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13 novembre 2011 7 13 /11 /novembre /2011 03:37

?-rhombohedral boron

?-rhombohedral boron has a unit cell of twelve boron atoms. The structure consists of B12 icosahedra in which each boron atom has five nearest neighbors within the icosahedron. If the bonding were the conventional covalent type then each boron would have donated 5 electrons. Sony VAIO VGN-NR498E/S Battery

However, boron has only 3 valence electrons, and it is thought that the bonding in the B12 icosahedra is achieved by the so-called 3-center electron-deficient bonds where the electron charge is accumulated at the center of a triangle formed by 3 adjacent atoms.

The isolated B12 icosahedra are not stable; thus boron is not a molecular solid, but the icosahedra in it are connected by strong covalent bonds.Sony VAIO VGN-NR498E/T Battery

?-rhombohedral boron

?-rhombohedral boron has a subcell containing 105–108 atoms — or a unit cell of 320 atoms. Many atoms form B12 icosahedra, but there are also a large number of non-icosahedral atoms as well. For long time, it was unclear whether the ? or ? phase is most stable at ambient conditions;Sony VAIO VGN-NR498E/W Battery

however, gradually a consensus was reached that ? phase as the thermodynamically stable allotrope.

?-boron

The ?-phase can be described as a NaCl-type arrangement of two types of clusters, B12 icosahedra and B2 pairs.Sony VAIO VGN-NR50 Battery

It can be produced by compressing other boron phases to 12–20 GPa, heating to 1500–1800 0C and is quenchable to ambient conditions. There is evidence of significant charge transfer from B2 pairs to the B12 icosahedra in this structure;[6] in particular, lattice dynamics suggests the presence of significant long-range electrostatic interactions.Sony VAIO VGN-NR50B Battery

This phase was reported by Wentorf in 1965, however neither structure nor chemical composition were established. The structure was solved using ab initio crystal structure prediction calculations[6] and confirmed using single crystal X-ray diffraction.Sony VAIO VGN-NR51 Battery

Tetragonal boron phases

Two tetragonal phases have been reported, T-50 (or ?-tetragonal boron)[17] and T-192 (or ß-tetragonal boron) with 50 and 192 atoms in the unit cell, respectively. Whereas T-50 has been assigned to a compound (nitride (B50N2) or carbide (B50C2)),[8] T-192 is a genuine pure boron phase. Sony VAIO VGN-NR51B Battery

It was produced in 1960 by hydrogen reduction of BBr3 on hot tungsten, rhenium or tantalum filaments at temperatures 1270–1550 °C (i.e. chemical vapor deposition).[9] Further studies have reproduced the synthesis and confirmed the absence of impurities in this phase.Sony VAIO VGN-NR52 Battery

High-pressure superconducting phase

Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure. Contrary to other phases, which are semiconductors, this phase is a metal and becomes a superconductor with a critical temperature increasing from 4 K at 160 GPa to 11 K at 250 GPa.Sony VAIO VGN-NR52B Battery

This structural transformation occurs at pressures at which theory predicts the icosahedra to dissociate.

Amorphous boron

Amorphous boron contains B12 regular icosahedra that are randomly bonded to each other without long range order.Sony VAIO VGN-NR71B Battery

Pure amorphous boron can be produced by thermal decomposition of diborane at temperatures below 1000 °C. Annealing at 1000 °C converts amorphous boron to ?-rhombohedral boron. Amorphous boron nanowires (30–60 nm thick) or fibers[28] can be produced by magnetron sputtering and laser-assisted chemical vapor deposition, respectively;Sony VAIO VGN-NR71B2 Battery

and they also convert to ?-rhombohedral boron nanowires upon annealing at 1000 °C.

Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms. Sony VAIO VGN-NR72B Battery

The hexagonal form corresponding to graphite is the most stable and softest among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic (sphalerite structure) variety analogous to diamond is called c-BN. Its hardness is inferior only to diamond, but its thermal and chemical stability is superior. Sony VAIO VGN-NS10E/S Battery

The rare wurtzite BN modification is similar to lonsdaleite and may even be harder than the cubic form.

