19 2 Carbocation rearrangements

Dehydration Reactions of 3,3-Dimethyl-2-butanol

Overview of the Reaction

• Dehydration reactions may lead to unexpected products.

• Starting compound: 3,3-dimethyl-2-butanol.

• Reaction involves:

  • Protonation of the alcohol.

  • Deprotonation of the beta carbon.

  • Notable aspect: Observing beta hydrogens on carbons for elimination.

Expected vs. Actual Products

• Expected Product: Terminal mono-substituted alkene formed from the elimination reaction.

  • General structure:

    • 2 carbons with a t-butyl group.

• Actual Products: Two unexpected products:

  • Major product: Tetra-substituted alkene.

  • Minor product: Alkene with methyl on one side and isopropyl on the other side.

  • Reason for discrepancy: Rearrangement of carbocations, particularly evident with secondary substrates.

Carbocation Formation and Rearrangement

• Step 1: Proton transfer to alcohol → generates a protonated alcohol and a molecule of water.

• Step 2: Leaving group departs → forms a carbocation intermediate (likely a secondary carbocation).

• Stability Consideration: Tertiary carbocations are favored over secondary ones due to stability.

  • Rearrangement of the secondary carbocation to a tertiary carbocation occurs:

    • Methyl group migrates to adjacent carbon, resulting in a more stable tertiary carbocation.

Product Formation

• Following the formation of the tertiary carbocation:

  • Elimination reactions with beta hydrogens can now occur:

    • Pathway A: Pulling off a beta hydrogen leads to the formation of the major product (the tetra-substituted alkene).

    • Pathway B: Pulling off an alternative beta hydrogen leads to the minor product (disubstituted alkene).

Stability of Alkene Products

• Tetra-substituted alkene (major product) is more stable than disubstituted alkene (minor product) due to alkene stability trends.

Carbocation Migratory Shifts

• Common Types of Shifts:

  • Methyl Shift (Methanide Shift): Methyl group migration to form a more stable tertiary carbocation.

  • Hydride Shift (Hydride Migration): Hydrogen migration also resulting in a tertiary carbocation from a secondary one.

    • In cases where both shifts are possible, the shift leading to the more stable carbocation will occur.

Clarification on Alcohol Dehydration

• Primary Alcohols and E2 Mechanisms:

  • Although primary alcohols generally do not favor carbocation formation, dehydration can still lead to rearranged products through E2 mechanisms.

  • In E2, a less substituted alkene is initially formed.

  • Following protonation of the alkene, a stable carbocation (tertiary) can form, leading to further rearrangement and production of a more highly substituted alkene.

Conclusion

• Understanding carbocation stability and potential rearrangements is crucial in predicting the outcome of dehydration reactions, even with primary alcohols.

Dehydration Reactions of 3,3-Dimethyl-2-butanol

Overview of the Reaction

Dehydration reactions, a vital aspect of organic chemistry, may lead to unexpected products, particularly when alcohols are involved. In the case of 3,3-dimethyl-2-butanol, the reaction showcases complex behavior due to carbocation rearrangements.

Starting Compound

• 3,3-Dimethyl-2-butanol

  • An alcohol with two methyl groups attached to the tertiary carbon and a hydroxy group on the secondary carbon.

Reaction Mechanism

1. Protonation of the Alcohol: The hydroxyl group (-OH) of the alcohol is protonated using a strong acid (often sulfuric acid), leading to the formation of a protonated alcohol which enhances the leaving group ability.

2. Elimination of Water: The resultant protonated alcohol undergoes deprotonation at the beta carbon, leading to the departure of a water molecule and the formation of a carbocation. This carbocation is likely to be secondary due to the alcohol's structure.

3. Notable Aspect: When predicting elimination reactions, it is crucial to assess the beta hydrogens present on adjacent carbons as they play a significant role in the formation of alkenes.

Expected vs. Actual Products

Expected Product

• Terminal Mono-Substituted Alkene:

  • Anticipated outcome from the elimination reaction typically forms a terminal alkene consisting of two carbons bonded to a t-butyl group.

Actual Products

• Two Unexpected Products:

  • Major Product: Tetra-Substituted Alkene:

    • Resulting from the rearrangement of the carbocations, leading to a more stable product.

  • Minor Product: Disubstituted Alkene:

    • Formed when elimination occurs at an alternative beta hydrogen site, resulting in less stable compound.

Reason for Discrepancy

The variance between expected and actual products is attributed to the rearrangement of carbocations, particularly when secondary substrates are involved, which facilitates access to more thermodynamically stable carbocation intermediates.

Carbocation Formation and Rearrangement

Steps of Carbocation Formation

1. Proton Transfer: Proton transfer occurs, generating a protonated alcohol and a molecule of water, enhancing the leaving capacity of the -OH group.

2. Carbocation Intermediate Formation: Water leaves, resulting in a secondary carbocation indicative of the alcohol's structure.

Stability Considerations

• Carbocation Stability Hierarchy: Tertiary carbocations are more stable than secondary due to hyperconjugation and inductive effects, prompting rearrangement:

  • Rearrangement of Secondary Carbocation: A methyl group from the adjacent carbon can migrate to the positively charged secondary carbon, resulting in the more stable tertiary carbocation formation.

Product Formation

Pathways of Elimination Reactions

• Following formation of the stable tertiary carbocation, two pathways of elimination reaction can initiate:

  • Pathway A: Elimination (loss) of a beta hydrogen to yield the major product - the tetra-substituted alkene, characterized by maximum substitution.

  • Pathway B: Elimination of an alternative beta hydrogen leads to a less favorable minor product, which is a disubstituted alkene.

Stability of Alkene Products

• Tetra-Substituted Alkene (Major Product): This product is notably more thermodynamically stable compared to the disubstituted alkene due to the principles governing alkene stability (Zaitsev's rule), favoring more substituted alkenes.

Carbocation Migratory Shifts

Common Types of Shifts

• Methyl Shift (Methanide Shift): Methyl group migration to create a more stable tertiary carbocation from a secondary one.

• Hydride Shift: Migration of a hydrogen atom can also lead to stability enhancement, contributing to forming a tertiary carbocation.

Mechanism of Shifts

In scenarios where both shifts can occur, the migratory shift that leads to the formation of the most stable carbocation will predominate, thus influencing the major product formation.

Clarification on Alcohol Dehydration

Primary Alcohols and E2 Mechanisms

• Although primary alcohols typically do not favor carbocation formation due to steric hindrance, dehydration can still result in rearranged products through E2 mechanisms.

  • In the E2 pathway, the formation of a less substituted alkene may occur initially; however, once protonation happens on the alkene, the formation of a stable tertiary carbocation can induce rearrangement, yielding a more substituted and stable alkene as the ultimately observed product.

Conclusion

This detailed analysis emphasizes the pivotal role of carbocation stability and the potential for rearrangements when forecasting the outcomes of dehydration reactions involving alcohols, regardless of their classifications as primary, secondary, or tertiary. Understanding these fundamentals is essential for predicting reaction pathways and product formation with accuracy.