21.1 - Nucleophilic Substitution Reactions

Nucleophiles are electron-pair donors

A nucleophile is an electron-rich species that contains a lone pair of electrons. It can be neutral or negatively charged.

Examples of nucleophiles include:

• Anions - Halide ions (Cl^-, Br^-, I^-), hydroxide ion (OH^-), cyanide ion (CN^-).
• Neutral molecules - Water (H₂O), ammonia (NH₃), methylamine (CH₃NH₂).

Nucleophiles donate an electron pair to electron-deficient species called electrophiles, forming a covalent coordination bond.

The strength of a nucleophile depends on how readily it can donate its electron pair to an electrophile. Stronger nucleophiles donate their electrons more easily.

Nucleophilic substitution reactions of halogenoalkanes

In nucleophilic substitution reactions, the nucleophile donates an electron pair to the electrophile, forming a new covalent bond. At the same time, one of the existing bonds in the electrophile breaks heterolytically, releasing a small molecule or ion called a leaving group (X^-).

Where:

• Nu^- = nucleophile
• R = electron-deficient atom in the electrophile
• X = leaving group

Halogenoalkanes (R-X) are ideal substrates for nucleophilic substitution because:

• They contain a polar C–X bond due to the high electronegativity of the halogen (X) - this polarises the bond, making the C slightly electron-deficient (C^δ+).
• The electron-deficient C atom is susceptible to attack by electron-rich nucleophiles.

For example, when bromoethane reacts with hydroxide ions, the nucleophilic OH^- attacks the electron-deficient C atom, displacing the bromide leaving group:

Heterolytic fission

Heterolytic fission is the unsymmetrical cleavage of a covalent bond, where the bonding electron pair is unevenly distributed between the two atoms. One atom receives both electrons, becoming an anion, while the other receives none, becoming a cation.

In halogenoalkanes, heterolytic fission of the carbon–halogen bond forms a halide anion (X^-) and an alkyl cation (R⁺), also known as a carbocation. The equation for this heterolytic fission is:

The ease of heterolytic bond fission and the stability of the resulting carbocation intermediate determines whether nucleophilic substitution occurs via an S_N1 or an S_N2 mechanism.