Question

Ammonia is best described as: A. a Lewis acid B. a Lewis base C. an electrophile D. an aromatic compound

Short answer

B. a Lewis base

Step-by-step solution

- Understanding the Options

Review the definitions of the terms provided in the options: A. Lewis acid: A substance that can accept a pair of electrons. B. Lewis base: A substance that donates a pair of electrons. C. Electrophile: A substance that is attracted to electrons and tends to acquire them. D. Aromatic compound: A compound that contains a conjugated planar ring system with delocalized π-electrons.

- Properties of Ammonia (NH₃)

Recall that ammonia (NH₃) has a lone pair of electrons on the nitrogen atom. This makes NH₃ capable of donating this lone pair.

- Matching Ammonia's Properties to Definitions

Match the properties of ammonia to the given definitions:A. NH₃ does not accept electron pairs; thus, it is not a Lewis acid.B. NH₃ donates a lone pair, fitting the definition of a Lewis base.C. NH₃ is not electrophilic as it is not seeking electrons but rather donating them.D. NH₃ is not an aromatic compound as it does not have a planar ring system with delocalized π-electrons.

- Conclusion

Based on the matching, the best description for ammonia (NH₃) is option B, a Lewis base.

Key concepts

Lewis Acid

In chemistry, a Lewis acid is any substance that can accept a pair of electrons. This usually involves atoms or molecules that have an empty orbital which enables them to accept an electron pair.
For example:

• Boron trifluoride (BF₃) accepts an electron pair from ammonia (NH₃), forming an adduct.
• Hydrogen ion (H⁺) is a simple and common Lewis acid because it can accept a pair of electrons from a Lewis base.

Lewis acids are essential in many reactions, often helping stabilize the formation of bonds by accepting electrons from Lewis bases. They are usually involved in catalysis and in reaction mechanisms where electron pair donation and acceptance are critical aspects.

Lewis Base

A Lewis base is defined as a substance that can donate a pair of electrons. This electron pair donation usually occurs from atoms or molecules with lone pairs of electrons.
Key examples include:

• Ammonia (NH₃), which donates a lone electron pair to form bonds with other substances in chemical reactions.
• Hydroxide ion (OH⁻), which frequently donates electrons in various chemical processes.

Lewis bases are significant in forming Lewis adducts, where the electron pair from the Lewis base gets shared with a Lewis acid. For instance, in ammonia (NH₃), the nitrogen atom has a lone pair that makes it a classical Lewis base. This ability to donate a lone pair is why ammonia best fits the description of a Lewis base in various chemical contexts.

Electrophile

An electrophile, which means 'electron-loving,' is a substance that is attracted to electrons and tends to acquire them. Electrophiles are usually positively charged or neutral species with empty orbitals.
Classic examples include:

• Hydrogen ion (H⁺), as it lacks electrons and readily accepts them from electron-rich species.
• Alkyl halides like CH₃Cl, where the carbon atom attached to the halogen is electron-deficient and seeks electron pairs from nucleophiles.

Electrophiles play a key role in reactions like electrophilic aromatic substitution, where they attack electron-rich aromatic rings. However, since ammonia (NH₃) donates electrons rather than accepting them, it is not considered an electrophile.

Aromatic Compound

Aromatic compounds are a special class of molecules characterized by having a conjugated planar ring system with delocalized π-electrons, conforming to Hückel's rule (4n + 2 π-electrons).
Some common examples are:

• Benzene (C₆H₆), which has a six-membered carbon ring with alternating double bonds, creating a stable delocalized electron cloud.
• Naphthalene, consisting of two fused benzene rings, also exhibiting aromaticity.

These properties give aromatic compounds unique stability and reactivity profiles. On the other hand, ammonia (NH₃) lacks this planar ring structure and delocalized electrons, and hence, it doesn't qualify as an aromatic compound. Understanding these differences is crucial in correctly identifying and classifying compounds in various chemical reactions.