The Boron Group

The cooling to induce precipitation is effective because H3BO3 is much less soluble in cold water (47 g/L) than in boiling water (275 g/L). Recrystallizations are employed to increase the purity of H3BO3. This compound is the starting point for all other B compounds. 

In general, most alkali borates are soluble, whereas most other borates are insoluble. Table 7.1 presents the names, formulas, states, solubilities, and standard free energies of a number of the more important simple inorganic species of B. All borate salts are white unless they contain a colored cation. 

If H3BO3 is treated with HE, tetrafluoroboric acid HBF4 is slowly produced in solution. This strong acid has not been isolated from solution, but numerous salts, tetraflouroborates, have been prepared. These salts are similar in solubilities to perchlorates (salts C104 ). 

Name Formula State Color Solubility AG° (kJ/mole) 

Group 13 elements, also called the boron group, include a wide variety of properties. Some of the importamt properties about this group include  

They readily form Lewis acids, three coordinate compounds that are capable of accepting an electron pair and increasing the coordination number. 

All the Group 13 elements have the outer configuration of ns npL (Chapter 2 covers electron configurations if need a refresher.) 

Most of the boron group elements exhibit a tripositive (3+) oxidation state however, they can be occasionally found in a unipositive (1+) state (with the exception of boron itself, which we describe in more detail later in this chapter). Keep reading to find out the details of five of the Group 13 (the 13th column on the periodic table) elements boron, aluminum, gallium, indium, and thallium. 

Boron (B) can be identified by its characteristic green flame, which was first reported by Geoffroy the Younger in 1732 but it was not until 1808 that elemental boron was first crudely isolated by three different scientists  

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This provides a mechanism for migration of the boron group along the carbon chain by a series of eliminations and additions. 

The Boron Group (Metallics to Semimetals) Periods 2 to 6, Group 13 (IIIA)... 

The boron group (group 13 IIIA) consists of the elements boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Ti). All have three electrons in their outer valence shell. A few exhibit metal-like characteristics by losing one or more of their outer electrons. For example, aluminum can lose one or three of its valence electrons and become a positive ion just as do other metals, but other elements in this group have characteristics more hke metalloids or semiconductors. 

Hydroboration is thermally reversible. At 160°C and above, B—H moieties are eliminated from alkylboranes, but the equilibrium is still in favor of the addition products. This provides a mechanism for migration of the boron group along the carbon chain by a... 

The direct cycloaddition adduct was oxidized, resulting in the hydroxylated isoxazoline product (316). Better selectivities were obtained in 1,3-dipolar cycloadditions of 204 with nitrile oxides (317,318). The 1,3-dipolar cycloadditions proceeded with concomitant loss of the boron group to give the isoxazoline products in up to 74% ee (318). The alkene 204 was also tested in reactions with nitrones. The reactions proceeded with poor yields, but high selectivities were observed in two cases (318). Gilbertson et al. (319) investigated the use of chiral ot,p-unsaturated hexacarbonyldiiron acyl complexes 205 as dipolarophiles in reactions with nitrones. Selectivities of up to >92% de were observed. The iron moiety was removed oxidatively after the cycloaddition and the thioester was hydrolyzed. 

Passing back to the boron group, if it is desired to form anhydrous chloride, it is necessary either to heat the element, or its oxide mixed with charcoal, in a current of chlorine, or, except in the case of boron, to prepare a double salt of the chloride with ammonium chloride, and to volatilise the latter after driving off the water the aqueous chlorides are formed by dissolving the oxides or hydroxides in hydrochloric acid. Thallium forms monohalides, sparingly soluble in cold water, and thereby attaches itself to the copper group. 

As mentioned previously, the addition of borane, or an alkyl-substituted borane, to a carbon-carbon double bond is very sensitive to steric hindrance. The preceding vinylborane does not add a second boron because of the steric bulk of the ethyl group and the boron group on the end of the double bond. If the boron is considered to be about the same size as a carbon, then this vinylborane corresponds to a trisubstituted alkene. 

Later, he accepted the elemental nature of the rare earths and he tried to place them in the periodic table according to a homologous accommodation methodology, as Mendeleev did. In 1876, Meyer placed cerium, erbium, and yttrium in the boron group as trivalent elements, but he placed lanthanum in the column of the tetravalent elements. 

Using a different approach, the research gronps of Fabre and Freund have synthesized boronate-functionalized conjugated polymers, which serve as electrochemical sensors. For example, a conjugated redox-active film of polypyrrole (174) was electrodeposited onto a platinum electrode from acetonitrile solution. Addition of fluoride anions led to a new redox system that showed an anodic shift relative to polypyrrole itself, which was attributed to fluoride binding to the boronate group. A related poly(aniline boronic acid) (175) was also reported and studied for saccharide detection. " ... 

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