3
$\begingroup$

My education in chemistry goes as far as what I research on the internet. I have two different acid/base reactions for which I've tried to predict the resulting product. Can anyone here verify if I did this right or have an alternative prediction?

Reaction 1: p-toluenesulfonic acid (anhydrous) $\ce{C7H8O3S}$ reacts with potassium hydroxide $\ce{KOH}$. From this I calculated I should expect toluene $\ce{C7H8}$, potassium sulfate $\ce{K2SO4}$ and water $\ce{H2O}$:

$$\ce{C7H8O3S + 2KOH -> C7H8 + K2SO4 + H2O}\tag{1}$$

Reaction 2: p-toluenesulfonic acid (anhydrous) $\ce{C7H8O3S}$ reacts with sodium bicarbonate $\ce{NaHCO3}$:

$$\ce{C7H8O3S + 2NaHCO3 -> C7H8 + Na2SO4 + 2CO2 + H2O}\tag{2}$$

If correct, then both of these reactions result in toluene. Does anyone think I got it right (or wrong)?

If the p-toluenesulfonic acid is dissolved in hexane, and I wish to neutralize it, neither base is soluble in hexane. If I first dissolved the base in ethanol (anhydrous) which is miscible with hexane (if I'm using the term correctly), will it neutralize the acid in the hexane, and does the fact that hexane and ethanol are involved change the reactions above? Any prediction if the ethanol was 95% instead of anhydrous?

So separating ethanol from hexane is easy if you want to keep the hexane -- just add water and separate. But if I actually have hexane/ethanol/toluene and add water and dispose of the ethanol/water solution, I'm assuming I'm left with a hexane/toluene mix. The toluene has a much higher polarity than hexane, but much lower one than ethanol. Toluene has a boiling point higher than water, and approx. 40 degrees higher than hexane.

Under this circumstance it appears removing toluene from the hexane and solute is not realistic, and would leave me distilling the toluene from my solute at a too high a temperature for the solute ($\pu{111^\circ C}$) whereas the boiling point of hexane is more like $\pu{68^\circ C}$.

If I'm correct, is there any thoughts on how to neutralize the acid in hexane without creating toluene which is not easy to separate from the hexane or am I stuck with toluene as long as I use this particular acid?

Improve this question
$\endgroup$
3
  • $\begingroup$ You're trying to neutralize tosic (toluenesulfonic) acid in hexane as a post-reaction workup? $\endgroup$
    – electronpusher
    Commented Dec 20, 2016 at 4:04
  • 2
    $\begingroup$ Welcome to Chemistry.SE! You seem to assume that the neutralization of p-toluene sulfonic acid with potassium hydroxide (or sodium hydrogencarbonate generates) toluene. This is wrong! The neutralizations only lead to the potassium (or sodium) salts of the sulfonic acid. With other words, the (most) acidic proton is replaced with a potassium (or sodium) cation. There's no further degradation that would lead to toluene. $\endgroup$
    – Klaus-Dieter Warzecha
    Commented Dec 20, 2016 at 5:49
  • $\begingroup$ Thanks for your responses! -- based upon everyone's comments thus far I had another go at it (see comments in response to AK's answer). $\endgroup$
    – Christian B
    Commented Dec 20, 2016 at 6:24

3 Answers 3

Reset to default
6
$\begingroup$

You will not get toluene in either reaction, you will get the potassium and sodium salts of p-toluene sulfonic acid for the respective reactions. This is because 1.) the toluene sulfonic acid is not a salt of toluene and sulfuric acid and 2.) aromatic rings (unless special conditions are applied) undergo electrophilic aromatic substitution. i.e. positive groups cause substitution on the ring, which sodium and potassium are not electrophilic ("electron loving") enough to effect such reaction. If you wanted toluene from these reactions you would have to use a dilute mineral acid in water and the $\ce{H+}$ ions from the acid would cause a substitution, removing the sulfonyl group and yielding toluene and sulfuric acid.

