Depending on the solvent, ROH reacts with SOCl2 to give RCl by two pathways, each of which invol

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Depending on the solvent, ROH reacts with SOCl2 to give RCl...

Depending on the solvent, $\mathrm{ROH}$ reacts with $\mathrm{SOCl}_2$ to give $\mathrm{RCl}$ by two pathways, each of which involves formation of a chlorosulfite ester, <smiles>O=[Se](=O)Cl</smiles> along with $\mathrm{HCl}$. Use the following stereochemical results to suggest mechanisms for the two pathways: In pyridine, a $3^{\circ}$ amine base, $$ \text { (R)- } \mathrm{CH}_3 \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_2 \mathrm{CH}_3 \longrightarrow(S)-\mathrm{CH}_3 \mathrm{CHClCH}_2 \mathrm{CH}_3 $$ and, in ether, the same $(R)-\mathrm{ROH} \longrightarrow(R)-\mathrm{RCl}$. Pyridine (Py) reacts with the initially formed $\mathrm{HCl}$ to give $\mathrm{PyH}^{+} \mathrm{Cl}^{-}$and the free nucleophilic $\mathrm{Cl}^{-}$attacks the chiral $\mathrm{C}$ with inversion, displacing the $\mathrm{OSOCl}^{-}$as $\mathrm{SO}_2$ and $\mathrm{Cl}^{-}$. With no change in priority, inversion gives the $(S)$ $\mathrm{RCl}$. Ether is too weakly basic to cause enough dissociation of $\mathrm{HCl}$. In the absence of $\mathrm{Cl}^{-}$, the $\mathrm{Cl}$ of the $-\mathrm{OSOCl}$ attacks the chiral $\mathrm{C}$ from the side to which the group is attached. This internal nucleophilic substitution $\left(\mathbf{S}_{\mathrm{N}} \mathbf{i}\right)$ reaction proceeds through an ion-pair and leads to retention of configuration.

Depending on the solvent, $\mathrm{ROH}$ reacts with $\mathrm{SOCl}_2$ to give $\mathrm{RCl}$ by two pathways, each of which involves formation of a chlorosulfite ester,
<smiles>O=[Se](=O)Cl</smiles>
along with $\mathrm{HCl}$. Use the following stereochemical results to suggest mechanisms for the two pathways: In pyridine, a $3^{\circ}$ amine base,
$$
\text { (R)- } \mathrm{CH}_3 \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_2 \mathrm{CH}_3 \longrightarrow(S)-\mathrm{CH}_3 \mathrm{CHClCH}_2 \mathrm{CH}_3
$$
and, in ether, the same $(R)-\mathrm{ROH} \longrightarrow(R)-\mathrm{RCl}$.
Pyridine (Py) reacts with the initially formed $\mathrm{HCl}$ to give $\mathrm{PyH}^{+} \mathrm{Cl}^{-}$and the free nucleophilic $\mathrm{Cl}^{-}$attacks the chiral $\mathrm{C}$ with inversion, displacing the $\mathrm{OSOCl}^{-}$as $\mathrm{SO}_2$ and $\mathrm{Cl}^{-}$. With no change in priority, inversion gives the $(S)$ $\mathrm{RCl}$. Ether is too weakly basic to cause enough dissociation of $\mathrm{HCl}$. In the absence of $\mathrm{Cl}^{-}$, the $\mathrm{Cl}$ of the $-\mathrm{OSOCl}$ attacks the chiral $\mathrm{C}$ from the side to which the group is attached. This internal nucleophilic substitution $\left(\mathbf{S}_{\mathrm{N}} \mathbf{i}\right)$ reaction proceeds through an ion-pair and leads to retention of configuration.

Key Concepts

Solvent and Base Effects on Reaction Pathways

Solvent and base effects play a crucial role in determining the reaction mechanism. In a reactive medium with a strong or basic solvent, acid by-products may be sequestered (e.g., by forming ion pairs), which can promote external nucleophilic substitution (S_N2). In contrast, a less basic or non-reactive solvent minimizes such interactions, potentially favoring intramolecular pathways like S_Ni. The solvent’s nature, therefore, can shift the reaction mechanism and influence the stereochemical outcome.

S_Ni Reaction Mechanism

The S_Ni reaction mechanism, or internal nucleophilic substitution, involves the nucleophilic part of a molecule attacking the electrophilic center from within the same molecular entity. This internal attack often occurs through a cyclic or ion-pair intermediate and results in retention of the configuration at the chiral center, providing an alternative pathway to external nucleophilic substitutions.

Reaction Intermediates in Organic Synthesis

Formation of key intermediates, such as chlorosulfite esters in this context, is essential for facilitating nucleophilic substitution reactions. These intermediates are transient species that enable further reaction steps—either by being attacked by an external nucleophile via S_N2 or undergoing an internal attack leading to S_Ni. Understanding intermediate species helps in predicting both the mechanism and the final stereochemistry of the reaction.

Stereochemical Outcome in Nucleophilic Substitution

This concept involves how the spatial arrangement of atoms at a chiral center changes during a substitution reaction. Mechanisms such as backside attack in an S_N2 reaction lead to inversion of configuration, while internal nucleophilic substitution (S_Ni) can result in retention of configuration. The stereochemical outcome is a key diagnostic tool to infer which reaction mechanism is operative in a given chemical process.

S_N2 Reaction Mechanism

The S_N2 reaction mechanism is a bimolecular nucleophilic substitution process where the nucleophile attacks the electrophilic carbon from the opposite side of the leaving group, leading to inversion of stereochemistry. This concerted single-step mechanism is highly sensitive to the strength of the nucleophile and the steric environment around the substrate.

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