11.15
Bereiding en reacties van sulfiden
Sulfiden zijn het zwavelanaloog van ethers, net zoals thiolen het zwavelanaloog van alcohol zijn. Net als ethers bestaan sulfiden ook uit twee koolwat…
Recall that in sulfides or thioethers, the central sulfur atom is bonded to two hydrocarbon groups on either side. Symmetrical sulfides consist of identical groups, whereas asymmetrical sulfides have different groups.
Typically, when 2 moles of alkyl halide react with 1 mole of sodium sulfide in an SN2 manner, a symmetrical sulfide is produced.
The same reaction conditions can be applied to 1,4-dichlorobutane and 1,5-dichloropentane to synthesize five and six-membered cyclic sulfides, respectively.
On the other hand, when a thiol reacts with an alkyl halide in the presence of a base, an asymmetrical sulfide is produced.
For instance, the reaction of benzenethiol with methyl iodide and sodium hydroxide yields methyl phenyl sulfide.
Mechanistically, the reaction begins as the base deprotonates the thiol to produce a thiolate ion.
Subsequently, the thiolate ion acts as a nucleophile and attacks the alpha carbon of the alkyl halide, displacing the halide ion and forming the sulfide in an SN2 reaction.
As the reaction involves the alkylation of the thiolate ion, it is referred to as the sulfur analog of Williamson ether synthesis and works well with methyl and primary alkyl halides.
Unlike ethers that are non-oxidizable at the oxygen, sulfides can readily oxidize at sulfur to produce sulfoxides, which further oxidize to sulfones.
Typically, one equivalent of hydrogen peroxide at room temperature oxidizes methyl phenyl sulfide to methyl phenyl sulfoxide, which upon further treatment with a peroxy acid, oxidizes to methyl phenyl sulfone.
However, methyl phenyl sulfide can be directly converted to methyl phenyl sulfone in the presence of 2 equivalents of hydrogen peroxide.
Common examples of sulfoxides and sulfones include dimethyl sulfoxide and tetramethylene sulfone, which are excellent aprotic solvents.
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Q1: What is the difference between symmetrical and asymmetrical sulfides?
Symmetrical sulfides contain identical hydrocarbon groups bonded to the central sulfur atom, while asymmetrical sulfides have different groups attached to sulfur. Symmetrical sulfides form when 2 moles of alkyl halide react with 1 mole of sodium sulfide via SN2 mechanism. Asymmetrical sulfides are produced when a thiol reacts with an alkyl halide in the presence of a base, such as when benzenethiol reacts with methyl iodide and sodium hydroxide to yield methyl phenyl sulfide.
Q2: How does the thiolate ion mechanism work in sulfide synthesis?
The base first deprotonates the thiol to produce a thiolate ion, which acts as a nucleophile. The thiolate ion then attacks the alpha carbon of the alkyl halide, displacing the halide ion and forming the sulfide through an SN2 reaction. This reaction is called the sulfur analog of Williamson ether synthesis and works best with methyl and primary alkyl halides, preferring these substrates over tertiary halides.
Q3: What are the products when sulfides undergo oxidation?
Sulfides readily oxidize at the sulfur atom to produce sulfoxides, which can further oxidize to sulfones. One equivalent of hydrogen peroxide at room temperature converts a sulfide to a sulfoxide. Further treatment with a peroxy acid oxidizes the sulfoxide to a sulfone. Alternatively, two equivalents of hydrogen peroxide directly converts the sulfide to a sulfone in a single step.
Q4: How can cyclic sulfides be synthesized from dihalogenated compounds?
Cyclic sulfides form when dihalogenated compounds react with sodium sulfide under SN2 conditions. For example, 1,4-dichlorobutane produces a five-membered cyclic sulfide, while 1,5-dichloropentane yields a six-membered cyclic sulfide. The two halide groups are displaced sequentially by the sulfide nucleophile, forming the ring structure with sulfur as a ring atom.
Q5: What makes sulfoxides and sulfones useful as solvents?
Sulfoxides and sulfones are excellent dipolar aprotic solvents due to their chemical structure and polarity. Common examples include dimethyl sulfoxide (DMSO) and tetramethylene sulfone. These compounds can dissolve both polar and nonpolar substances effectively, making them valuable in organic synthesis, reactions, and laboratory applications where aprotic solvent properties are required.
Q6: Why do sulfides oxidize more readily than ethers?
Unlike ethers, which are non-oxidizable at the oxygen atom, sulfides can readily oxidize at the sulfur atom. This difference arises from sulfur's larger atomic size, lower electronegativity, and greater ability to expand its valence shell compared to oxygen. These properties make sulfur more susceptible to nucleophilic attack by oxidizing agents like hydrogen peroxide and peroxy acids.
Q7: What reaction conditions favor the formation of asymmetrical versus symmetrical sulfides?
Symmetrical sulfides form when 2 moles of alkyl halide react with 1 mole of sodium sulfide in an SN2 reaction without added base. Asymmetrical sulfides require a thiol starting material, an alkyl halide, and an external base like sodium hydroxide. The choice of starting material and reagents determines the sulfide structure, allowing chemists to selectively prepare either symmetrical or asymmetrical products.