Example of a chiral center (the carbon atom), with the two enantiomers shown.

Stereochemistry involves the study of the spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules.

Stereochemistry is the branch of chemistry concerned with the spatial arrangements of atoms in molecules. Stereoisomers are molecules which do not differ in the sequential arrangement of atoms, but the spatial arrangment. In our earlier chapter, the Conformations of Organic Molecules, we were concerned with one sub-class of stereoisomerism, conformational isomerism, in which interconversion between the differing forms may occur without breaking bonds. In contrast, however, configurational isomers, the other type of stereoisomers, may not be interconverted without breaking bonds. If two configurational isomers are mirror images, they are enantiomers. Configurational isomers which are not mirror images are enantiomers.

Stereochemistry is a crucial topic for the MCAT. Nearly every exam will contain at least one question dealing directly with stereochemistry and several questions dealing indirectly. An MCAT passage presenting a reaction mechanism will likely be followed by at least one question dealing with issues such as inversion of configuration or racemization. Students are often required to recognize meso- forms on the MCAT or predict the number of stereoisomers a structure may have. Watch for stereochemistry to be presented on the MCAT in an unusual context such coordination chemistry.

WikiPremed Resources




Stereochemistry Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Stereochemistry
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals

Proficiency 

Understand the factors making the staggered conformation of ethane more stable than the eclipsed conformation.

Be able to recreate the potential energy diagram for internal rotation around the central carbon-carbon bond in butane, and be capable of differentiating the two staggered conformations of butane, gauche and anti.

Understand the potential energy diagram for the various conformations of cyclohexane: chair, half-chair, skew boat, and boat and grasp the conformational analysis of mono- and di-substitution of cyclohexane.

Understand the distinctions between the various kinds of molecular isomerism beginning with geometric isomerism, stereoisomerism and the distinctions within these categories.

Be able to correctly determine whether two stereoisomers are enantiomers or diastereomers, and be aware of what it means for a molecule to be a meso form.

Understand the basics of the different systems of stereochemical nomenclature used in organic chemistry and biochemistry including R/S, E/Z and D/L.

Learn how to solve problems with molecules having multiple chiral centers such as predicting the number of stereoisomers and recognizing which are enantiomers.

Be able to predict which reactions produce racemic mixtures, for example, and be able to discuss laboratory techniques involving resolution of enantiomers.

Suggested Assignments

Get oriented in the important terminology for stereochemistry using the question server.

In In ExamKrackers Chemistry, read pp. 65-74. Perform practice items 41-48 on pp. 75-76.

Conceptual Vocabulary for Stereochemistry

Stereochemistry
Stereochemistry involves the study of the relative spatial arrangement of atoms within molecules.
Chiral
The term chiral is used to describe an object that is non-superimposable on its mirror image.
Cis-trans isomerism
Cis-trans isomerism is a form of stereoisomerism describing the orientation of functional groups typically around double bonds which cannot rotate.
Steric effects
Steric effects arise from the fact that if atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds.
Enantiomer
Enantiomers are stereoisomers that are nonsuperimposable complete mirror images of each other.
Racemic
A racemic mixture is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule.
Diastereomer
Diastereomers are stereoisomers that are not enantiomers.
Optical activity
Optical rotation or optical activity is the rotation of linearly polarized light as it travels through certain materials.
Meso compound
A meso compound is a chemical compound with molecules that contain 2 or more stereocenters but which is optically achiral because it contains an internal plane of symmetry.
Newman projection
A Newman projection visualizes chemical conformations of a carbon-carbon chemical bond from front to back, with the front carbon represented by a dot and the back carbon as a circle.
Staggered conformation
A staggered conformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 hybridised atoms as a conformational energy minimum.
Eclipsed conformation
An eclipsed conformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 hybridised atoms as a conformational energy maximum.
Stereocenter
A stereocenter is any atom in a molecule bearing groups such that an interchanging of any two groups leads to a stereoisomer.
Conformational isomerism
Conformational isomerism is a form of stereoisomerism involving molecules with the same structural formula existing as different conformers due to atoms rotating about a bond.
Van der Waals strain
Van der Waals strain results from van der Waals repulsion when two substituents in a molecule approach each other with a distance less than the sum of their van der Waals radii.
Cyclohexane
Cyclohexane is a cycloalkane containing 6 carbons and 12 hydrogens, which has the lowest angle and torsional strain of all the cycloalkanes.
Angle strain
The presence of angle strain in a molecule indicates that in a specific chemical conformation bond angles are deviating from the ideal bond angles required to achieve maximum bond strength.
Ring strain
Ring strain is an organic chemistry term that describes the destabilization of a cyclic molecule-such as a cycloalkane-due to the non-favorable high energy spatial orientations of its atoms.
Chiral resolution
Chiral resolution in stereochemistry is a process for the separation of racemic compounds into their enantiomers.
Prochiral
Prochiral molecules can be converted from achiral to chiral in a single step.
Hyperconjugation
Hyperconjugation is the stabilizing interaction that results from the interaction of the electrons in a sigma bond with an adjacent empty or partially filled non-bonding p-orbital or antibonding pi orbital leading to an extended molecular orbital that increases the stability of the system.
Anomeric effect
The anomeric effect or Edward-Lemieux effect describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the axial orientation instead of the expected, less-hindered equatorial orientation.
Enantiomeric excess
Enantiomeric excess exists where one enantiomer is present more than the other in a chemical substance.
Advanced terms that may appear in context in MCAT passages
Baeyer strain theory
Baeyer strain theory explains specific behaviour of chemical compounds in terms of bond angle strain.
Enantiomer self-disproportionation
Enantiomer self-disproportionation is a process describing the separation of a non-racemic mixture of enantiomers in an enantioenriched fraction and a more racemic fraction.
Homochirality
Homochirality is a term used to refer to a group of molecules that possess the same sense of chirality with similar groups are arranged in the same way around a central atom.
Chiral pool synthesis
Chiral pool synthesis is a strategy that aims to improve the efficiency of chiral synthesis by beginning from a stock of readily available enantiopure substances.
Asymmetric induction
Asymmetric induction describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment.
Chiral auxiliary
A chiral auxiliary is a chemical compound or unit that is temporarily incorporated into an organic synthesis so that it can be carried out asymmetrically with the selective formation of one of two enantiomers.
Circular dichroism
Circular dichroism is a form of spectroscopy based on the differential absorption of left- and right-handed circularly polarized light.