MCAT Organic Chemistry Review

Isomers

Concept Summary

Structural Isomers

·        Structural isomers share only a molecular formula.

·        They have different physical and chemical properties.

Stereoisomers

·        Conformational isomers differ by rotation around a single (σ) bond.

o   Staggered conformations have groups 60° apart, as seen in a Newman projection. In anti staggered molecules, the two largest groups are 180° apart, and strain is minimized. In gauche staggered molecules, the two largest groups are 60° apart.

o   Eclipsed conformations have groups directly in front of each other, as seen in a Newman projection. In totally eclipsed conformations, the two largest groups are directly in front of each other, and strain is maximized.

o   The strain in cyclic molecules comes from angle strain (created by stretching or compressing angles from their normal size), torsional strain (from eclipsing conformations), and nonbonded strain (from interactions between substituents attached to nonadjacent carbons). Cyclic molecules will usually adopt nonplanar shapes to minimize this strain.

o   Substituents attached to cyclohexane can be classified as axial (sticking up or down from the plane of the molecule) or equatorial (in the plane of the molecule). Axial substituents create more nonbonded strain.

o   In cyclohexane molecules with multiple substituents, the largest substituent will usually take the equatorial position to minimize strain.

·        Configurational isomers can only be interchanged by breaking and reforming bonds.

o   Enantiomers are nonsuperimposable mirror images and thus have opposite stereochemistry at every chiral carbon. They have the same chemical and physical properties except for rotation of plane-polarized light and reactions in a chiral environment.

o   Optical activity refers to the ability of a molecule to rotate plane-polarized light: d- or (+) molecules rotate light to the right; l- or (–) molecules rotate light to the left.

o   Racemic mixtures, with equal concentrations of two enantiomers, will not be optically active because the two enantiomers’ rotations cancel each other out.

o   Meso compounds, with an internal plane of symmetry, will also be optically inactive because the two sides of the molecule cancel each other out.

o   Diastereomers are non-mirror-image stereoisomers. They differ at some, but not all, chiral centers. They have different chemical and physical properties.

o   Cis–trans isomers are a subtype of diastereomers in which groups differ in position about an immovable bond (such as a double bond or in a cycloalkane).

·        Chiral centers have four different groups attached to the central carbon.

Relative and Absolute Configurations

·        Relative configuration gives the stereochemistry of a compound in comparison to another molecule.

·        Absolute configuration gives the stereochemistry of a compound without having to compare to other molecules.

o   Absolute configuration uses the Cahn–Ingold–Prelog priority rules, in which priority is given by looking at the atoms connected to the chiral carbon or double-bonded carbons; whichever has the highest atomic number gets highest priority. If there is a tie, one moves outward from the chiral carbon or double-bonded carbon until the tie is broken.

·        An alkene is (Z) if the highest-priority substituents are on the same side of the double bond and (E) if on opposite sides.

·        A stereocenter’s configuration is determined by putting the lowest priority group in the back and drawing a circle from group 1 to 2 to 3 in descending priority. If this circle is clockwise, the stereocenter is (R); if it is counterclockwise, the stereocenter is (S).

·        Vertical lines in Fischer diagrams go into the plane of the page (dashes); horizontal lines come out of the plane of the page (wedges).

o   Switching one pair of substituents in a Fischer diagram inverts the stereochemistry of the chiral center. Switching two pairs retains the stereochemistry.

o   Rotating a Fischer diagram 90° inverts the stereochemistry of the chiral center. Rotating 180° retains the stereochemistry.