Q10.96 CP - Chemistry: Molecular Nature Of Matter And Change

Question

Ethylene, ${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{4}}$ , and tetrafluoroethylene, ${\mathbf{C}}_{\mathbf{2}}{\mathbf{F}}_{\mathbf{4}}$, are used to make the polymers polyethylene and polytetrafluoroethylene (Teflon), respectively.

1. Draw the Lewis structures for ${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{4}}$ ,   and  ${\mathbf{C}}_{\mathbf{2}}{\mathbf{F}}_{\mathbf{4}}$, and give the ideal H-C-H and F-C-F bond angles.
2. The actual H-C-H and F-C-F bond angles are${\mathbf{117}\mathbf{.}\mathbf{4}}^{\mathbf{\circ }}$and $\mathbf{112}\mathbf{.}{\mathbf{4}}^{\mathbf{\circ }}$, respectively. Explain these deviations. 

Step-by-Step Solution

Answer

1. Lewis structure of Ethylene 

     Bond angles between H-C-H are  $\mathbf{117}\mathbf{.}{\mathbf{4}}^{\mathbf{o}}$.

     Lewis structure of tetrafluoroethylene 

        The bond angles between F-C-F are  ${\mathbf{120}}^{\mathbf{o}}$.

    b) Due to a greater difference in electronegativity in the C-F bond and F has 3 lone pairs               of electrons. These cause deviation in bond angle.

Step 1: Definition

The angle formed by three contiguous bonded atoms is known as the bond angle.

Step 2: Structures of ethylene and tetrafluoroethylene

The ideal bond angles in ethyne between H-C-H are$\mathbf{117}\mathbf{.}{\mathbf{4}}^{\mathbf{o}}$.

And the ideal bond angles in F-C-F are${\mathbf{120}}^{\mathbf{o}}$

Step 3: Deviations from the ideal bond angle

Due to a greater difference in electronegativity in C-F bond, the electron density is pulled more towards the fluorine atom and also there are 3 lone pair of electrons that causes extra repulsion. This reduces the bond angle between C-C from the expected 120 degrees.