Piperidine basic strength

The results in Table 2 show that the pyridine is less active than any of the X zeolites and Ge faujasite except the lithium form which shows slightly lower activity, whereas all Y zeolites show lower activity than pyridine. Piperidine, however, is more active than any of the zeolite samples studied here. From this comparison, it appears that, most of the basic sites of the zeolites must have pK<10.3. However, the fact that zeolites are also active for catalyzing the condensation of benzaldehyde with ethyl malonate, indicate that these samples have some basic sites with pK< 13.3. On a quantitative bases, and comparing the activity of zeolites for condensation with ethyl cyanoacetate, ethyl acetoacetate and ethyl malonate (Fig. 2), we can conclude that most of the basic sites of the zeolite have pK<9.0 with a sensible amount with 9.0basic strength of different solid base catalysts. 

Inspection of Tables XIII-XV shows that the basic strength of the probe molecules most used to determine the acid strength of metal oxides seems to be ammonia < n-butylamine pyridine < trimethylamine < piperidine <... 

When these bases are compared in terms of their respective proton affinities, the order of basic strength is ammonia < n-butylamine < pyridine < trimethylamine < piperidine < triethylamine, which is the same order observed with microcalorimetric measurements. In fact, plots of the initial differential heat of adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica-alumina and on silica as a function of the proton affinity give linear correlations, as can be seen in Fig. 7 (18, 105). 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

Pure alumina catalyst prepared either by hydrolysis of aluminum isopropoxide or by precipitation of aluminum nitrate with ammonia, and calcined at 600-800°, contains intrinsic acidic and basic sites, which participate in the dehydration of alcohols. The acidic sites are not of equal strength and the relatively strong sites can be neutralized by incorporating as little as 0.1 % by weight of sodium or potassium ions or by passing ammonia or organic bases, such as pyridine or piperidine, over the alumina. 

Jensen et al. [23], studied more than 60 different hydroxylated piperidine and hexahydropyridazines, and the measured stereoelectronic effects are summarized in Table 5.1. The value of each substituent is the basicity decreasing effect in pK units. As can be seen, the axial OH decreases the base strength of the amine by 0.5 and 0.2 units when it is in the p- and y-positirai, respectively, relative to the amine, while the equatorial OH decreases the pKa by 1.3 and 0.6 in those instances. Thus, the equatorial OH is significantly more electron-withdrawing than the axial OH. 

As early as 1933 Kilpatrick and Kilpatrick [499] were able to compare the relative acid strengths of a number of compounds in acetonitrile (AN) and to compare that order with the order of the pK s in water. One of the first quantitative determinations was a study by H.K. Hall Jr. in 1957 [190] that examined steric effects on the basicities of cyclic amines in water and AN. This study yielded the pK s of 15 methyl piperidines and some related amines at 30 and 59.6°C, plus information on the thermodynamic parameters. Additional early acidity studies in AN were carried out by Kolthofl and his co-workers [500-502] and by Coetzee and Padmanabhan [503]. 

---