Boron nitride is not found in nature and is therefore produced synthetically from boric acid or boron trioxide. The initial product is amorphous BN powder, which is converted to crystalline h-BN by heating in nitrogen flow at temperatures above 1500 °C. c-BN is made by annealing h-BN powder at higher temperatures, under pressures above 5 GPa. Sony VAIO VGN-NS10J/S Battery

Contrary to diamond, larger c-BN pellets can be produced by fusing (sintering) relatively cheap c-BN powders. As a result, c-BN is widely used in mechanical applications.

Because of excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of high-temperature equipment. Sony VAIO VGN-NS10L/S Battery

Boron nitride has a great potential in nanotechnology. Nanotubes of BN can be produced that have a structure similar to that of carbon nanotubes, i.e. graphene (or BN) sheets rolled on themselves, however the properties are very different: whereas carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius,Sony VAIO VGN-NS110E/L Battery

a BN nanotube is an electrical insulator with a wide bandgap of ~5.5 eV (same as in diamond), which is almost independent of tube chirality and morphology. Similar to other BN forms, BN nanotubes are more thermally and chemically stable than carbon nanotubes which favors them for some applications.Sony VAIO VGN-NS110E/S Battery

Structure

Boron nitride has been produced in an amorphous (a-BN) and crystalline forms. The most stable crystalline form is the hexagonal one, also called h-BN, ?-BN, or g-BN (graphitic BN). It has a layered structure similar to graphite. Sony VAIO VGN-NS110E/W Battery

Within each layer, boron and nitrogen atoms are bound by strong covalent bonds, whereas the layers are held together by weak van der Waals forces. The interlayer "registry" of these sheets differs, however, from the pattern seen for graphite, because the atoms are eclipsed, with boron atoms lying over and above nitrogen atoms.Sony VAIO VGN-NS115N/S Battery

This registry reflects the polarity of the B-N bonds. Still, h-BN and graphite are very close neighbors and even the BC6N hybrids have been synthesized where carbon substitutes for some B and N atoms.

As diamond is less stable than graphite, cubic BN is less stable than h-BN, but the conversion rate between those forms is negligible at room temperature. Sony VAIO VGN-NS11ER/S Battery

The cubic form has the sphalerite crystal structure, same as diamond structure, and is also called ?-BN or c-BN. The wurtzite BN form (w-BN) has similar structure as lonsdaleite, rare hexagonal polymorph of carbon. In both c-BN and w-BN boron and nitrogen atoms are grouped into tetrahedra, but the angles between neighboring tetrahedra are different.Sony VAIO VGN-NS11E/S Battery

The partly ionic structure of BN layers in h-BN reduces covalency and electrical conductivity, whereas the interlayer interaction increases resulting in higher hardness of h-BN relative to graphite. The reduced electron-delocalization in hexagonal-BN is also indicated by its absence of color and a large band gap.Sony VAIO VGN-NS11J/S Battery

Very different bonding – strong covalent within the basal planes (planes where boron and nitrogen atoms are covalently bonded) and weak between them – causes high anisotropy of most properties of h-BN.

For example, the hardness, electrical and thermal conductivity are much higher within the planes than perpendicular to them. On the contrary, the properties of c-BN and w-BN are more homogeneous.Sony VAIO VGN-NS11L/S Battery

Those materials are extremely hard, with the hardness of c-BN being slightly smaller and w-BN even higher than that of diamond.[10] Because of much better stability to heat and metals, c-BN surpasses diamond in mechanical applications.[11] The thermal conductivity of BN is among the highest of all electric insulators (see table).Sony VAIO VGN-NS11MR/S Battery

Boron nitride can be doped p-type with Be and n-type with boron, sulfur, silicon or if co-doped with carbon and nitrogen. Both hexagonal and cubic BN are wide-gap semiconductors with a band gap energy corresponding to the UV region. If voltage is applied to h-BN  or c-BN,   then it emits UV light in the range 215–250 nm and therefore can potentially be used as light emitting diodes (LEDs) or lasers.Sony VAIO VGN-NS11M/S Battery

Little is known on melting behavior of boron nitride. It sublimates at 2973 °C at normal pressure releasing nitrogen gas and boron, but melts at elevated pressure.

