Organic Chemistry MCAT Practice Exam

Which of the following is true about optical activity?

1. Chiral molecules are optically inactive

2. Optical rotation can be predicted by structure

3. Dextrorotatory compounds rotate light clockwise

4. All optical activity is identical regardless of structure

The correct answer is: Dextrorotatory compounds rotate light clockwise

Optical activity refers to the ability of a chiral compound to rotate the plane of polarized light. This property arises from the spatial arrangement of atoms in a chiral molecule, leading to two non-superimposable mirror images, known as enantiomers. The statement that dextrorotatory compounds rotate light clockwise is true. Dextrorotatory (often denoted as + or d) compounds specifically cause the plane of polarized light to shift to the right (clockwise) when it passes through a solution containing the compound. This is a recognized convention in organic chemistry, where the direction of rotation is measured with respect to an observer facing the light source. In contrast, the other options present misconceptions about optical activity. Chiral molecules, which are defined as having a non-superimposable mirror image, are, in fact, optically active. The prediction of optical rotation based solely on molecular structure is complex and not straightforward; while certain trends can be observed, it's not always accurate to predict the specific optical activity from structure alone. Lastly, the assertion that all optical activity is identical regardless of structure is incorrect because different chiral molecules can exhibit different degrees of optical activity and direction of rotation. Thus, the assessment of dextrorot