If you really need to neutralize this material, just add the mentioned bases as solids and allow them time to react. Bases don't need to be in solution to react with acids.

Improve this answer
$\endgroup$
5
  • $\begingroup$ Thank you AK and Electronpusher, so much for reminding me how little I know ;) I really appreciate you taking the time! – I took advice from the two prior answers and rethought a bit and here is what I got. First instead of looking at the p-Toluenesulfonic acid like C7H8O3S I look at it like CH3 C6H4 SO3 H So CH3 C6H4 SO3 H + 2KOH gives me: CH3C6H4OH (p-Cresol) K2SO3 (Potassium Sulfite) and H2O (Water) Do I have it right yet? $\endgroup$
    – Christian B
    Commented Dec 20, 2016 at 6:18
  • 1
    $\begingroup$ No, this would require a nucleophilic substitution which will not happen. You should watch some youtube videos on electrophilic aromatic substitution so that you may understand the underlying principles of substitution on aromatic rings. It is not like most other reactions. $\endgroup$
    – A.K.
    Commented Dec 20, 2016 at 19:57
  • $\begingroup$ Ok, so, I checked out some videos on KahnAcademy. Thank you for that. So basically all that is happening is the neutralization of the acid like initially shown in ElectronPusher's structure. Basically the H of the OH of the acid gets swapped with the K from the KOH, the H from the acid goes to the OH of the KOH making H2O. This leaves me with H2O and Potassium p-toluenesulfonate, the latter of which I assume will be a solid precipitate, insoluble in either hexane or ethanol? $\endgroup$
    – Christian B
    Commented Dec 22, 2016 at 6:04
  • 1
    $\begingroup$ Now you have the right idea for the acid-base neutralization. However, the potassium p-toluenesulfonate salt (AKA potassium tosylate), should be soluble in ethanol and maybe somewhat suble in hexane too. See all those C's and H's in the ring? That nonpolar structure is soluble in nonpolar solvents, like hexane. The ionic charges on the sulfonate side would be soluble in polar solvents. This could cause some precipitation in hexane, but ethanol can accommodate both domains and I would expect no precipitate in the alcohol solvent. $\endgroup$
    – electronpusher
    Commented Dec 23, 2016 at 4:56
  • $\begingroup$ How can I predict if, given a solution of hexane and ethanol which contains the dissolved potassium tosylate, if I were to add water to separate, if the potassium tosylate will go with the etoh/h2o or stay in the hexane? I would guess the polar sulfonate side would create a stronger bond with water than the bond between the non-polar side and the hexane, and perhaps the non-polar side would be satisfied by the alcohol at the same time the polar side is teamed up with the water, i.e. gone from the hexane? $\endgroup$
    – Christian B
    Commented Dec 28, 2016 at 22:44
2
$\begingroup$

A sulfonic acid substituent on a benzene ring could, under some conditions, undergo nucleophilic aromatic substitution by hydroxide to give a phenol (in this case, p-cresol). This type of reaction is more favorable with additional electron withdrawing groups on the benzene ring.

See Aromatic Nucleophilic Substitution Reactions. Joseph F. Bunnett and Roland E. Zahler, Chem. Rev., 1951, 49 (2), pp 273–412.

Improve this answer
$\endgroup$
2
$\begingroup$

This is more than a simple acid-base neutralization, you are hydrolyzing the $\ce{C-S}$ bond. Is this your intention, to create toluene, or is that a byproduct? Your balanced reactions look correct, but I would recommend you use formulas that show more detail to see what's going on (better yet, structures! see photo). I see no risk in aqueous conditions, since you are hydrolyzing tosic acid anyway. Is there a reason you can't use an organic-soluble base?

Edit: I erroneously went with the OP's assumption that the hydrolysis of the ring would take place. The other comments have reminded me that these conditions are not sufficient for substitution of the aromatic ring.

enter image description here

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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