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🎯 By the end of this guide:

 

• You’ll confidently determine the relative stability of conformations of different molecules.
• You’ll understand how torsional strain, steric strain, and hydrogen bonding impact stability.

 

 

Challenge 1

 

Question:

Rank each conformation in order of increasing stability from least stable to most stable.

 

 

Here’s how we’ll do it

 

Step 1: Is the conformation staggered or eclipsed?

 

The first thing to figure out is whether each conformation is staggered or eclipsed. Why does this matter? Because staggered conformations are more stable than eclipsed ones—always.

Here’s a quick breakdown:

 

• Staggered: Groups on the front and back carbons are spaced out at 60° angles. This reduces:
  • Torsional strain: repulsion caused by bonds overlapping in space.
  • Steric strain: repulsion between bulky groups.

 

• Eclipsed: Groups on the front and back carbons are lined up (0° apart), which increases torsional and steric strain. More strain means less stability.

 

Let’s label your options:

 

• Choice B and C: Both are staggered.

 

 

• Choice C and D: Both are eclipsed.

 

 

Step 2: Rank the eclipsed conformations

 

Eclipsed conformations are the least stable, but one of them will still be worse than the other. This depends on how close and how big the overlapping groups are.

 

Choice D: The largest groups, CH₃ and Br, are directly aligned (0° apart). This creates the maximum steric strain, making this the least stable conformation overall.

 

 

Choice A: The CH₃ and Br groups are 120° apart, so while there’s still strain, it’s not as bad as in Choice D.

 

 

Ranking so far:

D < A 

 

Step 3: Rank the staggered conformations

 

Now onto the staggered conformations. These are more stable than eclipsed, but we still need to rank them. The key here is to look for gauche interactions. A gauche interaction happens when two bulky groups are 60° apart, causing some steric strain.

 

• Choice B: There are no gauche interactions because CH₃ and Br are 180° apart. This is the most stable conformation overall.

 

 

• Choice C: There’s one gauche interaction between CH₃ and Br. This creates a bit of strain, but it’s still much more stable than any eclipsed conformation.

 

 

Ranking so far:

C < B

 

Step 4: Final stability order

 

Let’s combine everything:

 

• Choice D: Eclipsed with CH₃ and Br at 0° → Least stable.
• Choice A: Eclipsed with CH₃ and Br at 120° → More stable than D.
• Choice C: Staggered but with one gauche interaction → More stable than both eclipsed conformations.
• Choice B: Staggered with no gauche interactions → Most stable.

 

Final ranking of stability:
D < A < C < B

 

 

Quick recap:

 

1. Identify whether the conformation is staggered or eclipsed.
2. Rank eclipsed conformations based on the size of overlapping groups.
3. Rank staggered conformations based on gauche interactions.

 

 

Challenge 2

 

Question:

What is the order from most stable to least stable for these conformations of propylene glycol?

 

 

We’ll tackle this challenge following the same steps as before.

 

Step 1: Is the conformation staggered or eclipsed?

 

The first thing we always check is whether the conformation is staggered or eclipsed because this tells us a lot about stability.

 

In this case, we’re in luck—all the conformations we’re looking at are staggered! That’s great news because we don’t need to worry about eclipsed torsional strain this time.

 

Since they’re all staggered, let’s move straight to the next step!

 

Step 2: Rank the staggered conformations

 

We are going to look for gauche interactions. Let’s analyze each choice:

 

Choice A:

 

Contains two gauche interactions:

 

 

1. OH/OH interaction: Two hydroxyl groups are gauche.
2. OH/Me interaction: One hydroxyl group and a methyl group are gauche.

 

Both interactions involve large groups, creating a lot of steric strain. Even though the two OH groups are close enough to form a hydrogen bond, the strain outweighs any potential stabilization.

 

Choice B:

 

Contains one gauche interaction:

 

 

1. OH/OH interaction between the two hydroxyl groups.

The two OH groups are positioned close enough to form a hydrogen bond. This bonding interaction greatly increases stability because it counteracts the strain caused by the gauche interaction.

 

Choice C:

 

Contains one gauche interaction:

 

 

1. OH/Me interaction between a hydroxyl group and a methyl group.

 

There’s no hydrogen bonding here because the OH groups are too far apart. This conformation is more stable than Choice A (fewer gauche interactions) but less stable than Choice B (no hydrogen bonding).

 

Step 3: Final ranking

Let’s put it all together:

 

1. Choice B – Most stable. Has one OH/OH gauche interaction and stabilizing hydrogen bonding between OH groups.
2. Choice C – Moderately stable with one gauche interaction (OH/Me) and no hydrogen bonding.
3. Choice A – Least stable with two gauche interactions (OH/OH and OH/Me).

 

Final order of stability:
B > C > A

 

 

You did it! 🎉

 

Congrats on completing this guide! Understanding Newman projections and stability is no small feat, so give yourself a pat on the back. Got any lingering questions or thoughts? Drop them in the comments section below—I’d love to hear from you and help you out. Every step you take brings you closer to mastering organic chemistry. See you in the next post! 😊