Butyric acid, for example, has C3H7COOH or CH3CH2CH2CO2H or C4H8O2 (This one also applies to ethyl acetate). Are there different methods for writing formulas?
In: Chemistry
You use a simple formula as required for the situation. Make it so it is simple to read but provide enough information.
C3H7COOH implies that the C3H7 part is a change of carbon 3 carbon with hydrogen attached.
COOH implies a carbon atom with one O that have is doubled bounded and a single bounded O with an H on the other end. COOH is a common way to illustrate a carboxyl group, you can be even clear as (C(=O)OH)
CH3CH2CH2CO2H is the same chain expanded. I am not sure why there is a CO2H insted of COOH at the end..
If you just look at them the shorter that gives you all information if you have some knowledge about the subject is better. The longer is harder to read because you need to count the C in it to get the light.
Compare CH3CH2CO2H CH3CH2CH2CO2H and CH3CH2CH2CH2CO2H versus C2H5COOH C3H7COOH and C4H9COOH
The second is a lot simpler to tell apart to know you have Propionic acid, Butyric acid and Valeric acid
So if you care about chemical properties you need to know the structure
C4H8O2 is appropriate if you for example like to evaluate the mass of the molecule. If you want to know home many moved 10 grams of it is then that information is enough.
You can also look at if to get an idea of the result if you burn. You will get CO2 and H2O so just calculate the number that is needed for the C and H atoms
C4H8O2 + x O2 = 4 CO2+4 H2O
That is 2 O on the right and 12 on the right so you need to add 10 O = 5 O2
Now you know the amount of oxygen used for combustion and you do not need to know the physical strture
The chemical formulas and also the names of the chemicals are actually descriptions of the chemicals. And as any descriptions you can use varying levels of detail. In chemical formulas you can list the formulas of different groups of the chemical separately. But how big the groups are or even where you group them is up to the person writing the formula and often depends on what is important features.
It depends on where a person wants to put “emphasis”.
C3H7COOH draws a person’s attention to the COOH [functional group](https://en.wikipedia.org/wiki/Functional_group) that gives the chemical its carboxylic acid properties.
CH3CH2CH2CO2H is like a text version of the chemical structure.
C4H8O2 counts the atoms in the molecule but obscures the structure, which means it could also refer to the chemical isomer, isobutyric acid.
Technically all three are the same thing, but that’s like saying 6/3 and 2 are the same thing. One is definitely preferred, but the other one may be appropriate in certain circumstances. The first one is the form I’d expect since it gives information about the structure of the molecule. The C3H7 part is a hydrocarbon chain and the COOH is the organic acid.
The second one tells me the same thing, but (unnecessarily IMHO since the carbons all have the maximum number of hydrogens attached) explicitly shows that hydrocarbon chain. That form could be useful though if it wasn’t full and you needed to show where the hydrogens were missing, or if it was branched and you wanted to show where the splits were.
The third one is too vague. There are way too many possible ways to arrange those atoms to tell me anything useful. It could be butyric acid, ethyl acetate, or even some weird carbohydrate.
All three describe the same structure in different ways. The third is the most simple, counting up all the atoms in the molecule and just listing how many there are of each.
The second one is a little more complicated and describes the molecule in terms of the carbon backbone, listing which atoms are attached to each carbon. So 3 hydrogens on the first carbon, 2 on the second etc.
The first one is basically the same as the second but assumes that you don’t need to know how many hydrogens are on each carbon in the chain because its always pretty much the same. So only the functional group, the COOH is separate
These are all synonymous ways of describing [this same structure](https://www.carlroth.com/medias/3277-SF-1000Wx1000H?context=bWFzdGVyfGltYWdlc3wyNzIwNnxpbWFnZS9qcGVnfGltYWdlcy9oYjgvaGUxLzg5NDAxMDY0NDg5MjYuanBnfDBlYjg4YmUzNDFmMTcyZGM4ZjRhZWU1MWU0MzkwYTM2Yzg1OTljNzM4OGRhOGVmOTZmMzU2OTQ2NjFjOWMxMGQ). However, I would argue that the third is bad because it could easily refer to multiple different structures from those same atoms and the second is really just a more verbose way of saying the first.
The answers so far are pretty spot on but the lack of directly naming each bothers me so I’m adding in:
3 is the **“chemical (or molecular) formula”**, this is the term for the formula just basically just tells you what’s in it, and how much. No structural information is available from it but it’s a good quick overview to go ok this has so many oxygens in it and so many carbons. Good for calculating atomic weights quite quickly.
1/2 are both **”structural formula”**, this gives info about the structural makeup and what functional groups there are, and also importantly, where they are. 1 is probably what’s most likely to be used as a structural formula where you have only straight chains with 1 functionality. If there was a double bond, a carbon chain that splits off the main body, or any other non-alkane functional group etc., then 2 is what’s most likely to be used as it gives more precise information on the location.
**Personal opinion + not ELI5 after this**
You can go one further on from these with the skeletal formula which is probably the best way to do it (and I think they look nicer in general) given most circumstances as it helps you properly visualise it, but based off how/when my country teaches it, that’s a small bit further into chemistry than ELI5.
yes. particularly in organic chemistry, molecules are often made up of groups of atoms that are common to many chemicals. these groups often behave in similar ways regardless of what they’re stuck to.
“COOH” specifically is the carboxyl group. acetic acid is similarly CH3COOH
so for butyric acid, C4H8O2 is easier to calculate bulk properties, like molecular weight, from. this is also likely the least useful overall, because isomers (chemicals with the same formula but the atomcs arranged differently) exist and can have radically different properties.
C3H7COOH emphasizes that it is a carboxylic acid. in some cases, what the acid was specifically isn’t very relevant. the rest of the molecule doesn’t participate in the relevant reaction.
CH3CH2CH2CO2H has lots of information. butyric acid is a methyl group, two methyene bridges, and a carboxyl group. from that you also know what the molecule looks like. it’s long and mostly straight. but writing it out like that is rather cluttered, particularly if you don’t care about most of that.
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