvent codes are shown in Table 1 footnote b. Additional rate constants in s-' (not tabulated) are (1; Z = Me); 90A k = 1.02 x (1; Z = C1); 90A k = 4.9 x lC4; 90M k = 1.16 x 1C2. Additional literature data have been added to Table 3 to show the substituent effects on reactivity as well as the consistency of the experimental data. The activation parameters further support the reliability of the difficult measurements on solvolyses of (1; Z = OMe) (Table 1); the increase in AHz and ASs in highly aqueous alcohols has been observed for solvolyses of t-butyl chloride.' 1,1 More detailed evidence supporting the reliability of the relatively large rate constants (k > 2 s-') obtained by extrapolation of data at lower temperatures has been reported elsewhere." The highest reliable value of k that we have obtained by direct measurements is 2.5 ssl for (1; Z =Me) in 20E at 25 "C (Table 3 footnote h).Selectivities (S) are reported in Table 4. Trends for each substrate are more reliable than absolute values because of small (up to 5%) uncertainties in response calibration which at least partly accounts for the ca. 10% higher values reported previously for benzoyl ~hloride.~" Also rapid mixing is now known to be important for these product studies; less rapidly mixed solutions tend to give higher S values." Discussion Selectiuities.-Mechanistic changes for solvolyses of the three substrates (1; Z = C1 H Me) can be seen directly from the J.CHEM. SOC. PERKIN TRANS. 11 1989 Table 4. Pattern of selectivity (S)for product formation in solvolyses of para-Z-substituted benzoyl chlorides (1) in alcohol-water at 25.0 OC." Substituent Z r A Alcohol ("/,v/v) NO C1 H Me OMe Ethanol 98 0.7 0.9 1.3 1.6 0.6 95 0.9 1.3 1.7 1.7 0.7 90 1.3 1.8 1.9 1.7 0.7 80 2.0 2.6 2.3 1.3 0.6 70 2.5 3.4 2.1 1.1 0.7 60 3.1 3.7 1.6 1.o 0.7 50 3.6 3.8 1.3 1.o 0.7 40 4.2 3.0 1.1 1.o 0.7 20 5.2 1.5 0.7 0.8 0.8 Methanol 98 1.9 2.0 4.0 2.7 1.3 95 2.4 2.6 4.2 2.6 1.4 90 3.5 3.4 4.3 2.5 1.4 80 4.9 4.8 4.3 2.1 1.3 70 5.4 3.6 1.6 60 7.9 5.7 2.7 1.6 1.2 50 5.2 2.0 1.4 40 9.9 4.1 1.6 1.4 1.4 20 110.0 2.7 1.3 1.4 1.4 a Data from this work and references 8(a)and 10; experimental values have been rounded off to emphasise trends in a compact form.Values in bold show deviations from expected trends based on data for solvolyses of (1; Z = NO and (1; Z = OMe). 31 aJU .-L 0 ru -2 x0 t n 70M o/8 50E 50A 1 I I I I I -1 0 1 3 .? log (k/ko)forp-methoxybenzoyl- chloride -Figure 1. Correlation of logarithms of solvolysis rates for p-methyl-benzoyl chloride (1; Z = CH,) relative to rates in 80% v/v ethanol- water (k,) at 25 "C versus corresponding rates for p-methoxybenzoyl chloride (1; Z = OMe); solvent codes given in Table 1; kinetic data from Tables 1-3 and refs. 3,4 and 10. The hatched line is predicted for sN2-SNlsolvolyses from a least-squares calculation for points A B and C (see the text) and experimental data points for 20A 20E and 20M slope 1.01 k 0.02; correlation coefficient 0.9995.selectivity data when comparisons with solvolyses of (1; Z = OMe) and (1; Z = NO,) are made (Table 4). The expected trend for the carbonyl addition reaction channel is an increase in S as water is added to alcohol-see data for (1; Z = NO,) (Table 4).Departures from this trend (shown in bold typeface) occur in 40E and 50M (1; Z = Cl) in 70E and 80M (1; Z = H) and in 90E and 90M (1; 2 = Me). Although S values for the sN2-sN1 reaction channel are not known it appears that for / 20M/P 30M/O 2 0Ae?' 2 OE 40M/,O 30E x %I a,a L0- MeOH e 0 0 80E 0 ,%60A 90E /OC 70A// EtOH n / I I -1 / I 0 1 I 2 I 3 log (k/ko)for p-methoxybenzoyl chloride Figure 2.As for Figure 1 except the substrate is benzoyl chloride. For the hatched line slope 0.90 k 0.02; correlation coefficient 0.9986. ethanol-water mixtures an Svalue of 0.7_+ 0.1 can be reached as a lower limit (1; Z = OMe) in all compositions; (1; Z = H and Me) in 20E when the carbonyl addition processes becomes insignificant. Similar results are observed for methanol-water mixtures with S = 1.3 & 0.1 for (1; Z = OMe) in all composi- tions and for (1; Z = H and Me) in highly aqueous media. Rate-Rute ProJ1es.