Trihalide of boron is a trigonal planar molecule. Explain
What are different types of carbides? Explain each of them briefly.
Answers
Answer:
Boron Halides
Boron undergoes halogenation to form trihalides with the molecular structure BX3, as in the following:
[latex]2B + 3Br_2 \rightarrow 2BBr_3[/latex]
In its most familiar compounds, boron has the formal oxidation state III, or +3. These include oxides, sulfides, nitrides, and halides. The trihalides form planar trigonal structures and are Lewis acids because they readily form adducts with electron-pair donors, which are called Lewis bases. For example, fluoride (F–) and boron trifluoride (BF3) combine to give the tetrafluoroborate anion, BF4–. Boron trifluoride is used in the petrochemical industry as a catalyst.
The halides react with water to form boric acid:
[latex]BX_3 + 3H_2O \rightarrow B(OH)_3 + 3HX[/latex]
where X can be Cl, Br, or I.
Lewis Acidity of Boron Halides
All three lighter boron trihalides, BX3 (X = F, Cl, Br), form stable adducts with common Lewis bases. Their relative Lewis acidities can be evaluated in terms of the relative exothermicities of the adduct-forming reaction. Such measurements have revealed the following sequence for the Lewis acidity: BF3 < BCl3 < BBr3 (in other words, BBr3 is the strongest Lewis acid).
This trend is commonly attributed to the degree of π-bonding in the planar boron trihalide that would be lost upon pyramidalization (the conversion of the trigonal planar geometry to a tetrahedral one) of the BX3 molecule, which follows this trend: BF3 > BCl3 > BBr3 (that is, BBr3 is the most easily pyramidalized). The criteria for evaluating the relative strength of π-bonding are not clear, however. One suggestion is that the F atom is small compared to the larger Cl and Br atoms, and the lone pair electron in the 2pzorbital of F is readily and easily donated, and overlaps with the empty 2pz orbital of boron. As a result, the [latex]\pi[/latex] donation of F is greater than that of Cl or Br. In an alternative explanation, the low Lewis acidity for BF3 is attributed to the relative weakness of the bond in the adducts F3B-L.
Boron trifluorideBoron (III) trifluoride structure, BF3, showing the “empty” boron p orbital in pi-type coordinate covalent bonds.
Boron Tribromide
Boron tribromide (BBr3) is a colorless, fuming liquid compound that contains boron and bromine. The reaction of boron carbide with bromine at temperatures above 300 °C leads to the formation of boron tribromide. The product can be purified by vacuum distillation.