Acid-base reactions deprotonation

Tab. 13.4 Thermochemistry of Selected Acid/Base Reactions Deprotonation Enthalpies (kcal/mol) for Deprotonations of /PrOH with Various Organolithium Compounds and Lithium Amides...

Redman, E.W. Morton, T.H. Product-Determining Steps in Gas-Phase Broen-sted Acid-Base Reactions. Deprotonation of 1-Methylcyclopentyl Cation by Amine Bases. J. Am. Chem. Soc. 1986, 108,5701-5708. 

The acid-base reactions that occur after the amide bond is broken make the overall hydrolysis irreversible m both cases The amine product is protonated m acid the car boxylic acid is deprotonated m base... 

Most of the reactions of ester enolates described so far have centered on stabilized eno lates derived from 1 3 dicarbonyl compounds such as diethyl malonate and ethyl ace toacetate Although the synthetic value of these and related stabilized enolates is clear chemists have long been interested m extending the usefulness of nonstabilized enolates derived from simple esters Consider the deprotonation of an ester as represented by the acid—base reaction... 

The hydrogenation of simple alkenes using cationic rhodium precatalysts has been studied by Osborn and Schrock [46-48]. Although kinetic analyses were not performed, their collective studies suggest that both monohydride- and dihydride-based catalytic cycles operate, and may be partitioned by virtue of an acid-base reaction involving deprotonation of a cationic rhodium(III) dihydride to furnish a neutral rhodium(I) monohydride (Eq. 1). This aspect of the mechanism finds precedent in the stoichiometric deprotonation of cationic rhodium(III) dihydrides to furnish neutral rhodium(I) monohydrides (Eq. 2). The net transformation (H2 + M - X - M - H + HX) is equivalent to a formal heterolytic activation of elemental... 

In acid-base catalysis there is at least one step in the reaction mechanism that consists of a generalized acid-base reaction (a proton transfer between the catalyst and the substrate). The protonated or deprotonated reactant species or intermediate then reacts further, either with... 

It is interesting to note that in the reaction of (188) with [TcOCU] , the 0x0 group is replaced by a doubly deprotonated amino group. The mixed imido-amido Tc complex [Tc(app)Cl2(Happ)j (263) was synthesized and structurally characterized with Re. Neutral, mixed amino-phosphino ligands can stabilize the soft [Tc=N] + core in the expected way, whereas the harder [Tc=0] + core imposes subsequent acid/base reactions. Stabilization is then mainly achieved by deprotonation, in order to compensate for the relatively high charge. ... 

Perrone et al. (2001) modelled Ni(II) adsorp-tion to synthetic carbonate fluoroapatite (CaI0 ((P04)5(C03))(0H,F). The solid phase had a pHIEP of 6.3 and a ZPC of 6.4 with an SSA of 8.8m2/g, an estimated sorption site density of 3.1 sites/nm2. They conducted 8-day isotherms in closed vessels at Ni concentrations of 5 x 10-10 to 1 x 10 8 M, constant I (0.05, 0.1 or 0.5 M), constant solid phase concentrations of 10 g/dm3 at pH values of 4 to 12. As Ni sorption occurred, no significant release of Ca was seen. Sorption was reversible. Rather than precisely characterize surface functional groups, they elected to describe their sorbent surfaces using acid-base reactions for the average behaviour of all sites involved in protonation and deprotonation. Potentiametric titration data were used to estimate the constants with the FTTEQL computer code ... 

While the necessity of having both and acid and a base present in order to have an acid-base reaction is axiomatic, it is surprising how often this concept is neglected. However, if these conditions are met, then a wide variety of organic compounds can donate protons to appropriate bases (they are deprotonated) and a wide variety of compounds can accept protons from appropriate acids (they are protonated). 

Under different reaction conditions, esters still other than the ones shown in Figure 10.53 can be employed for the acylation of ester enolates. In such a case, one completely deprotonates two equivalents of an ester with LDA or a comparable amide base and then adds one equivalent of the ester that serves as the acylating agent. The acylation product is a /3-ketoester, and thus a stronger C,H acid than the conjugate acid of the ester enolate employed. Therefore, the initially formed /3-ketoester reacts immediately in an acid/base reaction with the second equivalent of the ester enolate The /3-ketoester protonates this ester enolate and thereby consumes it completely. 

