Q. Use of Halides of Boron?

Boron trihalides of the type BX3 exist for all the four halogens. Boron trifluoride can be prepared on a large scale by the fluorination of boric oxide or borax with fluoaspar and conc. H₂S0₄:

B₂0₃ + 3CaF₂ + 3H₂S0₄-------------------> - 2BF₃ + 3CaS0₄, + 3H₂0

Na₂B₄O₇ + 6CaF₂ + 8H₂S0₄ ---------------> 4BF₃ + 6CaS0₄, + 6NaHS0₄, + 7H₂0

In the laboratory, pure BF3 is best prepared by thermal decomposition of Gene diazonium tetrafluoroborate, PhN₂BF₄:

PhN₂BF₄-----------------------------------------> PhF + N₂ + BF₃

BCI₃, and BBr₃, are prepared on a large-scale by direct halogenation of B₂O₃ in the presence of C, e:g:

In the laboratory BCl₃ and BBr₃, are prepared by halogen exchange reaction between BF₃ and Al₂X₆:

2BF₃+ Al₂X₆----------------------> 2BX₃+ 2AlF₃ where X = Cl and Br

BI₃ is prepared in good yield by reacting-LiBH₄, or NaBH₄, with I₂ at 400 and 475 K, respectively:

LiBH₄, + 4l₂-------------------------------> Bl₃ + LiI + 4HI