Alkylamine hydrates

Although the topology of the hydrogen-bonded water host structure is closely related in the gas hydrates, the alkylamine hydrates, and the alkylammonium salt hydrates, with the pentagonal dodecahedron shown in Fig. 21.3 playing a prominent role, the interactions between the host and guest spedes are different. 

In the alkylamine hydrates, with one exception, the functional group of the amine is hydrogen-bonded to or included with the water host lattice. These compounds are not true clathrates i.e., they, are sometimes referred to as semi-clathrates [433]. 

Hydrates with related clathrate-like structures are formed by tetramethyl and tetraethyl ammonium salts. The tetramethyl and tetraethyl ammonium salts form hydrates in which the cations occupy voids in hydrogen-bonded structures formed by the anions and the water molecules. Unlike the gas hydrates, the alkylamine hydrates, and the butyl and isoamyl ammonium salt hydrates discussed in the next section, these cage-like structures do not feature the pentagonal dodecahedron as a common structural component. 

The alkylamine hydrate inclusion compounds form a large variety of structures. Phase diagram studies at the end of the last century [7651 (see Tkble 21.3), and an extensive series of phase diagram studies carried out between 1970 and 1981 [798], showed that aliphatic amines form a large variety of hydrates. Relatively few of these have been studied by X-ray crystal structure analysis and none by neutron diffraction [433, 799]. These hydrates differ from the gas hydrates and the alkyl ammonium salt hydrates in that there appear to be no definite structural types into which several hydrates can be classified. Hitherto, a different crystal structure has been observed for every alkylamine hydrate studied. In this respect, they resemble low hydrated crystals. 

The majority of alkylamine hydrates are semi-clathrates . As Ihble 21.4 indicates, in all alkylamine hydrates except tert-butylamine, the amine molecule is hydrogen-bonded to the water framework, which, nevertheless, retains the cagelike structure. For this reason, these compounds are described as semi-clathrates . 

Table 8. Hydration Free Energies of Some Protonated Alkylamines, Alkyldiamines, and Peptides3...

The dichalcogenides of group VI do not readily form intercalates with organic molecules. But n-alkylammonium compounds can be prepared by ion-exchange reactions of the hydrated sodium intercalation compounds such as Na j(H20)o g M0S2. Alkylammonium compounds prepared by this route show lattice expansions that depend on the alkyl chain length, very similar to the MX2-n-alkylamine systems described above. This raises the fundamental question whether protonated amines are the actual intercalated species in both the cases, involving reduction of the host. In fact, this constitutes the basis of the new model for the intercalation reaction in MX2 proposed by Schollhorn (1980). 

Pickering SU (1893) The hydrate theory of solutions. Some compounds of the alkylamines and ammonia with water. Thins Chem Soc 63 (I) 141 -195... 

Desnoyers, J.E. and Arel, M. Apparent molal volumes of n-alkylamine hydrobromides in water at 25°C hydrophobic hydration and volume changes. Can. J. Chem. 1967, 45, 359-366. 

The polarity of (012)2 has been determined in a molecular beam electric resonance spectrometer and is said to be consistent with either an L- or a T-shaped structure. Fluorescence spectra have been obtained from both monomeric and dimeric CI2 and Br2 molecules in inert-gas matrixes at 15 The i.r. spectra of the molecular complexes of CI2 and Brj with NH3 have been obtained at ca. 72 at or above 150 K reactions occur with the formation of ammonium salts. Raman spectra have also been reported by the same group of workers " " for CI2 and Br2 complexes with NH3, alkylamines, and pyridine. From the results, simple treatments of normal co-ordinates and of the extent of charge transfer were performed. Raman spectra in the range 20—600cm of the hydrates of CI2 and Br2, obtained at 77 K, show that the interaction between the donor and either acceptor is remarkably weak. ... 

If the hydrophilic interaction effect is significant (in low molecular alkylamines [4] and peralkylonium salts [29]), every particular guest builds its own" structure- When, however, the hydrophobic interaction prevails (in the molecules considered), hydrate formation, depending first of all on the molecule shydrophobic part size and configuration, becomes less sensitive to the hydrophilic interaction changes. 

The same problem arises to an even greater extent when alkyl substitution occurs very close to the acid-base centre, as in the alkylamines, since the separation into internal and external effects will be a poor approximation in this case. Thus in aqueous solution the sequence of acid strength is NH4 > Me3NH > MeNHj > Me2NHj. The unexpected position of trimethylamine cannot be explained in terms of an internal effect of the methyl groups, but could arise if the hydration of the ions or molecules introduces a complicating factor. This explanation is supported by the observed entropy changes (Table 5) and also by measurements in aprotic solvents we shall return to the problem in Chapter 10 in connection with basic catalysis by amines. 

The 4PMH is not stable in aqueous solution. After hydration, another proton is consumed, yielding a product which slowly hydrolyzes to an alkylamine. Methylamine has been detected after radiolysis of l-alkyl-4-carbamidopyridinium ions. (Eq. 8). 

A problem with sodium hypochlorite is its ability to chlorinate some compounds to form volatile reaction products. Ammonia and alkylamines, for example, can be chlorinated to form chloramines. The problem can be avoided by removing alkaline nitrogen compounds from the gas stream with an acid wash prior to the sodium hypochlorite scrubber. An alternative approach described by Valentin (1990) is the use of a catalyst in the scrubber liquid to promote oxidation. Hydrated nickelic oxide, which is formed in solution from nickel sulfate and sodium hypochlorite at a pH of 9 to lO.S, is recommended. A nickel concentration of 50-75 ppm is said to be effective, making removal of up to 95% of the odorous compounds possible in a single stage. 

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