Cyclohexylamine, basicity

The equilibrium shown m the equation lies to the right =10 for proton transfer from the conjugate acid of aniline to cyclohexylamine making cyclohexylamine 1 000 000 times more basic than aniline... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

This common technique, and many others, failed in reductive alkylation of 13 by 12 to 14, and, in an unusual variation, Campbell and Lavagnino 10) obtained satisfactory results only with an excess of aniline serving as both reactant and solvent. Normal procedures were satisfactory when the more basic cyclohexylamine was the reactant. 

Various amines find application for pH control. The most commonly used are ammonia, morpholine, cyclohexylamine, and, more recently AMP (2-amino-2-methyl-l-propanol). The amount of each needed to produce a given pH depends upon the basicity constant, and values of this are given in Table 17.4. The volatility also influences their utility and their selection for any particular application. Like other substances, amines tend towards equilibrium concentrations in each phase of the steam/water mixture, the equilibrium being temperature dependent. Values of the distribution coefficient, Kp, are also given in Table 17.4. These factors need to be taken into account when estimating the pH attainable at any given point in a circuit so as to provide appropriate protection for each location. 

AVT Barg BD BDHR BF BOF BOOM BOP BS W BSI BTA Btu/lb BW BWR BX CA CANDUR CDI CFH CFR CHA CHF CHZ Cl CIP CMC CMC CMC COC All-Volatile treatment bar (pressure), gravity blowdown blowdown and heat recovery system blast furnace basic oxygen furnace boiler build, own, operate, maintain balance of plant basic sediment and water British Standards Institution benzotriazole British thermal unit(s) per pound boiler water boiling water reactor base-exchange water softener cellulose acetate Canadian deuterium reactor continuous deionization critical heat flux Code of Federal Regulations cyclohexylamine critical heat-flux carbohydrazide cast iron boiler clean-in-place carboxymethylcellulose (sodium) carboxy-methylcellulose critical miscelle concentration cycle of concentration... 

For R = benzylamine (B) and R — cyclohexylamine (C) the amounts of BB and CC and of the BC cross product formed as the relative concentration of the reactants varies is shown in Figure 3. Clearly whilst for an approximately 50 50 reactant mixture the amount of CC formed has dropped to practically zero that of the cross product (BC) is still hi. The interpretation suggests that the less baisic atmine is more readily intercalated auid essentially remains urprotonated whilst the more basic amine, althou present in much smaller concentration, removes nearly all of the available protons. Hence the major reaction is between protonated B and non-protonated C, yielding BC (23,23). 

The aniline molecule is thus stabilised with respect to the anilinium cation, and it is therefore energetically unprofitable for aniline to take up a proton it thus functions as a base with the utmost reluctance (p Ka = 4 62, compared with cyclohexylamine, pKa = 10-68).Thebase-weakening effect is naturally more pronounced when further phenyl groups are introduced on the nitrogen atom thus diphenylamine, Ph2NH, is an extremely weak base (pKa = 0-8), while triphenylamine, Ph3N, is by ordinary standards not basic at all. 

The pathway of the Kabachnik-Fields reaction depends on the nature of the substrates. The amine and hydrophosphoryl compound form a complex in which either one of the partners may react with the carbonyl compound. Often, the basicity of the amine determines the reaction pathway. Weakly basic amines such as anilines, which can act as proton donors, favour the formation cf an imine, whereas alkylamines such as cyclohexylamines do not form imines ... 

There is an extra R.E. of 6 kcal but the basicity of aniline is also very much smaller than that of an aliphatic primary amine, such as cyclohexylamine. This special resonance does not exist any longer in the anilinium ion because the free pair has now become a bonding pair by proton addition. The base constant Kb is related to the (free) energy AF of the proton addition according to the expression AF = —RTlnKb. 

For example, when cyclohexylamine is treated with aqueous HCl, it is protonated, forming an ammonium salt. Because the ammonium salt is ionic, it is soluble in water, but insoluble in organic solvents. A similar acid-base reaction does not occur with other organic compounds like alcohols, which are much less basic. 

