Ketones base strength

A striking result of this reinvestigation (128, 129) is the observation that the ratio of the product ketone to the acetylene formed from a-bromo-p-aminostyrene is a function of the pH (Table Vll) but that the rate at which they are formed is not. As the pH increases from 3.9 to 13.1, the relative yield of acetylene increases from 16% to 85%. Therefore, the acetylene formation by elimination of a proton from the vinyl cation (path b in route D in Scheme XI) is more susceptible to an increase in base strength than is ketone formation via the enol (path a). This observation is a rare case of pH control over product composition in a 1-El reaction. 

Early in 1963 Culbertson and Pettit6 reported the base strengths of ten simple polycyclic benzenoid aldehydes and ketones and attempted to relate the pKa values to molecular structure. They were able to show the existence of a linear correlation between -pKa and the protonation it energy, that is, the gain in n energy of the protonated over the neutral compound. In their treatment... 

The observations that secondary amines, (Rf)2NH, do not react with boron trifluoride, hydrogen chloride or trifluoroacetic acid [13] also serve to indicate a lack of basic properties. Similarly, tertiary perfluoroalkylamines are quite without basic properties. Moreover, the oxygen atoms in perfluoroalkyl ethers and ketones are poor donors this is exemplified by the fact that hexafluoroacetone cannot be protonated by superacids in solution. Such findings parallel similar observations with unsaturated derivatives where the base strength is considerably reduced in, for example, perfluorop3Tidine or perfluoro-quinoline [14] in comparison with the parent compounds. 

A suitable solid base must have the appropriate base strength for the reaction under investigation. If the initial reaction step is the removal of a proton from a reactant of the form R1-CH2-R2 then the acidity of the proton to be removed depends on the identity of the R) and R2 groups (Table 1). The solid base selected should have sufficient base strength to carry out the reaction but should not have excessive base strength as this may lead to rapid catalyst deactivation or to side-product formation. For aldehyde and ketone condensation reactions therefore with a p.Ka of 19.7 - 20 a strong base is required but not a superbase material. Caustic can be used to carry out reactions with reactants with the removable proton having a pA a of up to around 20. 

The impetus for low temperature aldol processes in the absence or minimization of organic solvent has resulted in several new approaches. For example, Dewa et al. describe an aldehyde-ketone condensation catalyst that can be suspended in water, prepared by reacting lanthanum tris(isopropoxide) with anthracenebisresorcinol (ABR). A polymeric aquo complex with several 1,2-linked ABRs and two (LaOH) + groups is formed, coordinating 30 water molecules and relatively stable to decomposition. The base strength of this complex approached that of aqueous NaaCOs. 

As might be expected the base strengths of aldehydes seem to fall between those of the corresponding ketones and carboxylic acids. They are too weak to be titrated in acetic acid and the aliphatic members are too unstable to acid to have been studied successfully as Hammett bases. Most of our knowledge of pK s in this series comes from the careful studies of Schubert et al. (57,298,301-303) and the more recent report of Yates and Stewart (371) all of which are concerned with substituted benzaldehydes. 

Figure 11-10 Titration ot a mixture of acids with tetrabutylammonium hydroxide in methyl isobutyl ketone solvent shows that the order of acid strength is HCI04 > HCI > 2-hydroxybenzoic acid > acetic acid > hydroxybenzene. Measurements were made with a glass electrode and a platinum reference electrode. The ordinate is proportional to pH. with increasing pH as the potential becomes more positive. [D. B. Brass and G. E. A. Wyld. Methyl Isobutyt Ketone as a Wide-Range Solvent for Titration of Acid Mixtures and Nitrogen Bases," Anal Chem. 1957, 29.232.]...

Hull and Conant in 1927 showed that weak organic bases (ketones and aldehydes) will form salts with perchloric acid in nonaqueous solvents. This results from the ability of perchlonc aad in nonaqueous systems to protonate these weak bases. These early investigators called such a system a superacid. Some authorities believe that any protic acid that is stronger than sulfunc aad (100%) should be typed as a superaad. Based upon this criterion, fluorosulfuric arid and trifluoro-methanesulfonic acid, among others, are so classified. Acidic oxides (silica and silica-aluminai have been used as solid acid catalysts for many years. Within the last few years, solid acid systems of considerably greater strength have been developed and can he classified as solid superacids. 

Similar correlations between the acid-base properties of catalysts and activ-ity/selectivity were earlier observed in the rearrangement of simple oxiranes (refs. 5-8). In our case it seems reasonable to suppose that the observed changes are due to the different competing mechanisms discussed above. WO, with strong acidic sites in high concentration, is able to form the carbenium ion. Since the density and the strength of the basic sites on WO are low, formation of the double-bonded surface species depicted in Fig. 3 has only a low probability. The single-bonded open carbenium ion is then mainly transformed to ketone 3. In harmony with this, the isomers exhibit identical selectivity, a... 

The curves for log(kK/s ) and log(kK/s ) of acetophenone are parallel in the range pAT << PH << pAT and the vertical distance between them then equals pAtE = log(kK/s 1) — log /s-1). Most ketones are very weak bases, pAt < 0, so that the parameter does not affect the shape of the pH-rate profiles in the range pH > 1. Base catalysis of ketonization saturates at pH = pAr , while the rate of enolization continues to rise, so that the curves for kE and kK eventually cross at higher pH. At still higher pH, the rate constant kE exceeds that of kK = k o and kobs follows kK. The crossing point, for which kE = kK, lies at pH = pAT = 18.3 for acetophenone (Fig. 3), which is outside the accessible pH range when ionic strength I is limited to 0.1 m, but pA is readily calculated from Equation (2). 

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