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June 26

Carboxylic acids

So, I ran out of lime juice (containing citric acid) while cooking some fish, and started wondering if there were any easily obtainable/ common substitutes. Obviously vinegar (containing acetic acid) goes with fish, but it probably doesn't go with vodka and diet coke to reduce the sweetness (I haven't actually tried that yet). So I wondered how chemically similar citric and acetic acids are. I have a very basic knowledge of chemistry, (I scraped a C at O-level many moons ago), so please treat me as an interested layman whose lack of even fundamental mathematics at the time sadly prevented me from becoming an organic chemist.

Anyway, I started off with organic acids, where the lede lists some common carboxylic acids, but they don't seem to be in any sort of order.

(Answer, I think: Citric and acetic acids are somewhat alike, but only up to a point, Lord Copper). With some further hunting, I wondered about arranging the above list in order of complexity, with some extra additions: I hope I have got everything right. This exercise gave rise to some questions, included in the entries: I wonder if anyone could help me with these, please?

Carboxylic acids (one carboxyl COOH group)
- Formic acid CH₂O₂ (Question 1: why is it not written HCOOH like oxalic acid below?)
- Acetic acid CH₃COOH
- Butyric acid C₃H₇COOH, alternatively CH₃(CH₂)₂COOH (not in list)

Alpha hydroxycarboxylic acids (one COOH group plus hydroxyl group separated by one C atom)
- Lactic acid C₃H₆O₃
  - Glycolic acid C₂H₄O₃ (not in list) (Question 2: The list in Omega hydroxy acid#List includes it as matches the formula pattern C_nH_2nO₃. To which classes does this pattern refer, like the alkanes match C_nH_2n+2?)

Beta hydroxycarboxylic acids (one COOH group plus hydroxyl group separated by two C atoms)
- Salicylic acid C₇H₆O₃ (not in list)

Dicarboxylic acids (two COOH groups)
- Oxalic acid H₂C₂O₄, or alternatively (COOH)₂ - (Question 3: Why H₂ first and not C₂? See Q. 1)
- Malic acid C₃H₄O₄
- Malonic acid C₄H₆O₅ (not in list)

Tricarboxylic acids (three COOH groups)
- Tartaric acid C₄H₆O₆, alternatively HO₂CCH(OH)CH(OH)CO₂H
  - Aconitic acid C₆H₆O₆ (not in list)
- Citric acid C₆H₈O₇, alternatively HOC(CO₂H)(CH₂CO₂H)₂
  - Isocitric acid C₆H₈O₇ (not in list)- (Question 4: How can it be both a tricarboxylic and an Alpha hydroxycarboxylic acid? Is one a subset of the other?)
  - Agaric acid C₂₂H₄O₇ (not in list)

Not sure:
- Uric acid C₅H₄N4O₃
- Folic acid C₁₉H₁₉N₇O₆ - in Category:Dicarboxylic acids, otherwise no mention of it being one in the article. (Question 5: are they both really carboxylic? Are they not heterocyclic nucleic acids?)
- (Question 6: How many of these groups in main bullet points match a formula pattern like C_nH_2nO₃?)

Thanks for your patience, cheers, >MinorProphet (talk) 12:49, 26 June 2024 (UTC)[reply]

Food-grade lactic acid and tartaric acid are used to give a sour taste to foods and drinks, so they are clear candidates for being tried as substitutes. If you happen to have unripe grapes or plums, you could use their juices. --Lambiam 16:28, 26 June 2024 (UTC)[reply]

Malic acid gives the tart taste to plums and many other fruits, as well as rhubarb. --Lambiam 09:37, 27 June 2024 (UTC)[reply]

Acetic acid has a strong odour, and so will change the human experience. Many of the acids you listed above are toxic and so either should be avoided or only used in tiny amounts as they occur in foods, eg oxalic acid. Q1: Formic acid can be written written HCOOH. Q2: the pattern you give is for an alkane derive carboxylic acid with extra oxygen. An extra oxygen could go in between carbon and hydrogen so that it is a hydroxy acid, (alpha, beta, omega etc); it could form an ether between two carbons, or it could go next to another oxygen and make a peroxy acid. The alkane base compound could be a linear chain of carbon atoms, or it could be branched. Q3: compounds could be in more than one category, so that if the second carbon is the last, an alpha acid is an omega acid too. Q2: many of these compounds' formulae have been written using Hill notation, which has C first and H second then in alphabetical order. The formula you gave for oxalic acid is not in this form. However it does not matter as there is only one isomer with this combination of atoms. Often the formula can be written in a variety of ways for one compound. Q4: a compound can be in more than one class if it meets the membership requirements or multiple classes. Q5: some compounds are acidic even if they are not carboxylic acids. Folic acid is a dicarboxylic acid. Uric acid can form a tautomer that can lose an H⁺ to make urate. It is not a carboxylic acid. And yes, both of these are heterocylcic compounds. Graeme Bartlett (talk) 22:58, 26 June 2024 (UTC)[reply]

For Q2/6, other examples of C_nH_2nO₃ are methoxyacetic acid and peracetic acid Graeme Bartlett (talk) 23:02, 26 June 2024 (UTC)[reply]

Strictly speaking, other isomers of oxalic acid are possible. See C₂H₂O₄ search at ChemSpider which gives two others that have associated literature. Our organic acid article is very poor, probably because it is difficult to cover such a broad subject well. If the OP wants to delve deeper, a textbook would be a better place to look. We have recently started to link some on our Project Page at WP:CHEM#Good open access sources and the McMurry one has extensive coverage of organic acids. Mike Turnbull (talk) 13:20, 27 June 2024 (UTC)[reply]

Those general formulae above will actually have many more isomers by moving the oxygen or double bond around. But they would not be carboxlic acids any more. Graeme Bartlett (talk) 00:26, 28 June 2024 (UTC)[reply]

That's really helpful, thanks all very much indeed for some clear explanations. @Lambiam: would a bit of Cream of Tartar (in the kitchen cupboard) work? Not worried about the possible cloudy effect. @Graeme Bartlett: I wasn't thinking of trying all the acids I listed as lime juice substitutes, I was just hoping to make sense of the list in the Organic acid article. @Mike Turnbull: I found McMurry is also available at archive.org to borrow. Lots to think about. Exits left, clutching tripod, gauze mat, Liebig condenser and Bunsen burner. Favourite quote: Frederick Sanger described himself as "just a chap who messed about in a lab." MinorProphet (talk) 21:07, 28 June 2024 (UTC)[reply]

@MinorProphet That archive.org version is the fifth edition. The latest (10th) edition is the one available at Openstax. You can download .pdf by chapter or get the whole ~190 MB. Mike Turnbull (talk) 21:22, 28 June 2024 (UTC)[reply]

Here it is said that the tangy flavor of cream of tartar makes it an excellent addition to marinades, adding acidity and enhancing the taste of grilled or roasted meats. Whether it works in a fish recipe can only be determined experimentally. --Lambiam 09:57, 29 June 2024 (UTC)[reply]

And all this time I've been thinking that Tartar sauce contains some cream of tartar ... Abductive (reasoning) 05:55, 30 June 2024 (UTC)[reply]