Enthalpies of deprotonation

Arnett and Moe have reported enthalpies of deprotonation, hHdpnn, for 4-substituted phenols and for the alcohols XCH2OH (sets CRl and CR2, Table 9) with LiN(SiMe3)2 in THF at 25 °C. For the phenols only electrical effects were considered. Best results were obtained with the LD equation (equation 4). The regression equation is equation 25 ... 

Use the van t Hoff equation (see Section 9.10) to estimate the enthalpy of deprotonation of heavy water, D20, given the following data on its autoprotolysis ... 

Besides geometric and vibrational properties, identification of the relative energies of compounds, or the energy differences between points on the potential energy surface of a particular compound have also been undertaken.[70-73] Calculations of the bond dissociation energy, reaction energy, electron affinity, heat of formation, and enthalpy of deprotonization are practical examples of the type of properties that have been determined for salts by using quantum chemistry methods. 

Calorimetric determination of the enthalpies of deprotonation and protonation, reactions (1) and (2), involves addition of an acid or a base to suspension at certain (initial) pH. In such a case, neutralization also takes place... 

The proposed approach to the design of the calorimetry experiments, concerning charging of metal oxide surfaces, resulted in the difference of standard enthalpies of deprotonation and protonation reactions (2) and (1). As already demonstrated for anatase [5], the calorimetry result for hematite agrees with the corresponding measurements of the p.z.c. dependence on temperature. The obtained value also agrees with other published data [10, 11]. 

The sole representative of this class of compounds for which there is an experimentally measured enthalpy of formation is CH2CHCH=NH, alternatively called propenaldimine and vinylimine146. The literature value, 125 11 kJ mol 1, was obtained by measurement of the appearance energy for formation of its protonated ion from the radical cation of cyclopentylamine (reaction 56), and then bracketing experiments to deprotonate (reaction 57) this ion to the imine of interest. 

There are but two members of this class of compounds that have been studied with explicit interest as to their enthalpies of formation158. The first is benzaldimine (68a), studied by gas-phase ion-molecule reactions141 for which the alternative possibility of troponimine (68b) was considered plausible. It is unequivocal that the enthalpy of formation of PhCHX is significantly more negative than that of cyclo-(CH=CH)3CX for both the archival X = O and the newly generated value of X = CH2, cf Reference 8. X = NH is expected, as before, to interpolate the ketone and olefinic cases and so benzaldimine is most assuredly more stable than troponimine. However, this species was synthesized by deprotonation of the corresponding cation and protonated troponimine is probably more stable than protonated benzaldimine159. If it is the former ion that was seen in Reference 141, simple deprotonation is not expected to yield benzaldimine. 

Another relevant phenomenon is the reaction of carbon monoxide with the lithium trimethylsilyldiazomethanide (MesSiCNaLi) to form either an ynolate (MesSiC COLi) or a ketenide (Me3SiC(Li)=C=0) derivative. The corresponding neutral species Mes SiCHN2 reacts with CO, in the presence of Co2(CO)s, to form Me3SiCH=C=0 . Neither reaction was studied calorimetrically. Accordingly, they cannot be compared with the energetics of the reaction of HCNi with CO to form HCCO and N2, a quantity indirectly obtainable from the deprotonation enthalpies of diazomethane and ketene. ... 

Beryllium chemistry includes its S-diketonate complexes formed from dimedone (9), acetylacetone and some other S-diketones such as a,a,a-trifluoroacetylacetone. However, unlike the monomeric chelate products from acetylacetone and its fluorinated derivative, the enolate species of dimedone (9) cannot form chelates and as the complex is polymeric, it cannot be distilled and is more labile to hydrolysis, as might be expected for an unstabilized alkoxide. However, dimedone has a gas phase deprotonation enthalpy of 1418 9 kJmoD while acetylacetone enol (the more stable tautomer) is somewhat less acidic with a deprotonation enthalpy of 1438 10 klmoD Accordingly, had beryllium acetylacetonate not been a chelate, this species would have been more, not less, susceptible to hydrolysis. There is a formal similarity (roughly 7r-isoelectronic structures) between cyclic S-diketonates and complexes of dimedone with benzene and poly acetylene (10). The difference between the enthalpies of formation of these hydrocarbons is ca... 

There are several relevant aspects of rhodium enolate chemistry. Rh(I) catalyzes the isomerization of allylic aUtoxides to enolates. We welcome this reaction done in a direct thermochemical context analogous to the related isomerizations of allyl halides , ethers and allyl amines . From the enthalpies of formation of allyl alcohol and propanal (—185.6 and —124.5 kJmol ), and their respective gas phase deprotonation enthalpies (1564 and 1530 kJ mol 252,253 jj. jjg concluded that the rearrangement of the allyloxide to propen-1-olate is exothermic by 95 kJmoR. ... 

If the 2-pK concept is applied, one gets the difference in enthalpies of the deprotonation and protonation reactions from ... 

One might also have thought that ion chemistry would have provided the enthalpy of formation of an AT,A -dialkylated enamine. However, we know of no measured appearance potential of any immonium ion [R R C=NR R ] for which there is also a measured deprotonation enthalpy that must result in an enamine. That is, R and R cannot be H, and at most one of R and R may be. The simplest cation that could qualify is [MeCH=NMe2] for which a deprotonation enthalpy is known , but we know of no appearance potential of this ion from Mc2CHNMe2 or from any other source. That is, what is the energy of the simple fragmentation process... 

The tervalent tautomer (162) of dimethyl phosphonate (163) has an enthalpy 6.5 kcal mol higher than (163), as estimated from gas-phase studies of deprotonation, relative to dedeuteriation, of (164). The value, although lower than previous estimates, is high enough to explain why tautomers like (162) have not been observed. They exist, however, as ligands, e.g., in (165) and (166). ... 

---