When CH3 CH2 S̈ CH2 CH2 S̈ CH2 CH3 reacts with two equivalents of CH3 I, the following double su

step 1 Which of the following compounds can exist as pairs of cis trans isomeres? Draw each cis -trans

When CH3 CH2 S̈ CH2 CH2 S̈ CH2 CH3 reacts with two...

When $\mathrm{CH}_{3} \mathrm{CH}_{2} \ddot{\mathrm{S}} \mathrm{CH}_{2} \mathrm{CH}_{2} \ddot{\mathrm{S}} \mathrm{CH}_{2} \mathrm{CH}_{3}$ reacts with two equivalents of $\mathrm{CH}_{3} \mathrm{I},$ the following double sulfonium salt precipitates: (a) Give a curved-arrow mechanism for the formation of this salt. (b) Upon closer examination, this compound is found to be a mixture of two isomers with melting points of $123-124^{\circ} \mathrm{C}$ and $154^{\circ} \mathrm{C}$, respectively. Explain why two compounds of this structure are formed. What is the relationship between these isomers? (Hint: See Sec. $6.9 \mathrm{~B} .$ )

When $\mathrm{CH}_{3} \mathrm{CH}_{2} \ddot{\mathrm{S}} \mathrm{CH}_{2} \mathrm{CH}_{2} \ddot{\mathrm{S}} \mathrm{CH}_{2} \mathrm{CH}_{3}$ reacts with two equivalents of $\mathrm{CH}_{3} \mathrm{I},$ the following double sulfonium salt precipitates:
(a) Give a curved-arrow mechanism for the formation of this salt.
(b) Upon closer examination, this compound is found to be a mixture of two isomers with melting points of $123-124^{\circ} \mathrm{C}$ and $154^{\circ} \mathrm{C}$, respectively. Explain why two compounds of this structure are formed. What is the relationship between these isomers? (Hint: See Sec. $6.9 \mathrm{~B} .$ )

Key Concepts

Diastereomerism and Stereoisomer Relationships

Due to the generation of stereogenic centers at sulfur, the double sulfonium salt can exist as stereoisomers. These isomers are related as diastereomers, meaning they are not mirror images of each other. Depending on the relative configuration (e.g., both centers having the same configuration versus opposite configurations), the salt may exhibit differences in physical properties such as melting point. One of the isomers could be meso (possessing an internal plane of symmetry) while the other exists as a pair of enantiomers, explaining the observed distinct melting points.

Chirality at Sulfur

Although sulfur in simple sulfides is not chiral, when it becomes tetralkylated (or trialkylated in sulfonium salts) it can become a stereogenic center if the three substituents bound to it are different. In the given reaction, after alkylation each sulfonium center is attached to three different groups, rendering the sulfur atoms stereogenic. This leads to the possibility of different stereochemical configurations at each sulfonium center even in an overall symmetric molecule.

Nucleophilic Substitution Mechanism

This is a fundamental organic reaction mechanism where an electron?rich center (nucleophile) attacks an electron?poor center (electrophile) leading to the displacement of a leaving group. In the context of the reaction described, the lone pairs on the sulfurs in the starting dithioether attack the electrophilic carbon atoms of methyl iodide in an SN2-like pathway, with the curved-arrow formalism illustrating the simultaneous bond formation to carbon and bond cleavage of the carbon–iodine bond.

Formation of Sulfonium Salts

Sulfonium salts are formed when a sulfur atom, initially in a neutral sulfide, is alkylated to become a positively charged species with three alkyl substituents. This transformation is significant in organic synthesis because the resulting sulfonium ion is both synthetically versatile and exhibits unique reactivity. In this reaction, methylation of the sulfide centers by methyl iodide produces a double sulfonium salt, which precipitates under the reaction conditions.

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