Question

Decreasing order of reactivity of alkyl halides: RCl, RI, RBr.

Answer 1

The electronegativity is directly proportional to bond dissociation enthalpy. The bond dissociation enthalpy is inversely proportional to reactivity. Thus, more is the bond dissociation enthalpy lower is the reactivity.

Complete answer:
The periodic table consists of different elements arranged in vertical columns and horizontal rows.
These different elements are arranged in the increasing order of atomic numbers.
The periodic table consists of a total of $118$ elements.
The vertical columns are called groups and horizontal rows are called periods.
There are $7$ periods and $18$ groups in the periodic table.
The ${17^{th}}$ group elements are called halogens.
Fluorine (F), Chlorine (Cl), Bromine (Br) and Iodine (I) are halogens.
Halogens are non-metals and deficient of one electron to attain inert gases configuration.
The electronegativity decreases down the group.
It is defined as the tendency to attract a shared pair of electrons towards itself.
Fluorine is the most electronegative element in the periodic table.
The order of electronegativity of halogens are:
$F > Cl > Br > I$
Thus, the order of electronegativities of given alkyl halides will be $RF > RCl > RBr > RI$
Thus, the bond dissociation enthalpy order of halogens will be
$F > Cl > Br > I$
Thus, the bond dissociation enthalpy order of given alkyl halides will be
$RF > RCl > RBr > RI$
The bond dissociation enthalpy is defined as the energy needed to break the chemical bond present between two atoms or groups.
More is the amount of energy needed to break, lesser is the reactivity.
Thus, the order of reactivity of alkyl halides is inversely proportional to the order of bond dissociation enthalpy of alkyl halides.
$RI > RBr > RCl > RF$

Note:
The halogens electronegativity must be considered carefully, it was increasing down the group.
The bond dissociation energy is directly proportional to electronegativity; thus, the bond dissociation energies are also increasing down the group.
The bond dissociation energy is inversely proportional to reactivity. Thus, the reactivity of halogens decreases down the group.