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(Video) Structural formulas

by MSJChem

In this video, MSJChem walks you through the distinct types of chemical formulas. He compares the molecular, empirical, and structural formulas of three different organic molecules. Structural formulas are most common in organic chemistry. So next, he gives structural formula examples for more complex organic molecules. If you need help distinguishing between chemical formulas, this video can help.

Structural Formulas

What are structural formulas?

• Structural Formulas are also called Lewis structures/formulas.

• A structural formula is a way of drawing molecules.

• The structural formula is used to show the connections and order of atoms in a molecule.

As you can see, the structural formula shows how each atom connects to each other in the molecule. It also details the types of bonds (single, double, or triple bonds).

We've been learning about different chemical formulas. We started with molecular formulas. They tell us what atoms make up a molecule. They also specify the number of atoms. But, they don't tell us anything about bonding. For example, ethane's molecular formula would be $C_{2}H_{6}$. Next, we simplified it to an empirical formula. It told us the ratio of a molecule's atoms. Ethane's empirical formula is $C_{1}H_{3}$. Again, we learn nothing about bonding. Do we have any formulas that show bonds? We do! It's called a structural formula!

Lewis Structures

We've already seen examples of structural formulas. When we studied Lewis dot diagrams, we were making structural formulas. But, the molecules we examined were simple. They had a central atom with bonds and lone pairs. So, we could describe the bonding well enough. We could show multiple bonds based on the hybridization. Then, we could determine the molecular geometry using VSEPR. That's fine for general chemistry. But, as you take more advanced chemistry, the molecules become more complicated.

What is a Structural Formula?

A structural formula is an illustration of a molecule. It shows us information about bonding by drawing lines between atoms. That sounds pretty normal. But wait, there's more! It also shows us a three-dimensional arrangement. How? We use special lines to denote bonds that come into or out of the page. A straight line tells us a bond is in the same plane as our piece of paper. A solid black wedge tells us it is coming out of the page. A dashed wedge means it's behind our paper. So, we can draw some complicated molecules.

But when will you use this? Typically, you start to draw more complex structural formulas in organic chemistry. Why? Organic compounds can be massive. For example, since I love coffee, let's talk about caffeine. The molecular formula is $C_{8}H_{10}N_{4}O_{2}$. Its molar mass is 194 grams per mole! And honestly, that's really not that big. But, the molecular formula doesn't tell us much. We wouldn't know if it has carbon-oxygen single bonds or double bonds. Why does that matter? It turns out it has two carbon-oxygen double bonds. The carbon-oxygen double bonds compete with a molecule named adenosine. When adenosine binds to its receptor, we get drowsy. But, if it can't bind to the receptor, we feel very awake and alert. To understand what's going on, we need to know the structure.

Condensed Formulas

For some simple molecules, we can use a condensed formula. It lets us display a bonding by using a dash. Another option is placing empirical groups next to each other. For example, let's consider ethanol. We can write it has $CH_{3}-CH_{2}-OH$ or $CH_{3}CH_{2}OH$. Both are condensed formulas.

The disadvantage of condensed formulas is that they don't work for complicated molecules. For example, molecules that have ring structures. Chemists developed condensed formulas when it was difficult to publish graphics. Today, we have computer programs that can draw any molecule. You can copy and paste it into any report. But, early on, even simple molecules took a lot of time to draw. So, we wrote easy molecules as condensed formulas. We only drew out a molecule if it was necessary.

Skeletal Structures

Remember, chemists are lazy. So, we developed a less complex drawing method called a skeletal formula. Most structural formulas involve a lot of carbon-carbon and carbon-hydrogen bonds. We didn't want to write all of those. So, we created a rule that you only had to write the atomic symbol for other elements. We show a carbon-carbon bond as a line. If two lines meet, we assume there's a carbon there. If a line ends without another atomic symbol, that's also carbon. We also never write the carbon-hydrogen bonds. We assume the correct number of hydrogens without showing them. We also drop lone pairs on any element.

How do we Actually Know the Structure?

You might wonder how we know if our structures are correct. Analytical chemists have various experimental techniques for determining a compound's structure. Let's consider two standard methods.

X-ray Crystallography

Many different compounds form crystals. The most simple examples are ionic and metallic compounds. In them, the electrostatic interactions produce a lattice structure. But even organic compounds can crystallize at low temperatures. In organic molecules, Van der Waal interactions create the lattice.

X-ray crystallography is a technique that analyzes crystalline solids. X-rays have a very short wavelength. So, they interact with the electrons. When an x-ray hits an electron, it gets diffracted. The instrument measures the angles and intensities of the diffraction. With this information, the chemist determines the location of the atoms. Do you know the most famous use of x-ray crystallography? It was the discovery of the double-helix structure of DNA.

Electron Diffraction

The other tool chemists have for determining structure is electron diffraction. For solids, electron diffraction is similar to x-ray crystallography. A beam of electrons gets shot at the material. As the electrons interact with atoms, they scatter. From the wave-particle duality, we know that electrons act as waves. So, the waves will interfere with one another. A computer can analyze the interference pattern and determine the location of atoms.

Electron diffraction can also determine a molecule's molecular geometry. The sample gets vaporized into the gas phase. As a gas, the electrons interact differently than in solid-state electron diffraction. Now, the electrons bounce off of atoms in a molecule. Again, a computer interprets the diffraction to determine the atoms' locations. Electron diffraction is how we know the deviations from ideal bond angles. VSEPR is a basic model. It gives us approximations. But, electron diffraction helps us determine the exact bond angles.