Q78. - Organic Chemistry

Question

Question:  $\stackrel{\mathbf{+}}{\mathbf{C}}{\mathbf{H}}_{\mathbf{3}\mathbf{ }}\mathbf{and}\mathbf{ }\stackrel{\mathbf{-}}{\mathbf{C}}{\mathbf{H}}_{\mathbf{3}}$ are two highly reactive carbon species.

1. What is the predicted hybridization and geometry around each carbon atom?
2. Two electrostatic potential plots are drawn for these species. Which ion corresponds to which diagram and why?

                                                             A.

                                                              B.

Step-by-Step Solution

Answer

Answer

1. The carbon atom in  $\stackrel{+}{\mathrm{C}}{\mathrm{H}}_3$ is ${\mathrm{sp}}^2$  hybridized and has trigonal planar geometry, while the carbon atom in $\stackrel{-}{\mathrm{C}}{\mathrm{H}}_3$ is ${\mathrm{sp}}^3$   hybridized and has tetrahedral geometry.

 

1. The electrostatic potential plot A corresponds to $\stackrel{+}{\mathrm{C}}{\mathrm{H}}_3$   as the plot represents an electron-deficient species and plot B represents an electron-rich species; therefore, it corresponds to$\stackrel{-}{\mathrm{C}}{\mathrm{H}}_3$  .

Step-by-Step Solution Step 1: Hybridization

Hybridization is defined as combining/mixing atomic orbitals leading to the generation of hybrid orbitals.

 

The hybridization of an atom in a molecule can be determined by considering the atoms/groups surrounding it.

 

If the atom under consideration is surrounded by or bonded to four other groups, then it is said to be ${\mathrm{sp}}^3$  hybridized; if it is bonded to three other groups, then it is  ${\mathrm{sp}}^2$ hybridized; and if bonded to or surrounded by two different groups, then it is sp hybridized.

Step 2: Geometry

The three-dimensional (3D) arrangement of atoms in a molecule is termed its geometry.

 

The geometry of a molecule depends on the hybridization of the central atom in it.

 

• ${\mathrm{sp}}^3$  hybridization corresponds to tetrahedral geometry.
• ${\mathrm{sp}}^2$  hybridization corresponds to trigonal planar geometry.
• $\mathrm{sp}$   hybridization corresponds to linear geometry.

 

Step 3: Hybridization and geometry around each carbon atom in the given molecules

a. The structure of $\stackrel{+}{\mathrm{C}}{\mathrm{H}}_3$ is shown below:

                                                 

                                                            Structure of  $\stackrel{+}{\mathrm{C}}{\mathrm{H}}_3$

 

The carbon atom is bonded to three hydrogen atoms, i.e., three orbitals are involved in bonding. Therefore, the carbon atom in $\stackrel{+}{\mathrm{C}}{\mathrm{H}}_3$  is ${\mathrm{sp}}^2$  hybridized and will have a trigonal planar geometry.

 

The structure of  $\stackrel{-}{\mathrm{C}}{\mathrm{H}}_3$ is shown below:

                                                       

                                                       Structure of  $\stackrel{-}{\mathrm{C}}{\mathrm{H}}_3$

 

The carbon atom is bonded to three hydrogen atoms and has a lone pair of electrons on it, i.e., it is surrounded by a total of four groups (four orbitals are involved in bonding).

 

Therefore, the carbon atom in $\stackrel{-}{\mathrm{C}}{\mathrm{H}}_3$ is ${\mathrm{sp}}^3$  hybridized and will have a tetrahedral geometry.

Step 4: Analyzing the electrostatic potential plots

 b. The plots used to describe a molecule’s charge distribution three-dimensionally are electrostatic potential plots. 

Plot A represents lower electron density (blue-colored region indicates electron deficiency), and hence, it corresponds to $\stackrel{+}{\mathrm{C}}{\mathrm{H}}_3$  

Plot B represents higher electron density (pink/red colored region indicates electron richness), and hence, it corresponds to $\stackrel{-}{\mathrm{C}}{\mathrm{H}}_3$ .