Structural Formula - Online Tutor, Practice Problems & Exam Prep

Molecular formulas, like C₂H₆O, lack structural information, making it difficult to determine atom connectivity and orientation in organic compounds. Structural formulas illustrate how atoms bond without showing lone pairs, focusing on the connections between atoms. Understanding these representations is crucial for grasping concepts like isomerism and the behavior of organic molecules, which are foundational in organic chemistry.

Molecular Formula

Now, molecular formulas are insufficient for organic compounds because they do not convey structural information. Now, 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. Now connectivity is how atoms are connected to each other orientation. That's going to be discussed later on when we talk about stereo isomers. Well, for right now let's not worry about it.

Now, molecular representation, here we have our molecular formula of C2H6O. This structure can be drawn in a couple of ways. C2H6O potential way of drawing this is having our two carbons connected to each other and then we're going to give hydrogen to these, 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 on it the remaining hydrogen. So this is 1 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 3 hydrogens. Technically, either one of these structures are 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're connected to one another and what their orientation would be in terms of space. All right. So just remember, this is one of the limitations when it comes to molecular formulas.

Structural Formula Example

Here in this example question, it says which of the following statements is incorrect about molecular formulas and molecular structures. All right, so molecular formula and structures show which atoms are present in a molecule. That's true. If we have a formula of C2H6O, I know that there's two carbons, six hydrogens and one oxygen. So this is true.

Molecular structure cannot be used to calculate molecular weight. This is incorrect. If I know a 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 our molecular structure, right? So this is true. So out of all the options, option B is the one that's incorrect.

Structural Formula

Now a structural formula shows how atoms in a compound are connected or bonded to one another. Now it's the same as a Lewis structure, except we do not show lone pairs. So 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 on the bonds that are being created between our different atoms. We don't care about non-bonding 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 non-bonding electrons or lone pairs. Structural formulas don't really care about that.

Structural Formula Example

Here it is to draw a structural formula for C₂H₆. Alright, so this molecule or compound has two carbons within it. We can show these two carbons bonding to one another. Now we have 6 hydrogens and two carbons. To keep things fair, each one can get three 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 two carbons must go in the center.

Carbon ideally wants to make four bonds by drawing it this way. They're all making four bonds, which means we cannot draw a double bond between them or a triple bond, because that would exceed the four 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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• B.
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• D.

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

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• B.
• C.
• D.

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 compound but does not provide information on how these atoms are connected or their spatial orientation. For example, C₂H₆O can represent different compounds. A structural formula, on the other hand, shows the connectivity between atoms, illustrating how they are bonded to one another. This is crucial for understanding the compound's structure. Structural formulas do not typically show non-bonding electrons or lone pairs, unlike Lewis structures, which include these details.

Why are molecular formulas insufficient for organic compounds?

Molecular formulas are insufficient for organic compounds because they do not convey structural information. They only indicate the types and numbers of atoms present but do not show how these atoms are connected or their spatial orientation. For example, the molecular formula C₂H₆O can correspond to different structures, such as ethanol or dimethyl ether. Without structural information, it is impossible to determine the exact arrangement of atoms in the compound, which is essential for understanding its properties and reactivity.

How does a structural formula differ from a Lewis structure?

A structural formula shows how atoms in a compound are connected or bonded to one another, focusing on the bonds between atoms. It does not typically show non-bonding electrons or lone pairs. In contrast, a Lewis structure provides a more detailed depiction by including non-bonding electrons or lone pairs. For example, in a Lewis structure of water (H₂O), the lone pairs on the oxygen atom are shown, whereas in a structural formula, only the bonds between hydrogen and oxygen are depicted.

Can you provide an example of how a molecular formula can represent different structures?

Yes, the molecular formula C₂H₆O can represent different structures. One possible structure is ethanol, where two carbon atoms are connected, with an oxygen atom bonded to one of the carbons and the remaining hydrogens attached to the carbons and oxygen. Another possible structure is dimethyl ether, where each carbon atom is connected to the oxygen atom, and each carbon has three hydrogens attached. These different structures illustrate the limitation of molecular formulas in conveying the exact arrangement of atoms.

Why is structural information important in understanding organic compounds?

Structural information is important in understanding organic compounds because it reveals how atoms are connected and their spatial orientation. This information is crucial for determining the compound's properties, reactivity, and behavior in chemical reactions. Without structural information, it is impossible to accurately predict how a compound will interact with other substances. For example, the molecular formula C₂H₆O can represent both ethanol and dimethyl ether, which have different properties and uses despite having the same molecular formula.