Enantiomers, Diastereomers, Identical or Constitutional Isomers ... - Free Printable

This image presents a set of eight problems (a-h), each showing two chemical structures. The task is to determine the relationship between the two molecules in each pair. Common relationships include: identical, constitutional isomers, stereoisomers (which can be further broken down into enantiomers or diastereomers), or unrelated.

I will analyze each pair one by one to determine their relationship.

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Pair a)

- First look: Both structures are chair conformations of a disubstituted cyclohexane. In the first structure, both methyl groups are axial. In the second, both are equatorial. This suggests they are conformational isomers (conformers) of the same molecule.
- Analysis: Since these are different conformations of the same compound (cis-1,3-dimethylcyclohexane), they are identical molecules viewed in different conformations. Conformers are not considered different compounds; they interconvert rapidly at room temperature.
- Conclusion for a): Identical

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Pair b)

- First look: Both molecules have a cyclohexene ring with a side chain containing an SH group and an isopropyl group. The difference is in the stereochemistry at the chiral center bearing the SH group. One has the SH group pointing out of the plane (wedge), the other has it going into the plane (dash). This indicates they are non-superimposable mirror images.
- Analysis: These are enantiomers because they differ only in the configuration at a single chiral center and are mirror images.
- Conclusion for b): Enantiomers

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Pair c)

- First look: Both are alkenes with the same molecular formula. The double bond is in the same position, but the substituents around it are arranged differently. The first structure shows the ethyl groups on opposite sides of the double bond (trans), while the second shows them on the same side (cis).
- Analysis: These are geometric isomers (a type of diastereomer) of 3,4-dimethylhex-3-ene. They are not mirror images and have different physical properties.
- Conclusion for c): Diastereomers

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Pair d)

- First look: Similar to (c), these are alkenes. The first structure has the two ethyl groups on opposite sides (trans), and the second has them on the same side (cis). However, looking closely, the carbon skeleton is the same: 2,3-dimethylpent-2-ene. The double bond is between carbons 2 and 3, with methyl and ethyl groups attached.
- Analysis: These are also geometric isomers (E/Z isomers) of the same compound. They are diastereomers.
- Conclusion for d): Diastereomers

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Pair e)

- First look: Both are 2-butanol molecules. The first has the OH group on a wedge (coming out), and the second has it on a dash (going in). This indicates they are mirror images.
- Analysis: These are enantiomers of 2-butanol. The chiral center is the carbon with the OH group.
- Conclusion for e): Enantiomers

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Pair f)

- First look: The first structure is a Newman projection, and the second is a sawhorse or perspective drawing. Both show a central carbon bonded to Br, F, NHCH₃, and OH. The substituents are arranged identically in space in both drawings.
- Analysis: These are simply different ways of drawing the same molecule. The Newman projection is looking down the C-C bond, and the perspective drawing shows the same spatial arrangement. They represent the same stereoisomer.
- Conclusion for f): Identical

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Pair g)

- First look: Both molecules have a benzyl group attached to a carbon that also has a methyl and a bromine. The first structure has the bromine on a dash, and the second has it on a wedge. This suggests they are mirror images.
- Analysis: These are enantiomers of 1-bromo-1-phenylpropane (or 1-phenylpropyl bromide). The carbon with the Br is chiral.
- Conclusion for g): Enantiomers

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Pair h)

- First look: Both are 2-chloro-1-propanamine. The first has the NH₂ group on a wedge, and the second has it on a dash. This indicates they are mirror images.
- Analysis: These are enantiomers of 2-chloropropan-1-amine. The carbon with the NH₂ group is chiral.
- Conclusion for h): Enantiomers

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Final Answer

Here is the completed solution for each pair:

a) Identical
b) Enantiomers
c) Diastereomers
d) Diastereomers
e) Enantiomers
f) Identical
g) Enantiomers
h) Enantiomers