Water lone pairs

Generic process An addition and an elimination have occurred. Medium Definitely basic, predominant anion is hydroxide, plsTabH 15.7, whose pA"a would give a useful proton transfer A"eq up to about p Ta 26. Sources The carbonyl lone pair, water lone pair, and hydroxide anion. Best source Hydroxide anion, a lone pair source can behave as a nucleophile or as a base. Sinks Polarized multiple bond, the aldehyde carbonyl. Acidic Hs Water and the CH2 next to the aldehyde, pA a 16.7, are within range of hydroxide. Leaving groups None. Resonance forms ... 

Figure C2.14.2. The hydrogen bond in water. The oxygen lone pairs (shaded blobs) are the donors, and the hydrogen atoms the acceptors [ 177, 178].

The distortion due to the presence of lone pairs of electrons is more marked in water ... 

If the spatial arrangement of atoms is required this can be deduced from the basic structure by neglecting the positions occupied by lone pairs of electrons. Water, for example, can be described as a V shape whilst ammonia is a trigonal pyramid. 

The ability to act as a lone pair acceptor is not confined to Group III, and can occur wherever a quantum level is incomplete. This ability to accept electrons explains why covalent chlorides, with the exception of carbon tetrachloride, are readily hydrolysed, the apparently anomalous behaviour of carbon tetrachloride being readily explained by the fact that the carbon has a completed quantum level and is unable to form an intermediate complex with water. 

As an example of a really strong base, the hydride ion H (for example in NaH) is unique it has one lone pair, a negative charge and a very small size. Like O , it is too strong a base to exist in water ... 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Ammonia as a donor molecule. Because of the presence of the lone pair of electrons on the nitrogen atom, ammonia can behave as an electron pair donor. For example, ammonia abstracts a proton from a water molecule producing the tetrahedral ammonium, NH4, ion and forms the compounds HjN- AlClj and HjN- BClj. 

Phosphine is a colourless gas at room temperature, boiling point 183K. with an unpleasant odour it is extremely poisonous. Like ammonia, phosphine has an essentially tetrahedral structure with one position occupied by a lone pair of electrons. Phosphorus, however, is a larger atom than nitrogen and the lone pair of electrons on the phosphorus are much less concentrated in space. Thus phosphine has a very much smaller dipole moment than ammonia. Hence phosphine is not associated (like ammonia) in the liquid state (see data in Table 9.2) and it is only sparingly soluble in water. 

The covalently bonded oxygen atom still has two lone pairs of electrons and can act as an electron pair donor. It rarely donates both pairs (to achieve 4-coordination) and usually only one donor bond is formed. A water molecule, for example, can donate to a proton, forming H30, and diethyl ether can donate to an acceptor such as boron trifluoride ... 

FIGURE 13.5 Isosurface plots, (a) Region of negative electrostatic potential around the water molecule. (A) Region where the Laplacian of the electron density is negative. Both of these plots have been proposed as descriptors of the lone-pair electrons. This example is typical in that the shapes of these regions are similar, but the Laplacian region tends to be closer to the nucleus. 

Although isothiazole (pK = 1.90) is less basic than thiazole, its rale of quaternization by dinitrophenyl acetate in water at 52°C is approximately 2.5 times higher (447). This deviation from the Bronsted relationship (A log k - 0.ApK, with positive) is interpreted as a consequence of the or effect of the adjacent sulfur lone pair in isothiazole that is responsible for its higher nucleophilicity (448, 449). 

Bismuth Trifluoride. Bismuth(III) duoride is a white to grey-white powder, density 8.3 g/mL, that is essentially isomorphous with orthorhombic YF, requiring nine-coordination about the bismuth (11). It has been suggested that BiF is best considered an eight-coordinate stmcture with the deviation from the YF stmcture resulting from stereochemical activity of the bismuth lone-pair electrons. In accord with its stmcture, the compound is the most ionic of the bismuth haUdes. It is almost insoluble in water (5.03 0.05 x 10 M at pH 1.15) and dissolves only to the extent of 0.010 g per 100 g of anhydrous HF at 12.4°C. 

Pyrazolines substituted at position 4 or 5 with hydroxy or amino groups readily eliminate a molecule of water or amine yielding pyrazoles. The 4-substituted derivatives are relatively more stable than the 5-substituted ones, because for the last group the lone pair at N-1 assists the elimination (407) -> (408) -> (409). The sulfonyl group at position 1 is also easily eliminated and this property is taken advantage of in Dorn s elegant synthesis of 3-aminopyrazole (Section 4.04.3.3.1). 

Compare and contrast the electrostatic potential map of a typical detergent with that of a typical soap (stearate). Which part of each molecule will be most water soluble (hydrophilic) Draw a Lewis structure that describes each molecule s water-soluble group (make sure you indicate all necessary formal charges and lone pairs). Which part(s) of each molecule will be most grease soluble (lipophilic) What kinds of atoms and bonds are found in these groups ... 

There is little experience with the von Niessen method, but for most molecules the remaining three schemes tend to give very similar LMOs. The main exception is systems containing both a- and vr-bonds, such as ethylene. The Pipek-Mezey procedure preserves the cr/yr-separation, while the Edmiston-Ruedenberg and Boys schemes produce bent banana bonds. Similarly, for planar molecules which contain lone pairs (like water), the Pipek-Mezey method produces one in-plane cr-type lone pair and one out-of-plane yr-type lone pair, while the Edmiston-Ruedenberg and Boys schemes produce two equivalent rabbit ear lone pairs. 

Just as individual bonds are often polar, molecules as a whole are often polar also. Molecular polarity results from the vector summation of all individual bond polarities and lone-pair contributions in the molecule. As a practical matter, strongly polar substances are often soluble in polar solvents like water, whereas nonpolar substances are insoluble in water. 

In contrast with water, methanol, ammonia, and other substances in Table 2.1, carbon dioxide, methane, ethane, and benzene have zero dipole moments. Because of the symmetrical structures of these molecules, the individual bond polarities and lone-pair contributions exactly cancel. 

Water acts as a nucleophile, using a lone pair of electrons to open the bromonium ion ring and form a bond to carbon. Since oxygen donates its electrons in this step, it now has the positive charge. 

Q The nitrogen lone-pair electrons expel water, giving an iminium ion. 

Elimination of water by the lone-pair electrons on nitrogen then yields an intermediate iminium ion. 

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