Sciencemadness Discussion Board - Why are there two ways to express Acetate?

Why are there two ways to express Acetate?

NEMO-Chemistry - 1-9-2016 at 10:14

This might turn out to be a stupid question, so sorry in advance if the answer is obvious.

I have seen Acetate expressed as an equation (especially in books) in two different ways.

For example.

CH₃COO^-

And

C₂H₃O₂^-

Now both ways say exactly the same thing regarding the number of elements etc, but what i dont understand is why have and use two ways of expressing it?

The only thing i could come up with was, one shows how it splits up (cant think of the correct term!!). While the other is just an overall formula.

I havnt explained that very well sorry, so my question is what does one show/tell you about it, that the other one dosnt?

Or is it just a simple one way was the old way and now its done like this kind of thing?

Thanks and sorry if its a dopey question but i cant find an explanation in the books.

DraconicAcid - 1-9-2016 at 10:17

Same reason you can express pentane at C5H12 or CH3CH2CH2CH2CH3. One is simpler, the other gives more information (i.e., shows how the molecule is put together).

However, I would never write acetate as CH3COO(-), as that suggests that the two oxygens are different in some way. They aren't. CH3CO2(-) is more accurate.

Praxichys - 1-9-2016 at 10:31

There is actually a third way of expressing an acetyl group, as "Ac" (OAc or AcO when referring to acetic acid), which is frequently used in organic chemistry.

Example: Methyl acetate = MeOAc, acetic acid = AcOH or HOAc, 1-hydroxy-2-propanone can be "acetylmethanol" instead.

Basic notation like C₂H₃O₂^- doesn't give any indication to the structure or the functional group producing the charge. Writing the structural formula, or CH₃COO^-, at least indicates a methyl, a carbonyl, and a deprotonated hydroxyl are present.

Fulmen - 1-9-2016 at 10:43

Quote: Originally posted by NEMO-Chemistry

The only thing i could come up with was, one shows how it splits up (cant think of the correct term!!). While the other is just an overall formula.

That's basically it. For something like complete combustion you don't need to know the structure, the net composition is enough. In other reactions the structure and/or functional groups can be extremely important, not showing these obscures the actual reaction.

blogfast25 - 1-9-2016 at 11:08

Nothing really beats:

acetate.jpg - 2kB

NEMO-Chemistry - 1-9-2016 at 11:19

Quote: Originally posted by DraconicAcid

Same reason you can express pentane at C5H12 or CH3CH2CH2CH2CH3. One is simpler, the other gives more information (i.e., shows how the molecule is put together).

However, I would never write acetate as CH3COO(-), as that suggests that the two oxygens are different in some way. They aren't. CH3CO2(-) is more accurate.

Thanks for all the answers.

The CH3COO(-) is actually from 'principles of general chemistry by M. Silberg, one page ref is page 55 of the book, unsure page num of pdf.

I am reading it as one of the books to study, but for now i will skip that part until i go from 'general/elemental' Chemistry on to Organic Chemistry.

The answers have been really helpful, i can see why the different methods exist, but its a little tricky knowing when to use.

At the moment the first way i find more helpful as its easy to see whats in it, but later on i can see i will need to understand how the molecule fits together.

Cheers

DraconicAcid - 1-9-2016 at 11:28

Yes, I've seen it used in several textbooks. It's just wrong. The two oxygens are equivalent due to resonance, and writing them as if they were different is just silly.

NEMO-Chemistry - 1-9-2016 at 11:43

Quote: Originally posted by DraconicAcid

Yes, I've seen it used in several textbooks. It's just wrong. The two oxygens are equivalent due to resonance, and writing them as if they were different is just silly.

This is where the real value of a forum shines through, its peer reviewed on the spot and even if there is no consensus, you still get a good overview.

Books on the other hand while great, leave you at the mercy of the Author.

From a few books i have read lately (must stress mostly general chemistry types), seem to write organic equations in the way illustrated above, i think more specialised Organic chemistry Text books tend to be more careful in what formula is used.

Texium - 1-9-2016 at 12:38

Quote: Originally posted by DraconicAcid

Yes, I've seen it used in several textbooks. It's just wrong. The two oxygens are equivalent due to resonance, and writing them as if they were different is just silly.

What a coincidence, just learned that like half an hour ago in my ochem class. I hadn't ever thought of it that way before, but it makes sense. Especially since acetic acid then does not exhibit resonance, making CH3COOH more accurate than CH3CO2H.

j_sum1 - 1-9-2016 at 16:34

I have to admit that I have seen (and used) CH3COO(-) a lot more than CH3CO2(-). I think it is because visually it is similar to CH3COOH and so its relationship to the acid is more obvious.

