Trapping of the oxocarbonium ion intermediate in the hydrolysis of acetophenone dimethyl ketals

196

175

Rate and equilibrium constants for reactions of carbocations, and a few other electrophiles, with anionic and neutral nucleophiles in several solvents are summarized. Equilibrium constants for such reactions are a measure of the carbon basicities of the nucleophiles, and are discussed in the context of differences in proton and carbon basicities. The factors affecting the kinetic reactivities of nucleophiles are not well understood, even though the reactivity patterns are frequently the same toward widely different electrophiles.

Section: Kinetic Reactivities Of Nucleophilesmentioning

confidence: 99%

Cation–anion combination reactions. 26. A review

Ritchie¹

1986

196

175

“…[3]. However, recent measurements of the rate of reaction of alkoxy-substituted carbonium ions with water (8) suggest that this reaction, in the case of the cation formed from 9-methoxyoxacyclonon-2-ene, is likely to be a diffusion-controlled process. It therefore cannot be slower than reversion of the proton transfer step, but decomposition of the hemiacetal intermediate, eq.…”

Section: Discussion Reaction Mechanismmentioning

confidence: 99%

Vinyl ether hydrolysis. XIV. 2-Methoxy-2,3-dihydropyran: concurrent reaction of vinyl ether and acetal functional groups

Burt¹,

Chiang²,

Kresge³

1980

. Can. J. Chem. 58,2199Chem. 58, (1980. The hydrolysis of 2-methoxy-2,3-dihydropyran shows a normal isotope effect (kH/kD > 1) under catalysis by the hydrogen ion and gives an accurately linear dependence of reaction rate upon undissociated acid concentration in cyanoacetic acid and formic acid buffer solutions. This substrate, therefore, unlike its higher homolog, 9-methoxyoxacyclonon-2-ene, provides no evidence in support of an anything but a normal mechanism for vinyl ether hydrolysis. Analysis of the hydrogen isotope effect suggests that a 1 minor amount (8%) of this hydrolysis occurs via reaction of the acetal functional group. R. A. BURT, Y. CHIANG et A. J. KRESGE. Can. J. Chem. 58,2199Chem. 58, (1980. L'hydrolyse du methoxy-2 dihydro-2,3 pyranne catalysee parl'ion hydrogkne, montre un effet isotopique normal (kH/kD> 1) et sa vitesse de reaction depend d'une f a~o n precise et lineaire de la concentration en acide non dissocie dans des solutions tampons d'acides cyanoacktique et formique. Dans le cas de ce substrat, contrairement a son homologue superieur, le methoxy-9 oxacyclononbne-2, toutes les donnkes indiquent que la reaction se produit par un mtcanisme d'hydrolyse normal pour les ethers vinyliques. L'analyse de I'effet isotopique de I'hydrogkne suggkre qu'une partie infime (8%) de cette hydrolyse se produit par I'intermediaire de la reaction du groupe fonctionnel acttal.[Traduit par le journal]In order to gain information which might be of use in understanding the unusual hydrolytic behavior of 9-methoxyoxacyclonon-2-ene,3 1 (I), we have investigated the acid-catalyzed hydrolysis of the related substance, 2-methoxv-2.3-dihvdropyran, 2. The hydrolysis of a homoiog of 2, i-ethn oxy-2,3-dihydropyran, had been examined before (2), but that study was limited to HCl solutions. ether functional group. In carboxylic acid buffer solutions, however, its behavior deviates from that of all other vinyl ethers examined so far, and we were particularly interested, therefore, in studying a similarly substituted vinyl ether in these media. ExperimentalMaterials 2-Methoxy-2,3-dihydropyran was purchased from the Aldrich Chemical Company. Its proton nmr spectrum was consistent with its structure and showed no sign of impurities, but kinetic data for the hydrolysis of the commercial material nevertheless sometimes produced curved first-order plots. Samples freshly purified by gas chromatography, on the other hand, were well behaved, and kinetic measurements were therefore carried out on material purified in this way.Acids were best available commercial grades and were usedIn dilute aqueous HC1, 1 behaves normally in a without further treatment. Solutions were made using either manner characteristic of the hydrolysis of its vinyl deionized H 2 0 purified further by distillation from alkaline permanganate or D 2 0 as received (Merck); buffer solutions were prepared by adding requisite amounts of aqueous sodium hy-'Part XIII: Y. Chiang, W. K. Chwang, A. J. Kresge, and S. droxide to acid solutions. Szilagyi. Can. J. Chem. ...

