The structural formula is a graphical representation of the bonds of a molecule, shedding light on its structure once determined by spectroscopic methods. It is the most specific way when referring to a specific compound, and not to several isomers corresponding to the same molecular formula.
For example, butane, C 4 H 10, has two isomers: n-butane (linear) and 2-methyl-propane (branched). The molecular formula does not discriminate between either of the two; while if we resort to structural formulas, it will be seen precisely that one is linear and the other branched.
Structural formulas allow scrutinizing the molecular structures of compounds. Source: Pixabay.
Using structural formulas makes it easier to understand the changes that a molecule undergoes during a chemical reaction; which of its links are broken, how its structure is modified in the process and at the end of it. Learning to read these formulas is the same as superficially predicting the properties of molecules.
Structural formulas are 2D representations, although they can indicate some three-dimensional and geometric aspects. The more the structure of a compound is investigated, the more refined and faithful its structural formula ends up becoming. Otherwise, it leaves out essential aspects to understand the nature of the molecule.
Structural formula examples
Each compound has its respective structural formula, which can vary depending on the type of projection or perspective used. For example, condensed and skeletal formulas, Lewis structures, and stereochemical projections are all structural formulas, dedicated to graphing as much information as possible concerning molecular structure.
There are so many that only a few simple examples will be covered.
Glucose
Various representations of the structure of alpha-glucose. Source: Yikrazuul via Wikipedia.
Four representations of the glucose molecule are shown in the upper image. Each is a valid structural formula; but 2 (Haworth projection) and 3 (chair projection) are usually the most used in academic texts and in publications.
The 4 has the advantage that it indicates directly which OH groups are above (thick wedges) or below (dotted wedges) the hexagonal ring; that is, it facilitates the understanding of its stereochemistry. In contrast, the 1 (Tollens-Fisher projection) shows the linear character of glucose before converting to its cyclic form.
Methane
Structural formula of methane. Source: Gabriel Bolívar via MolView.
Above you have two structural formulas of methane, whose condensed molecular formula is CH 4. For those who lack knowledge of chemistry, they could interpret the formula CH 4 as if it were a molecule with a hydrogen atom in the center.
But in reality (and necessarily), the structural formulas make it clear that carbon is the central atom. Therefore, we have four CH bonds. Note also that the formula on the left creates the false impression that the molecule is flat, when in fact it is tetrahedral (formula on the right).
That is why in the structural formula on the right, the bonds are represented by wedges, indicating the relative spatial positions of each hydrogen atom (vertices of the tetrahedron).
Methanol
Structural formula of methanol. Source: NEUROtiker
The structural formula of methanol is practically the same as that of methane, with the difference that it has an H substituted by an OH. Its condensed or chemical formula is CH 3 OH, and the molecular CH 4 O. It is observed that it also consists of a tetrahedron.
Ethanol
Structural formula of ethanol. Source: Gabriel Bolívar via MolView.
Now we move on to ethanol, the next alcohol on the list. Its chemical or condensed formula is CH 3 CH 2 OH, which by itself already shows its linear structure. To be clear, the structural formula in the image above effectively demonstrates that ethanol is straight chain or backbone.
If you look closely, the surroundings of each carbon atom are tetrahedral.
Fructose
Structural formula of beta-D-fructofuranose. Source: NEUROtiker (talk • contribs)
Above we have the structural formula of fructose, more precisely the Haworth projection of its furanus ring (five-membered). Note how much the structural formula reveals, unlike the molecular one, C 6 H 12 O 6, which coincides with that of glucose, both being different sugars, however.
Water
Structural formula of water. Source: Benjah-bmm27 via Wikipedia.
The chemical formula of water is H 2 O, also corresponding to the condensed and molecular formulas. As with methane, those who do not know the water molecule (and have no notion of chemical bonds) may believe that its structure is OHH; but the structural formula in the image above clarifies the true structure.
Although it is not appreciated, the pairs of free electrons of oxygen and hydrogen atoms draw a tetrahedron around the oxygen; this is, the electronic geometry of water: tetrahedral. Meanwhile, the two hydrogen atoms establish a plane similar to a boomerang; this is, the molecular geometry of water: angular.
Although the structural formula of water is by far the simplest of the examples discussed, it hides more secrets and anomalies than it alone manages to represent.
Aspirin
Structural formula of aspirin. Source: Gabriel Bolívar via MolView.
We have one of the first "flaws" of structural formulas: their inability to represent the aromatic character of a structure; which in this case, corresponds to the aromaticity of the benzene (hexagonal) ring of aspirin (above).
If you look at this formula carefully, you will come to the conclusion that it is an essentially flat molecule; that is, almost all of its atoms "rest" in the same plane, with the exception of the methyl group, CH 3, to its left, where the tetrahedral environment of carbon is once again displayed.
Again, the structural formula provides much more information than its flat molecular formula, C 9 H 8 O 4; which corresponds to numerous structural isomers, totally different from aspirin.
Benzene
Structural formula of benzene. Source: Gabriel Bolívar via MolView.
Finally, we have above the structural formula of benzene. Its molecular formula is C 6 H 6, indicating that it effectively contains six carbon atoms and six hydrogen atoms. But it says nothing about the true structure of benzene.
The C = C double bonds are not static, since a pair of electrons, specifically the one located in the p orbitals of carbon, delocalizes within the ring. Consequently, benzene has several resonance structures, each with its own structural formula.
This delocalization is part of the aromatic character of benzene, not faithfully represented in the structural formula on the left. The closest thing is to replace the double bonds with a circle (called a donut by some) to indicate the aromaticity of the ring (right of the image).
And what about the skeletal formula? This is very similar to the structural one, differing only in that it does not represent hydrogen atoms; and therefore, it is more simplified and more comfortable to graph. The benzene ring on the right would be its skeletal formula.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
- Wikipedia. (2020). Structural formula. Recovered from: en.wikipedia.org
- Nissa Garcia. (2020). Structural Formula: Definition & Examples. Study. Recovered from: study.com
- Clark Jim. (2012). Drawing Organic Molecules. Recovered from: chemguide.co.uk
- William Reusch. (May 5, 2013). The Shape of Molecules. Recovered from: 2.chemistry.msu.edu