Halides of boron

Boron trifluoride is most important of all the halides of boron and is obtained as a colourless highly pungent gas by heating boron trioxide with CaF2 and Cone. H2S04. 

The mechanism of initiation of cationic polymerisations by metal halides was clarified and systematized to some extent by the discovery of the phenomenon of co-catalysis or co-initiation. But, whereas there was, by the mid-1960s, good evidence that at any rate in many systems the halides of boron, titanium, and tin required a co-initiator, the position with regard to the best-known and most popular initiator, and the one which was of greatest economic significance, aluminium chloride, remained obscure. Of the vast number of published experiments on the system, aluminium chloride + isobutylene, hardly any could provide evidence concerning the initiation reaction, because they were almost exclusively concerned with measurements of yields and degree of polymerisation (DP). 

Although it Wits long believed that most Friedel-Crafts acids, particularly halides of boron, titanium, and tin. require an additional cation source to initiate polymerization, recent results show that in many systems Friedel-Crafts acids alone are able lo initiate cationic polymerization. 

The other halides of boron and silicon, and also of, phosphorus, sulphur, selenium, tellurium, and iodine, react at once with water, forming hydrogen halide and an acid. The equations are as follows —... 

However, many species which are acids under Lewis definition cannot be termed so according to Bronsted definition. A few examples are sulphur trioxide and halides of boron, aluminium, iron (ferric) and zinc. The central atom in each is able to accept a pair of electrons to complete is octet. 

In relation to halides of boron, phosphine resembles ammonia, giving addition compounds of similar but not identical type such as 2BF3.PH3 and BC13.PH3, which are, however, more easily dissociated than the corresponding ammines. 

The simplest hydrides and halides of boron, silicon, germanium, phosphorus, and arsenic can be decomposed in electric discharges to form... 

The frequencies listed for the formate and acetate ions were obtained in aqueous solution. Vibrational spectra of metal salts of these ions are discussed in Sec. 3.8. Although not listed in this table, the IR spectra of binary mixed halides of boron [767] and aluminum [768] have also been reported. 

If one of the Y atoms of a planar XY3 molecule is replaced by a Z atom, the symmetry is lowered to C2U. If two of the Y atoms are replaced by two different atoms, W and Z, the symmetry is lowered 10 C. As a result, the selection rules are changed, as already shown in Table 1-12. In both cases, all six vibrations become active in infrared and Raman spectra. Table II-4c lists the vibrational frequencies of planar ZXYj and ZXYW molecules. Although not listed in this table, the infrared spectra of binary mixed halides of boron" and aluminum " have been measured. The frequencies listed for ihe formate and acetate ions were obtained in aqueous solution. These frequencies are important when we discuss the vibrational spectra of metal salts of these anions (Sec. HI-7). 

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... 

Halides of boron are Lewis acids, with an acceptor strength that follows the order BI3 > BBrg > BCI3 > BFj. 

The halides of boron are readily hydrolyzed by the moisture in air. They react with a wide variety of electron-pair donors to form coordination complexes of the type RsN iBXs. In general, the halogens are not progressively replaced by other groups to form mixed halides except by means of the Grignard reagent. 

On the other hand halides of boron, aluminium and titanium are not solvolysed by the pure solvent. 

Boron forms trihalides with all the four halogens. Boron trifluoride is the most important halide of boron. 

Typical examples of this class of Lewis acids are electron-deficient molecules such as the halides of boron, beryllium and aluminium, for example, BCI3, BeCl2 and AICI3. 

Reaction of TMSC(Li)N2 with Halides of Boron, Phosphorus, Sulfur, and Tin. TMSC(Li)N2 undergoes the reaction with halides of boron,phosphorus, and sulfur to give the corresponding diazo trimethylsilylmethyl derivatives (eqs 36-38), while tin(II) chloride affords the carbodiimide (eq 39). ... 

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