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Isomers

Chapter 2: Isomers

Updated Apr 10, 2026
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1. (2.1) Isomers are:
B. Same atoms, different arrangement. This gives compounds distinct properties despite identical empirical data.
2. (2.1) The two broad categories of isomers are:
D. Structural: different connectivity. Stereoisomers: same connectivity, different spatial arrangement.
3. (2.2) Structural (constitutional) isomers differ by:
A. Examples include chain, positional, and functional-group isomers.
4. (2.2) n-Butane and isobutane are:
C. n-Butane has a straight chain; isobutane has a branched skeleton. Different boiling points and reactivity.
5. (2.3) Stereoisomers share:
B. All stereoisomers have the same connectivity and the same molecular formula. They differ only in how atoms point in 3D space.
6. (2.3) Stereoisomers can be:
D. Enantiomers come in pairs; diastereomers include cis/trans and those with multiple stereocenters.
7. (2.4) A chiral molecule is:
A. Like left and right hands. Chirality is the origin of the R/S labels and optical activity.
8. (2.4) A carbon bonded to four different groups is called a:
C. Each stereocenter doubles the maximum number of stereoisomers (up to 2ⁿ, minus any meso).
9. (2.5) R/S configurations are assigned by:
B. R = rectus (right); S = sinister (left). R/S says nothing by itself about the sign of optical rotation.
10. (2.5) CIP priority ranks substituents mainly by:
D. Heavier atom wins. Double and triple bonds count as duplicate atoms at both ends.
11. (2.6) Enantiomers are:
A. Example: (R)- and (S)-thalidomide; one treated morning sickness, the other caused birth defects. Biological systems distinguish enantiomers.
12. (2.6) Diastereomers are stereoisomers that are:
C. Diastereomers have different physical properties (different bp, mp, etc.) and are generally separable by chromatography.
13. (2.7) An optically active compound:
B. A polarimeter measures this as specific rotation [α].
14. (2.7) A racemic mixture:
D. The two opposite rotations cancel. Achieving an enantiomerically pure product is a major challenge in synthesis.
15. (2.8) A meso compound:
A. Example: meso-tartaric acid. Two stereocenters, but internal symmetry makes the molecule achiral overall.
16. (2.8) Meso compounds are identified by:
C. The plane of symmetry makes one half of the molecule the mirror of the other half, so the rotations cancel internally.
17. (2.9) E/Z nomenclature for double bonds uses:
B. E/Z is a rigorous CIP-based upgrade of the older cis/trans labels, which work cleanly only when each carbon has two different substituents.
18. (2.9) A cis-alkene has:
D. Cis = same side; trans = opposite sides.
19. (2.10) Conformational isomers:
A. Strictly not considered "true isomers" because they interconvert. But they matter for reactivity.
20. (2.10) The most stable conformation of ethane (viewed along the C-C axis) is:
C. Staggered puts all H-H dihedrals at 60°, minimizing eclipsing interactions. Energy difference is ~3 kcal/mol for ethane.
21. (2.11) The chair conformation of cyclohexane:
B. Boat and twist-boat conformations are higher in energy by about 5-7 kcal/mol.
22. (2.11) A bulky substituent on cyclohexane prefers:
D. A tert-butyl group is essentially locked equatorial. This can be used to "lock" a conformation in synthesis.
23. (2.12) Fischer projections represent 3D structure in 2D by convention:
A. Fischer projections are common for sugars and amino acids, where the chirality center is the subject.
24. (2.12) To determine R/S from a Fischer projection:
C. Because the lowest priority is pointing the "wrong" way, the actual rotation is the opposite of what is drawn.

Your left hand and your right hand have the same parts: five fingers, same joints, same fingernails. They are connected the same way. And yet a left-handed glove will never fit on a right hand. Same pieces. Same connections. Different arrangement in space. That is chirality, and that is why isomers matter on the MCAT.

Isomers are molecules that share a molecular formula but differ in how the atoms are connected or arranged. Some differences are tiny - just a flip in 3D space. Others are dramatic enough to reverse a drug’s effect or turn a sugar your body can digest into one it cannot. A classic example: thalidomide, a sedative prescribed in the 1950s, had two enantiomers. One relieved morning sickness in pregnant women. The other caused severe birth defects. Same formula. Same connectivity. Opposite spatial arrangement. Tragic outcome.

This chapter gives you the decision tree to classify any two molecules the MCAT presents. You will learn to tell at a glance whether a pair is constitutionally different, stereochemically different, or just a conformational snapshot of the same molecule. You will assign R/S configurations, spot meso compounds by their internal mirror plane, and read Newman and Fischer projections without flinching.

The Central Analogy

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