Optically pure 2-octyl sulfonate, a compound with a leaving group comparable to the tosylate and mesylate groups discussed in this chapter, was treated with varying mixtures of dioxane and water as solvent. The optical purity of the resulting 2-octyl alcohol, a solvolysis product, varied with the ratio of dioxane to water according to the given table. Given that dioxane possesses fairly nucleophilic oxygen atoms (and, therefore, participates DIRECTLY in the mechanism of this reaction), provide a complete mechanism that explains the variation in the solvolysis product’s optical purity due to changes in solvent composition. 2-octyl sulfonate ---------------------------> OH-CH3-CH2CH2CH2CH2CH2CH3 dioxane Solvent Ratio Optical Purity of (water:Dioxane) 2-Octyl alcohol 25:75 77% 50:50 88% 75:25 95% 100:0 100%
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The mechanism of the solvolysis reaction involving the optically pure 2-octyl sulfonate can be explained by considering the nucleophilic substitution of the sulfonate leaving group by the solvent molecule (water or dioxane). Here is a step-by-step mechanism describing the variation in the solvolysis product's optical purity due to changes in solvent composition:
Step 1: Nucleophilic attack by the solvent molecule
In the presence of a nucleophilic solvent like dioxane or water, the solvent molecule can attack the carbon atom adjacent to the sulfonate group, initiating the solvolysis reaction. Let's consider the case where the solvent molecule is dioxane.
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R-CH2-CH2-CH2-CH2-CH2-CH2-SO3 + Dioxane --> R-CH2-CH2-CH2-CH2-CH2-CH2-O-R' + DSO3-
Step 2: Formation of a carbon-oxygen bond
In this step, the nucleophilic oxygen atom of the dioxane molecule attacks the carbon atom bonded to the sulfonate leaving group. As a result, a carbon-oxygen bond is formed, and the sulfonate group leaves as the corresponding sulfonate anion (DSO3-).
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R-CH2-CH2-CH2-CH2-CH2-CH2-SO3 + Dioxane --> R-CH2-CH2-CH2-CH2-CH2-CH2-O-R' + DSO3-
Step 3: Rearrangement of the alkoxide intermediate
After the carbon-oxygen bond formation, the intermediate alkoxide undergoes a rearrangement to increase stability. This rearrangement can occur via a C-C bond migration, forming a more stable tertiary carbocation.
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R-CH2-CH2-CH2-CH2-CH2-CH2-O-R' + DSO3- --> R-CH2-CH2-CH2-CH2-CH2-C(+)H2-O-R' + DSO3-
Step 4: Nucleophilic attack by the solvent molecule (intramolecular)
In this step, the nucleophilic oxygen from the dioxane molecule attacks the positively charged carbon to form a new C-O bond. This intramolecular nucleophilic attack leads to the formation of the 2-octyl alcohol product.
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R-CH2-CH2-CH2-CH2-CH2-C(+)H2-O-R' + DSO3- --> R-CH2-CH2-CH2-CH2-CH2-CH2-O-R' + DSO3-
Step 5: Rearrangement of the alcohol product
Finally, the 2-octyl alcohol product can undergo a rearrangement to achieve the most stable conformation. This rearrangement is responsible for the variation in the solvolysis product's optical purity.
The variation in optical purity depends on the rate of nucleophilic attack, which is influenced by the solvent composition. When the solvent is mostly dioxane (25:75 ratio of water to dioxane), the nucleophilic attack by dioxane is relatively slower compared to the case where the solvent is mostly water (75:25 ratio of water to dioxane). Consequently, a slower nucleophilic attack minimizes the opportunity for rearrangements, resulting in lower optical purity. Conversely, when the solvent is mostly water, the faster nucleophilic attack leads to a higher optical purity due to the predominance of the most stable conformation of the product.
Please note that the above mechanism is a simplified representation and additional considerations, such as stereoselectivity, may also play a role in determining the optical purity of the solvolysis product.
To understand the variation in the solvolysis product's optical purity due to changes in solvent composition, we need to consider the role of the solvent in the reaction mechanism. The given compound, optically pure 2-octyl sulfonate, undergoes solvolysis to form 2-octyl alcohol in the presence of varying mixtures of dioxane and water as solvent.
The solvent mixture of dioxane and water plays a crucial role in the solvolysis reaction. Dioxane possesses nucleophilic oxygen atoms, and it directly participates in the reaction mechanism. The reaction proceeds through an SN2 (substitution nucleophilic bimolecular) mechanism, where the dioxane molecule acts as the nucleophile attacking the carbon atom bearing the leaving group (sulfonate group) to displace it.
Here is a stepwise mechanism that explains the variation in the solvolysis product's optical purity with changes in solvent composition:
Step 1: Nucleophilic Attack
In the first step, a dioxane molecule acts as a nucleophile attacking the carbon atom bonded to the sulfur atom of the sulfonate group. This results in the displacement of the sulfonate group and the formation of a dioxane-sulfonium intermediate.
Step 2: Rearrangement
The dioxane-sulfonium intermediate then undergoes a rearrangement where a hydride shift occurs within the molecule. This rearrangement is critical in determining the stereochemistry of the solvolysis product.
Step 3: Deprotonation
In the third step, the rearranged intermediate is deprotonated by water molecules present in the solvent mixture. This deprotonation leads to the formation of the solvolysis product, 2-octyl alcohol.
The optical purity of the 2-octyl alcohol, which is a chiral molecule, depends on the stereochemistry established during the rearrangement step. The rearrangement can occur in a way that either retains or inverts the stereochemistry at the chiral center. The presence of dioxane and water in varying ratios affects the rate and selectivity of the rearrangement step, thereby impacting the optical purity of the final product.
By analyzing the given table, we observe that as the ratio of dioxane to water increases, the optical purity of the 2-octyl alcohol also increases. This suggests that a higher proportion of dioxane in the solvent mixture promotes a more selective and stereochemically controlled rearrangement step, resulting in a higher optical purity of the product.
It is important to note that the exact mechanism and microscopic details of the rearrangement step may depend on the specific structure and reactivity of the 2-octyl sulfonate compound used in this reaction. Additionally, further experimental evidence and analysis might be required for a more detailed understanding of this solvolysis reaction and the effects of the solvent mixture on the stereochemistry of the product.