# Boron trihalides acidity

A boron trihalide ($$\ce {BX3}$$) is a Lewis acid, because it can accept an electron pair, as per Gilbert N. Lewis' definition.

Now my question is-

Arrange $$\ce {BF3}$$, $$\ce {BCl3}$$, $$\ce {BBr3}$$ and $$\ce {BI3}$$ from the weakest to the strongest acid properties.

• Relative Lewis acidities parallel exothermicities of a Lewis base adduct-forming reaction. The adduct is a dative bond tetrahedral boron anion. The worst pi-bonding substituent has the highest energy versus product valley, maximizing exotherm difference. Order radii of the participating pi-orbitals by their worst matches. – Uncle Al Apr 27 '14 at 17:09
• hint; most important factor in BX3 is the bonding between X and B which is going to determine the acidic strength , and here we can clearly see that in BF3 there is the partial double bond character between atoms which is due to back bonding (2pi-2pi), and gradually back bonding weakens and lewis acidic character increases so your final order will be. $$\ce{BF3}<\ce{BCl3}<\ce{BBr3}<\ce{BI3}$$ – Jack Rod May 18 at 9:46

$$\ce{BF3}<\ce{BCl3}<\ce{BBr3}<\ce{BI3}$$
This stands in contrast to what is expected when the electronegativity of the halides is considered. A more electronegative halide should be able to stabilize the negative charge at $$\ce{B}$$ in the Lewis acid-base complex better, and this would suggest that $$\ce{BF3}$$ would be the strongest Lewis acid.
However, the nature of the $$\ce{B-X}$$ bond needs to be taken into account. The change of hybridization from $$\ce{sp^2}$$ in $$\ce{BX3}$$ to $$\ce{sp^3}$$ in $$\ce{[BX4]-}$$ involves the loss of partial $$\pi$$ bonding which results from the overlap of the $$\ce{p_z}$$ orbitals of $$\ce{B}$$ and $$\ce{X}$$. The $$\ce{p_z}$$ orbitals of $$\ce{B}$$ and $$\ce{F}$$ are of similar size, because both elements are members of the same period. Therefore, they have the best overlap and form the strongest $$\pi$$ bond. Because the reaction with a Lewis base would require some energy to break this bond, $$\ce{BF3}$$ is a comparably weak Lewis acid. The larger $$\ce{p_z}$$ orbitals of the heavier halides have increasingly less overlap with the smaller boron $$\ce{p_z}$$ orbital. Therefore, the $$\pi$$ bonding is weaker and formation of the tetrahedral Lewis acid-base adduct via rehybridization becomes energetically more favorable.