Solubility of amines

Once neutralization capacity is considered, the next factor is the solubility of amine carbonates formed by neutralization. This characteristic varies by product and temperature. 

Evidence exists that the relative solubility of amines and inhibitors in heterogeneous oil-water systems could be decisive in formation of nitrosamines and blocking these reactions, Nitrosopyrrolidine formation in bacon predominates in the adipose tissue despite the fact that its precursor, proline, predominates in the lean tissue (5,6,7). Mottram and Patterson (8) partly attribute this phenomenon to the fact that the adipose tissue furnishes a medium in which nitrosation is favored, Massey, et al, (9) found that the presence of decane in a model heterogeneous system caused a 20-fold increase in rate of nitrosamine formation from lipophilic dihexylamine, but had no effect on nitrosation of hydrophilic pyrrolidine. Ascorbic acid in the presence of decane enhanced the synthesis of nitrosamines from lipophilic amines, but had no effect on nitrosation of pyrrolidine. The oil-soluble inhibitor ascorbyl palmitate had little influence on the formation of nitrosamines in the presence or absence of decane. 

Extraction of the amine surfactant using pure CO2 at 65°C and 150 bar has been attempted, but it has been found from TGA that there is completely no extraction at all. In 1985, Dandge et al. [4] have reported that the solubility of amines in supercritical CO2 generally decreases with increasing basicity (Kb). The threshold basicity, above which the compound becomes insoluble in CO2, was found to be about 10 9. As such, it is a foregone conclusion that the surfactant dodecylamine (Kb 10 4) is insoluble in pure supercritical CO2. This explains the zero extraction observed and thereby justifies the need of a modifier (methanol in this case). 

EtOAc affords an excellent yield of 19, which still has two acid-labile protecting groups. The successful conversion is driven by the poor solubility of amine hydrochloride salts in EtOAc, and precipitation of the product limits any significant further reaction of the molecules. 

Amines possess higher boiling points than non-polar compounds (of the same molecular weight), but lower boiling points than alcohols or carboxylic acids. Amines of all types can form hydrogen bonds with water. Consequently, the solubility of amines depends on size smaller amines are quite soluble in water, with borderline solubility reached at about six carbon atoms. In less polar solvents such as ether, alcohol, and benzene, amines are soluble. 

The N—H bond in primary and secondary amines, like the O—H bond of an alcohol, participates in hydrogen bonding. The N—H bond is not as polarized as the O—H bond and this results in weaker hydrogen-bonding intermolecuiar attractions in amines compared to alcohols. Primary and secondary amines have higher boiling and melting points than hydrocarbons and ethers but lower than alcohols. The solubility of amines in water is comparable to that of alcohols and ethers. 

Compare the water solubilities of amines with their amine salts. 

The high water solubility of amine salts is important in biological systems. The body s buffer systems neutralize and solubilize amines by conversion to amine salts. Intravenous drugs containing the amino group are usually administered in the form of the amine salt instead of the amine to achieve faster absorption into the body. 

Some free amine may remain in equilibrium with the salt. Tertiary amines lack the tautomerizable proton required for this reaction and are thus soluble. In addition, several workers have not that reaction 3 does not occur for less basic amines such as aniline which are soluble in supercritical C02 (67) These results suggest the existence of a basicity limit for solubility of amines (and possibly other compounds) in supercritical (and subcritical) CO2 which is thought to be roughly pKb > 9. (68)... 

The basicity of amines and the solubility of amine salts in water can be used to separate amines from water-insoluble, nonbasic compounds. Shown in Figure 10.2 is a flowchart for the separation of aniline from anisole. Note that aniline is recovered from its salt by treatment with NaOH. 

The basicity of amines and the solubility of amine salts in water can be used to separate amines from water-insoluble, nonbasic compounds. 

Tables 10.2 and 10.3 lists the physical properties of the aliphatic amines. The lowest molecular weight amine, ethylamine, boils at only 16.6 C and is thus available only as a compressed gas or as an aqueous solution. The other fifteen aliphatic amines have boiling points in the range of 32.4-213 C. The flash points of these amines are low with the highest being only 70 F for the tributylamine. These short-chain amines are readily soluble in water and most organic solvents. The solubility of amines decreases with increasing temperature, for example, triethylamine is completely soluble in water below 18 C but is only partially soluble above this temperature. The lower alkylamines have the characteristic ammonia odor which decreases with increased substitution onto the nitrogen atom.

Amines extract rare earths in reversed order compared to organophosphates. Aliphatic amines form salts with acids in aqueous solution and precipitate the hydroxides of metals. The solubility of amines in water decreases with increase in molecular weight. Primary amines extract from sulfate solutions while the tertiary amines extract from nitrate solutions. The lighter rare earths are preferentially extracted by amines and when used with aqueous soluble aminocarboxylic acid, chelating reagents preferentially extract the heavy rare earths. 

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