Thermal stability

Hexagonal and cubic (and probably w-BN) BN show remarkable chemical and thermal stabilities. Sony VAIO VGN-NS11SR/S Battery

For example, h-BN is stable to decomposition in temperatures up to 1000 °C in air, 1400 °C in vacuum, and 2800 °C in an inert atmosphere. The reactivity of h-BN and c-BN is relatively similar, and the data for c-BN are summarized in the table below.

Chemical stability

Boron nitride is insoluble in usual acids, but is soluble in alkaline molten salts and nitrides, such as LiOH, KOH, NaOH-Na2CO3, NaNO3, Li3N, Mg3N2, Sr3N2, Ba3N2 or Li3BN2, which are therefore used to etch BN.Sony VAIO VGN-NS11S/S Battery

Thermal conductivity

The theoretical thermal conductivity of hexagonal Boron nitride nanoribbons (BNNRs) can approach 1700–2000 W/(m·K), which has the same order of magnitude as the experimental measured value for graphene, and can be comparable to the theoretical calculations for graphene nanoribbons.Sony VAIO VGN-NS11ZR/S Battery

Moreover, the thermal transport in the BNNRs is anisotropic. The thermal conductivity of zigzag-edged BNNRs is about 20% larger than that of armchair-edged nanoribbons at room temperature.

The resulting disordered (amorphous) boron nitride contains 92–95% BN and 5–8% B2O3. Sony VAIO VGN-NS11Z/S Battery

The remaining B2O3 can be evaporated in a second step at temperatures >1500 °C in order to achieve BN concentration >98%. Such annealing also crystallizes BN, the size of the crystallites increasing with the annealing temperature.

h-BN parts can be fabricated inexpensively by hot-pressing with subsequent machining.Sony VAIO VGN-NS12M/S Battery

The parts are made from boron nitride powders adding boron oxide for better compressibility. Thin films of boron nitride can be obtained by chemical vapor deposition from boron trichloride and nitrogen precursors. Combustion of boron powder in nitrogen plasma at 5500 °C yields ultrafine boron nitride used for lubricants and toners.Sony VAIO VGN-NS12M/W Battery

Boron nitride reacts with iodine fluoride in trichlorofluoromethane at ?30 °C to produce an extremely sensitive contact explosive, NI3, in low yield.

Intercalation of hexagonal BN

Similar to graphite, various molecules, such as NH3  or alkali metals, can be intercalated into hexagonal boron nitride, that is inserted between its layers. Both experiment and theory suggest the intercalation is much more difficult for BN than for graphite.Sony VAIO VGN-NS12S/S Battery

Preparation of cubic BN

Synthesis of c-BN uses same methods as that of diamond: Cubic boron nitride is produced by treating hexagonal boron nitride at high pressure and temperature, much as synthetic diamond is produced from graphite. Sony VAIO VGN-NS21M/P Battery

Direct conversion of hexagonal boron nitride to the cubic form has been observed at pressures between 5 and 18 GPa and temperatures between 1730 and 3230 °C, that is similar parameters as for direct graphite-diamond conversion. The addition of a small amount of boron oxide can lower the required pressure to 4–7 GPa and temperature to 1500 °C. Sony VAIO VGN-NS21M/W Battery

As in diamond synthesis, to further reduce the conversion pressures and temperatures, a catalyst is added, such as lithium, potassium, or magnesium, their nitrides, their fluoronitrides, water with ammonium compounds, or hydrazine. Other industrial synthesis methods, again borrowed from diamond growth, Sony VAIO VGN-NS21S/S Battery

use crystal growth in a temperature gradient, or explosive shock wave. The shock wave method is used to produce material called heterodiamond, a superhard compound of boron, carbon, and nitrogen.

Low-pressure deposition of thin films of cubic boron nitride is possible. As in diamond growth, the major problem is to suppress the growth of hexagonal phases (h-BN or graphite, respectively). Sony VAIO VGN-NS21S/W Battery

Whereas in diamond growth this is achieved by adding hydrogen gas, boron trifluoride is used for c-BN. Ion beam deposition, plasma-enhanced chemical vapor deposition, pulsed laser deposition, reactive sputtering, and other physical vapor deposition methods are used as well.Sony VAIO VGN-NS21Z/S Battery

