The enol content of simple carbonyl compounds: a thermochemical approach

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. Rate constants for oxygen-base-catalyzed enolization of di-and tricarbonyl compounds can be correlated by a Marcus equation, with hydroxide requiring a slightly different curve from other oxyanions. Data for monocarbonyl compounds fall on a slightly different, but similar, set of curves. The equilibrium constants for enolization for a set of monocarbonyl compounds were derived from recent work in this laboratory. These correlations permit estimation of pKaKel" for a number of other monocarbonyl compounds from kinetics data in the literature. Elaboration of methods previously developed permits estimation of pKaEnol. From the values of pKaEno' and pKaKC1", pKEFol can be derived. It is pointed out that pKaKel" for acetone and acetophenone can be determ~ned from the kinetics of the reaction with hypochlorite in base; the values so derived are in good agreement with those used in this work.

Section: Introductionmentioning

confidence: 99%

The enol content of simple carbonyl compounds: an approach based upon pK_a estimation

Guthrie¹

1979

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“…The pK,'s of a number of these transient species have been obtained experimentally, mainly through the elegant work of Kresge and co-workers (1-7).2 Guthrie, in elaborating a thermochemical approach for predicting the en01 content of a simple ketone, estimated the pK,'s of 24 enols by a LFER method (8,9).…”

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confidence: 99%

A study of thermodynamic acidities of enols with the semiempirical computational method AM1

David¹

1990

. Using the semiempirical computational method AM1 we have computed the enthalpies of ionization of enols 1-21 and correlated these values with solution andestimated pKa's. From this initial study it appears that, in general, it is possible to predict the pK's of acyclic enols within ? 1 pKa unit. On the basis of this correlation we predict the pKa's of 1-hydroxy-l,3-butadiene (3-butenal enol) (9.7) and 1,l-difluoroacetone en01 (9.6) and suggest that the previously estimated pKa of 1,l-dichloroacetone en01 (6.2) is low by approximately 3 pK units. We find computationally (this has been established experimentally) that the Z-en01 of phenylacetaldehyde is less acidic than the E-enol.Key words: enols, acidities, computational studies, AMl.NICK HENRY WERSTIUK et DAVID ANDREW. Can. J. Chem. 68, 1467Chem. 68, (1990. Utilisant la mCthode de calculs semi-empiriques AM1 , on a calculC les enthalpies d'ionisation des Cnols 1-21 et on a Ctabli une corrClation entre les rCsultats de ces calculs et les valeurs observCes en solution ainsi qu'avec les valeurs estimCes des pKa. Sur la base de cette Ctude initiale, il semble que, en gCnCral, il est possible de prCdire les valeurs des pK d'Cnols acycliques 1 * 1 unit6 de pK,. Sur la base de cette corrklation, on peut prCdire les valeurs des pKa du 1-hydroxy-buta-l,3-dikne (Cnol du but-3-tnal) (9,7) et de 1'Cnol de la 1,l-difluoroacCtone (9,6) et suggCrer que la valeur de 6,2, estimte anterieurement pour le pKa de I'tnol de la 1,l-dichloroacCtone, est environ 3 unitCs de pK, trop basse. Sur la base des calculs -qui ont Ct C confirmCs d'une facon expkrimentale -le Cnol-Z du phCnylacCtaldthyde est moins acide que I'tnol-E.Mots clks : [Traduit par la revue]Enols are important intermediates in organic chemistry. The pK,'s of a number of these transient species have been obtained experimentally, mainly through the elegant work of Kresge and co-workers (1-7).2 Guthrie, in elaborating a thermochemical approach for predicting the en01 content of a simple ketone, estimated the pK,'s of 24 enols by a LFER method (8, 9).To expand studies on the enthalpies of ionization394 with the semiempirical computational method AM1 (lo), we undertook a study of enols to establish whether the pK,'s of these transient species can be estimated computationally. We calculated heats of formation of 1-formylfluorene en01 (I), mandelic acid en01 (2), diphenylacetaldehyde en01 (3), phenylacetaldehyde en01 (4), methoxyacetophenone en01 (S), acetophenone en01 (6), isobutyraldehyde en01 (7), acetone en01 (S), and acetaldehyde en01 (9) and the corresponding enolates. To extend this study to the ionization of an anionic species, heats of formation of the en01 of pyruvate (CH2=CH(OH)CCO-+, neutralized by a unit positive charge, and its enolate (CH2=CH(O-)COO-+ were '~e c i~i e n t of a Natural Sciences and Engineering Research Council of Canada Summer-Student Research Scholarship, 1988.'~l s o A. J. Kresge, personal communication. We thank Professor Kresge for providing us with the pKa (1 1.57) of the en01 of sodium ...

“…' The method is limited to non-hydrogen bonding liquids, with normal boiling points below 200°C. We have had occasion to determine heats of formation (3)(4)(5)(6)(7)(8) which were used to calculate free energies of formation in aqueous solution. In the course of these calculations it was necessary to estimate the heat of vaporization.…”

Section: Introductionmentioning

confidence: 99%

“…[7], and the vapor pressure at 25°C was estimated. This value was compared to the value obtained by solving eq.…”

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confidence: 99%

Estimation of heats of vaporization for non associating organic liquids from the boiling points at various pressures

Guthrie¹

1986

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J. PETER GUTHRIE. Can. J. Chem. 64, 635 (1 986). At any pressure the heat of vaporization can be expressed as a quadratic function of the boiling point at that pressure. A seven parameter equation expressing the simultaneous dependence on boiling point and pressure can be fitted to the data; six pressures from 1 to 760 Torr (1 Torr = 133.3 Pa) were used. AH,,, = bll + b12 In (p) + b13p + (bzl + bz2 In (p))tbp + (b3[ + b32 In (p))tbp2. This relationship served as a guide for developing a relationship between vapour pressure at 25°C and the calorimetric heat of vaporization, and also a relationship between vapor pressure at 25OC and the boiling point at some other pressure. Parameters for both these relationships could be derived from the parameters obtained for AH,,, as a function of temperature and pressure. A third method was developed starting from an equation for vapor pressure and fitting to the heat of vaporization, the heat capacity of vaporization, and at least one t,p point. These methods allow the estimation of the vapor pressure at room temperature from very meager data. The problems of errors in estimated values are discussed.J. PETER GUTHRIE. Can. J. Chem. 64, 635 (1986). A toutes les pressions, la chaleur de vaporisation peut etre exprimte c o m e une fonction quadratique de son point d'kbullition a cette tempkrature. Toutes les donnkes peuvent Ctre accomodtes par une tquation h sept paramktres exprimant la dtpendance simultante sur le point d'tbullition et sur la pression; on a utilisk six pressions de 1 h 760 Torr. AH,,, = bl + b12 In (p) + b13p + (bZl + bZ2 In (p))tbp + (b31 + b32 In (p))tbp. Cette relation a servi de guide pour dtvelopper une relation entre la pression de vapeur 25°C et la chaleur de vaporisation calorimCtrique ainsi que le point d'tbullition h d'autres pressions. On a pu obtenir les paramktres pour ces deux relations a partir des paramktres obtenus pour le AH,,, en fonction de la temp6rature et la pression. On a dtveloppt une troisikme mtthode h partir d'une tquation pour la tension de vapeur et en y instrant la chaleur de vaporisation, la capacitk calorifique de vaporisatin et au moins un point de t,p. Ces mtthodes permettent d'tvaluer la tension de vapeur a la tempkrature ambiante h partir de trks peu de donntes. On discute des problkmes des erreurs dans les valeurs prkdites.[Traduit par la revue]