Question

Correct Lewis acidity order of $B{X_3}$ is-
A.$B{F_3} < BC{l_3} < BB{r_3} < B{I_3}$
B.$B{F_3} > BC{l_3} > BB{r_3} > B{I_3}$
C.$BC{l_3} < BB{r_3} < B{I_3} < B{F_3}$
D.$B{F_3} < B{I_3} < BB{r_3} < BC{l_3}$

Answer 1

Hint: Lewis gave the concept of acid and base according to electron pair acceptor and electron pair donor. According to Lewis: Lewis acids are the electron pair acceptor and Lewis base are electron pair donors.

Complete step by step solution:
Electron pair acceptor and donor: This concept was given by Lewis. According to Lewis: Lewis acids are those substances or solutions which can accept electron pairs and Lewis bases are those substances or solutions which can donate electron pairs.
The order of acidity of $B{X_3}$ is defined on the basis of $p - p$ back bonding. It is defined as when one atom has vacant p-orbitals and the atom to which it reacts has a lone pair then it donates its lone pair to the vacant p-orbitals of the atom, this process is known as $p\pi - p\pi $ back bonding. Now the boron and fluorine belong to the same period hence the size of these elements are not too much different i.e. their size are almost similar so here the greater amount if back bonding takes place. So now this compound will not have space to accept electron pairs from other compounds and hence it will be the least acidic. Now as we move down from chlorine to iodine then size increases so back bonding decreases (less back bonding will create more space so that they can accept electron pairs from the other compound) hence it will show high acidic character according to the Lewis concept.
Hence the order of Lewis acidity is as: $B{F_3} < BC{l_3} < BB{r_3} < B{I_3}$.

Therefore, option A is correct.

Note:
Electron affinity: This is defined as the property of elements to attract the electrons of other elements towards it. The higher the electron they will attract the higher will be their electron affinity. The halides atoms have high electron affinity in their respective periods.