Using AI for Chemistry
Various Large Language Models (LLM), such as ChatGPT and others, are all over the place nowadays. I’ve also noticed that students like using them as well for studying and, occasionally, for completing their homework and other assignments.
Someone asked me recently: “Victor, aren’t you afraid that soon an AI tutor will just replace you?”
Frankly, no, I’m not worried. And let me tell you why.
The glorified AI models are first and foremost language models that attempt to predict the next word in a phrase (very basically speaking). It doesn’t have much of reasoning skills no matter what they advertise. I myself like to use ChatGPT to refine my writing, edit grammar, summarize long articles, brainstorm ideas, etc. And, for the fun of it, I often ask it chemistry questions that do require actual understanding to give the correct answer.
For instance, here’s a good example of such a conversation. I asked ChatGPT (I used the 4o version) to answer the following question:
A meso compound will necessarily have at least two diastereomers. True/False?
Here’s our conversation:
Statement claims:
“A meso compound will necessarily have at least two diastereomers.”
This statement is false because a meso compound does not always have two diastereomers. Here’s why:
A meso compound is a specific type of molecule that has multiple stereocenters but is achiral due to internal symmetry. This internal symmetry means that even though the molecule has stereocenters, it can be superimposed on its mirror image, making it optically inactive.
Let’s break it down:
• Enantiomers are non-superimposable mirror images of each other.
• Diastereomers are stereoisomers that are not mirror images of each other.
Now, while a meso compound has stereocenters, its internal symmetry makes it achiral, so it does not have an enantiomer. Additionally, the number of possible diastereomers depends on the specific stereochemistry and number of stereocenters in the molecule.
Example:
Take 2,3-dibromobutane:
• It has two stereocenters (C2 and C3), and you might expect it to have several stereoisomers.
• There are two enantiomers: (2R,3R)-dibromobutane and (2S,3S)-dibromobutane.
• However, there is also a meso form: (2R,3S)-dibromobutane, which is achiral due to internal symmetry. This meso compound is not an enantiomer or a diastereomer of itself. It only has one diastereomer, not two.
Therefore, a meso compound does not necessarily have two diastereomers, and that’s why (m) is false. It can have fewer diastereomers, depending on the structure of the molecule.
If 2,3-dibromobutane has 3 stereoisomers: a pair of enantiomers and a meso compound, wouldn’t that mean that the meso compound indeed does have two diastereomers in this example?
You are absolutely right in this context! Let me clarify this further.
For 2,3-dibromobutane, we have:
• A pair of enantiomers: (2R,3R) and (2S,3S).
• A meso form: (2R,3S), which is achiral due to internal symmetry.
In this case, the meso compound does indeed have two diastereomers:
• The (2R,3R) stereoisomer.
• The (2S,3S) stereoisomer.
These two stereoisomers are diastereomers of the meso compound, since they are stereoisomers but not mirror images of it.
So, yes, in the case of 2,3-dibromobutane, the meso compound has exactly two diastereomers. However, the statement in (m) — “A meso compound will necessarily have at least two diastereomers” — is still generally false, because the number of diastereomers a meso compound can have depends on the number of stereocenters and the specific stereochemistry of the molecule. In some cases, a meso compound may have fewer or no diastereomers.
For example, in a system with only two stereocenters like 2,3-dibromobutane, the meso form will typically have two diastereomers. However, in more complex cases (with more stereocenters or different symmetry elements), the number of possible stereoisomers, including diastereomers, can vary, and a meso compound might not always have two diastereomers.
Thank you for pointing that out! Does this explanation clear things up?
Can you define a meso compound for me?
A meso compound is a molecule that contains two or more stereocenters (chiral centers) but is achiral due to an internal plane of symmetry. This symmetry causes one half of the molecule to be the mirror image of the other half, making the overall molecule superimposable on its mirror image. As a result, even though meso compounds have stereocenters, they are optically inactive because the internal symmetry cancels out any optical rotation.
Key characteristics of a meso compound:
1. Stereocenters: A meso compound must have at least two stereocenters (chiral centers).
2. Internal symmetry: The molecule has a plane of symmetry or a center of symmetry that makes it superimposable on its mirror image.
3. Achirality: Despite having stereocenters, a meso compound is achiral because of the symmetry.
4. Optically inactive: Since it is achiral, a meso compound does not rotate plane-polarized light.
Example:
Tartaric acid is a classic example of a meso compound. The meso form of tartaric acid (2R,3S-tartaric acid) has two stereocenters, but due to the internal plane of symmetry, the molecule is achiral and optically inactive.
In summary, a meso compound is a special case of a stereoisomer that has stereocenters but is achiral due to symmetry, making it optically inactive.
Suppose. Although it’s not an actual definition of a meso compound, but I’ll roll with it.
Now, give me an example where a meso compound *only* has 1 diastereomer and not two.
Thank you for the clarification — let’s adjust the approach for a more precise understanding!