Preparation of wurtzite BN

Wurtzite BN can be obtained via static high-pressure or dynamic shock methods.[33] The limits of its stability are not well defined. Both c-BN and w-BN are formed by compressing h-BN, but formation of w-BN occurs at much lower temperatures close to 1700 °C.Sony VAIO VGN-NS31M/P Battery

Production statistics

Whereas the production and consumption figures for the raw materials used for BN synthesis, namely boric acid and boron trioxide, are well known (see boron), the corresponding numbers for the boron nitride are not listed in statistical reports. An estimate for the 1999 world production is 300 to 350 metric tons. Sony VAIO VGN-NS31M/W Battery

The major producers and consumers of BN are located in the United States, Japan, China and Germany. In 2000, prices varied from about $75/kg to $120/kg for standard industrial-quality h-BN and were about up to $200–$400/kg for high purity BN grades.Sony VAIO VGN-NS31S/S Battery

Hexagonal BN

Hexagonal BN is the most widely used polymorph. It is a good lubricant at both low and high temperatures (up to 900 °C, even in an oxidizing atmosphere). h-BN lubricant is particularly useful when the electrical conductivity or chemical reactivity of graphite (alternative lubricant) would be problematic. Sony VAIO VGN-NS31Z/P Battery

Another advantage of h-BN over graphite is that its lubricity does not require water or gas molecules trapped between the layers. Therefore, h-BN lubricants can be used even in vacuum, e.g. in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads.Sony VAIO VGN-NS31Z/S Battery

Hexagonal BN was first used in cosmetics around 1940 in Japan. However, because of its high price, h-BN was soon abandoned for this application. Its use was revitalized in the late 1990s with the optimization h-BN production processes, and currently h-BN is used by nearly all leading producers of cosmetic products for foundations, Sony VAIO VGN-NS31Z/W Battery

make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products.

Because of its excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of high-temperature equipment. h-BN can be included in ceramics, alloys, resins, plastics, rubbers, and other materials, giving them self-lubricating properties. Sony VAIO VGN-NS38M/P Battery

Such materials are suitable for construction of e.g. bearings and in steelmaking. Plastics filled with BN have less thermal expansion as well as higher thermal conductivity and electrical resistivity. Due to its excellent dielectric and thermal properties, BN is used in electronics e.g. as a substrate for semiconductors, microwave-transparent windows, and as a structural material for seals.Sony VAIO VGN-NS38M/W Battery

Hexagonal BN is used in xerographic process and laser printers as a charge leakage barrier layer of the photo drum. In the automotive industry, h-BN mixed with a binder (boron oxide) is used for sealing oxygen sensors, which provide feedback for adjusting fuel flow. The binder utilizes the unique temperature stability and insulating properties of h-BN.Sony VAIO VGN-NS130E/L Battery

Parts can be made of h-BN by hot pressing. Union Carbide Corporation produces three grades of BN. HBN, with boron oxide binder, usable to 550–850 °C in oxidizing atmosphere and up to 1600 °C in vacuum, but due to the boron oxide content is sensitive to water. HBR uses calcium borate binder and is usable at 1600 °C. HBC grade uses no binder and can be used to 3000 °C.Sony VAIO VGN-NS130E/S Battery

Cubic boron nitride

Cubic boron nitride (CBN or c-BN) is widely used as an abrasive. Its usefulness arises from its insolubility in iron, nickel, and related alloys at high temperatures, whereas diamond is soluble in these metals to give carbides. Polycrystalline c-BN (PCBN) abrasives are therefore used for machining steel, whereas diamond abrasives are preferred for aluminum alloys, ceramics, and stone. Sony VAIO VGN-NS130E/W Battery

When in contact with oxygen at high temperatures, BN forms a passivation layer of boron oxide. Boron nitride binds well with metals, due to formation of interlayers of metal borides or nitrides. Materials with cubic boron nitride crystals are often used in the tool bits of cutting tools. Sony VAIO VGN-NS140E/L Battery

For grinding applications, softer binders, e.g. resin, porous ceramics, and soft metals, are used. Ceramic binders can be used as well. Commercial products are known under names "Borazon" (by Diamond Innovations), and "Elbor" or "Cubonite" (by Russian vendors).Sony VAIO VGN-NS140E/S Battery

Similar to diamond, the combination in c-BN of highest thermal conductivity and electrical resistivity is ideal for heat spreaders. Contrary to diamond, large c-BN pellets can be produced in a simple process (called sintering) of annealing c-BN powders in nitrogen flow at temperatures slightly below the BN decomposition temperature. Sony VAIO VGN-NS140E/W Battery

This ability of c-BN and h-BN powders to fuse allows cheap production of large BN parts.

