Acid Catalyzed Hydro-alkoxy Addition

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Acid Catalyzed Alkoxylation
4:37 — by Jules B.

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Key Questions

  • An acid catalyzed hydro-alkoxy addition is the addition of an alcohol to a C=C double bond to form an ether.

    An example is the addition of methanol to 2-methylpropene to form t-butyl methyl ether.

    This is an electrophilic addition reaction.

    In Step 1, a hydronium or oxonium ion is attacked by the π bond.

    In Step 2, the alcohol attacks the carbocation and forms an oxonium ion.

    In Step 3, the alcohol deprotonates the oxonium ion to form the ether.

    Here’s a video on the addition of alcohols to alkenes.

  • One adds an alcohol like H-OCH3 to result in an ether, one adds a water to add an alcohol group.

    For example, with double bond reactions, you can add H3O+ alone or you can add H2O using Hg(OAc)2, THF, followed up with NaBH4.

    For adding an alkoxy group to a double bond, you don't usally add a protonated HOCH3 but instead add it with Hg(OAcF3), THF. I have never seen acid catalyzed alkoxylation to a double bond but I suppose it is possible.

  • The acid-catalyzed alkoxylation is an analogous reaction to the acid-catalyzed hydration (Markovnikov addition of water via acid catalysis), and can go as follows for a substituted alkene and a generic alcohol:

    where the racemic mixture of the major or minor products can be written as a line bond instead of both the hash and wedge bonds.

    (Had it not been racemic but uneven, a squiggly bond would have been the way to write it.)

    The mechanism would go as follows:

    with Markovnikov addition giving the major product.

    1. Protonation of the alkene to create reaction conditions in which an alcohol can be a good nucleophile.
    2. Nucleophilic backside-attack of the carbocation intermediate.
    3. Removal of the attached alkoxide's proton (#"pKa" ~~ -3.6#) to regenerate the catalyst and form the product(s).