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🥼organic chemistry review

key term - $eta$-hydrogen

Citation:

Definition

$eta$-hydrogen refers to the hydrogen atom that is located on the carbon atom adjacent to the reaction center in organic chemistry. This term is particularly relevant in the context of understanding the reactivity patterns associated with SN1, SN2, E1, E1cB, and E2 reactions.

5 Must Know Facts For Your Next Test

  1. The presence or absence of $eta$-hydrogen can significantly influence the mechanism and outcome of organic reactions, particularly SN1, SN2, E1, E1cB, and E2 reactions.
  2. In SN1 reactions, the presence of $eta$-hydrogen can stabilize the carbocation intermediate, leading to a more favorable reaction pathway.
  3. In SN2 reactions, the presence of $eta$-hydrogen can hinder the approach of the nucleophile due to steric effects, making the reaction less favorable.
  4. In E1 reactions, the presence of $eta$-hydrogen can facilitate the formation of a carbocation intermediate, which can then undergo elimination to form an alkene product.
  5. In E1cB reactions, the presence of $eta$-hydrogen allows for the formation of an enolate intermediate, which can then eliminate to form an alkene.

Review Questions

  • Explain how the presence of $eta$-hydrogen can influence the mechanism of an SN1 reaction.
    • In an SN1 reaction, the presence of $eta$-hydrogen can stabilize the carbocation intermediate that forms during the first step of the mechanism. This stabilization occurs through hyperconjugation, where the $eta$-hydrogen can donate electron density to the partially positive charge on the carbocation. The more stable the carbocation, the more favorable the SN1 pathway becomes, as the rate-determining step involves the formation of this intermediate.
  • Describe the role of $eta$-hydrogen in the context of an E2 reaction.
    • In an E2 reaction, the presence of $eta$-hydrogen is essential for the elimination to occur. The $eta$-hydrogen is removed in a concerted step, where the nucleophile attacks the substrate and simultaneously removes the $eta$-hydrogen, forming the alkene product. The orientation of the $eta$-hydrogen relative to the leaving group and the nucleophile is crucial, as it must be in the correct position for the E2 mechanism to proceed.
  • Analyze the role of $eta$-hydrogen in the E1cB mechanism and how it differs from the E1 mechanism.
    • In the E1cB (Elimination, Conjugate Base) mechanism, the presence of $eta$-hydrogen allows for the formation of an enolate intermediate, which is a stabilized carbanion. This enolate intermediate is then able to eliminate, forming the alkene product. In contrast, the E1 mechanism involves the formation of a carbocation intermediate, which does not require the presence of $eta$-hydrogen. The E1cB mechanism is favored when the substrate has $eta$-hydrogen available and when a strong base is present to facilitate the initial deprotonation step.