As cubic boron nitride consists of light atoms and is very robust chemically and mechanically, it is one of the popular materials for X-ray membranes: low mass results in small X-ray absorption, and good mechanical properties allow usage of thin membranes, thus further reducing the absorption.Sony VAIO VGN-NS190J/L Battery

Amorphous boron nitride

Layers of amorphous boron nitride (a-BN) are used in some semiconductor devices, e.g. MISFETs. They can be prepared by chemical decomposition of trichloroborazine with caesium, or by thermal chemical vapor deposition methods. Thermal CVD can be also used for deposition of h-BN layers, or at high temperatures, c-BN.Sony VAIO VGN-NS190J/S Battery

Other BN forms -Boron nitride fibers

Hexagonal BN can be prepared in the form of fibers, structurally similar to carbon fibers, by thermal decomposition of extruded borazine (B3N3H6) fibers with addition of boron oxide in a nitrogen atmosphere at 1800 °C. Sony VAIO VGN-NS190J/W Battery

An alternative method is thermal decomposition of cellulose fibers impregnated with boric acid or ammonium tetraborate in an atmosphere of ammonia and nitrogen above 1000 °C. Boron nitride fibers are used as reinforcement in composite materials, with the matrix materials ranging from organic resins to ceramics to metals (see Metal matrix composites). Sony VAIO VGN-NS235J/L Battery

Boron nitride nanomesh

Boron nitride nanomesh is an inorganic nanostructured two-dimensional material. It consists of a single BN layer, which forms by self-assembly a highly regular mesh after high-temperature exposure of a clean rhodium  or ruthenium  surface to borazine under ultra-high vacuum. Sony VAIO VGN-NS235J/P Battery

The nanomesh looks like an assembly of hexagonal pores. The distance between 2 pore centers is 3.2 nm and the pore diameter is ~2 nm.

The boron nitride nanomesh is not only stable to decomposition under vacuum, air and some liquids, but also up to temperatures of 800 °C.Sony VAIO VGN-NS235J/S Battery

In addition, it shows the extraordinary ability to trap molecules  and metallic clusters  which have similar sizes to the nanomesh pores, forming a well-ordered array. These characteristics promise interesting applications of the nanomesh in areas like nanocatalysis, surface functionalisation, spintronics, quantum computing and data storage media like hard drives.Sony VAIO VGN-NS235J/W Battery

Boron nitride nanotubes

Boron nitride nanotubes were theoretically predicted in 1994 ] and experimentally discovered in 1995. They can be imagined as a rolled up sheet of boron nitride. Structurally, it is a close analog of the carbon nanotube, Sony VAIO VGN-NS240E/L Battery

namely a long cylinder with diameter of several to hundred nanometers and length of many micrometers, except carbon atoms are alternately substituted by nitrogen and boron atoms. However, the properties of BN nanotubes are very different: whereas carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius,Sony VAIO VGN-NS290J/L Battery

a BN nanotube is an electrical insulator with a bandgap of ~5.5 eV, basically independent of tube chirality and morphology. In addition, a layered BN structure is much more thermally and chemically stable than a graphitic carbon structure.

All well-established techniques of carbon nanotube growth, such as arc-discharge, Sony VAIO VGN-NS290J/S Battery

laser ablation and chemical vapor deposition, are used to synthesize BN nanotubes. BN nanotubes can also be produced by ball milling of amorphous boron, mixed with a catalyst: iron powder, under NH3 atmosphere. Subsequent annealing at ~1100 °C in nitrogen flow transforms most of the product into BN.Sony VAIO VGN-NS50B/L Battery

Electrical and field emission properties of the thus prepared nanotubes can be tuned by doping with gold atoms via sputtering of gold on the nanotubes. Doping rare-earth atoms of europium turns a BN nanotube into a phosphor material emitting visible light under electron excitation.Sony VAIO VGN-NS50B/W Battery