“…We have described previously an approach for the oxocarbocation intermediates of acetal and ortho ester hydrolysis, where rate constants measured in strong acids were extrapolated to water with the assumption that differences between pairs of cations remain constant as the acidity is changed (24). This procedure was supported by observation of the required parallelism, albeit over a limited range in many cases, and through agreement with rate constants independently estimated by a "clock" approach (25 has been conclusively established through direct measurement in water using the technique of laser flash photolysis (26,27). Figure 3 shows data for phosphonium ions that allow for such an extrapolation.…”

Section: Lifetimes Of Phosphonium Ions In Watermentioning

confidence: 95%

Kinetics and mechanism of the hydrolysis of a (5,6)-spirophosphorane. Thermodynamics of the hydrolysis of cyclic five-membered and six-membered phosphonium ions

McGall¹,

McClelland²

1991

Can. J. Chem. 69, 2064. The cyclic five-membered phosphonium ion 26 (2-(2'-hydroxyethoxy)-2-pheny1-1,3,2-dioxaphospholan-2-ylium) derived from ring-opening of the (5,5)-spirophosphorane 16 (5-phenyl-l,4,6,9-tetraoxa-5-phosphaspiro[4.4]nonane) has been observed in neat CF3S03H and at >85% H2SO4. The cation undergoes hydrolysis in the latter solutions, and an extrapolation has been carried out to obtain an estimate for reactivity in 100% water. Hydrolysis rate constants for phenyltrialkoxyphosphonium ions in water are lo7, lo0, and 5 x s-I for cyclic five-membered, cyclic six-membered, and acyclic derivatives respectively; these show an excellent correlation with rate constants for a similar series of phosphate esters. An investigation of the hydrolysis of the (5,6)-spirophosphorane 5 (5-phenyl-8,8-dimethyl-1,4,6,1O-tetraoxa-5-phosphaspiro[4,5]decane) provides a clue as to the origins of these rate differences. This phosphorane can in principle hydrolyze via two isomeric cyclic phosphonium ions, the six-membered 14 and the five-membered 15. The former is thermodynamically more stable, being the only cation observed under equilibrating conditions of strong acid. However, the hydrolysis of the spirophosphorane, as well as the hydrolysis of fully formed 14, channels through the cyclic five-membered 15. A thermodynamic breakdown reveals that the 9.5 kcal mol-I difference in activation free energy for the hydrolysis of five-and six-membered cyclic phosphonium ions is due to a combination of a higher free energy (2.5-4.5 kcal mol-') for the five-membered cation, and a lower free energy (7-5 kcal mol-l) for the pentacoordinate transition state with the five-membered ring. This analysis also shows that a ($6)-spirophosphorane is 6 8 kcal mol-I more stable than a (6,6)-spirophosphorane. Thus, a five-membered ring has a significant stabilizing effect on a pentacoordinated phosphorus structure. The accelerated hydrolysis of cyclic phosphonium ions and phosphate esters with five-membered rings is caused by a combination of this stabilizing effect in the transition state and a destabilizing effect in the ground state associated with ring strain.Key words: phosphorane, hydrolysis, phosphate, phosphonium. Une attribution thermodynamique rCvtle que la diffkrence de 9,5 kcal mol-' qui existe entre Cnergies libres d'activation pour les hydrolyses des ions phosphonium cycliques i cinq et i six chafnons est due 2 une combinaison d'une tnergie libre plus deve'e (2,5-4,5 kcal mol-') pour le cation a cinq chainons et une Cnergie libre plus faible (7-5 kcal mol-') pour 1'Ctat de transition pentacoordine du cycle i cinq chainons. Cette analyse permet aussi de montrer qu'un (5,6)-spirophosphorane est de 6 8 kcal mol-' plus stable qu'un (6,6)-spirophosphorane. Un cycle i cinq chainons a donc comme effet de stabiliser une structure phosphorte pentacoordinbe. L'hydrolyse plus rapide des ions phosphonium cycliques et des esters de l'acide phosphorique comportant des cycles B cinq chainons est causCe par une combinaison de cet effet stabilisan...