Question

What is the correct Lewis acid order for boron halide?
A.$BB{r_3} > BC{l_3} > B{I_3} > B{F_3}$
B.$B{I_3} > B{F_3} > BB{r_3} > BC{l_3}$
C.$B{F_3} > BC{l_3} > BB{r_3} > B{I_3}$
D.$B{I_3} > BB{r_3} > BC{l_3} > B{F_3}$

Answer 1

Hint: In the periodic table, if we move from top to bottom in a group then its acidic nature decreases i.e. tendency to lose the electrons increases because size increases rapidly as we move from top to bottom in a group.

Complete step by step answer:
Acids and bases according to Lewis.
Lewis acids: Lewis acids are those chemical species which can accept an electron easily. When we move down in a group of periodic tables then the size of elements increases, hence the ability to lose the electrons increases and ability to hold the electrons or the tendency to accept electrons from others decreases, hence the acidity decreases down the group.
Lewis base: Lewis acids are those chemical species which can lose electrons easily. When we move down in a group of periodic tables then the size of elements increases, hence the ability to lose the electrons increases and ability to hold the electrons or the tendency to accept electrons from others decreases, hence the acidity increases down the group.
Halides: Group $17$ elements, are known as halides.
In the fluorides of boron, boron is the center element and the general formula for the boron halides is $B{X_3}$ where $X$ is halides. In halides $p - $ orbitals are not completely filled. And boron has three electrons. After forming boron halides it has six electrons(by sharing three electrons from halides) and it needs two more electrons to complete its octet(to become stable). When halides are formed, then the halides which are bigger in size form pi$(\pi )$ back bonding. Due to which the order of acidity of boron halides is as follows:
$B{I_3} > BB{r_3} > BC{l_3} > B{F_3}$
Hence, the correct answer is option D.

Note:
Pi back bonding is formed due to presence of lone pairs present on the halides atoms. If there are lone pairs then pie back bonds can take place and if there are no lone pairs present then no pie back bond can take place.