Like BN fibers, boron nitride nanotubes show promise for aerospace applications where integration of boron and in particular the light isotope of boron (10B) into structural materials improves their radiation-shielding properties; the improvement is due to strong neutron absorption by 10B. Sony VAIO VGN-NS51B/L Battery

Such 10BN materials are of particular theoretical value as composite structural materials in future manned interplanetary spacecraft, where absorption-shielding from cosmic ray spallation neutrons is expected to be a particular asset in light construction materials.Sony VAIO VGN-NS51B/P Battery

Composites containing BN

Addition of boron nitride to silicon nitride ceramics improves the thermal shock resistance of the resulting material. For the same purpose, BN is added also to silicon nitride-alumina and titanium nitride-alumina ceramics.Sony VAIO VGN-NS51B/W Battery

Other materials being reinforced with BN are, e.g., alumina and zirconia, borosilicate glasses, glass ceramics, enamels, and composite ceramics with titanium boride-boron nitride and titanium boride-aluminium nitride-boron nitride and silicon carbide-boron nitride composition.Sony VAIO VGN-NS52JB/L Battery

Boron neutron capture therapy (BNCT) is an experimental form of radiotherapy that uses a neutron beam that interacts with boron injected into a patient. BNCT depends on the interaction of slow neutrons with boron-10 to produce alpha particles and lithium nuclei, without producing other types of ionizing radiation.Sony VAIO VGN-NS52JB/P Battery

Patients are first given an intravenous injection of a boron-10 containing chemical that preferentially binds to tumor cells. In clinical trials performed so far the neutrons are created in a nuclear reactor, but particle accelerators may also be used to collide protons into targets made of lithium or beryllium.Sony VAIO VGN-NS52JB/W Battery

The neutrons pass through a neutron moderator, which shapes the neutron energy spectrum suitable for BNCT treatment. Before entering the patient the neutron beam is shaped by a beam collimator. While passing through the tissue of the patient, the neutrons are slowed by collisions and become low energy thermal neutrons. Sony VAIO VGN-NS70B/W Battery

The thermal neutrons undergo reaction with the boron-10 nuclei, forming a compound nucleus (excited boron-11) which then promptly disintegrates to lithium-7 and an alpha particle. Both the alpha particle and the lithium ion produce closely spaced ionizations in the immediate vicinity of the reaction, with a range of approximately 5-9 micrometres, Sony VAIO VGN-NS71B/W Battery

or roughly the thickness of one cell diameter. This technique is advantageous since the radiation damage occurs over a short range and thus normal tissues can be spared.

BNCT has been experimentally tested primarily as an alternative treatment for malignant brain tumors called glioblastoma multiforme as well as recurrent, locally advanced head and neck cancer. Sony VAIO VGN-NS72JB/W Battery

Although there are reports of some successful outcomes, this approach has not yet been shown to be superior to other current therapies. Hence, BNCT has not entered routine clinical use.

Recent applications

On 19 December 2001, BNCT has been successfully used for the first time in Pavia (Italy) on a 42 years old man to treat liver cancer. His liver has been explanted, has been subjected to the treatment, and has been reimplanted.Sony VAIO VGN-NS90HS Battery

The first clinical trial of BNCT in Argentina were performed on 9 October 2003.  BNCT has been used in Japan for head and neck cancers in 2009.[3] Taiwan BNCT group also started treating head and neck cancers at the Tsing Hua Open-pool Reactor (THOR) of National Tsing Hua University on 11 August 2010.Sony VAIO VGN-NS92JS Battery

Early history of neutron capture therapy

After the initial discovery of the neutron in 1932 by Sir James Chadwick, a study by H. J. Taylor in 1935 showed the ability of the boron-10 nuclei to capture thermal neutrons. The neutron capture initiated the fission of the boron-10 nuclei into helium-4 alpha particles as well as lithium-7 particles. Sony VAIO VGN-NS92XS Battery

In 1936, Locher realised the potential of this discovery in the field of medicine and subsequently suggested that neutron capture could be used to treat tumours. A binary system uses two separate components for the therapy of cancer. Each component in itself is relatively harmless to the cells, but when combined together for treatment they produce a highly cytocidal effect which is lethal.Sony VAIO VGN-NW11S/S Battery

