Question

3. Arrange the following in increasing order of Lewis acidic character
BF3, BC13, BBr3, and Bl3

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Answer 1

hey

One thing your must remember, ie., Acidity increases down the group in a periodic table.

A species which is more electron deficient will attract electron easily and hence will be more acidic. Boron atom in boron halides, ie BX3, has six electrons in the outermost orbit and thus it can accept a pair of electrons form a donor molecule like NH3 to complete its octet.

The effective overlap between orbital's decreases due to large size of p orbital of halides. Hence,electron deficiency of boron atom in its halides follow the trend--BI3>,BBr3,>BCL3>BF3

Hence Acid character of boron trihalides is found to obey the order ; BI3>BBr3>BCl3>BF3.

Moreover, as per Ligand close packing model, it states that the distance between X atoms in AXn molecule is same always. Thus, F-F distance in BF3 & in BF4- is same. & the radius is considered half of the distance. But the B-F bond is longer in the tetrahedral geometry to maintain LCP model. Thus, if a lewis acid forms adduct with a lewis base, central atom takes tetrahedral geometry. Thus, the bond length increases. Now, this increase of length is easier in case of weaker B-I bond than stronger B-F bond. Thus, lewis acidity increases as BF3<BCl3<BBr3<BI3. Hope that you have got the point.

hope this answer helpful u

Answer 2

The correct order of Lewis acidic character is BF3 < BC13 < BBr3 < Bl3.

• A Lewis acid is defined as the chemical compound which contains an empty orbital that can add an electron pair from a Lewis base to form a Lewis adduct.

• Higher charges on the cations of the metals tend to have higher Lewis acidity characters.
• The smaller the size of the metal the higher Lewis acidity the compound can show.
• Due to the large size of the p orbital halides, the effective overlap between the orbitals will be decreased.
• The Higher size of the anion implies higher power to pull the electron pairs.
• The anions with lower anion size also have a back bonding effect which decreases the Lewis acidity.
• Considering all the factors mentioned above, the Lewis acidic character order of the compounds BF3, BC13, BBr3, and Bl3 is BF3 < BC13 < BBr3 < Bl3.

Hence, the correct order is BF3 < BC13 < BBr3 < Bl3.