Structural Formula - Online Tutor, Practice Problems & Exam Prep

Molecular formulas, like C₂H₆O, lack structural information, limiting our understanding of atom connectivity and orientation. Structural formulas illustrate how atoms bond without showing lone pairs, focusing on the connections between atoms. For example, both C₂H₆O structures are valid, but the molecular formula alone doesn't clarify their arrangement. Understanding structural representations is crucial in organic chemistry, particularly for recognizing functional groups and isomers, which influence chemical behavior and reactions.

Molecular Representations Concept 1

Now molecular formulas are insufficient for organic compounds because they do not convey structural information. Structural information itself gives us an idea about the connectivity of an organic compound, as well as the orientation elements take when they bond to each other within that organic compound. Connectivity is how atoms are connected to each other. Orientation will be discussed later when we talk about stereoisomers. But for right now, let's not worry about it.

Molecular representation: Here we have our molecular formula of C₂H₆O. This structure can be drawn in a couple of ways. One potential way of drawing this is having our two carbons connected to each other, and then we're going to assign hydrogens to these carbons. Remember, carbon ideally wants to make four bonds. So far, we've used both carbons and five hydrogens. What we have left is an oxygen and a hydrogen. We could then write an oxygen connected to this carbon, and then place the remaining hydrogen on it. So, this is one potential structure from the molecular formula.

Now, there's just not enough information, just from this molecular formula because we could also draw it as our two carbons, each connected to the oxygen, and each carbon having three hydrogens. Technically, either one of these structures is correct if we're only given the molecular formula. As you can see, we don't have enough information to know for sure which one of these two structures the person might want. Right? So, that's the limitation when it comes to molecular formulas. We might know the number of elements, but we don't know how they are connected to one another, and what their orientation would be in terms of space.

Just remember, this is one of the limitations when it comes to molecular formulas.

Molecular Representations Example 1

Here in this example question, it asks, "Which of the following statements is incorrect about molecular formulas and molecular structures?" Alright. So molecular formulas and structures show which atoms are present in a molecule. That's true. If we have a formula of C₂H₆O, I know that there are 2 carbons, 6 hydrogens, and 1 oxygen. So this is true. Molecular structure cannot be used to calculate molecular weight. This is incorrect. If I know the number of elements from C₂H₆O, I can calculate the weight of that compound. This is our answer. Molecular formula is sufficient to show the composition of a compound. Yes, because you'll know the number of each element. From that, you'll know the percentage each one represents within that given organic compound. Molecular structure shows how atoms are connected to each other. Yes. You can draw out how those atoms are connected to one another through the use of molecular structure. Right? So this is true. So out of all the options, option B is the one that's incorrect.

Structural Formula Concept 2

Now a structural formula shows how atoms in a compound are connected or bonded to one another. It's the same as a Lewis structure, except we do not show lone pairs. If we took a look here at this structure, if we wanted to draw the Lewis formula, we'd remember that the oxygen has 2 lone pairs on it. When it comes to our structural formula, we don't need to show the presence of those lone pairs on oxygen, because we're more concerned with the bonds that are being created between our different atoms. We don't care about nonbonding electrons, which is typical with a Lewis dot structure. Right? So just remember, both of these are showing how atoms are connected to one another or bonded to one another, but Lewis does a little bit more by showing us our nonbonding electrons or lone pairs. Structural formulas don't really care about that.

Structural Formula Example 2

Here it says to draw a structural formula for C₂H₆. Alright. So this molecule or compound has 2 carbons within it. We can show these 2 carbons bonding to one another. Now we have 6 hydrogens and 2 carbons. To keep things fair, each one can get 3 hydrogens apiece. Now, here this carbon would have 3 hydrogens, and this one would have 3 hydrogens. This comes out to be our structural formula for C₂H₆, and in fact, this is the only structural formula we could draw. Remember, hydrogens can only make one bond, so we couldn't put any of them in the center, which means our 2 carbons must go in the center. Carbon ideally wants to make 4 bonds. By drawing it this way, they're all making 4 bonds, which means we cannot draw a double bond between them or a triple bond because that would exceed the 4 bonds that carbon ideally wants to make. Right? So this will represent our structural formula for C₂H₆.

Propane (C₃H₈) is a component of liquified petroleum gas. Draw a structural formula for propane. (Hint: Carbon makes four bonds.)

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Methylamine (CH₅N) is a gaseous compound with a fishy smell. Draw this molecule using structural formula.

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Here’s what students ask on this topic:

What is the difference between a molecular formula and a structural formula?

A molecular formula indicates the types and numbers of atoms in a molecule, such as C₂H₆O, but it does not provide information about how these atoms are connected or arranged. In contrast, a structural formula shows the specific connections between atoms, illustrating how they are bonded. For example, C₂H₆O can represent both ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃), but the structural formula clarifies which isomer is being referred to by showing the actual bonding arrangement.

Why are structural formulas important in organic chemistry?

Structural formulas are crucial in organic chemistry because they provide detailed information about the connectivity and arrangement of atoms within a molecule. This information is essential for understanding the molecule's properties, reactivity, and behavior in chemical reactions. For instance, structural formulas help identify functional groups and isomers, which can significantly influence a compound's chemical characteristics and biological activity.

How do structural formulas differ from Lewis structures?

Structural formulas and Lewis structures both show how atoms are connected in a molecule, but they differ in detail. Structural formulas focus on the bonds between atoms and do not display lone pairs of electrons. In contrast, Lewis structures include both bonding and non-bonding electrons (lone pairs), providing a more comprehensive view of the electron distribution within the molecule. For example, a structural formula for water (H₂O) would show H-O-H, while the Lewis structure would also depict the two lone pairs on the oxygen atom.

Can a single molecular formula represent multiple structural formulas?

Yes, a single molecular formula can represent multiple structural formulas. This phenomenon occurs because the same set of atoms can be connected in different ways, leading to different structures known as isomers. For example, the molecular formula C₂H₆O can correspond to both ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃). These isomers have distinct structural formulas and different chemical properties.

What limitations do molecular formulas have in representing organic compounds?

Molecular formulas have significant limitations in representing organic compounds because they do not provide information about the connectivity or spatial arrangement of atoms. For example, the molecular formula C₂H₆O can represent both ethanol and dimethyl ether, which are structurally and functionally different. Without structural information, it is impossible to determine the specific arrangement of atoms, which is crucial for understanding the compound's properties and reactivity.