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Question: which is better lewis acid SiBr4 ot SiI4...

which is better lewis acid SiBr4 ot SiI4

Answer

SiI4

Explanation

Solution

A Lewis acid is a species that can accept an electron pair. Silicon in silicon tetrahalides (SiX₄) acts as a Lewis acid due to the presence of vacant 3d-orbitals, which can accept electron pairs. The Lewis acid strength of SiX₄ compounds is determined by a combination of two factors:

  1. Electronegativity of the Halogen (X): A more electronegative halogen withdraws more electron density from silicon, making silicon more electron-deficient and thus a stronger Lewis acid. Based on electronegativity (F > Cl > Br > I), SiF₄ would be expected to be the strongest Lewis acid.

  2. pπ-dπ Back-bonding: The halogens (X) have lone pairs of electrons in their p-orbitals, which can be donated into the vacant 3d-orbitals of silicon. This back-bonding reduces the positive charge and electron deficiency on the silicon atom, thereby decreasing its Lewis acidity. The effectiveness of this back-bonding depends on the size and energy match between the p-orbital of the halogen and the 3d-orbital of silicon.

    • For fluorine (2p), the overlap with silicon's 3d-orbitals is quite effective due to comparable size and energy. This leads to significant back-bonding in SiF₄.
    • As we move down the halogen group to Cl (3p), Br (4p), and I (5p), the size of the halogen's p-orbital increases, and the energy difference between the halogen's p-orbital and silicon's 3d-orbital also increases. This results in progressively less effective pπ-dπ back-bonding.
    • Therefore, the extent of back-bonding decreases in the order: SiF₄ > SiCl₄ > SiBr₄ > SiI₄.

In silicon tetrahalides, the back-bonding effect is generally dominant over the inductive effect (electronegativity). Since stronger back-bonding reduces the Lewis acidity (by making silicon less electron-deficient), the compound with the least effective back-bonding will be the strongest Lewis acid.

Based on this, the order of Lewis acid strength for silicon tetrahalides is: SiI₄ > SiBr₄ > SiCl₄ > SiF₄

Comparing SiBr₄ and SiI₄: SiI₄ has less effective pπ-dπ back-bonding compared to SiBr₄ because iodine's 5p orbitals are much larger and less energetically compatible with silicon's 3d orbitals than bromine's 4p orbitals. This means that the silicon atom in SiI₄ is more electron-deficient than in SiBr₄.

Therefore, SiI₄ is a better (stronger) Lewis acid than SiBr₄.