Lewis boron

C, b.p. 91 C, and has very similar properties to boron chloride but is a stronger Lewis acid. Diboron tetrabromide BjBr, is also known. 

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. 

The BF4 ion has a regular tetrahedral configuration. The most important property of boron trifluoride is its great capacity to act as an electron pair acceptor (Lewis acid). Some examples of adducts are ... 

Cl increases the Lewis acidity of boron making it a more reactive reagent... 

Nitration has also been effected with the complexes from dinitrogen tetroxide and Lewis acidsin the case of boron trifluoride the complex appears to be a mixture of nitronium and nitrosonium tetrafluoroborates. ... 

When mixed with Lewis acids, dinitrogen pentoxide yields crystalline white solids, which were identified as the corresponding nitronium salts by their infra-red spectra. The reaction with boron trifluoride can be formulated in the following way... 

Nitronium tetrafluoroborate was first prepared by adding a mixture of anhydrous hydrofluoric acid and boron trifluoride to a solution of dinitrogen pentoxide in nitromethane. Nitric acid can be used in place of dinitrogen pentoxide, and by replacing boron trifluoride by other Lewis-acid fluorides Olah and his co-workers prepared an extensive series of stable nitronium salts. ... 

Boron trichloride, usually in conjunction with an additional Lewis acid, effects o-chloroacetylation of anilines. The resulting products are converted to indoles by reduction with NaBH4.[l], The strength of the Lewis acid required depends upon the substitution pattern on the ring. With ER substituents no additional... 

Boron tnfluonde etherate (Lewis acid Lewis base complex)... 

The product of this reaction a Lewis acid Lewis base complex called informally boron tnfluonde etherate may look unusual but it is a stable species with properties different from those of the reactants Its boiling point (126°C) for example is much higher than that of boron tnfluonde—a gas with a boiling point of — 100°C—and diethyl ether a liquid that boils at 34°C... 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

Uses. Boron triduoride is an excellent Lewis acid catalyst for numerous types of organic reactions. Its advantages are ease of handling as a gas and the absence of undesirable tarry by-products. As an electrophilic molecule, it is an excellent catalyst for Friedel-Crafts and many other types of reactions (63-65) (see Friedel-craftsreactions). 

Fluorochloro, fluorobromo, and fluoroiodoalkanes react selectively with aromatics under boron trifluoride catalysis to provide chloro-, bromo- and iodoalkylated products (48). The higher reactivity of the C—F bond over C—Cl, C—Br, and C—I bonds under Lewis acid catalysis results in the observed products. 

In addition, boron, aluminum, and gallium tris(triduoromethanesulfonates) (tridates), M(OTf)2 and related perduoroalkanesulfonates were found effective for Friedel-Crafts alkylations under mild conditions (200). These Lewis acids behave as pseudo haUdes. Boron tris(tridate) shows the highest catalytic activity among these catalysts. A systematic study of these catalysts in the alkylation of aromatics such as benzene and toluene has been reported (201). 

The inactivity of pure anhydrous Lewis acid haUdes in Friedel-Crafts polymerisation of olefins was first demonstrated in 1936 (203) it was found that pure, dry aluminum chloride does not react with ethylene. Subsequentiy it was shown (204) that boron ttifluoride alone does not catalyse the polymerisation of isobutylene when kept absolutely dry in a vacuum system. However, polymers form upon admission of traces of water. The active catalyst is boron ttifluoride hydrate, BF H20, ie, a conjugate protic acid H" (BF20H) . 

Metal Alibis and Alkoxides. Metal alkyls (eg, aluminum boron, sine alkyls) are fairly active catalysts. Hyperconjugation with the electron-deficient metal atom, however, tends to decrease the electron deficiency. The effect is even stronger in alkoxides which are, therefore, fairly weak Lewis acids. The present discussion does not encompass catalyst systems of the Ziegler-Natta type (such as AIR. -H TiCl, although certain similarities with Friedel-Crafts systems are apparent. 

With the improvement of refining and purification techniques, many pure olefinic monomers are available for polymerization. Under Lewis acid polymerization, such as with boron trifluoride, very light colored resins are routinely produced. These resins are based on monomers such as styrene, a-methylstryene, and vinyltoluene (mixed meta- and i ra-methylstyrene). More recently, purified i ra-methylstyrene has become commercially available and is used in resin synthesis. Low molecular weight thermoplastic resins produced from pure styrene have been available since the mid-1940s resins obtained from substituted styrenes are more recent. 

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. 

Cationic polymerization of coal-tar fractions has been commercially achieved through the use of strong protic acids, as well as various Lewis acids. Sulfuric acid was the first polymerization catalyst (11). More recent technology has focused on the Friedel-Crafts polymerization of coal fractions to yield resins with higher softening points and better color. Typical Lewis acid catalysts used in these processes are aluminum chloride, boron trifluoride, and various boron trifluoride complexes (12). Cmde feedstocks typically contain 25—75% reactive components and may be refined prior to polymerization (eg, acid or alkali treatment) to remove sulfur and other undesired components. Table 1 illustrates the typical components found in coal-tar fractions and their corresponding properties. 

Lewis Acid Complexes. Sulfolane complexes with Lewis acids, such as boron trifluoride or phosphoms pentafluoride (17). For example, at room temperature, sulfolane and boron trifluoride combine in a 1 1 mole ratio with the evolution of heat to give a white, hygroscopic soHd which melts at 37°C. The reaction of sulfolane with methyl fluoride and antimony pentafluoride inhquid sulfur dioxide gives crystalline tetrahydro-l-methoxythiophenium-l-oxidehexafluoroantimonate, the first example of an alkoxysulfoxonium salt (18). 

The boron tnhahdes are strong Lewis acids, however, the order of relative acid strengths, BI > > BCl > BF, is contrary to that expected... 

Sodium borobydride reacts with Lewis acids in nonprotic solvents to yield diborane [19287-45-7] 2 6 which can then be used to generate other useful organoboranes such as amine boranes, alkyl boranes, and boron hydride clusters. 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

The epoxidation is generally conducted in two steps (/) the polyol is added to epichlorohydrin in the presence of a Lewis acid catalyst (stannic chloride, boron triduoride) to produce the chlorohydrin intermediate, and (2) the intermediate is dehydrohalogenated with sodium hydroxide to yield the aliphatic glycidyl ether. A prominent side-reaction is the conversion of aliphatic hydroxyl groups (formed by the initial reaction) into chloromethyl groups by epichlorohydrin. The aliphatic glycidyl ether resins are used as flexibilizers for aromatic resins and as reactive diluents to reduce viscosities in resin systems. 

Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process. The Lewis acid catalysts frequently employed are complexes of boron trifluoride with amines or ethers. 

Catalysts. The choice of the proper catalyst for an esterification reaction is dependent on several factors (43—46). The most common catalysts used are strong mineral acids such as sulfuric and hydrochloric acids. Lewis acids such as boron trifluoride, tin and zinc salts, aluminum haHdes, and organo—titanates have been used. Cation-exchange resins and zeoHtes are often employed also. 

Ethers are weakly basic and are converted to unstable oxonium salts by strong acids such as sulfudc acid, perchlodc acid, and hydrobromic acid relatively stable complexes ate formed between ethers and Lewis acids such as boron trifluodde, aluminum chlodde, and Gtignatd reagents (qv) (9) ... 

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