Question

(racem?c) Using your roadmaps as a guide, show how to convert ethylene into 2 -methyl1,3 -dioxolane. You must use ethylene as the source of all carbon atoms in the target molecule. Show all reagents and all molecules synthesized along the way.

Short answer

Question: Outline the steps to convert ethylene into 2-methyl-1,3-dioxolane using appropriate reagents. Answer: The synthesis of 2-methyl-1,3-dioxolane from ethylene can be achieved through the following steps: 1. Convert ethylene into an epoxide (ethylene oxide) using peroxybenzoic acid (PBA). 2. Open the epoxide ring using methylmagnesium bromide (MeMgBr) to form an alcohol with three carbon atoms. 3. Form a hemiacetal species by protonating the alcohol group using hydrochloric acid (HCl). 4. Treat the hemiacetal with acetic anhydride to form an acetoxonium ion intermediate. 5. Carry out an intramolecular nucleophilic substitution reaction to yield the target molecule, 2-methyl-1,3-dioxolane.

Step-by-step solution

Formation of Epoxide from Ethylene

First, ethylene (\(C_2H_4\)) can be converted into an epoxide, also known as ethylene oxide (\(C_2H_4O\)). This can be done by reacting ethylene with peroxybenzoic acid (PBA). The reaction forms an epoxide, which is required for the eventual synthesis of the target molecule.

Epoxide Ring Opening

The epoxide (\(C_2H_4O\)) can be opened using a suitable nucleophile to form a more functionalized carbon skeleton. In this case, we will use methylmagnesium bromide (MeMgBr) as the nucleophile. This Grignard reagent will attack the less hindered carbon of the epoxide and form a new carbon-carbon bond. The product of this step will be an alcohol with three carbon atoms and one alcohol group: \(\ce{CH3-CHOH-CH2-MgBr}\).

Formation of Hemiacetal

The next step is to form a hemiacetal species. To do this, add an acid, such as hydrochloric acid (HCl), which protonates the alcohol group (\(\ce{-OH}\)) in the product from Step 2, making it a good leaving group. Upon leaving, the chloride ion attacks the adjacent carbon atom from the less hindered side, forming a cyclic hemiacetal species.

Conversion to Acetoxonium Ion

To incorporate the second oxygen atom of the target molecule, treat the hemiacetal from Step 3 with acetic anhydride (\(\ce{(CH3CO)2O}\)). This results in an acetoxonium ion intermediate.

Intramolecular Nucleophilic Substitution

Now, an intramolecular nucleophilic substitution reaction can occur. The oxygen atom of the acetoxonium group attacks the adjacent carbon, releasing a molecule of acetic acid. This yields the target molecule, 2-methyl-1,3-dioxolane.

Key concepts

Ethylene Transformation

Transforming ethylene into more complex organic structures is a foundational concept in organic chemistry, often involving multiple reaction steps. Ethylene, or ethene, is a simple hydrocarbon with the formula \( C_2H_4 \) featuring a double bond between carbon atoms. This double bond is highly reactive and can be used to create a range of different compounds.

One common transformation is the synthesis of epoxides, such as ethylene oxide. The process typically involves the use of an oxidizing agent, like peroxybenzoic acid, which reacts with ethylene to form the three-membered epoxide ring. This reaction is an example of an electrophilic addition where the oxygen from the peroxide adds across the ethylene's double bond, leading to the formation of a new ring structure, setting the stage for further chemical modifications that can be achieved through subsequent reactions.

Epoxide Ring Opening

Epoxide ring opening is a crucial step in many synthetic pathways that allows for the expansion of an organic molecule by inserting new atoms into the three-membered epoxide ring. It involves the attack of a nucleophile on one of the carbon atoms within the ring, which results in the breakage of an already strained C-O bond due to the ring's high angle strain.

In the case of opening an ethylene oxide ring, a Grignard reagent, such as methylmagnesium bromide (MeMgBr), acts as the nucleophile. The Grignard reagent first attacks the less hindered carbon atom in the epoxide, effectively 'opening' the ring structure. This opens the gateway for further chemical transformations as it introduces a new carbon chain to the molecule while also preserving functional groups that offer additional reactive sites.

Grignard Reagent Reactions

Grignard reagents, named after French chemist Victor Grignard, are organomagnesium compounds typically represented as R-Mg-X where R is an organic group and X is a halogen. These reagents are highly nucleophilic, meaning they are great at attacking electrophilic carbon atoms, especially those within polar bonds like C-O.

Reactions with Grignard reagents are pivotal in forming new carbon-carbon bonds in organic synthesis. When a Grignard reagent reacts with an epoxide, such as ethylene oxide, it breaks open the ring and attaches the R group from the reagent to the molecule. This is followed by a protonation step, often with a mild acid, to yield the corresponding alcohol. The versatility of Grignard reagents opens up a plethora of synthetic possibilities to chemists manipulating organic molecules.

Hemiacetal Formation

The formation of hemiacetals is an essential step in synthesizing various cyclic organic molecules, like 2-methyl-1,3-dioxolane. Hemiacetals result from the reaction between an aldehyde or ketone and an alcohol under acidic conditions.

In a reaction sequence that includes hemiacetal formation, an alcohol group (\(-OH\)) within a molecule reacts with a neighboring carbonyl carbon atom, forming a cyclic structure. Acidic conditions, typically provided by an acid like HCl, facilitate this process by protonating the \(-OH\) group, making it a better leaving group. This increased electrophilicity allows the adjacent carbon to be attacked easily, leading to the formation of a ring structure known as a hemiacetal.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental class of reactions in organic chemistry involving the replacement of an atom, or group of atoms, in a molecule by a nucleophile. This process is critical for creating a diverse array of chemical structures and for making modifications to existing molecules.

There are two main types of nucleophilic substitution reactions: SN1 and SN2. The type of reaction depends on factors like the structure of the organic molecule, the nucleophile, and the leaving group. In the synthesis of 2-methyl-1,3-dioxolane, an intramolecular nucleophilic substitution plays a central role where a nucleophile within the molecule attacks an electrophilic center, leading to the formation of a new bond and the expulsion of a leaving group. This type of reaction is essential in closing rings and adding new functionalities to the organic framework.