Organic Chemistry – Stereochemistry: Enantiomers, Diastereomers, Identical

Question

For the following compounds, identify whether they are enantiomers, diastereomers, or the same compounds.

  1. To the right is a six carbon ring structure with a methyl group connected to carbon 1 by a diagonal line and a methyl group connected to carbon 2 by a solid filled triangle. To the left is a six carbon ring structure with a methyl group connected to carbon 1 by a solid filled triangle and a methyl group connected to carbon 2 by a diagonal line.
    Compounds A, Enantiomers Diastereomers Identical 1
  2. In the image, are two line structures with four carbons. The structure to the right, has a solid filled triangle off carbon 2 connected to the letter "Cl" and off carbon 3 is a diagonal line connected to a striaght line. The structure to the left, has a diagonal line off carbon 2 connected to the letter "Cl" and off carbon 3 is a diagonal line connected to a straight line.
    Compounds B, Enantiomers Diastereomers Identical 2
  3. In the image, are two line structures with four carbons. The structure to the right, has a solid filled triangle off carbon 2 connected to the letter "OH" and off carbon 3 is a diagonal line connected to the letter "OH". The structure to the left, has a diagonal line off carbon 2 connected to the letter "OH" and off carbon 3 is a diagonal line connected to the letter "OH".
    Compounds C, Enantiomers Diastereomers Identical 3
  4. To the right is a five carbon ring structure with a methyl group connected to carbon 1 by a diagonal line and a methyl group connected to carbon 2 by a diagonal line. To the left is a five carbon ring structure with a methyl group connected to carbon 1 by a solid filled triangle and a methyl group connected to carbon 2 by a solid filled triangle.
    Compounds D, Enantiomers Diastereomers Identical 4

 

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  1. Enantiomers
  2. Diastereomers
  3. Diastereomers
  4. Same compound

Refer to Section 8.1: Types of Isomers (1).

Strategy Map

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Check out the strategy map.

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Table 1: Strategy Map
Strategy Map Steps 
1. Use your knowledge on compound sketches to “flip” 1 of the compounds.

Show/Hide Hint

When we say “flip” a compound, we are referring to rotating its orientation. This causes dashed-back atoms to become wedged forward and vice versa. From this, we can see if the 2 compounds are the same and how they differ.
2. Identify if both compounds have the same shape or are optical isomers.
3. If they are optical isomers, identify if they are mirror images of each other.

Solution

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a. Compounds A

These compounds are mirror image optical isomers.

Answer: Enantiomers

b. Compounds B

These compounds are non-mirror image optical isomers.

Answer: Diastereomers

c. Compounds C

These compounds are non-mirror image optical isomers.

Answer: Diastereomers

d. Compounds D

These compounds have the same chemical formula and connectivity.

Answer: Same compound

Guided Solution

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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.

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Table 2: Guided Solution
Guided Solution Ideas
This question is a theory type problem that requires you to identify if 2 compounds are optical isomers and what type.

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Refer to Section 8.1: Types of Isomers (1).
Recall what these terms mean:

  • Enantiomer
    Show/Hide Don’t Forget!

    Enantiomers are non-superimposable mirror images. A common example of a pair of enantiomers is your hands. Your hands are mirror images of one another but no matter how you turn, twist, or rotate your hands, they are not superimposable.

  • Diastereomer
    Show/Hide Don’t Forget!

    Diastereomers are non-mirror image optical isomers. Diastereomers have a different arrangement around one or more atoms while some of the atoms have the same arrangement.
    Refer to Section 8.1: Types of Isomers (1).
Recall how you know the 2 compounds are the same.

Show/Hide Think About This!

If 2 compounds are the same, they will have the same formula and connectivity. When you “flip” the compound, you should be able to superimpose it on the other.
Recall how to “flip” the compound.

Show/Hide Think About This!

When we say “flipping” compounds, we refer to rotating its orientation. This causes dashed-back atoms to become wedged forward and vice versa. From this, we can see if the 2 compounds are the same and how they differ.
Table 3: Complete Solution
Complete Solution
a. Compounds A

These compounds are mirror image optical isomers.

These compounds are nonsuperimposable. The 2 methane groups have opposing orientations. On one compound, the first methane group is wedged and the second is dashed; on the other compound the first methane group is dashed and the second is wedged.

Answer: Enantiomers
b. Compounds B

These compounds are non-mirror image optical isomers.

These compounds are nonsuperimposable. On one compound, the chlorine is dashed back; on the other, the chlorine is wedged forwards. All other connectivity is the same.

Answer: Diastereomers
c. Compounds C

These compounds are non-mirror image optical isomers.

These compounds are nonsuperimposable. On one compound, the hydroxyl group is dashed back; on the other, the hydroxyl group is wedged forwards. All other connectivity is the same.

Answer: Diastereomers
d. Compounds D

These compounds have the same chemical formula and connectivity. They are superimposable.

Answer: Same compound

Check Your Work

Summary of what we would expect based on the related chemistry theory

Does your answer make chemical sense?

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It is important to be able to identify the different types of isomers. The easiest way to check optical isomers is to imagine yourself trying to superimpose them (or doing so with a molecular model kit). This will allow you to see which bonds are different. Ensure that all your answers follow the rules.

Refer to Figure 8.1.1 Different Types of Isomers (1).

PASS Attribution

Media Attributions

All figures are by Ashlynn Jensen, from LibreTexts PASS Chemistry Book CHEM 1500 (2) by Blackstock et al., and are used/updated under a CC BY-NC 4.0 license.

References

1. Farmer, S.; Kennepohl, D.; Sharrett, Z.; Reusch, W. 8.1: Types of Isomers. In CHEM 1500: Chemical Bonding and Organic Chemistry; LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM_1500%3A_Chemical_Bonding_and_Organic_Chemistry/08%3A_Organic_Chemistry_II_-_Stereochemistry/8.01%3A_Types_of_Isomers.

2. Blackstock, L.; Brewer, S.; Jensen, A. PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500.

3. Blackstock, L.; Brewer, S.; Jensen, A. 8.4 Question 8.4.E.2 PASS – Enantiomers, Diastereomers, Identical. In PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500/08%3A_Organic_Chemistry_II_-_Stereochemistry/8.Re%3A_Question_8.Re-6.11(other)_PASS_-_enantiomers_diastereomers_identical.

4. Sacramento City College. 6.14: Additional Exercises. In SCC: Chem 420 – Organic Chemistry I; LibreTexts, 2019. https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Chem_420_-_Organic_Chemistry_I/06%3A_Stereochemistry_at_Tetrahedral_Centers/6.14%3A_Additional_Exercises.

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