SEMI-DEVELOPED FORMULA: WHAT IT IS AND EXAMPLES (METHANE, PROPANE, BUTANE ...) - CHEMISTRY - 2025

The semi-developed formula, also known as the semi-structural 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, this one does not show the CH bonds, omitting them to simplify the representation. From this formula, any reader will be able to understand what the backbone of a molecule is; but not its geometry or any of the stereochemical aspects.

2-methylheptane semi-developed formula. Source: Gabriel Bolívar.

To clarify this point we have above the semi-developed formula of 2-methylheptane: a branched alkane whose molecular formula is C ₈ H ₁₈, and which obeys the general formula C _n H _{2n + 2}. Note that the molecular formula says absolutely nothing about the structure, while the semi-developed formula already allows us to visualize what this structure is like.

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

Examples

Methane

The molecular formula of methane is CH ₄, as it has four CH bonds and is tetrahedral in geometry. These data are provided by the structural formula with wedges outside or below the plane. For methane, the condensed formula also becomes CH ₄, as do the empirical and semi-developed. This is the only compound for which this uniqueness is true.

The reason why the semi-developed formula for methane is CH ₄ is because its CH bonds are not written; otherwise, it would correspond to the structural formula.

Propane

The semi-developed formula for propane is CH ₃ -CH ₂ -CH ₃, having only two CC bonds. Its molecule is linear, and if you notice, its condensed formula is exactly the same: CH ₃ CH ₂ CH ₃, with the only difference that the CC bonds are omitted. For propane it is true that both the semi-developed 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-developed formula of butane is CH ₃ -CH ₂ -CH ₂ -CH ₃. Note that it can be written on the same line. This formula strictly speaking corresponds to that of n-butane, indicating that it is the linear and unbranched isomer. The branched isomer, 2-methylpropane, has the following semi-developed formula:

Semi-developed formula of 2-methylpropane. Source: Gabriel Bolívar.

This time it can no longer be written or represented on the same line. These two isomers share the same molecular formula: C ₄ H ₁₀, which does not serve to discriminate one from the other.

Pentane

Again we have another alkane: pentane, whose molecular formula is C ₅ H ₁₂. The semi-developed formula for n -pentane is CH ₃ -CH ₂ -CH ₂ -CH ₂ -CH ₃, easy to represent and interpret, without the need to place the CH bonds. The CH ₃ groups are called methyl or methyls, and the CH ₂ are the methylenes.

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

Semi-developed formulas of the two branched isomers of pentane. Source: Gabriel Bolívar.

The a isomer is 2-methylbutane, also called isopentane. Meanwhile, the isomer b 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, structural formulas and 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 a semi-developed formula: CH ₃ -CH ₂ -OH. Note that a CO bond is now represented, but not the OH bond. All hydrogen bonds are neglected.

Linear alcohols are easy to represent as are alkanes. In short: all semi-developed formulas for linear molecules are easy to write.

Dimethyl ether

Ethers can also be represented with semi-developed formulas. In the case of dimethyl ether, whose molecular formula is C ₂ H ₆ O, the semi-developed formula is: CH ₃ -O-CH ₃. 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-developed formulas for branched compounds are more tedious to represent than linear ones; but even more so are cyclic compounds, such as cyclohexane. Its molecular formula corresponds to the same as for hexene and its structural isomers: C ₆ H ₁₂, since the hexagonal ring counts as an unsaturation.

To represent cyclohexane, a hexagonal ring is drawn at whose vertices the methylene groups, CH ₂, will be located, just as seen below:

Semi-developed formula of cyclohexane. Source: Gabriel Bolívar.

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 H ₃ PO ₃. For many inorganic compounds, the molecular formula is sufficient to give you an idea of ​​the structure. But there are several exceptions, and this is one of them. With the fact that H ₃ PO ₃ is a diprotic acid, the semi-developed formula is: HPO (OH) ₂.

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

The semi-developed 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 in principle simpler, 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, in general, chemistry notes are crammed with skeletal-type structural formulas; That is, not only are the CH bonds omitted, but time is also saved by ignoring the C.

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

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Wikipedia. (2020). Semi-developed formula. Recovered from: es.wikipedia.org
3. Siyavula. (sf). Organic Molecular Structures. Recovered from: siyavula.com
4. Jean Kim & Kristina Bonnett. (June 05, 2019). Drawing Organic Structures. Chemistry Libretexts. Recovered from: chem.libretexts.org
5. Teachers. MARL and JLA. (sf). Introduction to carbon compounds.. Recovered from: ipn.mx