MCAT Organic Question — Plane Polarized Light | Next Step Test Prep MCAT Organic Question — Plane Polarized Light | Next Step Test Prep

Fields marked with an * are required

Schedule a Free Tutoring Consultation

A Next Step Academic Manager will contact you within 1 business day or call us now at 888-530-6398.

Contact us

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Plane-polarized light will NOT be rotated by which of the following molecules:

A)   I only

B)   III only

C)   II and III only

D)   I and III only

 

Explanation

 

A)   I only

B)   III only

C)   II and III only

D)  I and III only

 

Plane-polarized light is not rotated if a molecule is meso (like I) or if it has no chiral centers (like III). To see how structure I is meso, remember that in a Newman projection, groups can be rotated freely, since free rotation is possible around a single bond. If the front carbon is spun 180º, each of the substituents lines up with the same group on the rear carbon. Thus, the molecule has a plane of symmetry and is meso.

 

For structure III, note that the front central carbon is bound to two CHO groups, while the rear central carbon is attached to two hydrogen atoms. Thus, no carbon in the compound is bound to four different substituents, meaning that this compound has no chiral centers. As a result, we know that structures I and III will not rotate plane-polarized light, and D is our answer.

 

Want more practice?

Fields marked with an * are required

Sign up for our MCAT Question of the Day!

X