-Mechanistic changes for solvolyses of benzoyl chloride have previously been predicted from mass law effects (NB solvent effects ignored),6b and from solvent effects e.g.Grunwald-Winstein plots 2",7 and comparisons of logar- ithms of rate constants for solvolyses of various acid chlorides versus Yc,.3bThese predictions can now be modified slightly to improve further the allowance made for solvation effects." Of the alternative model substrates for which data suitable for rate- rate profiles are available," we chose solvolyses of (1; Z = OMe) because a wider range of data was accessible. The new rate-rate profiles are shown in Figures 1-3. Discussion of the hatched lines will be given later. Considering results for indi- vidual binary mixtures changes in slopes for (1; Z = Me) (Figure 1) occur close to 80M and close to 80E,and for benzoyl chloride (Figure 2) close to 60M and between 60E and 70E. These are the solvent compositions where a change of reaction channel is predicted to occur i.e.at that composition the two reaction channels both contribute 50% towards the observed rate of reactions. These predicted values for solvent compositions are all of slightly higher alcohol content than previous predictions based on Ycl;3b e.g. for benzoyl chloride in methanol-water we previously predicted 3b the change to be at 50M whereas 60M is now predicted (see Figure 2). Interpretation of kinetic data for (1; Z = C1) and (1; Z = NO,) is now known8" to be more complex than that given previo~sly.~~In less aqueous media rates and products (Table 4)of solvolyses of (1; Z = Cl) resemble those of (1; Z = NO,). For solvolyses of (1 Z = NO,) over the whole range of aqueous alcohol mixtures rates fit the third-order equation (2) in which ka is the rate constant for attack by two alcohol molecules k refers to water k refers to alcohol as nucleophile and k refers to water as nucleophile.Products can be calculated directly if the (kaW+ k,,) term is assumed to lead only to ester product; 8" i.e. the k term leading to acid product is relatively small. In more aqueous media (e.g. 60M -+ water) rates of solvolyses 1388 J. CHEM. SOC. PERKIN TRANS. II 1989 Table 5. Comparisons of third-order rate parameters [equation (2)] for solvolyses of p-nitrobenzoyl chloride (1; Z = NO,) and p-chlorobenzoyl chloride (1; Z = Cl)." Parameter Z = NOzb z = c1 Ratio k (for MeOH)/mol-' dm6 s-' 6.69 x 10-5 9.95 x 6.7 k (in water)/mol-' dm6 s-l k (for EtOH)/mol-' dm6 s-l 2.18 x 3.95 x 10-5 10-5 2.00 x 4.16 x 1e6 10.9 9.5 m -0.176 -0.168d " Summarised in the text but full details given in ref.8(a). From ref. 8(a).'Extrapolated from a plot (Figure 4) of third-order hydrolysis rate constants in 90A 80A and 70A for (1;Z = Cl). Slope of plot described in footnote c; standard error in slope 0.003 correlation coefficient 0.99986. / 20M' 30M O@' 40M 0 ,E20A60M 'iM 0 e/ /30EBOM 0 ,/30A MeOH 0 0 70E 0 0 80E e 90E 60A o e 70A two reaction channels are predicted from the kinetic data (Figure 2)to occur equally. Using the two assumptions stated above S can be calculated for solvolyses of (1; Z = H). For 60E the observed ester/acid concentration ratios are 1.44 for (1; Z = NO,)*" and 0.32for (1; Z = OMe)." If 50% of the total reaction goes by each reaction channel then 29.5% of the total product will be ester formed by the carbonyl addition channel and 12.1% will be formed by the sN2-sN1 reaction channel.From these yields of ester the S value is calculated to be 1.54 in good agreement with the experimental result of 1.6 for (1; Z = H) in 60E. For methanol-water the mechanistic change is predicted to occur close to 60M (Figure 2)and the calculated S value is 2.8,again in good agreement with the experimental value of 2.7. The above results illustrate that in the region of mechanistic change the S values can be calculated quite well. It would be possible to use only the S values (Table 4) with the assumptions given above to predict the solvent composition when the mechanistic change occurs (i.e.50% of each reaction channel). However this is probably less reliable than the analysis given above based on the