Boron halide

Boron tnhahdes, BX, are trigonal planar molecules which are sp hybridized. The X—B—X angles are 120°. Important physical and thermochemical data are presented in Table 1 (8—14). Additional thermodynamic and spectroscopic data may be found in the hterature (1 5). 

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

primary and secondary amines, and lower alcohols, BCl, BBr, and BI react to hberate the corresponding hydrogen hahde. Tertiary alcohols and the boron tnhahdes yield the alkyl hahde and boric acid. The boron tnhahdes hydrolyze readily in water or moist air to produce boric acid and hydrogen hahdes. 

and BI undergo exchange reactions to yield mixed boron hahdes. Exchange reactions also occur with trialkyl, triaryl, trialkoxy, or triaryloxy boranes and with diborane. Anhydrous metal bromides and iodides can be prepared by the exchange reaction of the metal chloride or oxide and BBr or BI (21)- 

Boron tnhahdes can be reduced to elemental boron by heating and presence of alkah metals, alkaline-earth metals, or (22—26) such reductions 

Boron halides, such as BF3 or BC13, are electron-deficient molecules because they do not have an octet of electrons surrounding the boron atom. In accord with this property, they tend to act as strong Lewis acids by accepting electron pairs from bases to form stable acid-base adducts. Such electron donors as pyridine or ether can be used  

Because they are strong Lewis acids, the boron halides act as acid catalysts for several important organic reactions. 

Boron halides also form some complexes of the type BX4. For example, 

The BCLf ion is tetrahedral because the addition of the fourth Cl- means that an additional pair of electrons must be accommodated around the boron atom. 

Bond Type Sum of Covalent Single Bond Radii (pm) rAB Calculated from Eq. (8.6) (pm) Expt l rAB (pm) 

Boron Halides.—An ab initio MO calculation on BF2, using a near-Hartree-Fock atomic basis, predicts a bond angle of 120° and a bond length of 1.22 A for the X2(Aj) ground state.250 

A relative acidity scale based on the proton shifts of the diethyl etherates of BC13 and BF3 has been extended to a number of trifluorovinylchloro-boranes.254 The following sequence was observed, with approximate values as shown  

These are consistent with a lack of interaction between the 7r-electrons of the trifluorovinyl group and the boron p,-orbital. 

Preliminary results of a 13C n.m.r. study of BF3 and BC13 complexes with a number of ethers have been reported.255 At low temperatures it was possible to detect separate 13C signals for free and co-ordinated ether molecules. The 13C shifts at the a-carbon are to lower field in the complexes, and decrease in the order THF Et20 PrzO — Bu20. 

Ab initio calculations of the heats of formation of a series of BF3 addition compounds (with, for example, NH3, H20, F , CO, CF, H2S, Ne, or Ar) reflect the trends in the experimental data, where these are available.256 Complexes which are unknown (Ne, Ar, H2S, and CO) all have negative heats of formation, but complex formation is not favoured by the entropy changes. 

Boron Halides.—N.m.r. data have been recorded for the adducts formed by (XC6H4)3 PC1 with boron trihalides. A correlation was found between the shielding of the orfho-protons and the stability of the adducts. 

Mixed boron trihalide adducts of McsP, McaPO, and MesPS are formed by halogen redistribution from the BX3 adducts themselves. For the MesPS 

The reaction with BF3 of LiAlX4 (X = SMe, SeMe, or NMe2), which is generated in situ, by a reaction such as (31), provides a convenient synthetic route to BX3.  

LMO calculations have been reported for the boron fluorides BF, BHjF, BHF2, BF3, BF2NH2, B4F4, and B2F4. The LMO valence structures obtained by the Boys procedure are typically (33) and (34), where the solid line originates at the atom donating an electron pair, and becomes dotted toward the atom which is electron deficient (indicating bond polarity). These are described as fractional bonds . 

Ion-molecule studies reveal that HBF2 and BF3 are protonated by H3, but not by CHj. This sets the boundaries to their proton affinities as 5.10 0.40eV.  

Boron Halides.— Timms has published a short review on the preparation and properties of boron subhalides.  

Resonance fluorescence has been observed from diatomic BF produced in a microwave discharge.  

Ab initio SCF-MO calculations have been performed on BFg, and alterations in predicted geometries with different gaussian-orbital basis sets noted. Further non-empirical SCF calculations on this molecule have been reported by Rothenberg and Schaeffer. Values of the dipole moment, quadrupole moment, octupole moment, second and third moments of the electronic charge distribution, diamagnetic susceptibility, and electric field gradient were calculated. 

