Same Chemical Formula! But Different Substances!?

Hello.

This is Komiya, in charge of Kawagoe Minami & Gaku-Pura, Saitama Western Area, at Gakushu Kukan.

Today, I would like to talk about something you learn in the field of organic chemistry in high school: isomers.
It might be a little—or rather, very—difficult for junior high school students.

Let’s study chemistry!!

So, what exactly are isomers? Well, according to the organic chemistry textbook “Bruice” I have here,

Compounds that have the same chemical formula but different structures

That’s how it is described.

In organic chemistry, if you’re only given a chemical formula, it’s often very difficult to determine exactly what compound it is.

For example, if the formula is C2H6O, two compounds can be considered: ethanol and dimethyl ether.

One belongs to the alcohol group, the other to the ether group. So, while they share the same chemical formula, their properties are completely different.
(I won’t go into details here. If you’re curious, please look it up!)

When the way the atoms are connected in the molecule differs, the resulting isomers are called structural isomers.

Other times, the difference lies only in the three-dimensional structure.

Perhaps you’ve heard of trans fats in daily life? That’s a good example. They have the same chemical formula and connectivity as regular fatty acids, but because their three-dimensional arrangement differs, their properties also change.

Isomers that differ only in spatial arrangement are called stereoisomers, and among them, the ones like trans fats are called geometrical isomers.
(Please look up more details if you’re interested)

However, in stereoisomers, there’s also a type where one is a mirror image of the other and cannot be superimposed. These are called enantiomers.
A common way to explain this is by comparing them to a “right hand” and a “left hand.” Just as you can’t wear a right-handed glove on your left hand, these molecules cannot overlap.

To summarize:

There are mainly two types of isomers:
→ Structural isomers and stereoisomers

・Structural isomers: Compounds with different atom-to-atom connections

・Stereoisomers: Compounds with the same connections but different 3D arrangements
→ Includes geometrical isomers and enantiomers

Now, at this point you might be wondering:

Structural isomers make sense, but do stereoisomers—especially enantiomers—really make such a difference?

That’s a very reasonable question. With structural isomers, they’re basically completely different compounds, so it’s easier to understand why their properties differ.
But stereoisomers can look identical when written as a molecular formula, so it’s harder to imagine.

Still, my blunt answer to this question would be: “Of course they make a difference!”

But throwing complicated explanations at you would only cause more confusion, so let’s look at some real-world examples.

As I mentioned earlier, stereoisomers differ in their 3D arrangements. This becomes especially important in substances like proteins, which consist of countless repeating units.
That’s because proteins are made of 20 different amino acids strung together—not in a straight line, but folded into specific shapes.

The way they fold, such as where bends occur or where folding is easier, depends on their 3D structure.

If this folding goes wrong, it can sometimes cause serious diseases. A classic example is Bovine Spongiform Encephalopathy, or BSE.

Some of you parents may remember this from over 20 years ago. BSE caused cows to act strangely before dying, and their brains were found to be spongy and full of holes.

This disease was not caused by bacteria or viruses, but by proteins misfolding into abnormal structures. The exact trigger is still unclear, but once a misfolded protein appears, it can cause surrounding normal proteins to misfold as well.
In other words, unless abnormal proteins enter the body, infection doesn’t occur.

… That’s why so many cows were culled back then. Scary stuff, isn’t it?

Anyway, back to the main point. Even stereoisomers must be carefully separated, because they can cause massive problems. A famous example involving enantiomers is the “Thalidomide tragedy.”

In the 1950s, the drug thalidomide was sold as a sleeping pill. However, when pregnant women took it, some of their babies were born with severe birth defects.
It was later discovered that one enantiomer of thalidomide had strong teratogenic effects, meaning it caused malformations in fetuses. This led to a worldwide recall of the drug.

The tragedy happened because both the right-handed and left-handed forms of the molecule were sold together. Truly frightening…

So as you can see, even when the difference lies only in 3D arrangement, the consequences can be enormous. That’s why researchers in organic chemistry pay close attention to stereoisomers, especially when synthesizing compounds.

I was going to introduce the machines used to measure this, but if I continue, this will get way too long, so let’s stop here.

Adios!!

posted by本橋(埼玉西部エリア)
入間扇台教室＆桶川西教室

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