Diethyl ether, atom polarization

Tetrameric [MeLi]4 (1), [EtLi]4 (2) and [t-BuLi]4 (3) are white pyrophoric powders. While methyllithium is soluble only in polar solvents like diethyl ether, the two others are soluble even in non-polar hydrocarbons like hexane. In non-donating solvent the tetrameric aggregation is retained. Each of the four U3 triangles is /ra-capped by a Ca atom above the center of the equilateral metal triangle. Even in the solid-state none of the three tetramers adopts ideal symmetry (Figure 6). 

The saturated five-membered cyclic ether resembles furan but has four additional hydrogen atoms. Therefore, it is called tetrahydrofuran (THF). One of the most polar ethers, tetrahydrofuran is an excellent nonhydroxylic organic solvent for polar reagents. Grignard reactions sometimes succeed in THF even when they fail in diethyl ether. 

A reaction corresponding to Eq. (5-30) is the addition of nitrosyl chloride to alkenes such as cyclohexene or styrene [84, 85]. The reaction seems to be faster in polar solvents e.g. nitrobenzene and trichloromethane) than in less polar solvents e.g. toluene and tetrachloromethane). This is consistent with the view that the reaction involves an electrophilic attack of NO —Cl . The reaction was, however, also found to be very slow in diethyl ether. Presumably, this is due to strong bonding of the NO+ cation to the ether oxygen atom [84]. 

Ethers are relatively unreactive substances, which is why diethyl ether and tetrahydrofuran are widely used as solvents for organic reactions. However, the lone pairs on the oxygen atom are a source of reactivity. The oxygen atom may be protonated, and it reacts with Lewis acids. The increased polarity of the C-O bond then makes the neighbouring carbon atoms sensitive to nucleophilic attack. 

The compound [Mo(NO)2(CH3CN)4][BF4]2 is a moisture-sensitive dark crystalline emerald green substance. It is very soluble in acetonitrile and nitromethane but virtually insoluble in less polar organic solvents such as diethyl ether, chloroform, and benzene. The H NMR spectrum in CD3NO2 exhibits two singlets of equal intensity at 2.65 and 2.55 ppm, respectively, which is consistent with a cis arrangement of the two NO groups on the met atom. The IR spectrum of its Nujol mull shows absorptions at 2330 and 2300 cm (coordinated C=N), 1862,1755, and 1724 cm (coordinated NO) and may show weak absorptions at 2040 and 1957 cm" due to coordinated CO (impurity). 

A polar and protic solvent such as water can be contrasted with an aprotic solvent such as ether. The oxygen atom is 6-, and it can certainly coordinate to cations. The 6+ atom is a tetrahedral carbon, however, and, due to steric repulsion between the atoms, it is difficult for a negative anion or a negatively polarized atom to approach the carbon. Therefore, an aprotic solvent such as diethyl ether may solvate cations but not anions (see Figure 11.7) and there can be no separation of charges (ions). A simple experiment to examine the difference between these two solvents is to observe that sodium chloride will dissolve in water, but not in diethyl ether. The more polar the solvent is in... 

If the solvent is changed to diethyl ether, the negatively charged nucleophile is not well solvated, and it is easier to collide with the carbon atom. This suggests that an aprotic solvent is preferred in S 2 reactions because the initial reaction with the nucleophile and the carbon atom occurs more readily. The most common polar aprotic solvents used in Sn2 reactions are diethyl ether, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO Me2S=0), and dim-ethylformamide (DMF). An example that uses DMF as a solvent is the reaction of 1-chlorophenylmethane (benzyl chloride, 19) with potassium iodide to give benzyl iodide, 20. 

Silicones have been extracted from environmental samples with solvents such as hexane, diethyl ether, methyl isobutylketone, ethyl acetate, and THF, using either sequential or Soxhlet techniques (690-695). Silicones of a wide range of molecular weights and polarities are soluble in THF. This feature, coupled with its volatility and miscibility with water, makes THF an excellent solvent for the extraction of silicones from wet samples, ie, soils and sediments. Trace levels of silicones extracted from environmental samples have been measured by a number of techniques, including atomic absorption spectroscopy (AA), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), pyrolysis GC-MS, as well as H and Si NMR spectroscopy (674,684,692,696-700). The use of separation techniques, such as gel permeation and high pressure liquid chromatography interfaced with sensitive, silicon-specific AA or ICP detectors, has been particularly advantageous for the analysis of silicones in environmental extracts (685,701-704). 

This type of interaction occurs between the hydrogen atom that is part of a polar bond and an electronegative atom with a lone pair of electrons such as O and N. The interactions between proton donors and a proton acceptor, which is the case for both acids dissolved in diethyl ether and bases dissolved in chloroform, are typical examples of hydrogen bonding. 

However, some rather unrealistic fragmental values decrease the interest of this method. In diethyl ether, for example, the oxygen atom is found to be slightly hydrophobic (+0.04) and the adjacent carbon atoms are strongly hydrophilic (—0.95). We consider that this method seriously underestimates the polarity of heteroatoms such as oxygen or nitrogen. 

Act, however, as hydrogen-bond acceptors by using the lone pairs of electrons on the O atom -> Useful solvents for other organic compoimds because they have low polarity and low reactivity —> Quite flammable and must be handled with care Examples Diethyl ether (CH3CH2-O-CH2CH3) ... 

Even though both compounds have polar C-0 bonds, the electron pairs around the sp hybridized O atom of diethyl ether are more crowded and less able to interact with electron-deficient sites in other diethyl ether molecules. The O atom of the carbonyl group of 2-butanone extends out from the carbon chain making it less crowded. The lone pairs of electrons on the O atom can more readily interact with the electron-deficient sites in the other molecules, resulting in stronger forces. 

The Wittig reaction is regiospecific. The double bond forms between the carbonyl carbon atom and the carbon atom bonded to the phosphorus atom of the ylide. However, the reaction is not stereospecific. Thus, if geometric isomers are possible, both isomers form. The Wittig reaction is carried out in polar aprotic solvents such as diethyl ether, tetrahydrofuran, or dimethyl sulfoxide. The Wittig reaction can be carried out in the presence of alkene, alkyne, halogen, ether, or ester functional groups. 

The coordinate link is therefore unlike a normal covalent bond where the two atoms each contribute one electron to the bonding pair. Sidgwick recognized that the coordinate link was covalent insofar as it was not ionic. He was also aware that coordinate links often were weak compared to normal covalent bonds and that coordination complexes like those formed from BeBr2 or AICI3 with diethyl ether were very polar. He felt, therefore, that coordinate links were sufficiently different from normal covalent bonds to warrant a separate name. 

---