As development on neutron capture therapy continued, other radioactive isotopes such as uranium-235 were researched. However, studies in the late 1950s by Lussenhop et al. showed that the amounts of uranium needed for successful neutron capture therapy was too toxic for human use.Sony VAIO VGN-NW11S/T Battery

A boronic acid is an alkyl or aryl substituted boric acid containing a carbon–boron bond belonging to the larger class of organoboranes. Boronic acids act as Lewis acids. Their unique feature is that they are capable of forming reversible covalent complexes with sugars, amino acids, Sony VAIO VGN-NW11Z/S Battery

hydroxamic acids, etc. (molecules with vicinal, (1,2) or occasionally (1,3) substituted Lewis base donors (alcohol, amine, carboxylate)). The pKa of a boronic acid is ~9, but they can form tetrahedral boronate complexes with pKa ~7. They are occasionally used in the area of molecular recognition to bind to saccharides for fluorescent detection or selective transport of saccharides across membranes.Sony VAIO VGN-NW11Z/T Battery

Boronic acids are used extensively in organic chemistry as chemical building blocks and intermediates predominantly in the Suzuki coupling. A key concept in its chemistry is transmetallation of its organic residue to a transition metal.

The compound bortezomib with a boronic acid group is a drug used in chemotherapy. The boron atom in this molecule is a key substructure because through it certain proteasomes are blocked that would otherwise degrade proteins.Sony VAIO VGN-NW21EF/S Battery

Synthesis

Boronic acids can be obtained via several methods. The most common way is reaction of organometallic compounds based on lithium or magnesium (Grignards) with borate esters. For example phenylboronic acid is produced from phenylmagnesium bromide and trimethyl borate followed by hydrolysis .Sony VAIO VGN-NW21JF Battery

Another method is reaction of an arylsilane (RSiR3) with boron tribromide (BBr3) in a transmetallation to RBBr2 followed by acidic hydrolysis.

A third method is by palladium catalysed reaction of aryl halides and triflates with diboronyl esters in a coupling reaction. An alternative to esters in this method is the use of diboronic acid or tetrahydroxydiboron ([B(OH2)]2).Sony VAIO VGN-NW21MF Battery

Saccharide recognition

The covalent pair-wise interaction between boronic acids and 1,2- or 1,3-diols in aqueous systems is rapid and reversible. As such the equilibrium established between boronic acids and the hydroxyl groups present on saccharides has been successfully employed to develop a range of sensors for saccharides.Sony VAIO VGN-NW21MF/W Battery

One of the key advantages with this dynamic covalent strategy[24] lies in the ability of boronic acids to overcome the challenge of binding neutral species in aqueous media. If arranged correctly, the introduction of a tertiary amine within these supramolecular systems will permit binding to occur at physiological pH and allow signalling mechanisms such as photoinduced electron transfer mediated fluorescence emission to report the binding event.Sony VAIO VGN-NW21ZF Battery

Potential applications for this research include systems to monitor diabetic blood glucose levels. As the sensors employ an optical response, monitoring could be achieved using minimally invasive methods, one such example is the investigation of a contact lens doped with boronic acid based sensors to monitor glucose levels within ocular fluid.Sony VAIO VGN-NW31EF/W Battery

In organic chemistry, the hydroboration–oxidation reaction is a two-step organic reaction that converts an alkene into a neutral alcohol by the net addition of water across the double bond. The hydrogen and hydroxyl group are added in a syn addition leading to cis stereochemistry.Sony VAIO VGN-NW31JF Battery

Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction was first reported by Herbert C. Brown in the late 1950s[2] and it was recognized in his receiving the Nobel Prize in Chemistry in 1979.Sony VAIO VGN-NW320F/B Battery

where THF is tetrahydrofuran, the archetypal solvent used for this reaction. In the first step, borane (BH3) adds to the double bond, transferring one hydrogen from itself to the adjacent carbon. The second step substitutes the boron group BH2 with the hydroxyl group, creating the final product.Sony VAIO VGN-NW320F/TC Battery

The reaction described in the original 1957 Brown publication 1-hexene is converted to 1-hexanol.

