MCAT Organic Chemistry Review
Isomers
Answers to Concept Checks
· 2.1
1. Structural isomers share a molecular formula, and not necessarily anything else.
2. Cyclopropanol, acetone, and prop-2-ene-1-ol are all structural isomers of each other with the chemical formula C3H6O. 2-propanol has the chemical formula C3H8O.
3. Physical properties are aspects of a compound that do not play a role in changing chemical composition. Examples include melting point, boiling point, solubility, odor, color, and density.
4. Chemical properties are aspects of a compound that change chemical composition; in organic chemistry, chemical properties are usually dictated by the reactivity of various functional groups.
· 2.2
1. Conformational isomers are stereoisomers with the same molecular connectivity at different points of rotation around a single bond. Configurational isomers are stereoisomers with differing molecular connectivity.
2. 
3. A meso compound contains chiral centers, but also has an internal plane of symmetry. This means that the molecule is overall achiral and will not rotate plane-polarized light.
· 2.3
1. (E) and (Z) configurations refer to arrangement of groups around a double bond. (Z) describes when the two highest-priority groups are on the same side; (E) describes when the two highest-priority groups are on opposite sides.
2. Priority is assigned by atomic number: the atom connected to the stereocenter or double-bonded carbon with the highest atomic number gets highest priority. If there is a tie, one works outward from the stereocenter or double-bonded carbon until the tie is broken.
3. Switching a pair of substituents inverts stereochemistry; switching two pairs retains it. Rotating the molecule 90° inverts stereochemistry; rotating 180° retains it.