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🥼organic chemistry review

Primary Carbocation

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

A primary carbocation is a positively charged carbon atom that has three single-bonded substituents and one hydrogen atom attached to it. These carbocations are the least stable type of carbocation due to the limited ability to delocalize the positive charge.

5 Must Know Facts For Your Next Test

  1. Primary carbocations are the least stable type of carbocation due to their limited ability to delocalize the positive charge.
  2. The formation of primary carbocations is disfavored in Markovnikov's rule, which states that the hydrogen addition occurs at the carbon atom that can best stabilize the resulting carbocation.
  3. According to the Hammond Postulate, primary carbocations are higher in energy and more reactive than the corresponding transition states, making them less likely to form.
  4. Carbocation rearrangements, such as the Wagner-Meerwein rearrangement, can occur to convert a primary carbocation into a more stable secondary or tertiary carbocation.
  5. In electrophilic additions to conjugated dienes, the formation of allylic carbocations is favored over primary carbocations due to their increased stability.

Review Questions

  • Explain how the stability of primary carbocations relates to Markovnikov's rule in electrophilic addition reactions.
    • Markovnikov's rule states that in electrophilic addition reactions, the hydrogen addition occurs at the carbon atom that can best stabilize the resulting carbocation. Primary carbocations are the least stable type of carbocation due to their limited ability to delocalize the positive charge. Therefore, the formation of primary carbocations is disfavored, and the hydrogen addition will occur at the carbon that can form a more stable secondary or tertiary carbocation, in accordance with Markovnikov's rule.
  • Describe the role of primary carbocations in the context of the Hammond Postulate and its implications for reaction mechanisms.
    • The Hammond Postulate states that the transition state of a reaction resembles the higher-energy reactant or product, depending on the reaction progress. In the case of primary carbocations, they are higher in energy and more reactive than the corresponding transition states, making them less likely to form. This suggests that the formation of primary carbocations is not favored, and the reaction mechanism may proceed through alternative pathways that involve the formation of more stable secondary or tertiary carbocations.
  • Analyze the significance of primary carbocations in the context of carbocation rearrangements and their impact on the mechanism of electrophilic addition reactions.
    • Carbocation rearrangements, such as the Wagner-Meerwein rearrangement, can occur to convert a primary carbocation into a more stable secondary or tertiary carbocation. This is important because the formation of primary carbocations is disfavored, and the reaction mechanism will favor pathways that lead to the formation of more stable carbocations. Additionally, in the context of electrophilic additions to conjugated dienes, the formation of allylic carbocations is favored over primary carbocations due to their increased stability, which can influence the regio- and stereochemistry of the final product.