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.