0
$\begingroup$

This question already has an answer here:

I am quite new to chemistry, but I would like to think that I have mastered the basics. This would include naming and writing out chemical formulas. However, once I had seen the formula for vinegar ($\ce{CH3COOH}$), I was bewildered.

It would have made sense to me that the formula would be more, say, "compacted" into just $\ce{C2H4O2}$. What is the reasoning behind this odd way of writing such formulas and when should I use it (it seems like this could be a huge gap in my knowledge)

$\endgroup$

marked as duplicate by Jan, Klaus-Dieter Warzecha organic-chemistry Feb 12 '17 at 20:18

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Related: chemistry.stackexchange.com/questions/33135/… $\endgroup$ – Nilay Ghosh Feb 12 '17 at 5:07
  • 3
    $\begingroup$ When expressing a chemical formula within a text line three different forms are used - Empirical formula, molecular formula and condensed formula. Chemists also use structural formulas (drawings) to show a 2D representation of a 3D molecule. en.wikipedia.org/wiki/Chemical_formula $\endgroup$ – MaxW Feb 12 '17 at 7:08
  • 1
    $\begingroup$ IUPAC, functional groups, and ACID! and no molecule would be worth without its function, chemical properties that is shaped by its structure. How would anyone know if C2H4O2 is a ketone, aldehyde, ester...ether or an acid by just looking at that? As you dive into organic chemistry, there will be thousands of compunds and structures. This naming standard gives wings and freedom to a student, a chemist to do pattern recognition and to know who we are dealing with. In days and years to come you will appreciate this little tables and conventions much =) $\endgroup$ – bonCodigo Feb 12 '17 at 9:05
  • $\begingroup$ Also relevant: chemistry.stackexchange.com/q/58910 $\endgroup$ – Jan Feb 12 '17 at 19:49
3
$\begingroup$

CH3COOH is an accepted and common form of writing the structure of acetic acid (commonly known as vinegar when diluted in water). You are right in saying the formal notation should be used - which is C2H4O2 - since it gives the reader the types of atoms and their quantities, and these two characteristics define the exact composition of the molecule, and there's nothing wrong with that.

However, the notation CH3COOH takes you one step further and tells you something about the actual structure of the molecule, since it groups together the atoms that comprise a functional group, and a very important one at that. Grouping together COOH is an accepted and convenient form of communicating to the reader: "there is a carboxylic group here", or "this molecule is actually a carboxylic acid", which is something you'd like to know when dealing with a certain substance.

To summarize, C2H4O2 can be the molecular structure for acetic acid or formate (formic ester, as mentioned in another answer). In order to avoid confusion and, more importantly, indicate that there is a carboxylic group in the molecule, it is more convenient and straightforward to note it in the formula than letting readers figure it out themselves.

$\endgroup$
  • $\begingroup$ Plus one @Don_S simple and conscise. $\endgroup$ – bonCodigo Feb 12 '17 at 9:07
1
$\begingroup$

Carboxylic acid and ester are functional group isomers which means that both of them has the same formula but different functional groups.

Lets take the example of $\ce{C2H4O2}$.

Carboxylic acid : $\ce{CH3COOH}$
enter image description here

Ester : $\ce{HCOOCH3}$
enter image description here

To avoid the confusion formulas like $\ce{CH3COOH}$ are used.

$\endgroup$
  • 1
    $\begingroup$ Structral diagrams are at the heart of organic Chemistry. Nice one. $\endgroup$ – bonCodigo Feb 12 '17 at 9:11
1
$\begingroup$

The short answer is that CH3COOH indicates (by convention) that this compound has both a methyl group and a carboxylic acid group. (The group COOH is generally understood to be the C(O)OH group or C(=O)OH where the double bond "=" and the parentheses are implied.) Note that the organic acid in vinegar is acetic acid, but acetic acid is NOT vinegar!!! Vinegar is an aqueous mixture, not a chemical compound, vinegar is biologically derived and generally includes hundreds of other chemical compounds. As far as the basics of chemistry, no you haven't mastered them. Consider the difference between red phosphorus and white phosphorus. White phosphorus ignites at 30°C, red at 300°C. Or consider graphite, buckyballs, diamond, and graphene - all are "just" carbon, but their properties vary enormously. You haven't identified a chemical compound until you know both its atomic formula AND the bonding between its atoms. The common example is dimethyl ether H3C-O-CH3 and ethyl alcohol CH3CH2OH. Their atomic formula is identical, but one is a gas, and the other is a liquid (at STP). One is fermented in our guts and the other is used as a propellant for aerosol cans. To say that vinegar "is" acetic acid is like saying beer "is" ethyl alcohol.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.