Question

Decreasing order of reactivity of alkyl halide is: RCI, RI, RBr

Answer 1

Hint: Reactivity of Alkyl halide depends upon the size of the halogen atom joined to alkyl chain and also depends upon the length and complexity of the alkyl chain. Induction effect plays an important role in this.

Complete answer:
Alkyl halides (also known as haloalkanes) are compounds where one or more hydrogen atoms of an alkyl chain have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine).
The carbon atom, which is bonded to the halogen atom, has $s{{p}^{3}}$ hybridized bonding orbitals and exhibits a tetrahedral shape.
There is an electronegativity difference between the carbon atom to which halogen is bonded and halogen atom itself, the covalent bond between these atoms is polarized, with the halogen atom partially negative and the carbon atom becoming slightly positive. Hence creating an electronegativity difference.
Down the group in a periodic table, size of halogen atom increases and electronegativity of the halogen atom decreases.
Hence the bond length of carbon and halogen atom increases from fluorine to iodine. (Since fluorine is smallest atom and Iodine is largest atom of group $17$ of the periodic table)
That is why during the Elimination or substitution reaction, since C-I bond is weaker than C-Br and C-Cl, it is easier to remove Iodine atoms than compared to Bromine and Chlorine atoms.
Hence decreasing order of reactivity of alkyl halide is RI > RBr > RCl.

Note:
Reactivity of Haloalkane is also dependent upon the Alkyl chain, if the number of carbon atoms is more in an alkyl chain, then its more reactive. For an example ${{C}_{3}}{{H}_{7}}X$ is less reactive than ${{C}_{5}}{{H}_{11}}X$.Similarly if alkyl chain is tertiary then it is more reactive as compared to secondary or primary alkyl chain.