Question

why substitution reaction called as the very fast reaction

Answer 1

For bimolecular reactions, two molecules collide and react with each other. Their merger is called chemical synthesis or an addition reaction. Another possibility is that only a portion of one molecule is transferred to the other molecule. This type ofreaction occurs, for example, in redox and acid-base reactions

Answer 2

AHOY MATEY !!!

SUBSTITUTION REACTION :

a reaction in which an atom or a group of atoms in a given compound is replaced by other atom or group of atoms is called a substitution reaction..

A good example of a substitution reaction is halogenation. When chlorine gas (Cl-Cl) is irradiated, some of the molecules are split into two chlorine radicals (Cl.) whose free electrons are strongly nucleophilic. One of them breaks a weak C-H covalent bond and grabs the liberated proton to form the electrically neutral H-Cl. The other radical reforms a covalent bond with the CH3. to form CH3Cl (methyl chloride).

it is very fast because :

here the tertiary substrate reacts faster followed by the primary substrate

for example, CH₄ reacts with Cl in the presence of diffused sunlight. Hydrogen atoms of  CH₄ are replaced by Cl atoms

CH₃Cl+Cl₂-------> CH₃Cl + HCl

CH₂Cl₂+Cl₂------->CHCl₃ + HCl

CHCl₃+Cl₂ ---------> CCl₄ + HCl

WISH IT HELPS !!!

@__@