In conjunction with a study of bonding in the nitrate ion, Maria etal. have reported the electronic absorption spectra of all four boron halides 6X3. Vacuum-u.v. spectra of BF3, BCI3, and BBrg have been obtained, and tentative assignments made. The lowest-frequency band in BCI3 and BBrg is assigned to a valence-shell transition of the type. 

Photoelectron spectra have been observed for BX3 and BX2Y species (where X, Y = F, Cl, Br, I, NMeg, or Me). These were assigned, for the trihalides, on the basis of (i) line shapes, (ii) calculations of spin-orbit 

Boron Halides.—Non-empirical valence-electron (NEVE) M.O. calculations have been performed on all of the monohalides of Group m (B to In, F to I), llie trends in calculated properties are as expected, and the results agree well with available all-electron ab initio results, which are much more costly in computer 

Equilibria have been investigated in the system LjBF4-H20. Two crystalline modifications of hydrated LiBF4 were found. 

A number of novel complexes have been reported in which boron halides are co-ordinated to Pt° or Pt . Reaction (13) is an e mmple. This is apparently a true 

Mossbauer studies on (C5Hs)Fe(C5H4BX2), where X = F, Cl, Br, or I, show that the quadrupole splitting AE° decreases with increasing Lewis acidity of the dihalogenoboryl group, although the isomer shift remains constant. 

He (I) photoelectron spectra of mixed halides of boron, BX X 3 , where X, X = Q or F, n = 1 or 2, have been recorded and assigned, using the results of studies of mixtures of BX3+BX3. The mixed halides were present in essentially 

The interaction of boron fluoride with water was the object of a very careful study by Clayton and Eastham . They found that in ethylene chloride two hydrates, 

BF3 HjO and BF3 - 2 H2O, are formed at 25 C in equilibrium with free boron fluoride. In the same paper, the isomerisation of butene-2 by this catalytic mixture was examined kinetically. The rate of isomerisation was found to be proportional to the free BF3 as well as to its water-complex concentration, and to the olefin concentration. No complex between boron fluoride and mcmomer was identified, but in their conclusions the authors noted that a mechanism involving attack of the complex BF3 - H2O the complex butene-BF3 would satisfy the kinetics, but seems to have little else to recommend it . In fact we feel that precisely that mechanism was operative, through an Ad 3 intermediate, the Lewis acid-olefin complex being formed, but as in many other instances being too weak to be detected. In the light of present-day knowledge, this conclusion seems quite acceptable, but twenty years ago it probably seemed far-fetched. In any case, the authors clearly realised this possibility, but were too cautions about proposing it outri t. 

Bjrwater and Worsfdd examined the role of water cocatalysis in the dimerisation of 1,1-diphenylethylene catalysed by boron fluoride at 20 °C in methylene chloride. A temperature-dependent equilibrium between the carbenium ions produced and the reactants was observed and the quantitative relationship expressing this correlaticm was given as  

Kennedy and coworkers recently stoidied the different catalytic behaviour of boron fluoride, chloride and bromide in the polymerisation of isobutene cocatalysed by water With BF3—H2O, the monomer is readily pcJymerised and the product yield 

The ionic product can either rapidly collapse into the t-butyl halide by reaction with the anion (termination) or propagate with the monomer. When X = F, the anion is stable and the sustained ionisation favours polymerisation. When X = Cl, the anion is bulky and tends strongly to release a chloride ion to the carbenium ion, unless its stability is enhanced by a hi er polarity of the medium. When X = Br, the instability of the anion is so marked, that formation of BBraOH and Br is practically instantaneous and the carbenium ion pairs collapses without any chance to propagate, even in a polar surrounding. 

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These molecules exist in the solid halides, explaining the low melting points of these halides, and also in the vapour phase at temperatures not too far above the boiling point. At higher temperatures, however, dissociation into trigonal planar monomers, analogous to the boron halides, occurs. 

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

Sodium borohydride reacts with boron halides to form diborane [19287-45-7] 2 6 more conveniendy handled as the monomer... 

From Boron Halides. Using boron haUdes is not economically desirable because boron haUdes are made from boric acid. However, this method does provide a convenient laboratory synthesis of boric acid esters. The esterification of boron haUdes with alcohol is analogous to the classical conversion of carboxyUc acid haUdes to carboxyUc esters. Simple mixing of the reactants at room temperature or below ia a solvent such as methylene chloride, chloroform, pentane, etc, yields hydrogen haUde and the borate ia high yield. 

Chemistry of Organic Fluorine Compounds II Table 27. Alkylation of Boron Halides... 

The main chemical products produced from these minerals are (a) boron oxides, boric acid and borates, (b) esters of boric acid, (c) refractory boron compounds (borides, eu .), (d) boron halides, (e) boranes and carbaboranes and (f) organoboranes. The main industrial and domestic uses of boron compounds in Europe (USA in parentheses) are ... 

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