kinetic data (Figures 1-3).Because S is based on product ratios [equation (l)] contributions of 50% from a reaction having for example S = 9 (i.e. 90% of the product formed by this pathway will be ester when [ROH] = [H,O]) and of 50% from a reaction having S = 1 (50% ester when [ROH] = [H,O]) will give an overall value of 70% of the total product for the yield of ester i.e. S = 2.3,not the average S of 5.0. Hence the maximum S value is not the point of mechanistic change and appropriate S values cannot be pre- dicted simply by taking an average of the two S values for the competing reaction channels.Precise Quantitative Analysis of Rate Data-Further inform-ation can be obtained from the kinetic data about the changeover from the carbonyl addition reaction channel to the sN2-sN1 reaction channel. Logarithms of rate constants for the sN2-sN 1 reaction channel should plot linearly against logarithms of rate constants for solvolyses of (1;Z = OMe) an appropriate model compound for correlating reactions occurring via the sN2-sN 1 channel.' Solvolyses of benzoyl chloride in highly aqueous media (50M-+ 20M;40E -20E;see Figure 2) show a linear region. Fortuitously other linear plots are also observed for MeOH +70M and EtOH -+ 70E. Extrapol- ated correlation lines for MeOH -70M and 50M -20M intersect close to the point for 60M. The exact intersection point is calculated to be at y = 1.10,x = 1.65 valqes which represent logarithms of solvolysis rates relative to rates ob- served in the standard solvent 80E.".'3 If these extrapolations are reliable then for the solvent composition corresponding to this point on the x axis the two reaction channels each con- tribute 50% to the overall reaction.The sN2-sN1 channel is then contributing half of the observed rate (kobs);i.e. / / log (k/ko) for p-methoxybenzoyl chloride Figure 3. As for Figure 1 except the substrate is p-chlorobenzoyl chloride (1; Z = C1); the hatched line is predicted for SN2-S,1 solvolyses (see text and Figure 5).of (1; Z = NO,) decrease in contrast with the rate increase observed for solvolyses of (1; Z = C1) (Figure 3). Relative rates of solvolyses for (1;Z = NO,)/(l; Z = C1) are between 6 and 13 for solvolyses in less aqueous media (e.g.pure alcohols Table 5). In more aqueous media these relative rates decrease sharply so that in 20E the rate ratio is < 1. A mechanistic change appears to be taking place but the plot of the data (Figure 3)provides no clearly defined evidence that this change has been completed in 20E and 20M. These conclusions can be compared with the independent predictions based on changes in trends in the selectivity data (Table 4) [i.e. for (1; Z = H) mechanistic change appears to be in the region 70E-+40E and 80M -+ 40M; for (1; Z = Me) 90E -60E and 90M -+ 60M1.These predictions are in good agreement with those predicted from Figures 1 and 2. For (1; Z = C1) the lowest S values are 1.5 in ethanol-water and 2.7in methanol-water (Table 4) significantly greater than the lower limits of 0.7 2 0.1 and 1.3 & 0.1 for ethanol-water and methanol-water respectively. This may be another indication that the change of the reaction channel for solvolyses of (1; Z =Cl) is incomplete in 20% v/v alcohol-water (an alternative and preferred explanation is discussed later). Towards a More Quantitative Analysis of Rate and Product Data.-The pattern of results (Table 4) suggests a high level of consistency in the S values. For (1; Z = C1 NO,) S values for methanol-water and trends in S values for ethanol-water (Table 4) are very similar when carbonyl addition is dominant. Slightly higher S values are observed for (1; Z = H) (98E-80E) and (1;Z = Me) (98E--+ 95E).Hence as first approxim- ations we assume (a) data for (1; Z = NOz) (Table 4) provide reasonable estimates of the S values expected for the carbonyl addition reaction channel in each solvent and (b) data for (1; Z = OMe) provide solvent-independent S values of 0.70 in ethanoi-water and 1.3 in methanol-water for the sN2-sN1 reaction channel.For solvolyses of (1; Z = H) in 60E-70E the Hence log kobs(at x = 1.65) -log 2 = log kSN2-SN1 J. CHEM. SOC. PERKIN TRANS. 