My teacher likes to use household materials to do experiments. Today, we did an experiment that showed the reaction of vinegar and eggshell.

When we were doing the analysis of the experiment, I realized that my teacher used the term acetic acid ($\ce{CH3COOH}$) instead of ethanoic acid. So, my questions are:

  1. What is the accepted usage for the terms acetic acid and ethanoic acid?

  2. Which formula is the accepted representation of acetic acid, $\ce{CH3COOH}$ or $\ce{C2H4O2}$?


Carboxylic acids that can be isolated from a natural source were commonly given trivial names. These trivial names are usually related to the biological origin of the material. Various traditional names are retained for use in IUPAC nomenclature, though the number of retained names has been reduced with each succeeding edition of the IUPAC recommendations. For example, the trivial name “caproic acid” (for hexanoic acid) was abandoned in the 1979 recommendations, and the name “valeric acid” (for pentanoic acid) was no longer mentioned in the 1993 recommendations.

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), various trivial names are still retained, but only for general nomenclature (i.e. they are not preferred IUPAC names). Many of these names may not be used when the structure is substituted at any way, for example “propionic acid” and “butyric acid”.

According to Subsection P-, only the following five retained names are preferred IUPAC names:

  • formic acid
  • acetic acid
  • benzoic acid
  • oxalic acid
  • oxamic acid

Therefore, the retained name “acetic acid” is still the preferred IUPAC name; however, the systematically formed name “ethanoic acid” may be used in general nomenclature.

The choice of formula depends on the context.

Generally, the molecular formula is not suitable to unambiguously identify organic compounds. For example, the molecular formula $\ce{C2H4O2}$ for acetic acid could as well represent methyl formate, hydroxyacetaldehyde, 1,2-dioxetane, or 1,3-dioxetane. However, you could use the molecular formula as shorthand if the concerned compound is clear from the context.

Simple chemical structures can often be represented by a line formula*, such as $\ce{CH3COOH}$. They rely on the fact that many elements have consistent and well-understood bonding patterns. (Recommendations for contracted labels that represent multiple atoms can be found in the Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008).)

However, complex chemical structures depend on structural representations exhibiting the atom–atom connectivity, including the order and stereochemistry of the bonds. Therefore, it may be preferable to visualize the structure using a chemical structure diagram.

* According to IUPAC recommendations concerning nomenclature of organic as well as inorganic chemistry, a line formula is a formula that is written on a single line, as is text. Unfortunately, however, there are also other definitions of line formula.

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While I agree that the systematic name ethanoic acid makes a lot more sense, the name acetic acid has been used for so long that it's the accepted IUPAC name and is used almost universally. Both refer to the same compound.

Molecular formulas like $\ce{C2H4O2}$ are often seen when doing elemental analysis and the like, but give no structural information. $\ce{CH3COOH}$ makes it clear that it's an acid and shows the carbonyl group.

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