Question

A carbanion is a species that contains a negatively charged, trivalent carbon. (a) What is the electronic relationship between a carbanion and a trivalent nitrogen compound such as \(\mathrm{NH}_{3} ?\) (b) How many valence electrons does the negatively charged carbon atom have? (c) What hybridization do you expect this carbon atom to have? (d) What geometry is the carbanion likely to have?

Short answer

(a) Isoelectronic with \\(\mathrm{NH}_3\\); (b) 5 valence electrons; (c) sp³ hybridization; (d) Trigonal pyramidal geometry.

Step-by-step solution

Understanding Electronic Relationships

A carbanion has a negatively charged carbon with three bonds and one lone pair, making it an isoelectronic species. An isoelectronic compound has the same number of electrons as another compound. Here, both a carbanion and \(\mathrm{NH}_3\) have the same number of electrons.

Counting Valence Electrons

In its neutral state, carbon has four valence electrons. In a carbanion, carbon gains an additional electron due to the negative charge, making it five valence electrons.

Determining Hybridization

With three bonds and one lone pair, the negatively charged carbon in a carbanion is sp³ hybridized. This is similar to the hybridization of nitrogen in ammonia (\(\mathrm{NH}_3\)).

Predicting the Geometry

The sp³ hybridization typically leads to a tetrahedral geometry. However, due to the lone pair-bond pair repulsion, the actual shape of a carbanion is trigonal pyramidal, similar to ammonia.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial for determining how an atom bonds and interacts with other atoms. In a carbanion, the carbon atom carries a negative charge because it possesses more electrons than protons. Under normal circumstances, carbon has four valence electrons. However, when it becomes a carbanion, it gains an extra electron, bringing the total to five valence electrons. This additional electron is what imparts the negative charge, and it is important to note that the presence of five valence electrons differs significantly from carbon's neutral state.

• Neutral carbon: 4 valence electrons
• Carbanion: 5 valence electrons (due to the extra negative charge)

Understanding valence electrons helps predict bonding behavior and reactivity. They essentially hold the key to understanding the core differences in electronic configuration between neutral atoms and ions like a carbanion.

Hybridization

Hybridization is a concept used to explain the mixing of atomic orbitals to form new hybrid orbitals. These hybrid orbitals help describe the bonding properties of atoms. For a carbanion, the negatively charged carbon atom with three bonds and a lone pair undergoes sp³ hybridization. This process involves the mixing of one s orbital and three p orbitals from the carbon, producing four equivalent sp³ hybrid orbitals. Each of these orbitals can hold electron pairs, making them suitable for forming sigma bonds or accommodating lone pairs. In the case of the carbanion, three of these hybrid orbitals are used for bonding with other atoms, while the fourth contains the lone pair.
The hybridization determines both the geometry and reactivity of the carbon atom in the carbanion, reflecting properties similar to nitrogen in ext{NH}_3.

Molecular Geometry

The molecular geometry of a molecule refers to the three-dimensional arrangement of its atoms. In the context of a carbanion, the carbon atom, which is sp³ hybridized, would have a geometric arrangement that resembles a tetrahedron based on the types of orbitals present. However, due to electron pair repulsion, the actual observable geometry of this negatively charged carbon-containing compound is trigonal pyramidal. This trigonal pyramidal structure occurs because the lone pair of electrons needs more space than bonding pairs, thus pushing the bonds closer together. The central carbon atom is bonded to three other atoms with one lone pair remaining, influencing the geometry akin to a tripod-like shape, rather than a perfect tetrahedron. This is similar to the arrangement seen in ammonia ( ext{NH}_3), providing a clear example of how lone pairs affect molecular shape.

Isoelectronic Species

Isoelectronic species are different chemical species that have the same number of electrons or the same electronic structure. For example, a carbanion and ammonia ( ext{NH}_3) are considered isoelectronic because they both possess eight electrons surrounding their central atom when we consider the expanded electron counts with lone pairs and bonding pairs. This concept offers useful insights into predicting and comparing properties of different molecules. It allows for the inference of similar behavior in terms of structure and reactivity between seemingly different species. Additionally, understanding isoelectronic relationships can help in anticipating how changes in one atom or molecule might reflect in another, further explaining similarities in molecular geometry as seen in carbanions compared to ext{NH}_3.