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) Electronically similar to \(\mathrm{NH}_3\); (b) 5 valence electrons; (c) \(sp^3\) hybridization; (d) Trigonal pyramidal geometry.

Step-by-step solution

Compare Electronic Configurations

A carbanion has a negatively charged carbon with an extra electron, making it similar in electron count to a trivalent nitrogen compound like \(\mathrm{NH}_3\). Both have the same number of valence electrons due to the negative charge on carbon compensating for the missing proton compared to nitrogen.

Determine Valence Electrons

The neutral carbon atom has 4 valence electrons. In a carbanion, it gains an extra electron due to the negative charge, so it has a total of 5 valence electrons.

Identify Hybridization

The presence of a lone pair and bonds suggests \(sp^3\) hybridization, as one of the hybrid orbitals contains a lone pair of electrons, similar to the structure of \(\mathrm{NH}_3\).

Predict Geometry

With \(sp^3\) hybridization and one lone pair, the geometry of the carbanion is trigonal pyramidal, similar to ammonia.

Key concepts

Electronic Configuration

To understand the electronic configuration of a carbanion, we must consider its negative charge. A carbanion is a carbon atom that has an extra electron, resulting in it holding a negative charge. This extra electron alters its electronic configuration compared to a neutral carbon atom. In a neutral carbon atom, the electronic configuration is normally \(1s^2 2s^2 2p^2\). However, in a carbanion, the electronic configuration becomes \(1s^2 2s^2 2p^3\) because it has gained an extra electron in the 2p orbital.
The presence of this extra electron causes the carbanion to have the same electronic configuration as a nitrogen atom in ammonia (\(\mathrm{NH}_3\), which also has \(1s^2 2s^2 2p^3\)). Hence, even though they are different elements, the carbanion's electronic configuration allows it to mimic the behavior of a trivalent nitrogen compound.

Valence Electrons

Valence electrons are the electrons in the outermost shell and are critical in determining an atom's bonding behavior. In a neutral carbon atom, there are 4 valence electrons. However, in a carbanion, due to the negative charge, there is an addition of one extra electron, bringing the total to 5 valence electrons.
This increase plays a vital role in how the carbanion interacts with other atoms. It equips the carbanion with an electron configuration similar to nitrogen in \(\mathrm{NH}_3\). This extra electron is stored in a p orbital, increasing its bonding capacity and affinity for reactions involving electron sharing or transfer.

Hybridization

Hybridization is a concept used to describe the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding. In the carbanion, the carbon atom undergoes \(sp^3\) hybridization. This is similar to what happens in ammonia (\(\mathrm{NH}_3\)), where the nitrogen atom also undergoes \(sp^3\) hybridization.
The carbanion achieves \(sp^3\) hybridization because it has three sigma bonds and one lone pair of electrons.
The lone pair occupies one of these hybrid orbitals, influencing the molecular shape and electronic properties of the carbanion. The presence of this lone pair, which does not take part in bonding, gives the carbanion particular reactive characteristics distinct from a neutral carbon atom.

Molecular Geometry

The molecular geometry of a compound closely relates to its hybridization and the arrangement of bonds and lone pairs around the central atom. For many molecules or species with \(sp^3\) hybridization, such as the carbanion, the molecular geometry is trigonal pyramidal.
This is because the lone pair on the carbon atom, similar to ammonia, takes up more space and pushes the bond pairs closer together, arranging the bonds into a three-sided pyramid shape around the carbon. This geometry, while asymmetric, is crucial for understanding the compound's reactivity and interactions with other substances.
In summary, while carbanions are structurally similar to their nitrogen counterparts, their unique combination of lone pairs and bond angles in trigonal pyramidal geometry influence their chemical behavior significantly.