Q.Explain why $\text{SiCl}_4$ is readily hydrolysed by water while $\text{CCl}_4$ is not, even though both molecules are tetrahedral $\text{MCl}_4$ species. What role do vacant $3d$ orbitals on silicon play?

[!TLDR] SiCl4 hydrolyses because silicon has empty, accessible 3d orbitals that let water attack via a 5-coordinate transition state; carbon, with no d orbitals available, cannot do this. [!ANSWER] Silicon's vacant $3d$ orbitals let an incoming water molecule attack via a temporary 5-coordinate transition state, so hydrolysis proceeds readily; carb …

$\text{SiCl}_4$ and $\text{CCl}_4$ are both simple tetrahedral $\text{MCl}_4$ molecules, yet their reactivity towards water is completely different: $\text{SiCl}_4$ hydrolyses rapidly, while $\text{CCl}_4$ is essentially inert to water even on prolonged contact. The difference lies not in thermodynamics (hydrolysis of $\text{CCl}_4$ would actually be thermodynamically favourable) but in kinetics, specifically in the mechanism by which hydrolysis must proceed. Nucleophilic attack by an incoming water molecule's oxygen lone pair on the central atom requires that atom to be able to expand its coordination number, at least temporarily, to five in the transition state, before a chloride ion finally leaves. Carbon's valence shell is $n=2$, containing only $2s$ and $2p$ orbitals with no low-lying, energetically accessible $d$ orbitals — carbon simply has nowhere to accommodate a fifth incoming group, so this five-coordinate transition state cannot form, and hydrolysis is blocked kinetically (even though the products would be more stable). Silicon, however, has vacant, accessible $3d$ orbitals in its valence shell, which can participate in bonding …

Contrast the mechanistic requirement (a five-coordinate transition state) for hydrolysis with the orbital availability on each central atom, explaining why sil …

Attributing the difference to a thermodynamic rather than kinetic cause; vaguely citing 'silicon is more reactive' without identifying …