While we are at it, I have seen acetate as both Ac- and OAc- depending on whether the Ac refers to an acetyl group or the acetate ion. The difference probably relates to whether one is working in OC or performing elementary acid-base work.

This link is relevant and presents all of these ambiguities
https://en.wikipedia.org/wiki/Acetic_acid#Nomenclature

Magpie - 1-9-2016 at 18:24

The representation below may be the most accurate but I will not be using it routinely:

120px-Acetate-anion-resonance-hybrid-2D-skeletal.png - 3kB

120px-Acetate-anion-resonance-hybrid-2D-skeletal.png - 3kB

NEMO-Chemistry - 2-9-2016 at 02:59

Where is the Carbon meant to sit on that one? I assumed it was the center point but looking at blogfasts it would put it on the end of the bottom left side?

gdflp - 2-9-2016 at 04:10

Quote: Originally posted by NEMO-Chemistry

Where is the Carbon meant to sit on that one? I assumed it was the center point but looking at blogfasts it would put it on the end of the bottom left side?

There are two carbons atoms in acetate, so both places. Blogfast's drawing depicts one carbon atom, while Magpie's depicts none.

NEMO-Chemistry - 2-9-2016 at 04:46

Quote: Originally posted by gdflp

Quote: Originally posted by NEMO-Chemistry

Where is the Carbon meant to sit on that one? I assumed it was the center point but looking at blogfasts it would put it on the end of the bottom left side?

There are two carbons atoms in acetate, so both places. Blogfast's drawing depicts one carbon atom, while Magpie's depicts none.

Ok thanks, i can see how that fits, no idea how the fuck your supposed to know there is Hydrogen with Magpies one. No wonder organic chemistry gives so many the shivers..

Edit

The BRACKETS!!!!!

[Edited on 2-9-2016 by NEMO-Chemistry]

j_sum1 - 2-9-2016 at 05:10

It is a compact notation. It is efficient but takes a little getting used to.

Hydrogens are not shown and neither are their bonds.
Carbons are not shown but their bonds are. The number of carbon in a chain is indicated by the number of nodes. Thus ethane is indicated by a short straight line. Propane is two short lines at an angle. Longer chains are shown by a zigzag. (I have no idea how methane is denoted. I guess it isn't.)
All other elements are shown with bonds indicated as single, double or triple as appropriate.

Resonance is shown as a dotted line to indicate the delocalised electrons.

In general, the most convenient unambiguous form is used in chemical notation. And if isomerism is unimportant in a particular application then an even simpler form may be used -- for example, C2H7OH might represent both isomers of propanol.

[edit]
The brackets indicate that the whole thing is a species in its own right -- in this case an ion. The negative charge belongs to the unit as a whole and no attempt is made to show where that negative charge is concentrated. (So even the most thorough notations are a simplification.)

[Edited on 2-9-2016 by j_sum1]

NEMO-Chemistry - 2-9-2016 at 05:51

Thanks thats useful info, i am dreading the organic chemistry section on the course!

So many ways to write the same thing, i have books with just lines and others with symbols......It gets really hard to relate one to another even of the same chemical, i will keep at it though.

aga - 2-9-2016 at 08:19

OC is crazy complicated IMHO, probably because it is such a vast field, and things keep getting discovered.

Doesn't help when there's more than 1 Name for a substance (and there often is), e.g.

Propan-2-one
Dimethyl ketone
Dimethyl carbonyl
β-Ketopropane
Propanone
2-Propanone
Dimethyl formaldehyde
Pyroacetic spirit
Ketone propane

and more than one way to write the formula :

C₃H₆O
(CH₃)₂CO

and even more than 1 way to draw the thing :

acetone.png - 786B

All of these are exactly the same substance, i.e. Acetone.

[Edited on 2-9-2016 by aga]

Magpie - 2-9-2016 at 09:28

Quote: Originally posted by NEMO-Chemistry

Ok thanks, i can see how that fits, no idea how the fuck your supposed to know there is Hydrogen with Magpies one. No wonder organic chemistry gives so many the shivers..

Whoaa! You can't expect to jump into the middle of a complicated situation and have everything be obvious at first glance! That's what courses in OC are for - to introduce nomenclature gradually and systematically.

Don't get me wrong: I admire your ambition to learn quickly.