Hydroboration mechanism

Borane exists as a toxic, colorless gas called diborane (B2H6). In diborane, two hydrogen atoms are each bonded to both boron atoms by single pairs of electrons ("three-center two-electron bonds"). Sony VAIO VGN-NW35E Battery

This delocalization satisfies the octet around each boron and reduces the electrophilicity. That said, even diborane is intensely Lewis acidic, because of its vacant p orbitals. Because dimerization happens instantaneously, it is not possible to isolate pure borane.Sony VAIO VGN-NW380F/S Battery

However, when diborane is treated with an ether or amine, a stable complex is formed, as the lone pair from the Lewis basic oxygen or nitrogen atom is donated to the borane. These complexes act chemically like borane. Solutions of BH3 complexes in THF or diethyl ether are commercially available and more easily handled than diborane gas, and so are the more common form found in laboratories. Sony VAIO VGN-NW380F/T Battery

For simplicity in illustration, borane will be used instead of the borane–ether complex in this article.

The addition of BH3 to the alkene is a concerted reaction, with multiple bond formation and breaking occurring simultaneously. The intermediate step can be visualized more clearly by a theoretical transition state.Sony VAIO VGN-NW50JB Battery

Knowing that the group containing the boron will be replaced by a hydroxyl group, it can be seen that the first step determines the regioselectivity. The hydroborane will add to the alkene so that the boron always ends up on the lesser substituted carbon. In the transition state, the more substituted carbon bears a partial positive charge (a partial carbocation). Sony VAIO VGN-NW51FB/N Battery

As a general rule, carbocations that are more substituted tolerate positive charge better than those that aren't. Had the hydroborane attacked with the opposite orientation, the lesser substituted carbon would bear the (partial) positive charge, which is electronically unfavorable. Hydroborane also tends to add to the lesser substituted carbon because it is less sterically hindered.Sony VAIO VGN-NW51FB/W Battery

Hydroborations are also stereoselective, with the new constituents adding in syn (on the same face of the alkene). Thus 1-methylcyclopentene reacts with diborane predominantly to the trans-alkane.

Until all hydrogens attached to boron have been transferred away, the boron group BH2 will continue adding to more alkenes.Sony VAIO VGN-NW70JB Battery

This means that one equivalent of hydroborane will conduct the reaction with three equivalents of alkene. Furthermore, it is not necessary for the hydroborane to have more than one hydrogen. Therefore, BH3 can be better represented as R-BH, where R can represents the remainder of the molecule. Sony VAIO VGN-NW71FB/N Battery

A widely used hydroboration reagent is 9-BBN which has just one hydrogen at boron and the same applies for catecholborane.

Hydroboration–oxidation

In the second hydroboration–oxidation step, the nucleophilic hydroperoxide anion attacks the boron atom.Sony VAIO VGN-NW71FB/W Battery

Alkyl migration to oxygen gives the alkyl borane withretention of stereochemistry (in reality, the reaction occurs via the trialkyl borate B(OR)3, rather than the monoalkyl borinic ester BH2OR).

Oxymercuration–reduction is another reaction that converts an alkene into an alcohol. Unlike hydroboration–oxidation, oxymercuration-reduction is not stereospecific. Sony VAIO VGN-NW91FS Battery

Further, oxymercuration–reduction is a Markovnikov reaction. Therefore, oxymercuration–reduction and hydroboration–oxidation are complementary because they add with opposite regiochemistry.

Alkyne hydroboration

A hydroboration reaction also takes place on alkynes. Sony VAIO VGN-NW91GS Battery

Again the mode of action is syn and secondary reaction products are aldehydes from terminal alkynes and ketonesfrom internal alkynes. In order to prevent hydroboration across both the pi-bonds, a bulky borane like disiamyl (di-sec-iso-amyl) borane is used.Sony VAIO VGN-NW91VS Battery

Organoborane reactions

Amines can be obtained from the intermediate organoborane by action of chloramine. Reaction with iodine or bromine afford the corresponding alkyl halides. Acarboxylic acid simply replaces the borane group by a proton.Sony VAIO VGN-SR11M Battery,Sony VAIO VGN-SR11MR Battery,Sony VAIO VGN-SR140D Battery,Sony VAIO VGN-SR140D/B Battery,Sony VAIO VGN-SR140D/P Battery

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