Boron mechanism

Boron Mechanism. Boron functions as a flame retardant in both the condensed and vapor phases. Under flaming conditions boron and halogens form the corresponding trihalide. Because boron (rihalides are effective Lewis acids, they promote cross-linking, minimizing decomposition of the polymer into volatile flammable gases. These trihalides arc also volatile thus they vaporize into the (lame and release halogen which Ihen functions as a Maine inhibitor. 

Both boron and aluminium chlorides can be prepared by the direct combination of the elements. Boron trichloride can also be prepared by passing chlorine gas over a strongly heated mixture of boron trioxide and carbon. Like boron trifluoride, this is a covalent compound and a gas at ordinary temperature and pressure (boiling point 285 K). It reacts vigorously with water, the mechanism probably involving initial co-ordination of a water molecule (p, 152). and hydrochloric acid is obtained ... 

A mixture of an acid anhydride and a ketone is saturated with boron trifluoride this is followed by treatment with aqueous sodium acetate. The quantity of boron trifluoride absorbed usually amounts to 100 mol per cent, (based on total mola of ketone and anhydride). Catalytic amounts of the reagent do not give satisfactory results. This is in line with the observation that the p diketone is produced in the reaction mixture as the boron difluoride complex, some of which have been isolated. A reasonable mechanism of the reaction postulates the conversion of the anhydride into a carbonium ion, such as (I) the ketone into an enol type of complex, such as (II) followed by condensation of (I) and (II) to yield the boron difluoride complex of the p diketone (III) ... 

Stereoselectivities of 99% are also obtained by Mukaiyama type aldol reactions (cf. p. 58) of the titanium enolate of Masamune s chired a-silyloxy ketone with aldehydes. An excess of titanium reagent (s 2 mol) must be used to prevent interference by the lithium salt formed, when the titanium enolate is generated via the lithium enolate (C. Siegel, 1989). The mechanism and the stereochemistry are the same as with the boron enolate. 

Cera.micA.bla.tors, Several types of subliming or melting ceramic ablators have been used or considered for use in dielectric appHcations particularly with quartz or boron nitride [10043-11 -5] fiber reinforcements to form a nonconductive char. Fused siHca is available in both nonporous (optically transparent) and porous (sHp cast) forms. Ford Aerospace manufactures a 3D siHca-fiber-reinforced composite densified with coUoidal siHca (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see COMPOSITE MATERIALS, CERAMIC MATRIX). 

Aluminum bromide and chloride are equally active catalysts, whereas boron trifluoride is considerably less active probably because of its limited solubiUty in aromatic hydrocarbons. The perchloryl aromatics are interesting compounds but must be handled with care because of their explosive nature and sensitivity to mechanical shock and local overheating. 

Both ( )- and (Z)-l-halo-l-alkenes can be prepared by hydroboration of 1-alkynes or 1-halo-l-alkynes followed by halogenation of the intermediate boronic esters (244,245). Differences in the addition—elimination mechanisms operating in these reactions lead to the opposite configurations of iodides as compared to bromides and chlorides. 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

The second control mechanism is the soluble reactor poison boric acid [10043-35-3] Natural boron contains 20% boron-10 [14798-12-0] ... 

Lower Oxides. A number of hard, refractory suboxides have been prepared either as by-products of elemental boron production (1) or by the reaction of boron and boric acid at high temperatures and pressures (39). It appears that the various oxides represented as B O, B O, B22O2, and B23O2 may all be the same material ia varying degrees of purity. A representative crystalline substance was determined to be rhombohedral boron suboxide, B12O2, usually mixed with traces of boron or B2O3 (39). A study has been made of the mechanical properties of this material, which exhibits a hardness... 

Properties. Under nitrogen pressure hexagonal boron nitride melts at about 3000°C but sublimes at about 2500°C at atmospheric pressure. Despite the high melting point, the substance is mechanically weak because of the relatively easy sliding of the sheets of rings past one another (3). The theoretical density is 2.27 g/mL and the resistivity is about 10 H-cm. 

Polyhedral Expansion. The term polyhedral expansion is used to describe a host of reactions in which the size of the polyhedron is increased by the addition of new vertex atoms whether boron, heteroelements, or metals. In the case of the boranes, the pyrolysis of B2H has been used to obtain B H and industrially. Although a subject of much study, the mechanism of such pyrolytic expansions is not well understood. 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

Chemical erosion can be suppressed by doping with substitutional elements such as boron. This is demonstrated in Fig. 14 [47] which shows data for undoped pyrolitic graphite and several grades of boron doped graphite. The mechanism responsible for this suppression may include the reduced chemical activity of the boronized material, as demonstrated by the increased oxidation resistance of B doped carbons [48] or the suppressed diffusion caused by the interstitial trapping at boron sites. 

As an illustration of the results of the measurements just described, the mechanical properties for four unidirectionally reinforced composite materials, glass-epoxy, boron-epoxy, graphite-epoxy, and Kevlar 49 -... 

Thermal cleavage of boron tris(trifluoromethane)sulfonate and boron tris(pen-tafluoroethane)sulfonate was studied. Careful identificadon of the products has led to the proposal of a mechanism and an account for this process [121] (equation 93). 

There appears to be no end to the structural ingenuity of boron and, whilst it is true that many regularities can now be discerned in its stereochemistry, much more work is still needed to unravel the reaction pathways by which the compounds are formed and to elucidate the mechanisms by which they isomerize and interconvert. 

Cava and Schlessinger have reported the synthesis of 1,2,3-triphenyl-isoindole (65) in 78% yield from 1,3-diphenylisobenzofuran (68) hy reaction with thionylaniline (69) and boron trifluoride. The mechanism proposed for this remarkable transformation involves reaiTangement of the adduct (70) derived from thionylaniline and the isobenzofuran, to the tricyclic intermediate (71). This presumably collapses to the S-sultam (72), which yields the isoindole (65) upon extrusion of sulfur dioxide. Loss of sulfur dioxide, both from S-sultones and unsaturated S-sultams, is well documented. ... 

The spiro aziridine 202 (Scheme 3.74) reacts with a catalytic amount of boron trifluoride etherate to give rise to ring-expanded compound 205 in almost quantitative yield [55]. A possible mechanism involving intermediates 203 and 204 was proposed. 

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