Question

Which of the following molecules or ions may function as a nucleophile in a nucleophilic substitution reaction: (a) \(\mathrm{OH}^{-}\); (b) \(\mathrm{NH}_{4}^{+}\); (c) \(\mathrm{NH}_{2}^{-}\); (d) \(\mathrm{H}_{2} \mathrm{O}\) ?

Short answer

Molecules or ions that can function as nucleophiles in a nucleophilic substitution reaction are (a) \(\mathrm{OH}^{-}\), (c) \(\mathrm{NH}_{2}^{-}\), and (d) \(\mathrm{H}_{2}\mathrm{O}\).

Step-by-step solution

Understand Nucleophilic Substitution

A nucleophilic substitution reaction involves the replacement of an atom or a group of atoms in a molecule with a nucleophile. A nucleophile is a species that donates an electron pair to form a chemical bond. Nucleophiles are typically negatively charged or neutral species with a lone pair of electrons that they can share.

Analyze the Candidates

Examine each of the given species to determine if they have lone pairs of electrons or carry a negative charge that would allow them to function as a nucleophile.

Identify Nucleophiles

(a) \(\mathrm{OH}^{-}\) is a nucleophile because it is negatively charged and has a lone pair of electrons that it can donate. (c) \(\mathrm{NH}_{2}^{-}\) is a nucleophile for the same reasons as \(\mathrm{OH}^{-}\). (d) \(\mathrm{H}_{2}\mathrm{O}\) can act as a nucleophile because, although it is neutral, it possesses lone pairs of electrons. However, it is a weaker nucleophile compared to negatively charged species.

Eliminate Non-Nucleophiles

(b) \(\mathrm{NH}_{4}^{+}\) cannot function as a nucleophile because it is positively charged and has no lone pairs available to donate; all its electrons are involved in covalent bonding.

Key concepts

Nucleophile

In chemistry, a nucleophile is a substance that has a strong affinity for positive centers in other molecules or ions. Important to note, nucleophiles are fundamentally electron pair donors. This means they can offer a pair of electrons to form a new chemical bond, which is a central feature in nucleophilic substitution reactions.

Imagine a nucleophile as a 'chemical helping hand' that reaches out to a molecule in need of electrons. This is often depicted as an arrow in chemical equations, pointing from the nucleophile to the positively charged atom it seeks to bond with. Examples of nucleophiles include molecules such as hydroxide (OH^{-}), ammonia (NH_{3}), or water (H_{2}O), which all possess lone pairs of electrons that they're willing to share.

Electron Pair Donor

The term 'electron pair donor' quite literally describes a moiety that donates a pair of electrons. In chemical bonding, an electron pair donor interacts with an electron pair acceptor, typically a positively charged or electron-deficient site.

When analyzing reactions, identifying electron pair donors is crucial because they are potential nucleophiles. Think of an electron pair donor as a generous friend ready to provide something valuable—in this case, a pair of electrons for bond formation. By understanding the concept of electron pair donors, students can better predict and comprehend chemical reactivity and mechanism pathways.

Chemical Bonding

Chemical bonding is the process that enables atoms to combine and form molecules. There are several types of bonds, such as ionic, covalent, and metallic bonds. However, it's the covalent bond that's of interest in nucleophilic substitution reactions.

In a covalent bond, atoms share electron pairs, forming a strong connection between them. The strength of this bond and its characteristics play a significant role in determining the behavior of the compound, especially in reactions. Bonds can be polarized, making part of the molecule slightly positive and another part slightly negative. This polarization can make molecules more susceptible to attack by nucleophiles, setting the stage for a reaction.

Lone Pair of Electrons

A lone pair of electrons consists of two electrons that are paired but are not involved in bonding. They reside in the outermost shell of an atom and are vital to the formation of chemical bonds because they can be donated or shared during interactions.

In the context of nucleophilic substitution reactions, the availability of a lone pair is a fundamental requirement for a molecule or ion to act as a nucleophile. For instance, the lone pair on the hydroxide ion (OH^{-}) is what allows it to attack other molecules, making it a classic example of a nucleophile in action. Understanding the role of lone pairs can help students to predict the outcome of chemical reactions.