A-(2,3-epoxypropyliden)cyclohexylamine-A-oxide reacts with 1 equiv. Ic at low temperatmes to form a nitrosonium ion which imdergoes a new type of cyclization reaction on treatment with olefins then KCN, leading to cw-fused cycloadducts in moderate to high yield (Sch. 28) [56], Further basic and acidic treatment furnishes a-methylidene-y-lactones. 

The results with cyclopropyl-substituted amines also reveal the operation of several effects. Cyclopropylamine (5) is significantly less basic than cyclohexylamine in solution , as expected due to -withdrawal from the protonated ammonium ion. However, there is also evidence for significant stabilization of the free amine by u-electron donation to cyclopropyl (see Section III.D) so the resultant low basicity is evidently due to a combination of these effects, which operate in the same direction. p-Cyclopropylaniline is somewhat more basic than the p-isopropyl analog S and this effect evidently arises from 7c-donation to the ring, stabilizing the protonated conjugate acid. The gas phase basicity of 5 is discussed in Section V.I. 

Arylamines have a larger positive AG° for protonation and are therefore less basic than alkylamines, primarily because of resonance stabilization of their ground state. Electrostatic potential maps show that lone-pair electron density is delocalized in aniline compared to cyclohexylamine but that the corresponding ammonium ions localize charge in the same way. 

The gas-phase exothermicity is due to the enhanced resonance stabilization of aniline compared to benzene for the nonzwitterionic amino acids, as found in the gas phase. On the other hand, the aniline resonance stabilization is lost in the zwitterionic amino acids of the solid phase and thus the reaction is essentially thermoneutral. This is, of course, related to the weak basicity of aniline compared to related nonaromatic bases such as cyclohexylamine, as exhibited by the ca 50 kJ mol-1 exothermicity of the formal reaction 29. 

A second factor which obviously plays a role in the adsorption of molecules on the hydrogenation sites is the Jt electron density. This factor was considered to be predominant in the study of Moreau et al. [10]. The role of the x electrons can explain the fact that, in the study of Nagai et al [53], cyclohexylamine and piperidine did not fit the linear correlation between the adsorption coefficient and the gas phase basicity the adsorption coefficient of these saturated nitrogen compounds was about twice lower than expected on the basis of the above mentioned correlation. In our experiments, the high adsorption coefficient of the pyrrolic ring can be related to the very high n character of this heterocycle [59]. 

Chan et al.11 preferred cyclohexylamine as the basic modifier in the analysis of drugs of abuse on pellicular silica gel or aluminium oxide columns. Volatile amines were less suited, because the solvent composition changed with time, and secondary and tertiary amines absorbed UV-light at 254 nm. 

The catalysts were characterized by using various techniques. X-ray diffraction (XRD) patterns were recorded on a Siemens D 500 diffractometer using CuKa radiation. The specific surface areas of the solids were determined by using the BET method on a Micromeritics ASAP 2000 analyser. Acid and basic sites were quantified from the retention isotherms for two different titrants (cyclohexylamine and phenol, of p/Ta 10.6 and 9.9, and L ,ax 226 and 271.6 nm, respectively) dissolved in cyclohexane. By using the Langmuir equation, the amount of titrant adsorbed in monolayer form, Xm, was obtained as a measure of the concentration of acid and basic sites [11]. Also, acid properties were assessed by temperature-programmed desorption of two probe molecules, that is, pyridine (pKa= 5.25) and cyclohexylamine. The composition of the catalysts was determined by energy dispersive X-ray analysis (EDAX) on a Jeol JSM-5400 instrument equipped with a Link ISI analyser and a Pentafet detector (Oxford). 

Consistent with their low surface areas, magnesium orthophosphates contain few acid and basic sites (Table 1). All of them contain more acid sites than basic sites on their surface, as determined by the retention isotherms of cyclohexylamine and phenol, respectively. Only in the solid MgOc the population of basic sites is larger than that of acid sites. 

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