# Hard (I think) chemistry question. **URL:** \ **Category:** Factual Questions\ **Created:** [August 19, 2016, 1:39am UTC](https://boards.straightdope.com/t/hard-i-think-chemistry-question/763262 "2016-08-19T01:39:26Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Washoe](https://avatars.discourse-cdn.com/v4/letter/w/ecae2f/32.png) [@Washoe](https://boards.straightdope.com/u/Washoe)\ **Post date:** [August 19, 2016, 1:39am UTC](https://boards.straightdope.com/t/hard-i-think-chemistry-question/763262/1 "2016-08-19T01:39:26Z")
When tianeptine sodium (C21H24ClN2NaO4S) is reacted with DMSO (dimethyl sulfoxide; C2H6OS) the resulting solution is a bright yellow color. Can anybody explain why this is? I apologize for not being able to figure this out for myself, but I’ve only taken one undergraduate lab course in chemistry. All I know are the rudimentary basics. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [August 19, 2016, 12:27pm UTC](https://boards.straightdope.com/t/hard-i-think-chemistry-question/763262/2 "2016-08-19T12:27:50Z")
Why is tartrazine yellow ? > **[Tartrazine](https://en.wikipedia.org/wiki/Tartrazine)** > > Tartrazine is a synthetic lemon yellow azo dye primarily used as a food coloring. It is also known as E number E102, C.I. 19140, FD&C Yellow 5, Yellow 5 Lake, Acid Yellow 23, Food Yellow 4, and trisodium 1-(4-sulfonatophenyl)-4-(4-sulfonatophenylazo)-5-pyrazolone-3-carboxylate). > Tartrazine is a commonly used color all over the world, mainly for yellow, and can also be used with brilliant blue FCF (FD&C Blue 1, E133) or green S (E142) to produce various green shades. > Many foods contain tartraz... ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [August 19, 2016, 1:01pm UTC](https://boards.straightdope.com/t/hard-i-think-chemistry-question/763262/3 "2016-08-19T13:01:11Z")
> [@Isilder](#): > > Why is tartrazine yellow ? > > [Tartrazine - Wikipedia](https://en.wikipedia.org/wiki/Tartrazine) The azo chromophore? @ Washoe: do you know for a fact that they react (like, form a covalent bond)? Under what conditions does this reaction take place? ---
**Author:** ![Washoe](https://avatars.discourse-cdn.com/v4/letter/w/ecae2f/32.png) [@Washoe](https://boards.straightdope.com/u/Washoe)\ **Post date:** [August 19, 2016, 4:41pm UTC](https://boards.straightdope.com/t/hard-i-think-chemistry-question/763262/4 "2016-08-19T16:41:33Z")
No, sorry—I was being stupid. They don’t react; they form a solution. This is evidenced by the fact that tianeptine, which is a psychoactive drug, retains its psychoactive properties when dissolved in DMSO. ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [August 19, 2016, 11:09pm UTC](https://boards.straightdope.com/t/hard-i-think-chemistry-question/763262/5 "2016-08-19T23:09:49Z")
No, you’re the smart one. It’s just the organic chemist in me who insists that a “reaction” between two compounds must involve the formation of a covalent bond. Actually, I know that there’s plenty of substances whose color in solution depends on the qualities of the solvent. Take, for example, iodine: it is brownish-yellow in polar solvents (alcohol, acetone, ether) but purple in apolar (dichloromethane, pentane, cyclohexane). I bet it’s something like that going on with your tianeptine. DMSO is, indeed, polar like water, but unlike water it is aprotic. This might account for the change in color! But as for _why_, I’m at a loss. As you said, this is a \*hard \*chemistry question! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 20, 2016, 12:21am UTC](https://boards.straightdope.com/t/hard-i-think-chemistry-question/763262/6 "2016-08-20T00:21:20Z")
For what it’s worth, dissolution is often a chemical change, not just a physical one. Yes, I know that most textbooks say otherwise. They’re wrong.