THE BASICITY OF ETHERS

Ethers are weakly basic. The basicity is due to the presence of the unshared electrons that are present on the oxygen atom of ethers. 

---

Tackling the basics of ethers Finding out what ethers do... 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

Checking out alcohols How they look and what they re called Taking a look at the synthesis and reactions of alcohols Tackling the basics of ethers Finding out what ethers do Clarifying the spectra of alcohols and ethers... 

In the present paper, we examine the influence of structural variation within series of crown ether carboxylic acid and crown ether phosphonic acid monoalkyl ester carriers upon the selectivity and efficiency of alkali metal transport across three types of liquid organic membranes. Structural variations within the carriers include the polyether ring size, the lipophilic group attachment site and the basicity of ethereal oxygens. The three membrane types are bulk liquid membranes, liquid surfactant (emulsion) membranes and polymer-supported liquid membranes. 

These show marked similarities to their acyclic counterparts, e.g. tetrahydrofuran closely resembles diethyl ether. The minor differences which arise between these two types of compounds are due to the less sterically hindered nature of the heteroatoms in the cyclic compounds. The basicities of tetrahydropyrrole (pHTa 10.4), tetrahydrofuran (-2.1) and... 

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

The total yield of products from alkanecarboxyhc acids increases, in most cases, by addition of anhydrous hydrogen fluoride. The optimum hydrogen fluoride concentration is much higher than catalytic and is related to the basicity of a carbonyl group. A mechanism for the formation of both 1,1,1-trifluoroalkanes and bis(l,l-difluoroalkyl) ethers has been proposed [206] (equation 102)... 

The basicities of amines, ethers, and carbonyl compounds are invariably decreased by fluonnation. 2,2,2-Tnfluoroethylamine (p f = 3.3 [61]) and C 5NH2 = -0.36 [62]) are about 10 times less basic than CH3CH2NH2 and CgH5NH2, respectively, and (CF3)2CHNH2 (p j, = 1 22 [71]) is over 10 times less basic than 1-C3H2NH2. The relative gas-phase acidities in Table 15 illustrate the large effect of fluonnation. Perfluoro-rerf-amines (R )3N and ethers R Rf have no basic character m solution [, 74], and CF3COCF3 is not protonated by superacids [72]. 

Hydrogenation of carbonyls, or incipient carbonyls such as phenols (86), in lower alcohol solvents may result in the formation of ethers. The ether arises through formation of acetals or ketals with subsequent hydrogenolysis. The reaction has been made the basis of certain ether syntheses (45,97). Reaction of alcohols with carbonyls may be promoted by trace contamination, such as iron in platinum oxide (22,53), but it is also a property of the hydrogenation catalyst itself. So strong is the tendency of palladium-hydrogen to promote acetal formation that acetals may form even in basic media (61). 

This crude product is dissolved in 100 ml of dilute hydrochloric acid, the acid solution is extracted with ether, and the aqueous layer is made basic with sodium hydroxide solution (3N) in the presence of ether (approximately 250 ml). The ether layer Is separated, dried over potassium hydroxide and evaporated to a white solid. Additional purification by repeating the formation of the hydrochloric acid salt and reprecipitation of the base is carried out. When purified in this manner, followed by drying at 80°C in vacuo over phosphorus pentoxide, 2-chloro-11-(4-methyl-1-piperazinyl)dibenz[b,f] [1,4]oxazepine, li/IP 109° to 111°C, is obtained. 

In the case of the hexacarbonyls, the rate-expression contains not only the same type of first-order term but in addition one second-order overall. For good entering groups (but not CO, for example) the rate expression contains a term strictly first-order in both the complex and the entering nucleophile. The first-order rates of CO exchange are practically identical with the rates of substitution in hydrocarbon solvents, but there is nevertheless some acceleration in ether (THF, dioxan) solutions. This solvent-dependence is not so well-marked ° as in the case of nickel tetracarbonyl. The second-order rate of substitution very strongly depends upon the basicity of the entering nucleophile... 

To synthesize new surfactants, having incorporated both structural elements, the known siloxanyl modified halogenated esters and ethers of dicyclopentadiene [5] were treated with different amines according to the reaction scheme. Triethylamine yielded quaternary ammonium salts directly. Alternatively, after reaction with diethylamine or morpholine, the isolated siloxanyl-modified tertiary amines were also converted to quaternary species. To obtain anionic surfactants, the halogenated precursors were initially reacted with n-propylamine. In subsequent reaction steps the secondary amines formed were converted with maleic anhydride into amides, and the remaining acid functions neutralized. Course and rate of each single reaction strongly depended on the structure of the initial ester or ether compound and the amine applied. The basicity of the latter played a less important role [6]. 

Although transition metal-catalyzed allylic alkylation has become one of the most powerful methods in chemical synthesis, the formation of ether bonds using this process has been slow to evolve.119-121 The main reasons for this disparity are the lower nucleophilicity and higher basicity of oxygen nucleophiles, particularly those derived from aliphatic alcohols, compared to their carbon or nitrogen analogs. However, this notion has rapidly been revised, as recent advances in the O-allylation area have largely addressed the issue of the reactivity mismatch between the hard alkoxide and the soft 7r-allylmetal species to provide a considerable body of literature. 

It has been known for some time that the basicities of a heteroatom decrease upon a-silyl substitution [12], For example, alkyl silyl ethers (R3Si-0-R ) are less basic than dialkly ethers. Silylamines are weak bases compared to alkylam-ines. This electron-withdrawing effect of silyl groups has been explained in terms of the interaction between low lying vacant orbitals such as 3d orbitals of silicon or a orbitals with the nonbonding p orbitals (lone pairs) of the heteroatom (Fig. 4). This interaction decreases the HOMO level which in turn lowers the basicity of the heteroatom. Such effect may also cause the increase of the oxidation potentials, but little study has been reported on the electrochemical properties of this type of compounds. 

See, for a basic bibliography T. Shono in Supplement E The Chemistry of Ethers, Crown Ethers, Hydroxyl Groups and their Sulfur Analogues (Ed. S. Patai), Wiley, Ghichester,... 

ESI has been used to study the effect of the basicities of the side arms on the abilities of these lariat ethers to extract metal ions from solution. A series of compounds of structure 9 were studied and the authors conclude that the lariat ether with the least basic side arm binds best to the smaller cations (Table 6). Specifically for Li+, it was found that it is best bound to the lariat ether with the ester side arm. It is proposed that electrostatic repulsion of the lariat ether side arm and the electron-rich jt cloud of the adjacent aromatic ring play a dominant role in determining cation selectivities. The least basic side arm has lower electron density and thus these compounds have smaller cavity sizes and are more selective for smaller ions. ... 

---