Explanation
B4C and SiC are covalent compounds which do not react with water. Salt like carbides such as CaC2 and Al4C3 react with water, the former giving acetylene and the latter evolving methane.
Al4Cl3 + 12H2O → 4Al(OH)3 + 3CH4
Because of large electronegativity difference between Al and F in AlF3 and Al and oxygen in Al2O3, Al and F in AlF3 and sulphate group in Al2(SO4)3 all these are ionic compounds. In case of AlCl3 the hydration energy of Al is unable to overcome the larger value of ionization energy and therefore AlCl3 in anhydrous state is covalent.
In BCl3, the pπ − pπ overlap between the 2p – orbital of B and 3 p–orbital of Cl is less effective because of dissimilar sizes but in AlCl3, pπ − pπ overlap between the 3p – orbital of Al and 3p – orbital of Cl is more effective because of similar sizes. As a result, back donation is more and electrondeficiency of Al si reduced realative to B and hence BCl3 is a stronger Lewis acid than AlCl3.
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