11 1989 20A-L.5 et 90A 40A e 50A e -5.5 -'... I I 1 I 1 Figure 4.Correlation of logarithms of calculated third-order rate constants for solvolyses of p-chlorobenzoyl chloride (1; Z =C1) in acetone-water uersus Grunwald-Winstein Y values (ref. 13). The hatched line is an extrapolation to predict the rates of solvolysis occurring uia the carbonyl addition reaction channel in the more aqueous media (see also Table 4 footnotes c and d). d4 d 20E 0020MU YaJ1 20A 0 0 30E OM a 50M Q 040E 60M 8 40A 0 50E 0 50A log (k/kglfor p-methoxybenzoyl chloride Figure 5. As for Figure 1 except the substrate isp-chlorobenzoyl chloride (1; Z =C1) and the rate constants are the dissected contributions to the S$/sN1 reaction channel; least-squares analysis slope 0.83 0.03,correlation coefficient 0.994.The logarithm of the rate constant kobs (at x =1.65) was estimated by linear interpolation between the logarithms of observed rate constants for the two closest solvent compositions for which experimental data had been obtained. The dissected contribution (log k sN2-sN1) can be plotted on the graph (Figure 2)at point A,y =(1.15 -log 2),x =1.65. Rate constants for the carbonyl addition channel in highly aqueous media cannot be obtained by extrapolation of the MeOH-70M line because after 60M the rate decreases-see Figure 3 and par- ticularly data published elsewhere for (1; Z = This decrease introduces a small uncertainty into the location of point A.From expected trends it can be estimated that in 20M the sN2-sN1 channel contributes over 90% of the observed rate and we approximate this to 100%.A line through the data point for 20M (Figure 2) and point A should then be the dissected contribution from the sN2-sN1 reaction channel. If this line is both interpolated and extrapolated dissected contributions in other solvent compositions and corresponding S values could be predicted. The above arguments can be strongly supported by the following additional evidence. Calculations repeated for the ethanol-water data (Figure 2)give point B well separated from point A. Different aqueous binary mixtures may give slight 'dispersion' of correlation lines and to check that this effect was minor data for acetone-water mixtures have been added (point C Figure 2) to the results communicated previou~ly.~' The correlation line shown in Figure 2 is a least-squares fit to the experimental data points for 20M 20E and 20A and the corresponding calculated points A,B,and C respectively. These calculations were repeated for solvolyses of (1;Z =Me) (Figure 1).Extrapolation of the lines through points A,B and C to x = 0 permits calculation of the rates of sN2-sN1 solvolyses in 80E; this is calculated to be 0.5 /cobs for (1; z =Me) and 0.22 kobs for (1;Z =H).The self consistency of the above results for (1; Z =H and Me) prompted a more difficult analysis of the data for (1; Z = Cl) (Figure 3) for which no convincing linear plots through the data points were seen. As most of these solvolyses resemble those of (1;Z =NO,) more closely than those of (1;Z =OMe) (Figure 3) allowance can be made for the carbonyl addition channel [equation (2)]as in previous work.8a Third-order rate constants for solvolyses of (1; Z =C1) in pure alcohols [k, equation (2)] are calculated from the observed pseudo-first order rate constants and the molar concentrations of alcohol (Table 5).Third-order rate constants in the less aqueous compositions of acetone-water can be extrapolated to pure water (Figure 4)to obtain the k term [equation (2)],and the medium effect (m).Strong similarities with solvolyses of the more reactive substrate (1;Z =NO,) can be seen from the rate ratios and m values (Table 5). Following our earlier work,8a we have retained the use of Y values (Figure 4) for the small medium effect on rate constants.Very similar rate-product correlations to those previously reported 8a have now been obtained by replacing Y values with logarithms of rate con- stants for solvolyses of (1; Z =OMe).8b For solvolyses of (1;Z =NO,) a plot 8a analogous to Figure 4 is linear over a wide range of acetone-water