Synthesis of Propylene Oxide (2-methyloxirane, 2-epoxypropane, etc.) via Chlorohydrin Intermediate
Hello my fellow chemists.
My ultimate goal here is to produce propylene oxide. I’ve read some patents (and used the search engine) and I have settled on one of two variations
of the chlorohydrin process. I was hoping for some input or insights into potential pitfalls or improvements.
To those unfamiliar, the chlorohydrin-based synthesis of propylene oxide (methyloxirane, 1,2-epoxypropane, it is a compound of many names) is as
follows:
Propene will be produced continuously via the acid catalyzed dehydration of 2-propanol with conc. sulfuric acid.
Now this can be performed continuously, forming hypochlorous acid (HClO) in situ, or batchwise by preforming the hypochlorous acid either via
acidification of a solution of hypochlorite (Reaction 3), or by bubbling chlorine gas (Cl2) into an alkaline solution (Reaction 4). The problem with
the latter process is the unavoidable formation of HCl which is subsequently neutralized to NaCl. This is problematic as the presence of chlorine
anions drives the equilibrium of the following reaction to the left (Reaction 4):
NaClO- + H+ <-------> HClO + Na+
3
Cl2 + H2O <------> HClO + HCl
4
Additionally, the presence of chloride leads the the unwanted byproduct of 1,2-dichloropropane
The problem that is chloride anion presence has been resolved in a few ways in the literature, though I am only going to discuss one method. HClO can
be extracted using methyl ethyl ketone, acetone, or ethyl acetate relatively efficiently, leaving the ionic salts behind in the aqueous phase. This
solution can be stored for short periods (on the order of hours), though from the perspective of both safety and yield, it is highly recommended to
use it immediately. Its use as an organic solution will be discussed later.
There are two practical possibilities to make hypochlorous acid. The first is to acidify commercial bleach (contains only NaClO and NaOH according to
the SDS) with dilute sulfuric acid and extract it with acetone to use in a second reaction vessel. The second possibility is to acidify calcium
hypochlorite Ca(ClO)2 with dilute sulfuric acid. The advantage of this option is that the insolubility of calcium sulfate means I could
potentially avoid the solvent extraction and directly bubble propene into this solution. Or if I decide to use a secondary reaction vessel, I could
simply filter off the precipitate and similarly avoid the solvent extraction. In all the outlined approaches, the pH is to be made 5.0 as this pH
minimizes the amount of Cl2 and ClO- present and maximizes [HClO] (Figure 1).
Figure 1
The chlorohydrin produced will be distilled and then dripped into calcium hydroxide solution to yield the epoxide. The worry here is that the
hydroxide (or what) present can open the epoxide ring up to yield the diol. However, due to its low boiling point (~40C) it can be removed
continuously as it forms with sufficient application of heat.
I've read a few patents on the matter (Attachment: chlorohydrinpatent.pdf (1MB)
This file has been downloaded 195 times) and haven't come across any method that exploits the insolubility of CaSO4. Was wondering if anybody had any insights into the topic?
Will be posting an update after attempting this later in the week.
[Edited on 2-8-2023 by SyntheticFunk]
[Edited on 2-8-2023 by SyntheticFunk]
[Edited on 2-8-2023 by SyntheticFunk]
[Edited on 2-8-2023 by SyntheticFunk]
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