Substrate and Reagent

In organic chemistry, reactions rarely involve free ions. Instead, whole molecules interact. To keep track of what is happening, we name one reactant the substrate and the other the reagent. The substrate is the molecule whose carbon atom forms a new bond. The reagent is the other molecule that attacks it. When a carbon–carbon bond is formed, the choice is arbitrary — it depends on which molecule you are focusing on.

Example (i):

$CH_2=CH_2$ (substrate) $+ Br_2$ (reagent) $\rightarrow BrCH_2–CH_2Br$ (product)

Example (ii):

$C_6H_6$ (benzene, substrate) $+ CH_3Cl$ (reagent) $\rightarrow C_6H_5CH_3$ (methylbenzene, product) $+ HCl$ (product)

Nucleophiles and Electrophiles

Reagents attack the reactive site of the substrate. The reactive site can be an electron-deficient region — for example, an atom with an incomplete octet or the positive end of a dipole. If the attacking species is electron-rich, it goes for these electron-deficient spots. If the attacking species is electron-deficient, it seeks out an electron-rich part of the substrate, such as the $\pi$ electrons in a double bond.

A reagent that brings an electron pair to the reactive site is called a nucleophile ($Nu:$) — it is nucleus-seeking. The reaction is called nucleophilic.

A reagent that takes away an electron pair from the reactive site is called an electrophile ($E^+$) — it is electron-seeking. The reaction is called electrophilic.

During a polar organic reaction, a nucleophile attacks the electrophilic centre of the substrate — that specific atom or part of the substrate which is electron-deficient. Conversely, an electrophile attacks the nucleophilic centre — the electron-rich centre of the substrate. The electrophile receives an electron pair from the substrate when they bond. A curved-arrow notation ($\curvearrowright$) shows the movement of an electron pair from the nucleophile to the electrophile.

Examples of Nucleophiles

Nucleophiles are often negatively charged ions with a lone pair of electrons:

• Hydroxide ion: $HO^-$
• Cyanide ion: $NC^-$
• Carbanions: $R_3C:^-$

Neutral molecules can also act as nucleophiles because they possess a lone pair:

• Water: $H_2O:$
• Amines: $R_3N:$, $R_2NH$

Examples of Electrophiles

Electrophiles include positively charged species and neutral molecules with a polar bond that creates a partial positive charge on carbon.

• Carbocations: $\mathrm{CH_3^+}$ — the carbon has only six electrons (sextet configuration), so it is electron-deficient and can accept a pair of electrons from a nucleophile. …