What's the role of nucleophiles in organic reactions?

Nucleophiles in organic reactions act as electron donors, forming bonds with electrophiles by donating a pair of electrons.

In organic chemistry, nucleophiles play a crucial role in many types of reactions. The term 'nucleophile' comes from the Greek words for 'nucleus-loving', which is fitting as these species are attracted to positive charges. They are characterised by their ability to donate a pair of electrons to form a new covalent bond. This is because they have a lone pair of electrons or a negative charge, making them electron-rich.

Nucleophiles can be neutral or negatively charged, and they are often found in the form of ions or molecules. Common examples of nucleophiles include water (H2O), ammonia (NH3), and the hydroxide ion (OH-). These species are able to donate their lone pair of electrons to an electrophile, which is an electron-deficient species.

The role of nucleophiles in organic reactions is best illustrated through the mechanism of nucleophilic substitution reactions. In these reactions, a nucleophile replaces a leaving group in a molecule. The nucleophile donates its pair of electrons to form a bond with the electrophilic carbon, while the leaving group departs with the electrons that were originally shared with the carbon. This results in the substitution of the leaving group with the nucleophile.

Nucleophiles also play a key role in addition reactions, where they add to a molecule without replacing any existing atoms. This typically occurs with unsaturated molecules, such as alkenes and alkynes, where the nucleophile and another species can add across the double or triple bond.

In summary, nucleophiles are vital players in organic reactions. They facilitate the formation of new bonds by donating their electron pairs to electrophiles. Understanding the role and behaviour of nucleophiles is fundamental to mastering organic chemistry.