A meso compound is typically defined as a molecule with multiple stereocenters that, despite those stereocenters, is achiral due to an internal plane of symmetry or other symmetry elements, making it superimposable on its mirror image.
Example where a meso compound has only one diastereomer:
1,2-dichloro-1,2-difluoroethane is a great example of a meso compound that has only one diastereomer.
• This molecule has two stereocenters (the two carbons), but because of the internal plane of symmetry when both carbons have the same configuration (R,S or S,R), it forms a meso compound.
• The meso compound is achiral, since it has a mirror plane along the middle of the molecule.
Now, let’s look at its stereoisomers:
• There are two stereoisomers in total:
1. The meso form (R,S or S,R).
2. A pair of enantiomers (R,R and S,S).
So, the meso form has exactly one diastereomer (the enantiomeric pair R,R/S,S as a collective diastereomeric counterpart), because diastereomers are defined as stereoisomers that are not mirror images. Since meso forms are superimposable on their mirror image, they don’t have an enantiomer of their own but do have one diastereomer.
In this case:
• The meso form (R,S) has one diastereomer, which is the pair of enantiomers (R,R and S,S).
Conclusion:
In 1,2-dichloro-1,2-difluoroethane, the meso compound has only one diastereomer and no enantiomer of its own. This example demonstrates that not all meso compounds must have two diastereomers.
How is this example any different from your first example? We still have a case of a meso compound + a pair of enantiomers. Thus, in this example we *again* have a meso compound with two diastereomers.
You’re absolutely right again — my previous explanation wasn’t quite addressing the difference I intended to highlight, and both examples do indeed present the meso compound with two diastereomers. Let me try this again with a clearer example where the meso compound has only one diastereomer.
Example: cis-1,2-dichlorocyclohexane
• This molecule has two stereocenters at C1 and C2.
• The cis form is a meso compound because it has a plane of symmetry through the cyclohexane ring, making the molecule achiral despite the presence of stereocenters.
Now, let’s consider the trans form:
• The trans-1,2-dichlorocyclohexane exists as two enantiomers: one with (R,R) and one with (S,S) configurations. These two enantiomers are not meso and are non-superimposable mirror images of each other.
Stereoisomers summary:
• cis-1,2-dichlorocyclohexane is a meso compound (due to internal symmetry and achirality).
• trans-1,2-dichlorocyclohexane has two enantiomers: (R,R) and (S,S).
How many diastereomers does the meso form have?
• The meso form (cis) has only one diastereomer, which is the pair of trans enantiomers. Both the (R,R) and (S,S) forms are diastereomers of the meso compound, but these two are a pair of enantiomers to each other.
Thus, in this case, the meso compound (cis form) has only one diastereomer — the trans pair — rather than two distinct diastereomers.
Key Difference:
The key difference here compared to previous examples (like 2,3-dibromobutane) is that the meso form (cis-1,2-dichlorocyclohexane) only has one stereoisomeric form that it is diastereomeric to: the trans form, which consists of a pair of enantiomers.
This is the kind of case where a meso compound has only one diastereomer, not two. Does this make things clearer?
Didn’t you just say that the trans isomer here is actually a pair of enantiomers, therefore the cis isomer still has 2 diastereomers?
You’re absolutely right again, and I see where I misunderstood the concept in my own explanation. You’re pointing out the core issue here, and I appreciate your patience. Let me clarify the situation one last time with the correct approach.
In both the case of cis-1,2-dichlorocyclohexane and 2,3-dibromobutane, we end up with the meso compound having two diastereomers (the pair of enantiomers). This means my examples so far were incorrectly stated.
So, to finally address your original request, I’ll explain why finding a meso compound with only one diastereomer is tricky in the classical cases we’ve discussed.
Here’s the corrected understanding:
• A meso compound will typically arise in molecules that have at least two stereocenters.
• Most of the time, if there are two stereocenters, you will end up with a meso form and a pair of enantiomers, which gives two diastereomers for the meso compound.
To truly satisfy your request:
It’s challenging to come up with an example where a meso compound has exactly one diastereomer unless you start increasing the complexity of the molecule by either involving more stereocenters or other types of structural restrictions that might reduce the possible stereoisomers.
After further reflection, I don’t believe the example you’re asking for — a meso compound with exactly one diastereomer and not a pair of enantiomers — can exist under simple stereocenters (two chiral centers, for example). If there are two stereocenters, you’ll always have either:
1. A meso form with two diastereomers (the enantiomeric pair),
2. Or other variations, but still resulting in two.
So, I stand corrected, and your initial observation holds: a meso compound in typical two-center cases will always have two diastereomers (a pair of enantiomers), as you astutely pointed out.
Let me know if you’d like to explore more nuanced cases or have further clarifications!
As you can see, while ChatGPT does know some definitions and can even pretend to reason, it hallucinates the answer and spits it back with total confidence, even when I correct it multiple times and point out the inconsistencies in its own logic. So, if you choose to use AI, don’t delegate all brain functions like, I dunno, actual thinking 😉 to it.
Have you ever used AI for learning? Let me know about your experiences in the comments!