NEMO-Chemistry - 2-9-2016 at 10:00

Quote: Originally posted by Magpie

Quote: Originally posted by NEMO-Chemistry

Ok thanks, i can see how that fits, no idea how the fuck your supposed to know there is Hydrogen with Magpies one. No wonder organic chemistry gives so many the shivers..

LOL dont blame me! I had no intention of jumping in at the middle, I was systematically going through several general chemistry books (first mistake).

Right near the beginning of one it had a page that mentioned Acetate, however later on the same page they had written the formula different.

The chapter and book had/has little if anything (at this point) of organic chemistry in it. But having seen two different ways to do it i got curious as to why.

The book is Martin Spilberg(sp?) principles of general chemistry. I am also reading the Linus Pauling one and the two from the course at school.

My second mistake is having a curiosity that tends to take me on detours!! I havnt posted the latest on things like my adventures with calcium chloride, mainly for fear of tooting

.

But a couple of odd things occurred and like a numpty i have gone wandering down avenues that are likely to lead to nowhere! But thats my nature, i get curious over even the most simple of things.

Its going to lead me to trouble one day for sure, my latest digging around is how to get from Citric acid to Acetone, all because i saw i single reference to it and have some!

Then i got to wondering about how or if you can get from chloroform to Acetone seeing as you can get from Acetone to Chloroform.

This is why i have taken Chemistry this year, i am hoping it will sort out my random ventures into a more organised and less chaotic style.

So far its just made it worse

Thanks aga i now know just about every name out there is actually for Acetone

DraconicAcid - 2-9-2016 at 10:00

Quote: Originally posted by aga

and even more than 1 way to draw the thing :

All of these are exactly the same substance, i.e. Acetone.

[Edited on 2-9-2016 by aga]

Half of those are incorrect.

The top one is a skeletal structure, in which you leave off the carbon atoms, the carbon-hydrogen bonds, and the hydrogen atoms. You can tell where the carbons are from where the bonds are, and you can tell how many hydrogens have been left off because you know that each carbon has to have four bonds, and that only the hydrogens are left off.

The bottom one is a condensed structure, in which you don't show the carbon-hydrogen bonds. It's simpler than the full Lewis structure.

The ones in the middle, where you have drawn in the carbon atoms, but not the hydrogens, is not an acceptable way of drawing an organic molecule. If you do that in any of my courses, I will laugh at you and cast you out of Valhalla.

NEMO-Chemistry - 2-9-2016 at 10:12

Why dont books put things like a Carbon has to have 4 bonds! That info alone would of made life simpler.

Thanks for that, and BTW how did you get rid of the 500g of TATP??

DraconicAcid - 2-9-2016 at 10:32

If you've learned about Lewis structure, you've surely learned that a carbon atom has to have a full octet. In organic chemistry, that generally means:
A carbon will have four bonds.
A nitrogen will have three bonds and a lone pair.
An oxygen will have two bonds and two lone pairs.
A halogen will have one bond and three lone pairs.
A hydrogen will have one bond. Period.

In organic chemistry, that will cover about 95% of all molecules. if you add "And sometimes, four bonds and a positive charge" to N, and "and sometimes one bond, three lone pairs and a negative charge" to O, that covers 99.5% of all organic molecules.

NEMO-Chemistry - 2-9-2016 at 10:39

So you set fire to the 500g then

.

Thanks for the info, Lewis structure i think is a chapter or two further on. I missed that week of the course as it was week 3, i have only caught upto week 4 (working backwards).

I know it seems a chaotic approach but its more me spotting something and going off at a tangent.

DraconicAcid - 2-9-2016 at 10:42

I'm not the one that had 500g of TATP, if that was directed towards me.

NEMO-Chemistry - 2-9-2016 at 10:54

Quote: Originally posted by DraconicAcid

I'm not the one that had 500g of TATP, if that was directed towards me.

No i havnt done acids in details yet. Looks like i got my acids mixed up sorry

aga - 2-9-2016 at 12:37

Quote: Originally posted by DraconicAcid

In organic chemistry, that generally means:
A carbon will have four bonds.
A nitrogen will have three bonds and a lone pair.
An oxygen will have two bonds and two lone pairs.
A halogen will have one bond and three lone pairs.
A hydrogen will have one bond. Period.

In organic chemistry, that will cover about 95% of all molecules. if you add "And sometimes, four bonds and a positive charge" to N, and "and sometimes one bond, three lone pairs and a negative charge" to O, that covers 99.5% of all organic molecules.

