The condensed formula is a purely textual representation of a molecule in which the bonds are omitted. Its purpose is to allow to know what is the ordering of the atoms. Generally, linear or branched molecules have condensed formulas, except for cyclic ones or those that are highly branched.
In organic chemistry there is often confusion as to what is the difference between this formula and the molecular one. In fact, it is common to find them as synonyms. Meanwhile, in inorganic chemistry, molecular formulas are used more, since in most cases they accurately describe covalent compounds; like water, H 2 O.
What is the condensed formula?
To define it more conveniently, the condensed formula is a simplified representation of the semi-expanded one. It is not necessary to write single links, nor does it have to span more than one line in a text. For example, 2-methylheptane, a branched alkane, can be represented on the same line as in the image above.
Condensed formulas are useful for representing some molecules without having to draw them. This is excellent when writing chemical equations where highly branched or cyclic molecules are not involved. It will be seen that these formulas use parentheses to further simplify the representation of a molecule.
Methane is the only chemical compound with a single formula: CH 4 . This corresponds to the empirical, molecular, condensed and semi-developed at the same time. That is why many times, to avoid confusion between all these concepts, people prefer to refer simply and inaccurately to the ‘chemical formula’ of a compound.
Ethane has the molecular formula C 2 H 6 . The condensed formula instead is CH 3 CH 3 . Note that the CC bond and CH bonds are omitted. The idea is to be able to write the formula as if it were a “word”, without having to draw lines or draw structures.
By definition, the molecular formula is C 2 H 6 and not CH 3 CH 3 ; however, for simple molecules both formulas are used to interchange, although it is still incorrect. This is a point of discrepancy between various sources, where apparently there are no specific rules in this regard.
Glucose and fructose
Here is an example where an error is immediately observed when wanting to exchange the molecular formula for the condensed one.
The sweet pair, glucose and fructose, have the same molecular formula: C 6 H 12 O 6 , and therefore this does not serve to differentiate them. In contrast, the structural and semi-developed formulas do visually establish that they are different molecules.
However, neither glucose nor fructose have a condensed formula. This is because regardless of its representation or projection, its structures cannot be written on the same line; or at least, in a way that is pleasant and simple in the eyes of the reader.
Butane has the molecular formula C 4 H 10 . It has two isomers: the linear, n- butane, and the branched, 2-methylpropane. Both can be represented by their condensed formulas. The n- butane is CH 3 CH 2 CH 2 CH 3 or CH 3 (CH 2 ) 2 CH 3 , and the 2-methylpropane is CH 3 CH (CH 3 ) 2 or (CH 3 ) 3 CH.
In 2-methylpropane we have a CH group surrounded by three CH 3 . Its two formulas are valid, and the parentheses are used to highlight the branches, the subscript being an indicator of how many groups make up said branch.
Ethanol has the condensed formula CH 3 CH 2 OH. Note how closely it resembles its semi-developed formula: CH 3 -CH 2 -OH. In the same way it is done with propanol, CH 3 CH 2 CH 2 OH, n- butanol, CH 3 CH 2 CH 2 CH 2 OH, and with all the other alcohols.
If they have a branch, it will be indicated within a parenthesis to the right of the atom to which it is linked. For example, 2-methylbutanol could be written as: CH 3 CH 2 CH (CH 3 ) CH 2 OH. Note that for many molecules it is becoming more convenient to use a structural or semi-developed formula.
The linear isomer of pentane, n- pentane, is easy to represent by its condensed formula: CH 3 CH 2 CH 2 CH 2 CH 3 . This formula can also be simplified using parentheses: CH 3 (CH 2 ) 3 CH 3 , indicating that between the two CH 3 there are three CH 2 .
The other two isomers of pentane, however, are a bit more complicated: CH 3 CH 2 CH (CH 3 ) 2 , for 2-methylbutane, and C (CH 3 ) 4 for 2,2-dimethylpropane or neopentane. Note that the carbon atom to the left of the parentheses is the one that forms bonds with the substituents or branches.
The linear isomer of octane, n -octane, due to its length, it is now appropriate to represent its condensed formula as CH 3 (CH 2 ) 6 CH 3 . It is at this point where something should be clear about condensed formulas: they seek to save time when representing molecules or compounds, without having to write so much on paper.
The case of cyclohexane is similar to that of glucose and fructose: it formally lacks a condensed formula because it is a cyclic compound. One way to try to represent it would be: (CH 2 ) 6 , which implies that six CH 2 groups must be linked, being possible only if they are closed in a hexagonal ring. However, it is better to draw the ring.
Acetone has the peculiarity of having a carbonyl group, C = O. Therefore, to write its condensed formula we have three options: (CH 3 ) 2 CO, CH 3 C (O) CH 3 or CH 3 (C = O) CH 3 .
In fact, in larger molecules the carbonyl group is usually represented as (O), taking into account that the carbon atom to its left is the one that forms the double bond with oxygen, C = O.
The condensed formula for acetic acid is CH 3 COOH or CH 3 CO 2 H. An important point arises here: functional groups written on the same line are parts of a condensed formula. This was the case with ethanol and acetone, and also applies to thiols (-SH), aldehydes (-CHO), esters (-CO 2 R or -COOR) and amines (-NH 2 ).
There is a lot of confusion between molecular and condensed formulas. Maybe it’s because these representations alone already provide a rough picture of the molecule, which is why we think of it as the molecular formula.
Likewise, formulas such as C 6 H 12 O 6 are also considered condensed, because in them it is simplified, the molecule is “condensed” into atoms and subscripts. That is why it is common to see how the two formulas are mentioned as if they were synonyms.
In many sources, including Wikipedia articles, the terms ‘chemical formula’ are used to refer to the molecular (type C 6 H 12 O 6 and others), and ‘formula’ to refer to the condensed.
- Whitten, Davis, Peck & Stanley. (2008). Chemistry . (8th ed.). CENGAGE Learning.
- Helmenstine, Anne Marie, Ph.D. (November 18, 2019). Condensed Formula Definition in Chemistry. Recovered from: thoughtco.com
- James Ashenhurst. (December 11, 2019). Condensed Formulas: Deciphering What the Brackets Mean. Master Organic Chemistry. Recovered from: masterorganicchemistry.com
- Co-engineering. (May 02, 2016). Empirical, Structural and Condensed Formula. Recovered from: quimiotecablog.wordpress.com
- Introduction to Chemistry: General, Organic, and Biological. (sf). Condensed Structural and Line-Angle Formulas. Recovered from: 2012books.lardbucket.org