SN1 vs SN2

1Introduction¶

Substitution reactions are very important in Chemistry. In general, a leaving group is replaced by a nucleophile.

Nucleophile

are electron rich species (lone pair of electrons) and are attracted to positive centers (nucleus loving). These guys donate their pair of electrons to form new covalent bonds in substitution reactions.

Electrophile

are electron deficient species. They are typically +'vely or $\delta$+'vely charged. These guys accept a pair of electrons from the nucleophile.

Substrate

is the starting compound which is to be substituted. This will generally be the electrophile.

Leaving Group

is/are the atom/atoms that leave the molecule and are replaced in the substrate.

Bases

are atoms with a lone pair of electrons.

Steric Hinderance

occurs when we have large groups in a molecule. When two groups of a molecule/between molecules come in close proximity there is repulsion between them which has an added energy cost.

1.1Bases Vs. Nucleophiles¶

How can we differentiate between Bases and Nucelophiles?
In general, bases use their lone pair to pull protons (hydrogen atoms) from a molecule while a nucleophile uses the lone pair to attack the substrate (i.e. donate its electrons to form a bond.

2Factors affecting Nucleophiles¶

• Good Nucleophiles are generally: negatively charged, large atoms, small molecules.
• If a nucelophile is sterically hindered. It will react more slowly. This is because these nucleophiles cannot approach to attack the substrate.

Warning: We speak of large atoms and small molecules. This is an important differentiation!