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What is the mechanism behind the drying of THF with sodium metal and benzophenone? When a deep color is seen, what does it imply? Where has the water gone to?
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Akinlade Akinlolu
What is the mechanism behind the drying of THF with sodium metal and benzophenone? When a deep color is seen, what does it imply? Where has the water gone to?
Well, water is quite soluble in THF (in fact the solvents are miscible) … On the other hand water irreversibly reacts with sodium metal as shown …
…the presence of benzophenone, i.e. [math]H_{5}C_{6}-C(=O)-C_{6}H_{5}[/math], serves as an indicator … it it reduced by sodium metal to give BLUE [math]H_{5}C_{6}-C(-O^{-}Na^{+})-C_{6}H_{5}[/math] …and when it gets completely dry it accepts another electron to give the purple [math]H_{5}C_{6}-stackrel{•}C(-O^{-}Na^{+})-C_{6}H_{5} [/math]... the which reacts with dioxygen...
…the presence of benzophenone, i.e. [math]H_{5}C_{6}-C(=O)-C_{6}H_{5}[/math], serves as an indicator … it it reduced by sodium metal to give BLUE [math]H_{5}C_{6}-C(-O^{-}Na^{+})-C_{6}H_{5}[/math] …and when it gets completely dry it accepts another electron to give the purple [math]H_{5}C_{6}-stackrel{•}C(-O^{-}Na^{+})-C_{6}H_{5} [/math]... the which reacts with dioxygen...
In addition to Robin Bedford's answer, I just wanted to add that it makes an incredibly handy glove box solvent indicator. Keep a vial of the ketyl radical in the box and you have a very sensitive instant way of telling if your solvent is dry.
When you are already dealing with very dry solvent, you get very sensitive differences from light blue (decent) → green (acceptable) → yellow (change the solvent) → colourless (severe panic accompanied by the public executions of the masters students).
In addition to Robin Bedford's answer, I just wanted to add that it makes an incredibly handy glove box solvent indicator. Keep a vial of the ketyl radical in the box and you have a very sensitive instant way of telling if your solvent is dry.
When you are already dealing with very dry solvent, you get very sensitive differences from light blue (decent) → green (acceptable) → yellow (change the solvent) → colourless (severe panic accompanied by the public executions of the masters students).
Well, water is quite soluble in THF (in fact the solvents are miscible) … On the other hand water irreversibly reacts with sodium metal as shown …
[math]Na(s) + H_{2}O(l) longrightarrow Na^{+}HO^{-} + dfrac{1}{2}H_{2}(g)uparrow[/math]
…the presence of benzophenone, i.e. [math]H_{5}C_{6}-C(=O)-C_{6}H_{5}[/math], serves as an indicator … it it reduced by sodium metal to give BLUE [math]H_{5}C_{6}-C(-O^{-}Na^{+})-C_{6}H_{5}[/math] …and when it gets completely dry it accepts another electron to give the purple [math]H_{5}C_{6}-stackrel{•}C(-O^{-}Na^{+})-C_{6}H_{5} [/math]... the which reacts with dioxygen...
Well, water is quite soluble in THF (in fact the solvents are miscible) … On the other hand water irreversibly reacts with sodium metal as shown …
[math]Na(s) + H_{2}O(l) longrightarrow Na^{+}HO^{-} + dfrac{1}{2}H_{2}(g)uparrow[/math]
…the presence of benzophenone, i.e. [math]H_{5}C_{6}-C(=O)-C_{6}H_{5}[/math], serves as an indicator … it it reduced by sodium metal to give BLUE [math]H_{5}C_{6}-C(-O^{-}Na^{+})-C_{6}H_{5}[/math] …and when it gets completely dry it accepts another electron to give the purple [math]H_{5}C_{6}-stackrel{•}C(-O^{-}Na^{+})-C_{6}H_{5} [/math]... the which reacts with dioxygen...
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In addition to Robin Bedford's answer, I just wanted to add that it makes an incredibly handy glove box solvent indicator. Keep a vial of the ketyl radical in the box and you have a very sensitive instant way of telling if your solvent is dry.
When you are already dealing with very dry solvent, you get very sensitive differences from light blue (decent) → green (acceptable) → yellow (change the solvent) → colourless (severe panic accompanied by the public executions of the masters students).
User-10084820961513694172's answer to What is the mechanism behind the drying of THF with sodium metal and benzophenone? When a deep color is seen, what does it imply? Where has the water gone to?
In addition to Robin Bedford's answer, I just wanted to add that it makes an incredibly handy glove box solvent indicator. Keep a vial of the ketyl radical in the box and you have a very sensitive instant way of telling if your solvent is dry.
When you are already dealing with very dry solvent, you get very sensitive differences from light blue (decent) → green (acceptable) → yellow (change the solvent) → colourless (severe panic accompanied by the public executions of the masters students).
User-10084820961513694172's answer to What is the mechanism behind the drying of THF with sodium metal and benzophenone? When a deep color is seen, what does it imply? Where has the water gone to?
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