Question

(a) Methyl chloromethyl sulfone reacts with strong alkali to yield ethylene and sodium sulfite. Write a mechanism for this reaction which is in accord with the reagents and the products. (b) Butadiene when heated with sulfur dioxide undergoes 1,4 -addition with formation of an unsaturated cyclic sulfone. How might this sulfone (in conjunction with the reaction which gives an alkene from an a-halosulfone) be used in a synthesis of cyclobutadiene?

Short answer

The mechanism for the reaction of methyl chloromethyl sulfone with a strong alkali involves a nucleophilic attack of the hydroxide ion on the sulfur atom, breaking the S-Cl bond, followed by an intramolecular S_N2 reaction forming a new C-S bond and releasing a chloride ion. Finally, the C-C bond breaks, forming ethylene and sodium sulfite as products. For the synthesis of cyclobutadiene, butadiene reacts with sulfur dioxide to form an unsaturated cyclic sulfone, which is then converted into an α-chlorosulfone intermediate. This compound reacts with a strong base, leading to a series of eliminations that release alkene molecules and ultimately yield cyclobutadiene.

Step-by-step solution

Identify the given molecules and products

We're given methyl chloromethyl sulfone, which has the formula ClCH_2SO_2CH_3. The strong alkali can be represented as OH^-. We need to find a mechanism that leads to the formation of ethylene (C_2H_4) and sodium sulfite (Na_2SO_3).

Write the mechanism for this reaction

The mechanism for this reaction can be described as follows: 1. The nucleophilic hydroxide ion (OH^-) attacks the sulfur atom of the ClCH_2SO_2CH_3 molecule, breaking the S-Cl bond. 2. A new bond is formed between the sulfur atom and the oxygen atom of the hydroxide ion, leading to an intermediate with a negative charge on the sulfur atom. 3. The intermediate molecule undergoes an intramolecular S_N2 reaction, in which the negatively charged sulfur atom attacks the methylene carbon (CH_2) adjacent to the sulfur, creating a new C-S bond and expelling a chloride ion. 4. The resulting intermediate further breaks the C-C bond on the other end of the molecule, and ethylene (C_2H_4) is released. What remains is the ion SO_3^2-, which can form sodium sulfite by binding with two sodium ions (Na^+). The final products are ethylene (C_2H_4) and sodium sulfite (Na_2SO_3). #Part (b)#

Understand the formation of the unsaturated cyclic sulfone

When butadiene (C_4H_6) is heated with sulfur dioxide (SO_2), it undergoes a 1,4-addition reaction to form an unsaturated cyclic sulfone. The reaction mechanism involves nucleophilic addition of sulfur dioxide to the conjugated double bonds, creating a cyclic sulfone intermediate with a double bond still present within the ring.

Reacting the unsaturated cyclic sulfone with the α-halosulfone reaction

To use the unsaturated cyclic sulfone in the synthesis of cyclobutadiene, we need to perform the α-halosulfone reaction, which converts α-halosulfone into an alkene. Since the unsaturated cyclic sulfone already contains an alkene, the reaction should go through the following steps: 1. Add a halogen atom (e.g., chlorine) adjacent to the sulfur atom in the sulfone group. This will form an α-chlorosulfone intermediate. 2. The α-chlorosulfone intermediate reacts with a strong base (e.g., an alkoxide, RO^-) in a manner similar to the reaction described in part (a). The base attacks the sulfur atom, breaking the S-Cl bond and forming a new S-O bond. This causes the ring to open and releases a molecule of alkene. 3. The elimination process is repeated once more, releasing another molecule of alkene as the product. 4. The final product obtained after these multiple elimination steps will be the desired compound, cyclobutadiene (C_4H_4).

Key concepts

Nucleophilic Substitution

In organic chemistry, nucleophilic substitution is a fundamental reaction mechanism where a nucleophile donates an electron pair to an electrophile, replacing a leaving group. The nucleophile is usually negatively charged or neutral with a lone pair of electrons. This kind of substitution is essential in the transformation of molecules in synthesis.
Consider methyl chloromethyl sulfone (ClCH₂SO₂CH₃) reacting with a strong base like hydroxide ion (OH⁻). Here, OH⁻ acts as the nucleophile that attacks the sulfur atom, taking the place of the weaker leaving group, which is chlorine. This reaction involves breaking the S-Cl bond and helps form valuable products like ethylene and sodium sulfite in later steps.

• Key players: nucleophile (OH⁻) and leaving group (Cl)
• Example: Both intramolecular and intermolecular substitutions can occur.

Understanding nucleophilic substitution is crucial for predicting and explaining a wide range of reactions in organic synthesis.

SN2 Reaction

An SN2 reaction is a type of nucleophilic substitution that occurs in a single concerted step. The term "bimolecular" in SN2 indicates that the reaction rate is dependent on the concentration of both the substrate and the nucleophile. In the case of methyl chloromethyl sulfone, an internal SN2 reaction is key.
The hydroxide ion initiates the reaction by attacking the sulfur, leading to the first step of substitution. This ion then facilitates the breaking of bonds and forms an intermediate. The sulfur atom, now negatively charged, engages in a "backside attack" on a methylene carbon to forge a new bond while displacing the chloride ion.
This straightforward concerted mechanism is distinctive because:

• Reaction proceeds with inversion of configuration
• Transition state exhibits simultaneous bond-making and bond-breaking

A comprehensive grasp of SN2 processes helps students predict outcomes in synthesis routes.

Alkene Formation

Alkene formation is a critical component in organic reaction mechanisms, particularly when aiming to synthesize certain compounds. In the reaction of methyl chloromethyl sulfone, the ultimate goal is to produce ethylene (C₂H₄).
Formation of the alkene involves the cleavage of the C-C bond within the intermediate molecule. As the remaining molecule rearranges, ethylene is successfully released. The entire process showcases how strategic bond breaking and forming can aid in the synthesis of desired alkenes.
Important aspects of alkene formation include:

• Double bond creation between carbon atoms
• Significance in polymerization and various chemical syntheses
• Serving as a building block for more complex molecules

Recognizing how alkenes are formed opens doors to understanding broader applications in organic reactions.

Cyclic Sulfone

Cyclic sulfones are an intriguing class of organic compounds, notable for their stability and utility in synthesis. When butadiene reacts with sulfur dioxide (SO₂), it undergoes a unique transformation to yield a cyclic sulfone, with a double bond remaining within the ring.
This process exemplifies how conjugated dienes, like butadiene, can capture and incorporate small molecules like SO₂ into a stable ring structure. The resultant cyclic sulfone retains a degree of unsaturation, evidenced by the double bond.
Features of cyclic sulfone formation include:

• 1,4-addition of SO₂ across conjugated double bonds
• Integral part of synthetic strategies
• Possibility of further transformations for fine-tuning properties

These characteristics make cyclic sulfones versatile intermediates in synthetic chemistry.

1,4-Addition Reaction

The 1,4-addition reaction, also known as conjugate addition, is a pivotal reaction in organic chemistry involving an electrophile being added across the 1,4-positions of a conjugated diene system. This type of addition is favored due to the ability of conjugated systems to stabilize charges more effectively.
In the example with butadiene and sulfur dioxide, the reaction leads to the creation of a cyclic sulfone. Sulfur dioxide adds across the diene, causing a distribution of electrons that allows ring closure while leaving an unsaturated bond in place.
This reaction is characterized by:

• Addition across conjugated systems like dienes
• Importance in forming rings or other polymeric structures
• Potential to introduce functional groups strategically in synthesis

The 1,4-addition is indispensable for chemists aiming to exploit the chemistry of conjugated systems for diverse applications.