Question

Epichlorotydrin (Section 11.10) is a valuable synthetic intermediate because each of its three carbons contains a reactive group. Following is the first step in its synthesis from propene. Propose a mechanism for this step.

Short answer

Answer: The proposed mechanism for the synthesis of Epichlorotydrin from propene involves two steps: 1. Electrophilic addition of a molecule of hypochlorous acid (H-O-Cl) to propene, forming 3-chloro-2-propanol: CH2=CH-CH3 + H-O-Cl → CH2(Cl)-CH(OH)-CH3 2. Intramolecular cyclization of 3-chloro-2-propanol to form Epichlorotydrin: CH2(Cl)-CH(OH)-CH3 → CH2(CH2)CH-Cl (with an epoxide ring between the first two carbons)

Step-by-step solution

Identify the reactants and products

For this reaction, we start with propene, which has the structure CH2=CH-CH3. We're looking for a mechanism that will transform propene to Epichlorotydrin, which contains three carbons with a reactive group at each.

Determine the reactive groups present in Epichlorotydrin

Examine the properties of Epichlorotydrin and identify possible reactive groups like an epoxide group, a chlorine atom, or a nucleophilic attack position. Understand that Epichlorotydrin is an epoxide with a chlorine atom bonded to one of the carbons. The structure will be: CH2(CH2)CH-Cl (with an epoxide ring between the first two carbons).

Propose a mechanism for the synthesis

Now that we know the structure of Epichlorotydrin, we can propose a mechanism for the formation of this compound from propene. Such a mechanism should typically involve the addition of reagents in multiple steps. A possible mechanism for the synthesis of Epichlorotydrin from propene is: 1. Electrophilic addition of a molecule of hypochlorous acid (H-O-Cl) to propene, forming 3-chloro-2-propanol: CH2=CH-CH3 + H-O-Cl → CH2(Cl)-CH(OH)-CH3 2. Intramolecular cyclization of 3-chloro-2-propanol to form the Epichlorotydrin: CH2(Cl)-CH(OH)-CH3 → CH2(CH2)CH-Cl (with an epoxide ring between the first two carbons) In conclusion, the mechanism proposed includes an electrophilic addition of hypochlorous acid to propene, followed by an intramolecular cyclization to form the Epichlorotydrin molecule, which contains three reactive carbons.

Key concepts

Organic Chemistry and Epichlorohydrin Synthesis

Organic chemistry is a vast field that involves the study of carbon-containing compounds. An important aspect of organic chemistry is the synthesis of valuable intermediates, such as Epichlorohydrin, from simple organic molecules like propene. Epichlorohydrin is a synthetic intermediate notable for its three reactive centers, which include an epoxide ring and a chloro substituent. This versatility makes it an essential building block in the production of various materials, including epoxy resins, pharmaceuticals, and water treatment chemicals.

The synthesis of Epichlorohydrin from propene involves a series of organic reactions that transform the simple alkene into a more complex molecule. Not only does this process provide a practical application of organic synthetic techniques, it also exemplifies how understanding organic reactivity can lead to the creation of compounds with desired functional groups.

Reaction Mechanism Fundamentals

The reaction mechanism is a detailed step-by-step description of how reactants are converted into products. It includes the sequence of bond formation and bond breaking, the intermediates, transition states, and the type of reaction that occurs during each step. In the case of Epichlorohydrin synthesis, the reaction mechanism includes an electrophilic addition reaction followed by intramolecular cyclization. Understanding each step of the mechanism allows chemists to predict the outcome of reactions and design strategies to synthesize complex molecules.

The proposed mechanism for converting propene to Epichlorohydrin starts with the attack of hypochlorous acid on propene. The intermediate product, 3-chloro-2-propanol, then undergoes a cyclization reaction to form Epichlorohydrin. Each step requires careful consideration of the movement of electrons and the stability of intermediates, which is critical for the success of the synthesis.

Electrophilic Addition in Synthesis

Electrophilic addition is a common reaction in organic chemistry where an electron-rich alkene (or alkyne) reacts with an electron-deficient molecule, often referred to as the electrophile. This type of addition is key to the synthesis of many organic compounds because alkenes are accessible and reactive starting materials. In the synthesis of Epichlorohydrin, electrophilic addition is initiated by the reaction between the pi bond of propene and hypochlorous acid (HOCl).

In this reaction, the alkene acts as a nucleophile, or electron donor, and the HOCl acts as an electrophile. The electrophilic chlorine atom is attracted to the electron-rich double bond, resulting in the formation of 3-chloro-2-propanol. This reaction is regioselective, meaning that the chlorine atom adds to the more substituted carbon, a concept known as Markovnikov's rule. This selectivity is important for the subsequent cyclization step that leads to the desired Epichlorohydrin.