Draw The Structure Of Two Alkenes That Would Yield 1-methylcyclohexanol

To understand which alkenes yield 1-methylcyclohexanol, we need to delve into the world of organic chemistry, specifically focusing on reactions that convert alkenes into alcohols. The key reaction here is hydration, the addition of water (H₂O) to an alkene. However, direct hydration often doesn't yield the desired product efficiently. A more common and controlled method involves a two-step process: oxymercuration-demercuration or hydroboration-oxidation. Let's break down the mechanism, explore the possible alkene structures, and understand the underlying chemistry.

Hydration Reactions: A Pathway to Alcohols

Alkenes, with their carbon-carbon double bonds, are reactive molecules. The double bond, composed of a sigma (σ) and a pi (π) bond, provides a region of high electron density, making alkenes susceptible to electrophilic attack. Hydration, the addition of water, follows this principle.

Direct Acid-Catalyzed Hydration

In the presence of a strong acid catalyst like sulfuric acid (H₂SO₄), an alkene can undergo direct hydration. Here's how it works:

Protonation: The alkene's pi electrons attack a proton (H⁺) from the acid, forming a carbocation intermediate. This protonation follows Markovnikov's rule, which states that the proton adds to the carbon with more hydrogen atoms, leading to the more stable carbocation.
Water Attack: A water molecule (H₂O) acts as a nucleophile, attacking the positively charged carbocation.
Deprotonation: Another water molecule removes a proton from the oxygen atom of the attached water molecule, regenerating the acid catalyst and forming the alcohol.

The Problem with Direct Hydration: Carbocations are prone to rearrangements. Methyl or hydride shifts can occur to form a more stable carbocation, leading to a mixture of products, not specifically 1-methylcyclohexanol.

Oxymercuration-Demercuration: A Regioselective Alternative

Oxymercuration-demercuration is a two-step reaction sequence that hydrates alkenes according to Markovnikov's rule without carbocation rearrangements.

Oxymercuration: The alkene reacts with mercuric acetate [Hg(OAc)₂] in water. The mercury(II) ion (Hg²⁺) acts as an electrophile, attacking the alkene's pi bond. A mercurinium ion intermediate forms, which is a three-membered ring containing mercury. Water then attacks the more substituted carbon of the mercurinium ion (Markovnikov's rule), opening the ring and forming a mercurated alcohol.
Demercuration: The mercurated alcohol is then treated with sodium borohydride (NaBH₄). NaBH₄ replaces the mercury atom with hydrogen, regenerating the alcohol.

Why No Rearrangements? The mercurinium ion intermediate shields the carbocation from rearrangement because mercury is already bonded to both carbons involved in the original double bond.

Hydroboration-Oxidation: Anti-Markovnikov Hydration

Hydroboration-oxidation provides a method for anti-Markovnikov hydration, meaning the hydroxyl group (OH) adds to the less substituted carbon of the alkene.

Hydroboration: Borane (BH₃) or a borane derivative (like disiamylborane or 9-BBN) adds to the alkene. Boron, being electron-deficient, acts as an electrophile. The boron atom and a hydrogen atom add across the double bond in a syn addition (meaning they add to the same side of the alkene). This process repeats until all three hydrogens on the boron are bonded to alkyl groups.
Oxidation: The trialkylborane is then treated with hydrogen peroxide (H₂O₂) in a basic solution (NaOH). The boron-carbon bond is cleaved, and a hydroxyl group (OH) replaces the boron atom. The overall result is the anti-Markovnikov addition of water.

Stereochemistry: Hydroboration-oxidation is a syn addition, meaning the boron and hydrogen add to the same face of the alkene. The oxidation step retains this stereochemistry, so the hydroxyl group ends up on the same face of the ring as the hydrogen that was originally added.

Identifying Alkenes that Yield 1-Methylcyclohexanol

Now that we understand the hydration reactions, we can work backward to identify alkenes that would yield 1-methylcyclohexanol. We need to consider both Markovnikov and anti-Markovnikov addition possibilities.

Markovnikov Addition (Oxymercuration-Demercuration or Acid-Catalyzed Hydration)

For Markovnikov addition, the hydroxyl group (OH) will add to the more substituted carbon of the alkene. Therefore, we need to look for alkenes where one carbon of the double bond is attached to the methyl group on the cyclohexane ring, and the other carbon has at least one hydrogen attached (to make it the less substituted carbon).

Alkene Structure 1: 1-Methylcyclohexene

This is the most straightforward possibility. If we hydrate 1-methylcyclohexene using acid-catalyzed hydration or oxymercuration-demercuration, the hydroxyl group will add to the more substituted carbon, which is the carbon bearing the methyl group. This directly yields 1-methylcyclohexanol.

Reaction:

      CH3                   OH
       |                     |
   ----C=CH----  + H2O  --> ----C-CH2----
  |          |          H+ |          |
  Cyclohexane      or Hg(OAc)2  Cyclohexane
                  NaBH4

Explanation: The double bond is located between carbon 1 (with the methyl group) and carbon 2 of the cyclohexane ring. Markovnikov addition dictates that the OH group adds to carbon 1, resulting in 1-methylcyclohexanol.

Anti-Markovnikov Addition (Hydroboration-Oxidation)

For anti-Markovnikov addition, the hydroxyl group (OH) will add to the less substituted carbon of the alkene. Therefore, we need to find an alkene where the carbon adjacent to the methyl-bearing carbon is the less substituted one.

