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Boron Halides

The boron halides are covalent. BF3 is gaseous, BCl3 liquid and BI3 is solid. They are all hydrolyzed by water. BF3 is hydrolyzed incompletely while BCl3 and BI3 are completely hydrolyzed.

Hydrolysis of BF3
In this, the HF produced reacts with the H3BO3 to form fluoroborate.
4BF3 + 12H2O 4H3BO3 + 12HF
12HF + 3H3BO3 3H+ + 3[BF4]- + 9H2O
4BF3 + 3H2O H3BO3 + 3H+ + 3[BF4]-

Hydrolysis of BCl3 (and BI3)
BCl3 + 3H2O H3BO3 + 3HCl

BF3 is a useful organic catalyst for Friedel-Crafts reactions. It is also used extensively as a polymerization catalyst.

The fluorides of Al, Ga, In and Tl are ionic while the other halides are generally covalent and exist as dimers, thus attaining an octet of electrons. The structure of dimer (AlCl3)2 is

This dimeric structure is retained in non-polar solvents such as benzene. In water, the high enthalpy of hydration is sufficient to break the covalent dimer into [M . 6H2O]3+ and 3X- ions.

Anhydrous aluminium chloride is used as a catalyst in the Friedel-Crafts reaction. It is also used in petroleum cracking and in the manufacture of dyes, drugs and perfumes.

The trihalides of the Group 13 elements are electron-deficient compounds. In BX3, the electron deficiency is removed byπbonding. The other elements in the group have larger atoms and cannot get effective π overlap, so they polymerize to remove electron deficiency.

Structure of Boron Trifluoride

Outer configuration of boron atom

Configuration of boron in BF3

The sp2 hybridization leads to the triangular planar geometry. The observed bond length of 130 pm is shorter than the sum of the covalent radii (B = 80 pm and F = 72 pm).
all the three B—F bond distances are identical.

Acidic character of Boron Tri -halides
Boron trihalides are electron-deficient compounds. They act as Lewis acids. Due to back bonding, the electron density on boron is increased. The tendency to form pp-pp bond is maximum in BF3 and falls rapidly on passing to BCl3 to BBr3 due to the increase in size of halogen 2p orbital. Thus, tendency to accept electron pair is minimum for BF3 and BCl3 and is maximum for BBr3. Hence, the increasing order of acid strength is
BF3 < BCl3 < BBr3

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