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Is Hydration An Addition Reaction? [SOLVED!]

In the mechanism for a 3º alcohol shown above, the red H is added to the least-substituted carbon connected to the nucleophilic double bonds (it has less carbons attached to it). This means that the carbocation forms on the 3º carbon, causing it to be highly stabilized by hyperconjugation—electrons in nearby sigma (single) bonds help fill the empty p-orbital of the carbocation, which lessens the positive charge. More substitution on a carbon means more sigma bonds are available to “help out” (by using overlap) with the positive charge, which creates greater carbocation stability 😉 In other words, carbocations form on the most substituted carbon connected to the double bond 🙌 Carbocations are also stabilized by resonance, but resonance is not a large factor in this case because any carbon-carbon double bonds are used to initiate the reaction, and other double bonded molecules can cause a completely different reaction 😊 [1]
In the mechanism for a 3º alcohol shown above, the red H is added to the least-substituted carbon connected to the nucleophilic double bonds (it has less carbons attached to it). This means that the carbocation forms on the 3º carbon, causing it to be highly stabilized by hyperconjugation—electrons in nearby sigma (single) bonds help fill the empty p-orbital of the carbocation, which lessens the positive charge. More substitution on a carbon means more sigma bonds are available to “help out” (by using overlap) with the positive charge, which creates greater carbocation stability. In other words, carbocations form on the most substituted carbon connected to the double bond. Carbocations are also stabilized by resonance, but resonance is not a large factor in this case because any carbon-carbon double bonds are used to initiate the reaction, and other double bonded molecules can cause a completely different reaction. If the carbocation does originally form on the less substituted part of the alkene, carbocation rearrangements occur to form more substituted products. (revised by Nancy Green from Tongchuan, China on May 2, 2020) [2]
Image #2
According to the analysts at chem.libretexts.org, in the mechanism for a 3º alcohol shown above, the red H is added to the least-substituted carbon connected to the nucleophilic double bonds (it has less carbons attached to it). This means that the carbocation forms on the 3º carbon, causing it to be highly stabilized by hyperconjugation—electrons in nearby sigma (single) bonds help fill the empty p-orbital of the carbocation, which lessens the positive charge. More substitution on a carbon means more sigma bonds are available to “help out” (by using overlap) with the positive charge, which creates greater carbocation stability. In other words, carbocations form on the most substituted carbon connected to the double bond. Carbocations are also stabilized by resonance, but resonance is not a large factor in this case because any carbon-carbon double bonds are used to initiate the reaction, and other double bonded molecules can cause a completely different reaction. If the carbocation does originally form on the less substituted part of the alkene, carbocation rearrangements occur to form more substituted products. (credit to Tenia Kohler for pointing this out to us). [3]
Image #3
Like other electrophilic addition reactions, the first step in the mechanism is the nucleophilic attack of the alkene π electrons onto the electrophile. In this case, the electrophile is an acidic proton. So, we draw an arrow from the alkene double bond to the acidic proton (in this example, a proton on sulfuric acid), and break the H-O bond, with the bond electrons going onto the oxygen (giving it a negative charge). However, we must decide which alkene carbon accepts the proton, and which becomes the carbocation. In the following scheme, if we protonate the carbon on the right, we get a tertiary carbocation (top arrow), while if we protonate the carbon on the’s had leaving left, we get a secondary carbocation (bottom arrow): [4]
Image #4

Article References

  1. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map:_Organic_Chemistry_(Smith)/Chapter_10:_Alkenes/10.12:_HydrationElectrophilic_Addition_of_Water
  2. https://chem.libretexts.org/Courses/Sacramento_City_College/SCC:_Chem_420_-_Organic_Chemistry_I/Text/09:_Reactions_of_Alkenes/9.04:_Hydration:_Acid_Catalyzed_Addition_of_Water
  3. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map:_Organic_Chemistry_(Wade)/09:_Reactions_of_Alkenes/9.04:_Addition_of_Water:_Hydration_of_Alkenes
  4. https://periodicchemistry.com/2018/08/21/organic-reactions-hydration-of-alkenes/
Mehreen Alberts

Written by Mehreen Alberts

I'm a creative writer who has found the love of writing once more. I've been writing since I was five years old and it's what I want to do for the rest of my life. From topics that are close to my heart to everything else imaginable!

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