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National Hazard
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Quote: Originally posted by Jenks  | | Phosphoric acid seems to be the obvious alternative to prevent reduction of the acid from being a problem. |
I already tried 75% phosphoric acid, but ethanol just boils unchanged from it.
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Jenks
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I'm surprised. This is useful to know. Maybe the dehydrating ability of sulfuric acid is driving the reaction. But Diachrynic wrote, "It can also be
seen that the dilution of water is not limiting the reaction, as it distills off alongside the ether and alcohol in more or less a stable equilibrium,
if it is allowed to do so." If the water is removed by distilling it as the azeotrope, the sulfuric acid is not needed for this. Maybe the ester
intermediates are the key, and the mechanism is not as simple as protonation of ethanol.
[Edited on 20-5-2024 by Jenks]
[Edited on 20-5-2024 by Jenks]
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Keras
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| Quote: Originally posted by Jenks |
I'm surprised. This is useful to know. Maybe the dehydrating ability of sulfuric acid is driving the reaction. But Diachrynic wrote, "It can also be
seen that the dilution of water is not limiting the reaction, as it distills off alongside the ether and alcohol in more or less a stable equilibrium,
if it is allowed to do so." If the water is removed by distilling it as the azeotrope, the sulfuric acid is not needed for this. Maybe the ester
intermediates are the key, and the mechanism is not as simple as protonation of ethanol. |
Yeah, there is obviously something specific about sulphuric acid.
I will try again in the following days and keep you posted.
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bnull
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| Quote: Originally posted by Jenks | | If the water is removed by distilling it as the azeotrope, the sulfuric acid is not needed for this. Maybe the ester intermediates are the key, and
the mechanism is not as simple as protonation of ethanol. |
Protonation of ethanol is the crucial step (see, for example, https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Or...); otherwise there is no reaction. Sulfuric acid is needed because it is a strong
acid and consequently a good proton donor, and is also non-volatile. Other common strong acids (HCl, HNO3) are volatile and boil off well
before the reaction temperature. Phosphoric acid is fixed but weak.
There are no ester intermediates. If that were indeed the case, phosphoric acid would work. Even boric acid would do.
[Edited on 20-5-2024 by bnull]
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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chornedsnorkack
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Indeed. Compare the boiling point of 85% phosphoric and 85% sulphuric acid. They have the same molar mass - both are 50% by mole. The boiling point of
85% H3PO4 is quoted as 158 degrees - cannot quickly find a quote for 85% H2SO4 boiling point but from
graphs looks around 215 degrees. Equimolar amount of sulphuric acid protonates water much more fully that phosphoric acid.
Strong monobasic acids like HClO4, CF3SO3H, ClSO3H and FSO3H are more volatile themselves
because they have less bonds for hydrogen bond network. (And ClSO3H and FSO3H hydrolyze in water anyway). The one acid whose
strength is close (slightly lower) and volatility similarly small is H2SeO4, and it is more oxidizing.
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