Q-1-1-51E - Organic Chemistry (Mcmurry)

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 NH₃?

(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? 

Step-by-Step Solution

Answer

(a) A carbanion is isoelectronic with a trivalent nitrogen compound.

(b) The negatively charged carbanion carbon has eight valence electrons.

(c) The carbon atom is ${\mathrm{sp}}^3$– hybridized.

(d) A carbanion is a pyramidal in geometry.

 

 

Definition of isoelectronic species

Isoelectronic refers to two atoms, ions, or molecules that have the same electronic structure and the same number of valence electrons. The term means equal electric or equal charge. Isoelectric chemical species typically display similar chemical properties. For example, carbanion and ammonia molecules are isoelectronic to each other but have different chemical aspects.

(a) Find the electronic relationship between a carbanion and a trivalent nitrogen compound such as NH 3

The electronic relationship between a carbanion and a trivalent nitrogen compound such as ${\mathrm{NH}}_3$ is isoelectronic means both have the same number of electrons or valence electrons.

So, therefore, both are isoelectronic with each other.

 

(b) Number of valence electrons on carbon atom with negative charge

A carbanion is a carbon atom bearing a negative charge. The given structure of the carbanion has three bond pairs, one lone pair, and a negative charge around it.

Every bond pair shares two electrons between the bonding atoms. The lone pair consists of two unshared electrons: six shared electrons and two unshared electrons around the carbon—the total count of eight electrons in carbanion.

Total number of electron count = 8

(c) Hybridization of carbon

Hybridization is a concept of mixing atomic orbitals into new hybrid orbitals, generally of lower energy and suitable for pairing electrons to form chemical bonds. The negatively charged carbon atom in methyl carbanion is  hybridized. There are three bond pairs and one lone pair of electrons, which show four electron clouds (number of bond pair and lone pair).

(d)Identifying the geometry of carbon atom with negative charge

The geometry of carbanion is pyramidal. The simplest localized carbanion is . It is isoelectronic with neutral molecule ammonia. The geometry of  ${{\mathrm{CH}}_3}^{-}$ ion is best represented by a pyramid with a carbon atom at the apex, a structure similar to the ammonia molecule. Both structures are shown below;

           Methide ion                                                             Ammonia