Hand written into the 'useful stuff' book.

Now all i got to do is properly get my head around the 'lone pair' thing.

DraconicAcid - 2-9-2016 at 12:44

Quote: Originally posted by NEMO-Chemistry

Quote: Originally posted by DraconicAcid

I'm not the one that had 500g of TATP, if that was directed towards me.

No i havnt done acids in details yet. Looks like i got my acids mixed up sorry.

Don't do acid while you're learning chemistry. It'll make more sense without it.

NEMO-Chemistry - 2-9-2016 at 13:22

Quote: Originally posted by DraconicAcid

Quote: Originally posted by NEMO-Chemistry

Quote: Originally posted by DraconicAcid

I'm not the one that had 500g of TATP, if that was directed towards me.

No i havnt done acids in details yet. Looks like i got my acids mixed up sorry.

Don't do acid while you're learning chemistry. It'll make more sense without it.

Dunno, i am starting to think mushrooms might be a good idea for Organic chemistry

.

Yes i have written it down in my book as well, snippets like that are really useful.

Found the Citric acid to Acetone Synth, well not exactly but the step before Acetone.

It looks like 20% free SO3 is needed, but i dont really see why. I have also read the threads on it and again yields seem low without the SO3.

I might try it at some point with 98% H2SO4 and anhydrous citric acid, but kind of going off the idea.

aga - 2-9-2016 at 14:02

Quote: Originally posted by DraconicAcid

Don't do acid while you're learning chemistry. It'll make more sense without it.

If he was still around, Shulgin would argue otherwise, maybe whilst gently prodding the air near your elbow and giggling.

chemrox - 2-9-2016 at 15:16

Quote: Originally posted by aga

Quote: Originally posted by DraconicAcid

Hand written into the 'useful stuff' book.

Now all i got to do is properly get my head around the 'lone pair' thing.

I think that means shutting the door ;^)

aga - 2-9-2016 at 15:35

Quote: Originally posted by chemrox

I think that means shutting the door

Not at all.

These are all stepping stones on the Path young chem walker.

Sadly we're aren't all Born with the most advanced knowledge of Chemistry, so it has to acquired by learning, step by step.

diddi - 2-9-2016 at 20:41

and that set of rules is a bit hard to use for aromatics in the case of those new to o-chem. every carbon has 4 bonds except when it has 3 and there is a delocalised cloud that is represented by a circle. or better still it is represented by the vertex of a hexagon.

NEMO-Chemistry - 3-9-2016 at 07:00

I know i shouldnt wonder off the path, but i couldnt help look at some OChem books.

I prefer the kind of older way of drawing mechanisms etc, they seem simpler which is good for now. On the other hand i am not sure its a good idea to learn something that seems to be outdated in newer books.

I should stick with inorganic for now.

DraconicAcid - 3-9-2016 at 07:25

Quote: Originally posted by diddi

and that set of rules is a bit hard to use for aromatics in the case of those new to o-chem. every carbon has 4 bonds except when it has 3 and there is a delocalised cloud that is represented by a circle. or better still it is represented by the vertex of a hexagon.

That's still four bonds, it's just that the fourth one is delocalized between two other carbons.

Alice - 3-9-2016 at 08:26

@DraconicAcid

Quote:

Yes, I've seen it used in several textbooks. It's just wrong. The two oxygens are equivalent due to resonance, and writing them as if they were different is just silly.

Following your argument the way carboxylates are drawn usually would be wrong everywhere:

But the point is, it's just a representation, otherwise would anybody assume a molecule is made of letters and lines? What's shown depends on the purpose. In case of the above image, resonance isn't part of the representation. If the purpose is talking about resonance, another representation is used.

Magpie - 3-9-2016 at 08:32

Dare we mention carbenes? Warning to NEMO: this could be a dangerous tangent leading to a multiplicity of other tangents.

DraconicAcid - 3-9-2016 at 09:15

Quote: Originally posted by Magpie

Dare we mention carbenes? Warning to NEMO: this could be a dangerous tangent leading to a multiplicity of other tangents.

Not stable in the least, which is why it's part of the 0.5%....

NEMO-Chemistry - 3-9-2016 at 09:29

Does look interesting though..............

Anyone read Organic Chemistry by Donald J Cram and George S Hammond 2nd edition?

Is it any good? I have seen a cheap copy in a charity shop

EDIT

I should have asked before parting with cash i guess, but there you go impulse buy. Also got 2 sterling silver rings for 25p each

, so silver nitrate on the way nice and cheap.