Alkene Structure 2: Methylenecyclohexane

In methylenecyclohexane, the double bond is between the cyclohexane ring and an exocyclic methylene group (CH₂). When subjected to hydroboration-oxidation, the hydroxyl group adds to the less substituted carbon, which is the methylene carbon (CH₂). This would not directly yield 1-methylcyclohexanol. Instead, it would yield (cyclohexylmethyl)methanol.

However: We need 1-methylcyclohexanol. Let's reconsider. Hydroboration-oxidation places the -OH on the less substituted carbon. This means that the carbon connected to the methyl group in the desired product (1-methylcyclohexanol) needs to be part of the alkene's less substituted carbon in the starting alkene.

Alkene Structure 3: 2-Methylcyclohexene

Consider 2-methylcyclohexene, where the methyl group is attached to the second carbon of the cyclohexene ring. Upon hydroboration-oxidation, the -OH group adds to the first carbon of the ring (the less substituted carbon), resulting in 2-methylcyclohexan-1-ol. This is not 1-methylcyclohexanol.

Conclusion Regarding Anti-Markovnikov: There is no simple alkene structure that, upon direct hydroboration-oxidation, will yield 1-methylcyclohexanol. Anti-Markovnikov addition always places the -OH group on the less substituted carbon, and to obtain 1-methylcyclohexanol, we need the -OH group on the carbon with the methyl group.

Summary of Alkenes that Yield 1-Methylcyclohexanol

Based on our analysis, the most direct and viable alkene precursor to 1-methylcyclohexanol is:

1-Methylcyclohexene (via Markovnikov addition)

While hydroboration-oxidation provides a route for anti-Markovnikov addition, it does not lead to the direct formation of 1-methylcyclohexanol from a simple alkene precursor. There are no other simple alkenes which will selectively give 1-methylcyclohexanol directly by hydroboration-oxidation.

Detailed Reaction Mechanisms

To further illustrate the formation of 1-methylcyclohexanol from 1-methylcyclohexene, let's look at the detailed reaction mechanisms for both acid-catalyzed hydration and oxymercuration-demercuration.

Acid-Catalyzed Hydration of 1-Methylcyclohexene

Protonation: 1-Methylcyclohexene reacts with a proton (H⁺) from the acid catalyst (e.g., H₂SO₄). The pi electrons of the double bond attack the proton, forming a carbocation intermediate. The proton adds to the carbon-2 of the ring (the carbon with more hydrogens), leading to a tertiary carbocation at carbon-1 (the carbon bearing the methyl group). This follows Markovnikov's rule and forms the more stable carbocation.
```
      CH3                    CH3   +
       |                      |
   ----C=CH----  + H+  --> ----C-CH2----
  |          |               |          |
  Cyclohexane                   Cyclohexane
```

Water Attack: A water molecule (H₂O) acts as a nucleophile and attacks the positively charged carbocation at carbon-1.

      CH3   +                CH3   +
       |                      |
   ----C-CH2----  + H2O  --> ----C-CH2----
  |          |               |   |      |
  Cyclohexane                   Cyclohexane
                                   OH2

Deprotonation: Another water molecule removes a proton from the oxygen atom of the attached water molecule, regenerating the acid catalyst and forming 1-methylcyclohexanol.

      CH3   +                CH3
       |                      |
   ----C-CH2----  + H2O  --> ----C-CH2----  + H3O+
  |   |      |               |   |      |
  Cyclohexane                   Cyclohexane
       OH2                       OH

Oxymercuration-Demercuration of 1-Methylcyclohexene

Oxymercuration: 1-Methylcyclohexene reacts with mercuric acetate [Hg(OAc)₂] in water. The mercury(II) ion (Hg²⁺) acts as an electrophile, attacking the alkene's pi bond, forming a mercurinium ion intermediate.

      CH3                      CH3
       |                        |
   ----C=CH----  + Hg(OAc)2 -->  ----C---CH----
  |          |                  |   \ /  |
  Cyclohexane                      Cyclohexane
                                      Hg(OAc)
                                        |
                                        OAc

Water Attack: Water attacks the more substituted carbon (carbon-1) of the mercurinium ion, opening the ring.

      CH3                      CH3
       |                        |
   ----C---CH----  + H2O  --> ----C-CH----
  |   \ /  |                  |   |   |
  Cyclohexane                      Cyclohexane
       Hg(OAc)                      OH Hg(OAc)
         |
         OAc

Demercuration: The mercurated alcohol is then treated with sodium borohydride (NaBH₄). NaBH₄ replaces the mercury atom with hydrogen, regenerating 1-methylcyclohexanol.

      CH3                        CH3
       |                          |
   ----C-CH----  + NaBH4      --> ----C-CH2----  + Hg + byproducts
  |   |   |                      |   |      |
  Cyclohexane                          Cyclohexane
       OH Hg(OAc)                         OH

Conclusion

In summary, 1-methylcyclohexene is the primary alkene that will yield 1-methylcyclohexanol via Markovnikov addition, using either acid-catalyzed hydration or oxymercuration-demercuration. Hydroboration-oxidation, which follows anti-Markovnikov's rule, does not directly lead to the formation of 1-methylcyclohexanol from a simple alkene precursor. Understanding the reaction mechanisms and the directing effects of substituents is crucial for predicting the products of alkene hydration reactions.