mixtures (90-2073,and the extrapolated line for (1;Z =CI) (Figure 4) from 60A to 20A gives the predicted rates in the various solvent mixtures for solvolyses of (1; Z =Cl) occurring via the carbonyl addition channel. The difference between the observed and predicted rate constants for solvolyses of (1; Z =Cl) is assumed to be the contribution from the s$-sN1 reaction channel. A very similar result for the dissected contribution from the two reaction channels could have been obtained by assuming that the k term [equation (2)]was 11 times greater in all solvents for (1; Z =NO,) than for (1; Z =C1) (see Table 5).This alternative approach gave good results for ethanol-water mixtures assuming a rate ratio of ten as an approximate weighted average of k, k, k, and k terms [equation (2)].A lower rate ratio of 7.5 was assumed for methanol-water mixtures-these solvolyses are dominated by the k term,*" which we cannot evaluate directly for (1; Z =Cl). The dissected rate constants calculated for solvolyses of (1; Z =Cl) uia the sN2-sN1 reaction channel give a satisfactory linear rate-rate profile uersus solvolyses of (1; Z =OMe) (Figure 5) considering the uncertainties in these calculated rate constants.The slope of a correlation line through all of the data points (Figure 5) is 0.83 rf 0.03 lower than for benzoyl chloride (0.90 rf 0.02,Figure 2). The correlation line (Figure 5) has been added to Figure 3 as a hatched line to provide a comparison with the undissected rate data. In retrospect it was seen that a similar line could have been constructed by linear extrapolations leading to points A B and C as described for the hatched lines in Figures 1 and 2. These results imply that the gradual increase in upward curva- ture in highly aqueous media (Figure 3) has ceased in 20A 20E and 20M. Calculation of Trendsin S Values.-A very demanding test of the reliability of the above evidence for the dissected sN2-sN1 contributions is the calculation of variations in S values as the sN2-sN1 reaction channel gradually supersedes the carbonyl addition channel.The calculated results are summarised in 1390 J. CHEM. SOC. PERKIN TRANS. II 1989 Table 6. Dissected contributions of two reaction channels to observed rates and products for solvolyses of p-Z-substituted benzoyl chlorides (1) in alcohol-water. Ethanol Alcohol (% v/v) % (S,2-S 1) Scdc p-Methylbenzoyl chloride 98 17 0.7 24 I .8 95 21 0.9 28 I .9 90 30 1.06 34 2.3 80 50 1.15 55 2.1 70 66 1.09 58 2.2 60 74 1.9 50 81 1.o 83 1.8 40 95 0.8 92 1.6 20 lood 0.7 lood 1.3 Benzoyl chloride 98 6 0.7 8 1.9 95 7 0.9 10 2.3 90 11 1.2 12 3.0 80 22 1.53 20 3.4 70 39 1.51 33 3.3 60 54 1.46 48 2.9 50 68 1.3 63 2.5 40 86 1.o 72 2.3 20 100d 0.7 lood 1.3 p-Chlorobenzoyl chloride 98 1 0.7 2 1.9 95 1 0.9 2 2.3 90 2 1.3 2 3.4 80 4 1.9 4 4.5 70 12 2.I 7 5.4 60 15 5.5 50 27 2.5 30 4.6 40 62 1.7 45 3.9 20 92 1.o 98 1.4 The contribution from the carbonyl addition channel is 100 -(SN2-SN1). Rate constants interpolated from data in pure alcohol and 90% vjv alcohol-water by comparison with rate data for (1; Z = OMe) (Table 2). Product data for (1; Z = NO,) in this composition ofmethanol-water were estimated from the trends in a data for other solvent compositions. dAssumed. Table 6 but one such calculation is now outlined. For benzoyl chloride in 90M the distance between the point for 90M and the extrapolated line (hatched Figure 2) is 0.94 log unit.For illustration we approximate this to 1 log unit so the sN2-sN1 process is calculated to contribute 10% of the observed rate. As before solvent dependent S values were obtained from data for (1; Z = NO,) to model the carbonyl addition reaction channel (S = 3.508") and S values for (1; Z = OMe) modelled the sN2- sN1 reaction channel (S= 1.3 in methanol-water). Ester/acid concentration ratios for the two reaction channels are 14 and 5.2 respectively corresponding to 93.3 and 83.9%ester. Taking 90% of 93.3 and 10% of 83.9 gives a total yield of ester of 92.4% and a calculated S value [equation (l)] of 3.0. The calculated results (Table 6) reproduce the trends in S values including the solvent compositions