11 julio, 2024

Semi-developed formula: what it is and examples (methane, propane, butane…)

The semi-developed formula, also known as the semistructural formula, is one of the many possible representations that can be given to the molecule of a compound. It is very recurrent in organic chemistry, especially in academic texts, since it shows the correct ordering of a molecule and its covalent bonds.

Unlike the developed formula, which is the same as the structural formula, it does not show the CH bonds, omitting them to simplify the representation. From this formula, any reader will be able to understand what the skeleton of a molecule is; but not its geometry or any of the stereochemical aspects.

To clarify this point we have above the semideveloped formula of 2-methylheptane: a branched alkane whose molecular formula is C8H18, and which obeys the general formula CnH2n+2. Note that the molecular formula says absolutely nothing about the structure, while the semi-developed formula already allows you to visualize what this structure is like.

Also, note that the CH bonds are omitted, highlighting only the CC bonds that make up the chain or carbon skeleton. It will be seen that for simple molecules, the expanded formula agrees with the condensed formula; and even, with the molecular.

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examples

Methane

The molecular formula of methane is CH4, since it has four CH bonds and has a tetrahedral geometry. These data are provided by the structural formula with wedges outside or below the plane. For methane, the condensed formula also becomes CH4, just like the empirical and semi-developed ones. This is the only compound for which said singularity holds.

The reason why the half-expanded formula for methane is CH4 is because its CH bonds are not written; if so, it would correspond to the structural formula.

Propane

The semi-expanded formula for propane is CH3-CH2-CH3, having only two CC bonds. Its molecule is linear, and if you notice, its condensed formula is exactly the same: CH3CH2CH3, with the only difference being that the CC bonds are omitted. For propane it is true that both the semi-developed formula and the condensed formula coincide.

In fact, this is true for all linear chain alkanes, as will continue to be seen in the following sections.

Butane

The semi-expanded formula for butane is CH3-CH2-CH2-CH3. Note that it can be written on the same line. This formula strictly speaking corresponds to that of the no-butane, indicating that it is the linear and unbranched isomer. The branched isomer, 2-methylpropane, comes to have the following semi-expanded formula:

This time it can no longer be written or represented on the same line. These two isomers share the same molecular formula: C4H10, which does not serve to discriminate one from the other.

pentane

Again we have another alkane: pentane, whose molecular formula is C5H12. The semi-developed formula of the no-pentane is CH3-CH2-CH2-CH2-CH3, easy to represent and interpret, without the need to place the CH bonds. The CH3 groups are called methyl or methyls, and the CH2 are the methylenes.

Pentane has other branched structural isomers, which can be seen in the image below, represented by their respective semi-developed formulas:

the isomer to is 2-methylbutane, also called isopentane. While the isomer b It is 2,2-dimethylpropane, also known as neopentane. Its semi-developed formulas make the difference between the two isomers clear; but it doesn’t say much about what such molecules would look like in space. For this, formulas and structural models would be needed.

Ethanol

Semi-developed formulas are not only used for alkanes, alkenes or alkynes, but for any type of organic compound. Thus, ethanol, an alcohol, has the semideveloped formula: CH3-CH2-OH. Notice that a CO bond is now represented, but not the OH bond. All hydrogen bonds are neglected.

Linear alcohols are easy to represent just like alkanes. In short: all semi-expanded formulas for linear molecules are easy to write.

dimethyl ether

Ethers can also be represented with semi-expanded formulas. In the case of dimethyl ether, whose molecular formula is C2H6O, the semi-developed formula becomes: CH3-O-CH3. Note that dimethyl ether and ethanol are structural isomers, since they share the same molecular formula (count the C, H, and O atoms).

cyclohexane

Semi-expanded formulas for branched compounds are more tedious to represent than linear ones; but they are even more so for cyclic compounds, such as cyclohexane. Its molecular formula corresponds to the same as for hexene and its structural isomers: C6H12, since the hexagonal ring counts as an unsaturation.

To represent cyclohexane, a hexagonal ring is drawn at the vertices of the methylene groups, CH2, just as shown below:

The developed formula for cyclohexane would show the CH bonds, as if the ring had television «antennas.»

phosphorous acid

The molecular formula of phosphorous acid is H3PO3. For many inorganic compounds, the molecular formula is enough to give you an idea of ​​the structure. But there are several exceptions, and this is one of them. Given that H3PO3 is a diprotic acid, the semi-expanded formula is: HPO(OH)2.

That is, one of the hydrogens is directly bonded to the phosphorus atom. However, the formula H3PO3 also admits a molecule with a semi-developed formula: PO(OH)3. Both are, in fact, what is known as tautomers.

The semi-expanded formulas in inorganic chemistry are very similar to the condensed ones in organic chemistry. In inorganic compounds, because they do not have CH bonds, and because they are simpler in principle, their molecular formulas are usually enough to describe them (when they are covalent compounds).

General comment

Semi-developed formulas are very common when the student is learning the rules of nomenclature. But once assimilated, chemistry notes are usually cluttered with skeletal-type structural formulas; that is, not only the CH links are omitted, but also time is saved by ignoring the C ones.

For the rest, in organic chemistry the condensed formulas are more recurrent than the semi-developed ones, since the former do not even need to write the bonds as in the latter. And when it comes to inorganic chemistry, these half-developed formulas are less used.

References

Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
Wikipedia. (2020). Semi-developed formula. Retrieved from: es.wikipedia.org
Siyavula. (nd). Organic Molecular Structures. Retrieved from: siyavula.com
Jean Kim & Kristina Bonnett. (June 5, 2019). Drawing Organic Structures. Chemistry Libretexts. Retrieved from: chem.libretexts.org
Teachers. MARL and JLA. (nd). Introduction to carbon compounds. [PDF]. Recovered from: ipn.mx

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