Thiols can be prepared from the reaction of thiourea with an alkyl halide, followed by hydroxide-ion-promoted hydrolysis.
a. Propose a mechanism for the reaction.

Verified step by step guidance

1. Step 1: Recognize the reaction components. Thiourea (NH2-CS-NH2) reacts with an alkyl halide (R-X) in the first step. The alkyl halide is an electrophile, and thiourea acts as a nucleophile due to the lone pair on the sulfur atom.
2. Step 2: Describe the nucleophilic substitution reaction. The sulfur atom in thiourea attacks the carbon atom of the alkyl halide (R-X), displacing the halide ion (X⁻) and forming an intermediate called an alkylisothiouronium salt (R-NH-CS-NH2). This is an SN2 reaction mechanism.
3. Step 3: Introduce the hydrolysis step. The alkylisothiouronium salt undergoes hydroxide-ion-promoted hydrolysis. The hydroxide ion (OH⁻) attacks the carbon atom of the C=S bond in the intermediate, leading to the cleavage of the C=S bond.
4. Step 4: Explain the formation of the thiol. The hydrolysis results in the release of the thiol (R-SH) and urea (NH2-CO-NH2) as byproducts. This step involves breaking the C=S bond and replacing it with a C-SH bond.
5. Step 5: Summarize the overall mechanism. The reaction proceeds via two main steps: (1) nucleophilic substitution of the alkyl halide by thiourea to form the alkylisothiouronium salt, and (2) hydroxide-ion-promoted hydrolysis of the intermediate to yield the thiol and urea.

Key Concepts

Thiourea Structure and Reactivity

Thiourea is a compound containing a sulfur atom bonded to a carbon atom and two amine groups. Its structure allows it to act as a nucleophile in reactions with alkyl halides, where the sulfur atom can attack the electrophilic carbon of the halide, leading to the formation of a thiol. Understanding the reactivity of thiourea is crucial for proposing the mechanism of thiol synthesis.

Nucleophilic Substitution Mechanism

Nucleophilic substitution is a fundamental reaction mechanism in organic chemistry where a nucleophile replaces a leaving group in a substrate. In the case of thiourea reacting with an alkyl halide, the sulfur atom acts as the nucleophile, attacking the carbon atom bonded to the halide. This process can follow either an SN1 or SN2 pathway, depending on the structure of the alkyl halide and reaction conditions.

Hydroxide-Ion-Promoted Hydrolysis

Hydroxide-ion-promoted hydrolysis involves the reaction of a compound with water in the presence of hydroxide ions, which can facilitate the breaking of bonds and the formation of new ones. In the context of thiol synthesis, after the nucleophilic substitution, hydroxide ions can help hydrolyze any intermediates formed, ultimately leading to the production of the desired thiol. This step is essential for completing the reaction mechanism.