SN2

[Ardo 2K]

Can someone please explain what a nucleophile is and how exactly do they replace certain atoms within a molecule to produce a completely new molecule?

[Xihui Yin 1I]

Nucleophiles have an extra pair of electrons (either lone pair or in a pi bond) and can attack an electrophile (an electron-deficient species, e.g. positively charged or partially positively charged) to form a bond. In a sense this means that something with the capacity to form a bond due to its extra pair of electrons donates a pair of electrons to one that needs a pair of electrons, and this occurs out of the electrostatic attraction between them. The electron pair is shared, and by definition of electron sharing, a covalent bond is formed.

However, for partially-positively charged species, they have a full octet set. The addition of two extra electrons means that they need to reform an octet, so another bond is broken in place and a bonded species leaves, thus it is essentially a substitution reaction.

[Bansi Amin 1D]

A nucleophile is a species that donates an electron pair, like a Lewis base. They replace another species that is more electronegative than the central atom that the nucleophile is being bound to. The substitution process occurs bc the leaving group is taking electrons from the central atom, and the central atom doesn't want that.