giving maximum S for (1; Z = C1 and H).The calculated S values are particularly good for solvolyses of (I; Z = Cl) in methanol-water (except for low values in 20M and 50M). Two deficiences in absolute values are discussed below. In less aqueous media (e.g. 98 -+ 90E) observed S values for solvolyses of (1; Z = C1 H Me) are higher than for (1; Z = NO,). However S values calculated from our rate-product correlation for (1; Z = assuming zero contribution from the k [leading to acid product equation (2)] are at least double the observed values. A few % contribution from the k pathway has a large effect on S values when as in these cases the total yield of acid is low (< 10%). Hence the increase in S values for Z = NO < Z = C1 < Z = H < Z = Me could be due to increasing (but small overall) contributions from the k pathway.A second deficiency is the choice of S values for solvolyses in highly aqueous media. For solvolyses of (1; Z = C1) in 20% vjv alcohol the SN2-S,1 mechanism is dominant but S has not reached such low values as those obtained for solvolyses of (1; Z = H Me and OMe) 4.g. 0.7-0.8 in 20% ethanol-water (Table 4).These results are consistent with greater sN2 character for solvolyses of (1; Z = CI) so that product formation does not occur from solvent-separated ion pairs (proposed when S < l)." Conversely for solvolyses of (1; Z = H and Me) in 20% vjv alcohol-water a prediction arising from this work is that it may be possible to trap a cationic intermediate; previous attempts were made using much less aqueous media (e.g.50% w/w acetone-~ater).~~'~" Dissections of kinetic data for solvolyses of benzoyl chloride in 30A -+ 95A were made by A. H. Fainberg and reported in a later doctoral thesis.14 For 50A the percentage of the sN2-sN1 pathway was calculated l4 to be 63% in very good agreement with our calculated value of 66%. However in 80A we predict 15% sN2-sN1 in comparison with the earlier prediction l4 of 3%. Although the exact procedure l4 was not reported calcul- ations were probably based on a correlation with Y values. No allowance could have been made for the recently discovered differences in solvation effects in less aqueous media.' J.CHEM. SOC. PERKIN TRANS. 11 1989 Table 7. Substituent effects on the dissected rate constants (k/10-3s-') for S,2-S,1 solvolyses of p-Z-substituted benzoyl chlorides ( Substituent Z A I Solvent c1 H Me OMe \ 80E 0.18 0.59 1.64 21.7' EtOH 0.014 0.04 0.07 0.99' 20M 99.4' 540' 3 100' 42 OOOb MeOH 0.10 0.31 0.79 10.7 'Calculated by extrapolation of the hatched correlation lines (Figures 1-3) except where noted otherwise. Observed rate constants. Table 8. Substituent effects on the dissected rate constants (k/10-3 s-l) for carbonyl addition solvolyses ofp-Z-substituted benzoyl chlorides (1)' Substituent Z AI ? Solvent NO* C1 H Me 80% EtOH 49.0' 4.3I 2.08 1.67 EtOH 11.6' 1.20 0.72 0.45 MeOH 40.8 ' 5.95 3.99 2.41 Calculated from the difference between observed rate constants and those calculated for the SN2-S,1 reaction channel (Table 7),except where noted otherwise.'Observed rate constants. Solvation effects were ignored in an independent published analysis of similar data in which an empirical mass law relationship was derived for contributions from unimolecular and bimolecular pathways.6b From the equation given for the two pathways,6h there is again agreement for 50A. However we calculate that in 60A there is a contribution of 69% from the bimolecular (presumably carbonyl addition) pathway signifi- cantly greater than the value of 47% obtained by our method 3c of dissecting the rate data. Also in 80A the bimolecular pathway is calculated to contribute 99.5% of the observed rate constant (cf our predicted value of 85%). It is difficult to devise experimental checks of these predictions for acetone-water mixtures.In contrast our studies of rates and products in alcohol-water mixtures have provided more experimentally accessible checks. Substituent Effects.