[Edited on 3-9-2016 by NEMO-Chemistry]

Darkstar - 3-9-2016 at 21:12

Quote: Originally posted by zts16

Especially since acetic acid then does not exhibit resonance

That's not actually true. If you really want to get technical, even protonated acetic acid will exhibit at least some degree of resonance. Like with acetate ions, the pi-electrons in acetic acid are also delocalized across the carboxyl group, providing a sort of stabilizing effect on the molecule. But unlike acetate ions, however, which benefit significantly from that electron delocalization via charge delocalization (the -1 charge isn't completely localized on any one oxygen atom, but is instead evenly distributed across both), the additional stability provided by resonance in acetic acid is rather marginal, coming from charge separation instead.

Thus a single acetic acid molecule would most accurately be represented in a fashion similar to how the acetate ion was depicted by Magpie above:

Note the partial-positive charge on the oxygen of the hydroxyl group and the partial-negative charge on the oxygen of the carbonyl group in the drawing on the right. One of the reasons COOH hydrogens are so acidic and easy to remove is because the oxygen they're connected to is actually somewhat electron-deficient. This causes the oxygen to not only pull even more electron density away from the hydrogen than it normally would, further increasing the O-H bond length and decreasing its strength, but it also creates an unfavorable electrostatic repulsion between the two positively-charged nuclei of oxygen and hydrogen. (the other reason for the unusual acidity is the fact that the hydrogen is only weakly attracted back to the oxygen atoms upon removal due to the delocalization of the resulting negative charge via resonance)

NEMO-Chemistry - 4-9-2016 at 16:29

I have a question on this, but i did to read more before i can ask it in a semi intelligent way.

Organic chemistry is a real struggle to get your head around, extremely interesting but alot of work just trying to get basics to stick.

j_sum1 - 4-9-2016 at 17:00

@ NEMO
I think there is an order to learning it all.

1. atomic electronic structures
2. covalent bonding and lewis diagrams
3. structure and nomenclature of organic compounds
4. elementary reactions -- addition, substitution, cracking, esterification, oxidation of alcohols
5. experimental techniques -- distillation, reflux, solvent extraction, acid-base extraction and so forth
6. reaction mechanisms, and more complex reactions
7. manipulating reaction conditions -- equilibria, kinetics, stability of products and side-products

I am up to #5 with the occasional glimpse into #6 having never read a dedicated organic chemistry book -- just general chemistry texts at high-school level with the occasional bit of undergraduate level reading thrown in.

In all honesty, I think you can get a long way into this stuff with some quite basic knowledge. Structure and nomenclature is just like Lego. But when it gets complex it ramps up significantly.

NEMO-Chemistry - 5-9-2016 at 00:36

The main problem is i like to experiment, but things can go very wrong when your not sure what your doing.

I have a part time job at the weekends in a cafe, i think i need to save up and get a safer working area.

I dont do anything that risky but a fume hood would go a long way in making things safer. Not just for me but everyone else, I can wear a respirator but convincing the entire family to sit and watch TV wearing one isnt likely.

I will keep plodding away

. Some very cool reactions in Organic chemistry, so many ways to make smelly black crud

.

[Edited on 5-9-2016 by NEMO-Chemistry]

j_sum1 - 5-9-2016 at 01:02

Definitely not an expert here but I think you can't go wrong doing a few esters and extractions. You will learn a lot of technique while your theory knowledge catches up.

And watch everything from chemplayer, doug'skab and nilered.

NEMO-Chemistry - 5-9-2016 at 01:09

Quote: Originally posted by j_sum1

Definitely not an expert here but I think you can't go wrong doing a few esters and extractions. You will learn a lot of technique while your theory knowledge catches up.

And watch everything from chemplayer, doug'skab and nilered.

I have most the morning in the library doing assorted 'study', perfect time for ear pones and some you tube then

j_sum1 - 5-9-2016 at 01:35

Aah... That was supposed to be "Doug's Lab". Sorry for the big thumbs on the phone.

NEMO-Chemistry - 5-9-2016 at 04:15

And there was me thinking Doug's skab was a skating boarding channel.

large thumbs and modern phones are like toilet paper filter paper

NEMO-Chemistry - 5-9-2016 at 10:31

4 vids explaining resonance, first one here

https://www.youtube.com/watch?v=tmeMhbi3Mi8

Might help someone else!

Has a fair few on Org chemistry.