Acid Catalyzed Hydration

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Hydration of Alkenes - Acid Catalyzed Reaction Mechanism
9:09 — by Leah F.

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Key Questions

  • A picture is worth a thousand words, but here goes...

    Let's take our old pal propene, the smallest non symmetrical double bond alkene. We have a secondary carbon and a primary carbon with a double bond, so we have a Markovnikov and non Markovnikov carbon in the double bond (respectively). We also have a lonely methyl danging off taking no part of the double bond.

    You treat the double bond as the Nucleophile, the electrons in the double bond are like a hidden lone pair (like undescended testicles). They act as the aggressors, to attack the electrophile or H+ from acidic water (H3O+). The double bond swings out and grabs off H+ from H3O+, creating a bond on the primary carbon, a plus charge on the secondary carbon, like pulling an arm off a fly the carbon lost a bond and has a plus charge.

    Now the secondary carbon can take a new bond, specifically, from H2O. H2O is now the nucleophile for the second step, which attacks the plus charged carbon (carbocation), thus forming a bond at the Markovnikov position with water. You still have both Hyrogens attached to the water so you gotta "mop up" the last step, have another water rip off the extra H+ (proton),

    You have a secondary alcohol.

    Dr. Robb
    www.passochem.com

  • Acid catalyzed dehydration is the removal of -OH moieties from a molecule in the presence of abundant H+.

  • Acid catalyzed hydration is a chemical reaction in which water adds to an unsaturated substrate under the influence of an acid catalyst.

    An example is the hydration of ethene.

    CH₂=CH₂ + H-OH → H-CH₂-CH₂-OH

    The common acid catalysts are sulfuric acid and phosphoric acid. They react with water to form hydronium ions, which is the strongest acid that can exist in aqueous solution.

    The reaction takes place in three steps.

    research.cm.utexas.edu

    Step 1. Protonation of the double bond.

    All the hydrogen atoms in the hydronium ion are fairly positively charged because they are attached to a very electronegative oxygen atom.

    One of these hydrogen atoms is strongly attracted to the carbon-carbon double bond.

    The π bond breaks, and the electrons in it move to make a new bond with the hydrogen atom.

    That forces the electrons in the hydrogen-oxygen bond entirely onto the oxygen.

    Step 2. Formation of an oxonium ion

    The carbocation reacts with one of the lone pairs on a water molecule.

    A bond forms between the positively-charged carbon atom and the oxygen atom of the water. The O atom gets a positive charge

    Step 3. Deprotonation of the oxonium ion

    Finally, reaction with water removes one of the hydrogen atoms on the oxygen.

    This regenerates the hydronium ion catalyst.

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