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Chapter 27: Problem 8

Discuss the structure of intermediates forms in \(\mathrm{S}_{\mathrm{N}^{1}, \mathrm{~S}_{\mathrm{N}} 2}\) and \(\mathrm{S}_{\mathrm{N}^{1}} \mathrm{CB}\) mechanism.

Short Answer

Expert verified
In the SN1 mechanism, the intermediate is a planar carbocation with sp2 hybridization. While in the SN2 mechanism, there is no intermediate as the mechanism is a concerted process. And in the SN1CB mechanism, the intermediate is a carbanion with sp3 hybridization carrying a negative charge.

Step by step solution

01

Nucleophilic Substitution SN1 Mechanism

In the SN1 mechanism, the formation of the intermediate is a two-step process. The first step is rate-determining involving the separation of leaving group from Carbon to form a Carbocation (C+). Step 1: The substrate molecule dissociates to form a carbocation and a leaving group. \[R-LG \rightarrow R^{+} + LG^{-}\]Step 2: The nucleophile attacks the carbocation.\[R^{+} + Nuc^{-} \rightarrow R-Nuc\]The carbocation intermediate in SN1 mechanism is a planar structure with sp2 hybridization.
02

Nucleophilic Substitution SN2 Mechanism

The SN2 mechanism follows a single-step mechanism, where the nucleophile displaces the leaving group in a simultaneous fashion, thus no intermediate structure is formed. Therefore, there are no intermediates for SN2 mechanism as it is a concerted process. \[R-LG + Nuc^{-} \rightarrow R-Nuc + LG^{-}\]
03

Nucleophilic Substitution SN1CB Mechanism

SN1CB stands for 'First-Order Nucleophilic Substitution with Conjugate Base'. It as a two-step mechanism. The first step is removing a proton from the molecule to form a conjugate base. Step 1: Proton abstraction - removal of a proton by the base to form a carbanion intermediate.\[R-H + B -> R^{-} + HB\]Step 2: The carbanion intermediate then captures the electrophile.\[R^{-} + E^{+} -> R-E\]The intermediate in an SN1CB mechanism is a carbanion which is a sp3 hybridization structure holding a negative charge.

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