Boron chemical reactivity

Crystalline boron is very inert. Low purity, higher temperatures, and changes in or lack of crystallinity all increase the chemical reactivity. Hot concentrated H2SO4—HNO at 2 1 ratio can be used to dissolve boron for chemical analysis but boron is not soluble in boiling HE or HCl. Boron is also unreactive toward concentrated NaOH up to 500°C. At room temperature, boron reacts with E2, but only superficially with O2. 

The chemical reactivity of boron itself obviously depends markedly on the purity, crystallinity, state of subdivision and temperature. Boron reacts with F2 at room temperature and is superficially attacked by O2 but is otherwise inert. At higher temperatures boron reacts directly with all the non-metals except H, Ge, Te and the noble gases. It also reacts readily and directly with almost all metals at elevated temperatures, the few exceptions being the heavier members of groups 11-15 (Ag, Au Cd, Hg Ga, In, Tl Sn, Pb Sb, Bi). 

The Group 13 metals differ sharply from the non-metallic element boron both in their greater chemical reactivity at moderate temperatures and in their well-defined cationic chemistry for aqueous solutions. The absence of a range of... 

Because the breadth of chemical behavior can be bewildering in its complexity, chemists search for general ways to organize chemical reactivity patterns. Two familiar patterns are Br< )nsted acid-base (proton transfer) and oxidation-reduction (electron transfer) reactions. A related pattern of reactivity can be viewed as the donation of a pair of electrons to form a new bond. One example is the reaction between gaseous ammonia and trimethyl boron, in which the ammonia molecule uses its nonbonding pair of electrons to form a bond between nitrogen and boron ... 

Anderson and coworkers [59-66] produced boron cluster cations Bj-B in molecular beams using laser vaporization and studied their chemical reactivity and fragmentation properties. The structures of B3 —IBI3 cations have been established computationally (see review [7] for details) represented in Figure 29.1. In this chapter, we are discussing stability and reactivity of Bj — B 3 cations on the basis of their multifold aromaticity, multifold antiaromaticity, and conflicting aromaticity. 

Dewar185-187 reported the first six-membered aromatic rings containing boron, oxygen, and nitrogen heteroatoms, and later Gronowitz188 prepared similar systems annelated with thiophene. The chemical reactivity and NMR data indicate electronic delocalization. 

Al, Ga, In and T1 differ sharply from boron. They have greater chemical reactivity at lower temperatures, well-defined cationic chemistry in aqueous solutions they do not form numerous volatile hydrides and cluster compounds as boron. Aluminium readily oxidizes in air, but bulk samples of the metal form a coherent protective oxide film preventing appreciable reaction aluminium dissolves in dilute mineral acids, but it is passivated by concentrated HN03. It reacts with aqueous NaOH, while gallium, indium and thallium dissolve in most acids. 

Chemical Reactivity of the Boron Hydrides and Related Compounds... 

Inverting the orientation of the C4-N3 imine unit of a 2,3,1-diheterabotine gives a boron heterocycle with a markedly different chemical reactivity. In effect, the weakly basic oxime- or hydrazone-type imine nitrogen in the 2,3,1-diheteraborine is replaced by a much more basic imidate- or amidine-type imine nitrogen in the 2,4,1-diheteraborine. Likely, the Lewis acid tendency of the boron is enhanced by the ready protonation of this basic N4, and the formation of a stable borate-based zwitterion becomes thermodynamically favored. 

As later work would show (47, 77), an entire series of polyhedral B Hn2-and isoelectronic C2B 2H carboranes were synthetically accessible for n = 6 through 12. The generally decreased chemical reactivity of these polyhedral species over that of the boron hydrides suggested that they... 

The a-rhombohedral variety is stable up to 1200°C. Above 1200°C., it transforms to the jS variety. The transformation is irreversible. The large chemical reactivity of a-boron, due to its crystalline structure and its state of division, makes this variety of boron an ideal material for the synthesis of borides. 

Stone, F. G. A. Chemical Reactivity of the Boron Hydrides and Related Compounds, in Emeleus and Sharpe s Advances in Inorganic Chemistry and Radiochemistry, Yol. II, 279-314, Academic Press, Inc., New York (1960). 

The structures of l,8-di(silyl)naphthalene and its mono- and di(p-anisyl) derivatives have been determined and are shown in Fig. 3-5. While the naphthalene part of the molecules appears to be largely undistorted, the two silyl groups are clearly bent away from each other in the molecular plane in order to avoid closer repulsive contacts. This steric crowding enhances the chemical reactivity of the molecule and makes the eompound a versatile starting material for numerous derivatives. For substitution control the conversion into the symmetrical dichlorosilane is possible using boron trichloride (Scheme 5). 

Fluorine is characterized by its extraordinary chemical reactivity— it is the most active of the elements. Non-metals, such as hydrogen, sulfur, iodine, and arsenic, and metalloids, such as silicon, boron, and carbon, combine spontaneously with fluorine, becoming incandescent. All metals are attacked by the gas. The alkali metals and alkaline-earth metals take fire in a stream of the gas at room temperature, whereas the more noble metals react with fluorine when warmed. Fluorine decomposes water, forming hydrogen fluoride and liberating a mixture of oxygen and ozone. 

NMR experiments on the reactions of halophosphates esters with pyridine showed that equilibria involving the formation of pyridinium salts in these reactions are almost entirely shifted to the left for chloro- and bromo-phosphates and to the right for the corresponding iodophosphates. This explains dramatic differences in chemical reactivity between these compounds. Substituted medium-sized and large N-heterocycles (117) have been prepared via an extension of the Suzuki reaction involving the palladium-catalysed coupling of vinyl-phosphates (118) with aryl or heteroaryl boronic acids (Scheme 27). ... 

The chemical reactivity of several heterocyclic derivatives is closely associated with a reactivity one might expect for open-chain unsaturated derivatives. The boron heterocycles are sensitive to oxygen or hydrogen peroxide 111, 169) and so apparently is 1,1,2,5-tetraphenylsilacyclo-pentadiene 17, 246). These reactions probably lead to ring cleavage but the products have not been characterized. Vinyl-metal bonds are frequently cleaved by electrophilic reagents and the heterocycles are no exception. 

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