Question

Which of the following is not a nucleophile? (A) \(\mathrm{H}_{2}\) (B) \(\mathrm{CH}_{3} \mathrm{OH}\) (C) \(\mathrm{H}_{2} \mathrm{O}\) (D) \(\mathrm{NH}_{3}\)

Short answer

The molecule that is not a nucleophile among the given options is \(\mathrm{H}_{2}\) (option A), as it does not have any lone pairs of electrons or any net negative charge to donate.

Step-by-step solution

Understand what is a nucleophile

A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in a reaction. They are often negatively charged or have an atom with a lone pair of electrons, and are attracted to positive charge.

Analyze option (A) - \(\mathrm{H}_{2}\)

Diatomic hydrogen, \(\mathrm{H}_{2}\), consists of two hydrogen atoms bound together. Each hydrogen atom contributes one electron, and the bond is formed with these two shared electrons. Hydrogen does not have any extra electrons to share or donate, so it cannot serve as a nucleophile.

Analyze option (B) - \(\mathrm{CH}_{3} \mathrm{OH}\)

Methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), is a molecule with a polar O-H bond. The oxygen atom also has two lone pairs of electrons. This makes methanol a potential nucleophile, as it can donate one of its lone pairs of electrons.

Analyze option (C) - \(\mathrm{H}_{2} \mathrm{O}\)

Water, \(\mathrm{H}_{2} \mathrm{O}\), has two hydrogen atoms and one oxygen atom. The oxygen atom has two lone pairs of electrons making it a powerful nucleophile as it can donate one of these electron pairs.

Analyze option (D) - \(\mathrm{NH}_{3}\)

Ammonia, \(\mathrm{NH}_{3}\), consists of a nitrogen atom bonded to three hydrogen atoms. The nitrogen atom has a free lone pair of electrons, making ammonia a potential nucleophile since it can donate this electron pair.

Conclusion

Among the given options, \(\mathrm{H}_{2}\) (option A) does not have an atom with a lone pair of electrons or any net negative charge. Therefore, \(\mathrm{H}_{2}\) is not a nucleophile.

Key concepts

Chemical Bond Formation

Chemical bond formation is a defining process in organic chemistry, where two atoms unite to create molecules. It involves the interaction of atomic orbitals, leading to electron sharing or transfer. Bonds hold the atoms in a molecule together and can be ionic, where electrons are transferred and atoms adopt charges, or covalent, where electrons are shared evenly between atoms.
When discussing nucleophiles in the context of bond formation, we refer to covalent bonding. Nucleophiles are electron pair donors that seek electrophiles, electron-loving species that accept these electrons to form a bond. They look for electrophilic centers, typically carbon atoms with a positive charge or a significant lack of electron density. The electron pair from the nucleophile is shared with this center, creating a stable chemical bond. This interaction is a cornerstone of reactions such as the SN2 mechanism, where the bond is made and broken in a single step.
Understanding the tendencies of various molecular entities to act as nucleophiles is crucial for predicting reaction outcomes. A good nucleophile must have readily available electrons and often carries a negative charge or has a neutral atom with lone electron pairs.

Lone Pair of Electrons

Lone pairs are pairs of valence electrons on an atom that are not used in bonding. These electrons are significant because they represent a source of negative charge that can be donated to an electrophile during chemical bond formation. In organic chemistry contexts, atoms with lone pairs, such as oxygen, nitrogen, and sulfur, often function as nucleophiles because they can provide these electrons to electrophilic atoms.
When looking at the structures of molecules, it's essential to identify atoms with lone pairs as potential sites of reactivity. For example, in water (H2O) and ammonia (NH3), oxygen and nitrogen exhibit lone pairs which can be donated. Hence, these molecules can participate in various chemical reactions, including acid-base reactions and nucleophilic substitutions. Lone pairs also explain the basicity of a compound, as they are responsible for binding protons (H+) from acids, making molecules with lone pairs basic in nature.

Electron Pair Donor

Being an electron pair donor is a defining feature of nucleophiles. The concept of electron pair donors is critical when exploring the properties of nucleophiles. These donors hold a pair of electrons that they are ready to share with an electrophile to form a new covalent bond. For a molecule to act as a nucleophile, it must possess a component like a lone pair or a pi bond that can be offered up in the bonding process.
Electron pair donors are not always negatively charged. Neutral molecules with lone pairs, such as methanol (CH3OH), can also act in the capacity of electron pair donors. The ability of a nucleophile to donate its electron pair depends on factors such as the nucleophile's basicity, solvent effects, and the presence of sterically encumbering groups that may hinder interaction with electrophiles. Mastering the identification of electron pair donors is vital to understanding many reaction mechanisms and predicting how molecules will behave under different conditions.