Redox reactions, i.e., when electron exchange occurs, are considered charge transfer reactions in solutions. It is frequently accompanied by the transfer of heavier species (ions). Processes in which charged species react in solutions but no electron exchange takes place, e.g., proton exchange in protonation-deprotonation or acid-base reactions are not called charge transfer reactions. 

In a first approximation supra-Cp metal complexes can be prepared the same way as normal or other Cp-metal and organo-metal bonds in general. The methods used most often (see Appendix) are the metathesis reaction [Eq. (1)] followed in number by oxidative additions (Eq. (2)] and metallation/deprotonation reactions [Eq. (3)]. The latter is especially important for the cyclpentadienyl alkali metal compounds. A useful variation of reaction (3) is the formation of CpTl in an acid/base reaction from cyclopentadiene and thallium ethoxide [Eq. (3b)]. This represents a convenient route to cyclopentadienylthallium compounds, which are also valued (in place of Cp alkalis) as mild Cp-transfer reagents for the synthesis of difficultly isolable cyclopentadienyl derivatives (77). 

Acid/base chemistry is also an obvious reaction path for selective detection of a variety of materials. For example, acid/base reaction can be made to occur within a molecularly imprinted pocket to assist in differentiating the molecule of choice before deprotonating it to produce an ion pair that then interacts with the evanescent field. This approach is currently being used to selectively detect TNT to levels in the low parts-per-trillion. 2,4,6-trinitrotoluene, a potent explosive, is also a weak acid having a p/sTa of approximately 14.5 [38]. A TNT derivative is synthesized with a tether to a silane. The silane is chosen so the group can be... 

Why is this acid-base reaction important After ingestion, aspirin first travels into the stomach and then the intestines. In the acidic environment of the stomach, aspirin remains in its neutral form, but in the basic environment of the small intestine, aspirin is deprotonated to form its conjugate base, an ion. 

What bases are used to deprotonate a carboxylic acid As we learned in Section 2.3, equilibrium favors the products of an acid-base reaction when the weaker base and acid are formed. Because a weaker acid has a higher pK, the following general rule results ... 

A similar acid-base reaction does not occur when cyclohexanol is treated with NaOH because organic alcohols are much weaker organic acids, so they can only be deprotonated by a very strong base such as NaH. NaOH is not strong enough to form significant amounts of the sodium alkoxide. 

Similar results have been obtained by Baciocchi for the deprotonation of a-substituted 4-methoxytoluenes by 2,6-lutidine and NOs in acetonitrile [145]. In this study, the same values of the Bronsted coefficient (a = 0.24), and of the deuterium kinetic isotope effect (kn/kD = 2.0 for 4-methoxytoluene radical cation) have been obtained with the two bases these results point again towards a highly asymmetric transition state with a very small amount of C-H bond cleavage. Moreover, values of 0.53 and 0.66 eV have been calculated for the intrinsic barrier of the reactions of the radical cations with NO3" and 2,6-lutidine, respectively, again comparable with those observed for acid-base reactions involving carbon acids [140, 141]. 

No example has so far been reported of a shuttling process controlled by electron transfer chemical reactions. There are, however, very interesting examples of shuttling processes controlled by acid/base reactions. One case is that of the previously discussed compound 13 " (see Figure 14), in which the shuttling of the macrocycle component can be controlled not only electrochemically, but also by protonation/ deprotonation of the benzidine unit [43]. 

Charge stabilisation of the anion, which is formed after deprotonation, is still important, but to a lesser extent than when the acid/base reaction is reversible. If butyllithium is used as a base, then very unreactive hydrogens may be removed, which result in unstabilised anions. However, the conjugate acid is butane, which is a very, very weak acid also, as it is a gas, it leaves the reaction vessel. Thus, if butyllithium is used, the deprotonation becomes effectively irreversible. 

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