-H aving separated the contribution from the sN2-sN1 reaction channel the rates of solvolyses in other solvents can be predicted by linear extrapolation of the correlation line (Figures 1-3) for the separated channel. Ex- ample calculations have been made for 80E ethanol methan- ol and 20M (Table 7). The relative rates depend on the slopes (m) of the correlation lines (Figure 1-3) and when m = 1.01 (Figure 1) relative rates are independent of solvent (e.g.for 1; Z = Me and OMe). As m decreases rates in less polar solvents drop less sharply (perhaps because of nucleophilic solvent assistance) and the magnitudes of substituent effects are re- duced. There is a consistent trend that rates of sN2-sN1 solvolyses are favoured by electron donation by the substituent (Z) particularly so when Z = OMe (Table 7). Also the dis- sected rate constants for the carbonyl addition reaction channel increase with electron withdrawal by the substituent Z parti-cularly when Z = NO (Table 8). General Implications.-Modern interpretations of reaction mechanisms often involve detailed kinetic or theoretical studies to characterise the structures of transition states which can vary so widely that three-dimensional contour diagrams may be required.5-1 These developments have made it increasingly difficult to establish the alternative possibility that competition between two distinct reaction channels can occur. This alter- native has now been strongly supported by studying rates products substituent effects and solvent effects for solvolyses of (1). However it may still be argued that instead of two or more different transition states at least one for each of the two reaction channels there is only one transition state of hybrid ~haracter.,~.~"As the contributions from the dual reaction channels change a single transition-state model would change in accordance with the weighted average of the two transition- state model.Because the single transition-state model is not predictive it is very difficult to disprove conclusively. However interpretation of the selectivity data (Table 4) would be very difficult without postulating several product- determining steps.18 Changes in rate-limiting step with the same product-determining step proposed for reactions of morpho- line with benzoyl halides," cannot explain all of these results adequately. Additional mechanistic studies are now required to characterise further the mechanistic changes occurring within each of the two proposed reaction channels. An important challenge is to explain why the carbonyl addition channel shows high Br/Cl rate Hence it should be emphasised that use of the term 'carbonyl addition reaction channel' is a convenient way to discuss the results in this paper not an explicit statement about exactly how these reactions occur.Related mechanistic studies of activated carboxylic esters 2c include consideraton of tetrahedral 'intermediates' with little or no barrier to decomposition. Conclusions Solvolyses of p-Z-substituted benzoyl chlorides (1; Z = Me H Cl) in aqueous ethanol and in aqueous methanol can be related to solvolyses of (1;Z = NO,) and (1; Z = OMe). Rate- rate profiles (Figures 1-3) are consistent with two competing reaction channels as discussed previ~usly.~~ The accuracy of these profiles has now been improved and rate constants for the sN2-sN1 reaction channel have been dissected from the ob- served rate constants; the dissected rate constants show the expected lo single correlation lines (Figures 1 2 and 5) for aqueous acetone aqueous ethanol and aqueous methanol.The carbonyl addition reaction channel is more complex 8a and shows a dispersion between plots of data for aqueous acetone ethanol and methanol (Figures 1-3). Dispersion may be caused by greater contributions from solvent nucleophi-li~ity,~~,~'and by possible contributions from general base catalysis.'" Changes in the contributions from the two reaction channels occurring as water is added to alcohol can be calculated (Table 6). Trends in product ratios and selectivities (S) [equation (l)] independently confirm the main conclusions from rate-rate profiles. For the carbonyl addition channel S increases as water is added to alcohol (data for 1; Z = NO,).For (1; Z = OMe) the model substrate for the