Question

In silicon tetra-halides the Lewis acid strength is maximum for-
A)${\text{Si}}{{\text{I}}_{\text{4}}}$
B)${\text{SiC}}{{\text{l}}_{\text{4}}}$
C)${\text{SiB}}{{\text{r}}_{\text{4}}}$
D)${\text{Si}}{{\text{F}}_{\text{4}}}$

Answer 1

Lewis acids are acids which easily accept electron pairs. The more their capability to accept electron pairs, the more is their acidic character. Here all silicon tetra-halides are Lewis acid. But their acid strength varies.

Complete step by step answer:
We know that the fluorine has the highest electro-negativity so the silicon element will be more attracted towards fluorine than any other halide. Similarly the electro-negativity in Iodine is lowest due to increase in atomic radius and shielding effect so ${\text{Si - I}}$ will have the least attraction. In ${\text{Si}}{{\text{F}}_{\text{4}}}$, Fluorine will attract the electrons of silicon toward itself which will make it easier for silicon to accept electron pair. Chlorine is the next most electronegative element so silicon will be attracted towards it, although the attraction will be less than silicon tetra-fluoride. So the order of strength of Lewis acid will be-
$\Rightarrow {\text{Si}}{{\text{F}}_{\text{4}}} > {\text{SiC}}{{\text{l}}_{\text{4}}} > {\text{SiB}}{{\text{r}}_{\text{4}}} > {\text{Si}}{{\text{I}}_{\text{4}}}$
This means that the maximum Lewis acid strength is in ${\text{Si}}{{\text{F}}_{\text{4}}}$.

Hence the answer is option D.

Additional Information:
Some facts about silicon tetra-halides-
1. ${\text{Si}}{{\text{F}}_{\text{4}}}$ is a colorless gas and has a strong odor. It is used to seal water out of oil-wells. It is also used in the production of silicon and fluosilicic acid.
2. ${\text{SiB}}{{\text{r}}_{\text{4}}}$ is used in chemical reduction with alkaline metals to prepare photo-luminescent silicon nano-crystal.
3. ${\text{SiC}}{{\text{l}}_{\text{4}}}$ is colorless, fuming liquid which has a pungent smell. It is corrosive to metals and tissue in presence of moisture. It is used in smoke screens. It is also used to make various silicon-containing chemicals.
4. ${\text{Si}}{{\text{I}}_{\text{4}}}$ reacts with light, air and moisture. It is used to prepare silicon amides. It is also used in etching of silicon in microelectronics.

Note:
In ${\text{SiB}}{{\text{r}}_{\text{4}}}$, due to back bonding, bromine donates its electron in vacant d-orbital of silicon more than fluorine and chlorine elements. Hence its acidic strength is less than ${\text{SiC}}{{\text{l}}_{\text{4}}}$ and ${\text{Si}}{{\text{F}}_{\text{4}}}$.