Question

Which factors determine the reactivity of halogens in substitution reaction?

Answer 1

Hint: Halogens form an important part and it has very important events in the reactions of alkanes. They substitute the hydrogen atom of alkanes.

Complete answer:
Substitution reactions are also known as the displacement reactions. The substitution reactions involve the process of replacement of one functional group of a chemical molecule or compound by the other. Halogens are widely used in the substitution reactions of the alkanes.
The substitution reactions of the alkanes generally occur in the presence of Sunlight and generate halo alkanes or alkyl halides. These substitution reactions follow a free radical mechanism.
There are mainly two factors which determine the reactivity of halogens in substitution reactions:
Activation energy - the activation energy is the smallest required amount of energy which enables the reactants to make collisions and form products. The activation energy of halogens increases with the increase in size and the number of collisions decreases. So, the reactivity order of the alkyl halides is
                           F > Cl > Br > I
 Iodine has the greatest size and largest activation energy among all halogens. Fluorine is the most reactive halogen in terms of substitution reactions.
2)Heat of reaction- the heat of reaction refers to the quantity of heat which should be removed or added from or to a chemical reaction. The substitution reaction of halogens is an exothermic process in which the heat is released and is called negative heat of reaction.
The heat of reaction increases from fluorine to Iodine. Iodine has maximum heat of reaction. So, the order of reactivity of alkyl halides is again the same according to heat of reaction.

Note: Iodine, although, is the least reactive halogen in substitution reactions but it is a very good leaving group, better leaving property of iodine as compared to other halogens is due to its large size and less electronegativity.