What is a nucleophile?

A nucleophile is best defined as an electron-rich species that donates electron pairs. In organic chemistry, this characteristic allows nucleophiles to participate in reactions by targeting electrophiles, which are electron-deficient species. The ability to donate electrons is crucial for forming new chemical bonds, as nucleophiles are often involved in substitution and elimination reactions.

In these reactions, nucleophiles provide pairs of electrons to electrophiles, facilitating the reaction process. This electron donation helps stabilize positive charges that may arise during the course of the reaction, emphasizing the nucleophile's role in driving the reaction forward.

Understanding the nature of nucleophiles is essential for grasping reaction mechanisms in organic chemistry, as their properties significantly influence how chemical transformations occur. The other choices do not accurately capture the essence of a nucleophile. For instance, a species that accepts electrons describes an electrophile, while positively charged ions are not necessarily nucleophiles unless they possess electron-rich characteristics. The mention of a catalyst refers to a different concept entirely, which is about enhancing the rate of a reaction rather than defining the nature of nucleophiles.