sN2-sN1 reaction channel S is constant. For solvolyses of (1;Z = Me H Cl) S increases then reaches a plateau and/or decreases. Deviations from expected trends occur in the range of solvent compositions predicted independently from rate-rate profiles. Taking S values from solvolyses of (1; Z = NO and OMe) as typical of the corre- sponding reaction channels combined with the calculated contributions from each reaction channel it was possible to calculate satisfactory trends in S for solvolyses of (1; Z = C1 H Me). Some of the calculated values were too low suggesting that the choice of S value has been oversimplified.A general implication of these results is- that the possibility of competing reaction channels should be considered in addition to variations in transition states within a particular reaction channel. Experimental Materials.-Acid chlorides were commercial samples purified by distillation under reduced pressure and shown to be pure ( 2 99.7%) by h.p.1.c. analyses of methanolysis products. Because of these small impurities corrections were required for solvents containing >90% alcohol. Acids and esters were obtained and purified by standard methods. Rates and Products.-The experimental methods were as described previously.’ Calculations.-Details of slopes of correlations and dissec- tions for (1 Z = C1) are available on request.Acknowledgements This work was supported by the SERC and by the Korean Science and Engineering Foundation. Preliminary product studies were carried out by Mr. N. Jones (Swansea 1986) and H. C. Harris. Helpful discussions with H. C. Harris and D. N. Kevill are gratefully acknowledged. References 1 Reviews (a) A. Williams and K. T. Douglas Chem. Rev. 1975 75 627; (6) A. Kivinen in ‘The Chemistry of Acyl Halides,’ ed. S. Patai Wiley New York 1972 ch. 6; (c) R. J. E. Talbot in ‘Comprehensive Chemical Kinetics,’ eds. C. H. Bamford and C. F. H. Tipper Elsevier Amsterdam 1972 vol. 10 ch. 3; (d) R. F. Hudson Chimia 1961 15 394. 2 (u) R. F. Hudson and G. E. Moss J. Chem. SOC.,1962 5157; (b) J. Miller and 0.-L. Ying J. Chem. SOC.,Perkin Trans.2 1985,323;(c) S. Ba-Saif A. K. Luthra and A. Williams J. Am. Chem. SOC.,1987,109 6362; (d)D. N. Kevill and C. B. Kim Bull. SOC.Chim. Fr. 1988,383; J. CHEM. SOC. PERKIN TRANS. II 1989 (e)D. N. Kevill and C. B. Kim J. Chem. SOC.,Perkin Trans. 2,1988 1353. 3 (a) T. W. Bentley G. E. Carter and H. C. Harris J. Chem. Soc. Perkin Trans. 2 1985 983; (b) T. W. Bentley and H. C. Harris J. Chem. Soc. Perkin Trans. 2 1986 619; (c) T. W. Bentley and 1. S. Koo J. Chem. SOC.,Chem. Commun. 1988,41. 4 (a) I. Lee I. S. Koo S. C. Sohn and H. H. Lee Buff.Korean Chem. SOC.,1982 3 92; (b) C. G. Swain R. B. Mosely and D. E. Bown J. Am. Chem. SOC.,1955,77,3731. 5 (a) S. Winstein E. Grunwald and H. W. Jones J. Am. Chem. SOC. 1951,73,2700;(b) F. L. Schadt T.W. Bentley and P. v. R. Schleyer J. Am. Chem. Soc. 1976,98,7667. 6 (a)V. Gold J. Hilton and E. G. Jefferson J. Chem. SOC.,1954,2756; (b) M. J. Kelly and G. M. Watson J. Phys. Chem. 1958,62,260. 7 T. W. Bentley G. E. Carter and H. C. Harris J. Chem. SOC.,Chem. Commun. 1984,387. 8 (a) T. W. Bentley and H. C. Harris J. Org. Chem. 1988 53 724; (b) H. C. Harris unpublished results. 9 B. D. Song and W. P. Jencks J. Am. Chem. SOC.,1987,109,3160. 10 T. W. Bentley H. C. Harris and I. S. Koo J. Chem. SOC.,Perkin Trans. 2 1988 783. 11 S. Winstein and A. H. Fainberg J. Am. Chem. SOC.,1957,79 5937. 12 For discussion of expected trends see T. W. Bentley and K. Roberts J. Chem. SOC.,Perkin Trans. 2 1989 1055. 13 A. H. Fainberg and S. Winstein J. Am. Chem. SOC.,1956,78,2770. 14 M. Cocivera Ph.D. Thesis University of California Los Angeles 1963 p. 120. 15 E. R. Thornton J. Am. Chem. Soc. 1967,89,2915. 16 R. A. More O’Ferrall J. Chem. SOC.B 1970,274. 17 W. P. Jencks Chem. Rev. 1972,72,705. 18 C. D. Ritchie J. Am. Chem. SOC.,1971,93,7324. 19 M. L. Bender and J. M. Jones J. Org. Chem. 1962,27,3771. 20 D. N. Kevill and G. M. L. Lin J. Am. Chem. SOC.,1979,101,3916. Received 14th July 1988; Paper 8/02844C

ISSN:1472-779X    

DOI:10.1039/P29890001385

出版商:RSC    

年代:1989

数据来源: RSC