Question

The order of reactivity of alkyl halides is as follows:
A. RCl < RBr < RI
B. RI < RBr > RCl
C. RBr > R I> RCl
D. RI > RCl > RBr

Answer 1

Hint: Halides belong to the Group 17 in the periodic table. In a Group, the size of the atom increases as we move down the group and hence, they are more reactive and can form bonds easily with the other atoms.

Complete step by step answer:
When we move down the group, we know that the size of the atom increases, which means additional shells are added and electrons reside farther than the Nucleus.
Nucleus is at the center of the atom which is made up of protons and neutrons which makes it positively charged and attract electrons towards it. Supposedly, the electrons which are in the s-orbital are held tightly by the nucleus, therefore, it wouldn’t let the electrons participate in the bonding easily.
However, in the 17 Group, where halogens are present are more electronegative as they all need only one more electron to complete their octet and stabilize. Hence, their natural tendency is to gain electrons and they attract electrons very strongly.
So, between the Nucleus attraction and bond-attraction, the deciding factor is bond-dissociation enthalpy which depends on the size and bond length.
As you move down the group, the size of the atom increases and attraction towards the nucleus is less and bond-length also increases, making it easier for the halogen to dissociate and form bonds. Increase in size also indicates an increase in reactivity of that particular halogen.
 So, the correct answer is “Option A”.

Note: The most electronegative atom is fluorine in the whole periodic table. Hence, the bond of R-F is very strong due to the more electronegativity, and small size of the atom. There is one more factor which affects the reactivity and that is R-F bond has very short bond length too. If it was included in the options, it would come at last.