What is the main product when an excess of ethyl alcohol vapour is passed over heated alumina at 250 °C?

I read that this reaction gives an ether (here, diethyl ether) at relatively lower temperature than when it gives an alkene (here, ethene). This didn't bother me much as I had also read that higher temperatures favour elimination reactions, whereas lower temperatures favour nucleophilic substitutions.

So I was pretty sure that at 250 °C it would give ethene. But according to the book, it's diethyl ether. Why does this happen?

One thought that I had was maybe it's because “excess of ethyl alcohol” is mentioned, it would follow the formation of ether by the reaction of two alcohol molecules. Is that right?

  • 3
    $\begingroup$ As you say the key here is excess ethanol. Enough ethanol that one ethanol molecule that is interacting with the alumina surface is captured by a second ethanol molecule before it loses H+ to become ethene $\endgroup$
    – Waylander
    Jun 11 '21 at 11:02
  • 1
    $\begingroup$ What book is this question from? $\endgroup$ Oct 25 '21 at 16:19

According to Paul Sabatier in "Catalysis in Organic Chemistry",

With ethyl alcohol [on alumina], ether is formed above 240° and at 290° pure ethylene is evolved regularly, this evolution becoming rapid at 340°.

so at 250°C the product should be primarily diethyl ether. I don't think "excess" has the normal organic chemistry meaning in this context of heterogeneous catalysis. My supposition is that the question is just lazily trying to get you to recall this exact statement from the preceding chapter, rather that testing your conceptual knowledge. I would be interested to know what book this is!


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