SUBSTITUTION REACTIONS

• In a substitution reaction, one atom or group of atoms in a molecule is replaced by another atom or group of atoms.

  • The atom or group of atoms that is replaced is called the leaving group.

  • The atom or group of atoms that replaces the leaving group is called the nucleophile.

The molecule that the leaving group and then the nucleophile is bonded to is called the substrate.

Nucleophiles donate lone pairs of electrons to form covalent bonds.

Examples of nucleophiles:

Strong:

I-

Br-

Cl-

HS-

H2S

RSH

HO-

RO-

N≡C-

Weak:

F-*

H2O

ROH

*Note: in polar protic solvents, the nucleophilicity of halides increases as you go down the table (I- is the strongest) because the charge-to-surface area allows the halide to be least surrounded by solvent molecules. In polar aprotic solvents, the opposite trend is observed (F- is the strongest) because of the trend in electronegativity.

SN1

SN2