Q10.86 CP - Chemistry: Molecular Nature Of Matter And Change

Question

Except for nitrogen, the elements of Group 5A(15) all form pentafluorides, and most form pentachlorides. The chlorine atoms of PCl5 can be replaced with fluorine atoms one at a time to give, successively, ${\mathbf{\text{PCl}}}_{\mathbf{\text{4}}}{\mathbf{\text{F,PCl}}}_{\mathbf{3}}{\mathbf{\text{F}}}_{\mathbf{2}}$ , …, ${\mathbf{\text{PF}}}_{\mathbf{\text{5}}}$ .

1. Given the sizes of F and Cl, would you expect the first two F substitutions to be at axial or equatorial positions? Explain.

2. Which of the five fluorine-containing molecules have no dipole moment?

Step-by-Step Solution

Answer

1. Due to the smaller size and high electronegativity, fluorine first occupies axial positions.

2. ${\mathbf{PF}}_{\mathbf{5}}$ and ${\mathbf{PCl}}_{\mathbf{3}}{\mathbf{F}}_{\mathbf{2}}$  are the fluorine-containing molecules that have no dipole moments. 

Step 1: Subpart (a) The Structures

Fluorine is the most electronegative atom and is smaller than the chlorine atom. The more electronegative a substituent is, the more likely it is to occupy axial positions as these positions require stabilizing a partial negative charge, and also the repulsion is minimum at axial positions. Therefore, the first two substitutions occur at axial positions and not at equatorial positions as shown below.

Step 2: The Dipole Moment.

The molecule with all fluorine substituents will be non-polar because symmetry is present in trigonal bipyramidal geometry so all the dipole moments are canceled out.

Similarly, the molecule with two axial fluorine atoms and three axial chlorine atoms is symmetrical and the dipole moments are canceled out.

Here, the dipole moment is zero, so the molecule is non-polar.

The rest of the fluorine substituted molecules have F in the corresponding position of chlorine, therefore their dipole is not canceled and are polar in nature as shown below.