Stereochemistry: distinguish connectivity, configuration, and identity

MCAT content library / Chemistry & physics / Stereochemistry: distinguish connectivity, configuration, and identity

Track atom identity and viewing direction to distinguish a new stereoisomer from a new drawing of the same molecule.

About 17 minutes, including the worked examples and checks.

Useful foundations

• Molecular geometry: connect bonding, shape, and polarity

On this page

• Concept: ask connectivity before handedness
• Rule: assign priorities, then orient the lowest group
• Application: compare two centers without losing atom identity
• Pattern: symmetry can reduce the expected count
• Changed application: assign alkene configuration locally
• Check your reasoning
• Continue learning
• Sources

Concept: ask connectivity before handedness

Two drawings can look different because they show different connections, different spatial arrangements, or simply different views of the same molecule. Start with the atom-to-atom connections. Compounds with the same molecular formula but different connectivity are constitutional isomers. If connectivity agrees, compare spatial arrangement before deciding that two separate substances are present.

Stereoisomers share connectivity but differ in spatial arrangement. Enantiomers are nonsuperimposable mirror images; diastereomers are stereoisomers that are not mirror images. A tetrahedral carbon attached to four different groups is a common stereogenic center. The groups must differ as complete paths through the molecule, not merely have different labels on a drawing.¹²

Rule: assign priorities, then orient the lowest group

For an R/S assignment, rank the four attached groups with the Cahn–Ingold–Prelog rules. Compare the directly attached atoms by atomic number. For isotopes of the same element, the heavier isotope has higher priority. When the first atoms tie, compare the next attached atoms in descending order until the first difference. Do not add their atomic numbers into a score. Multiple bonds are handled with the duplicate-atom convention for ranking.³

View the center with priority 4 pointing away. Follow priorities 1 → 2 → 3: clockwise gives R; counterclockwise gives S. If priority 4 instead points directly toward you, reverse the apparent clockwise/counterclockwise result. If it lies in the page, reorient the structure or use a carefully tracked exchange; do not apply the shortcut without checking the viewing direction.

An R/S descriptor states absolute configuration under the priority rules. A relative description compares arrangements without necessarily assigning every absolute center. Neither R nor S predicts the sign of optical rotation. That sign is a separately measured property under specified conditions.³

Application: compare two centers without losing atom identity

The examples and redrawings here are original instructional constructions. Consider CH₃–CH(OH)–CH(Br)–CH₂CH₃, numbered from the CH₃ end next to OH. It has centers at C2 and C3. The different ends and substituents prevent the internal symmetry that could otherwise make some apparent mirror-image assignments identical.

A Fischer projection represents horizontal bonds pointing toward the viewer and vertical bonds pointing away. It is a configuration convention, not a literal flat molecular conformation. The figure applies that convention to this example.⁴

At C2 in A, the priorities are OH first, the path through C3 second, CH₃ third, and H fourth. The C3 path outranks CH₃ because its next atoms include Br, whereas the methyl carbon is attached to three H atoms. On the page, left → down → up is counterclockwise. H points toward you, so invert that result: C2 is R.

At C3, Br ranks first. The path through C2 ranks second because its next atoms include O, outranking the ethyl path, which ranks third. H is fourth. Left → up → down is clockwise on the page; reversing for the forward H gives C3 as S. A is therefore (2R,3S).

Drawing	C2 configuration	C3 configuration	Relationship to A
A	R	S	Reference
B: invert both centers	S	R	Enantiomer
C: invert only C3	R	R	Diastereomer
D: rotate the complete drawing	R	S	Identical

The 180° redraw retains both configurations. Track the original atoms: the upper crossing in the rotated view is now C3, not C2. Renumbering a molecule from whichever end is physically at the top would create a bookkeeping error. A 90° rotation of a Fischer projection, reinterpreted with the same horizontal/vertical convention, changes its stereochemical meaning and is not an allowed identity shortcut.⁴

Pattern: symmetry can reduce the expected count

Two independent tetrahedral centers allow up to 2² = 4 configurations, but this is a maximum rather than an automatic count of distinct compounds. If internal symmetry makes a configuration superimposable on its mirror image, the molecule can be achiral despite containing stereogenic centers. Such a structure is called meso.⁵

That caveat does not collapse A and B in the unsymmetrical example above. Their atom identities and substituent patterns prevent the needed symmetry. More generally, “every listed R changes to S” is not sufficient by itself to prove a distinct enantiomer when equivalent centers can be interchanged by a symmetry operation. Test superimposability, not only the letter list.

Changed application: assign alkene configuration locally

A double bond can support E/Z stereochemistry when each alkene carbon has two different substituents. Rank the two substituents separately at each end. If the two higher-priority groups lie on the same side of the double bond, assign Z; if on opposite sides, assign E. If one end has two identical substituents, this double bond has no E/Z pair.⁶

For Cl–CH=CH–CH₃, Cl outranks H at the left carbon, and CH₃ outranks H at the right carbon. With the double bond drawn horizontally, compare the following supplied placements:

Arrangement	Left higher-priority Cl	Right higher-priority CH₃	Configuration
M	Above the bond	Above the bond	Z
N	Above the bond	Below the bond	E

M and N are diastereomers. They cannot become identical by rotating the entire page, because that preserves the relative sides of both substituents. Cis/trans descriptions can be useful when a comparable group is identified at both ends; E/Z is based on independently assigned priorities and should not be inferred from whichever groups look largest.

The AAMC stereochemistry scope joins these tasks: connectivity, spatial identity, absolute/relative configuration, and alkene descriptors.⁷ Work from the atom framework through priorities and viewing direction, then classify the pair. A familiar-looking drawing is only the beginning of the analysis.

Check your reasoning

Choose an answer before opening its feedback. Each option explains the reasoning behind that choice.

Continue learning

Related lessons

• Carbohydrate structures and glycosidic bonds

Browse chemistry & physics · MCAT content library

MCAT preparation and roadmap · Aim High, Score Higher

Sources

1. OpenStax Organic Chemistry — Enantiomers and the Tetrahedral Carbon

2. OpenStax Organic Chemistry — Diastereomers

3. OpenStax Organic Chemistry — Sequence Rules for Specifying Configuration

4. OpenStax Organic Chemistry — Representing Carbohydrate Stereochemistry: Fischer Projections

5. OpenStax Organic Chemistry — Meso Compounds

6. OpenStax Organic Chemistry — Alkene Stereochemistry and the E,Z Designation

7. AAMC — Content Category 5B