Question

Identify whether each of the following compounds exhibits a molecular dipole moment. If so, indicate the direction of the net molecular dipole moment:

(a) CH₃CH₂OCH₂CH₃                        (b) CO₂

Answer 1

Solution

(a) In order to determine whether the individual dipole moments cancel each other completely, we must first predict the molecular geometry. Specifically, we need to know if the geometry around the oxygen atom is linear or bent:

     

To make this determination, we use the method described in the previous SkillBuilder. The oxygen atom has two σ bonds and two lone pairs, so the steric number is 4. Since there are two lone pairs, VSEPR theory predicts bent geometry.

After determining the molecular geometry, now draw all dipole moments and determine whether they cancel each other. In this case, they do not fully cancel each other:

This compound does in fact have a net molecular dipole moment, and the direction of the moment is shown above.

(b) Carbon dioxide (CO₂) has two C=O bonds, each of which exhibits a dipole moment. In order to determine whether the individual dipole moments cancel each other completely, we must first predict the molecular geometry. Once again, we use the method described in the previous SkillBuilder. The carbon atom has two σ bonds and no lone pairs, so the steric number is two. Accordingly, VSEPR theory predicts linear geometry. Therefore, we expect the dipole moments to fully cancel each other:

In a similar way, the dipole moments associated with the lone pairs also cancel each other, and therefore CO₂ does not have a net molecular dipole moment.