What are the main types of organic reactions and their mechanisms?

(A) The main types of organic reactions are:
$i$. Substitution reactions: In these reactions,an atom or group of atoms in a molecule is replaced by another atom or group.
$ii$. Addition reactions: These involve the addition of two molecules to form a single product,typically occurring in unsaturated compounds.
$iii$. Elimination reactions: These involve the removal of two atoms or groups from a molecule,often resulting in the formation of a double or triple bond.
$iv$. Rearrangement reactions: These involve the migration of an atom or group within the same molecule to form a structural isomer.