Question

Ethylene oxide,is an important industrial chemical. Although most ethers are unreactive, ethylene oxide is quite reactive. It resembles \(\mathrm{C}_{2} \mathrm{H}_{4}\) in its reactions in that addition reactions occur across the C-O bond in ethylene oxide. a. Why is ethylene oxide so reactive? (Hint: Consider the bond angles in ethylene oxide as compared with those predicted by the VSEPR model.) b. Ethylene oxide undergoes addition polymerization, forming a polymer used in many applications requiring a nonionic surfactant. Draw the structure of this polymer.

Short answer

a. Ethylene oxide is more reactive than most ethers because of its three-membered ring, which creates considerable ring strain by forcing bond angles between C-O-C atoms to be smaller than the typical 109.5° predicted by the VSEPR model. This strain makes the molecule more susceptible to addition reactions. b. The structure of the ethylene oxide polymer, poly(ethylene oxide) or PEO, is represented as: \[ - \underset{n}{\underbrace{[-\mathrm{C}_{2}\mathrm{H}_{4}-\mathrm{O}-]}} - \] Where "n" represents the number of repeating ethylene oxide units in the polymer chain.

Step-by-step solution

a. Reactivity of Ethylene Oxide

To understand the reason behind the higher reactivity of ethylene oxide, we will examine its structure using the VSEPR model. Ethylene oxide is an epoxide, which has a three-membered ring consisting of two carbons and one oxygen atom. In this molecule, the oxygen atom forms single bonds with the two adjacent carbon atoms. According to the VSEPR model, a molecule with two bonding pairs should have bond angles of approximately 109.5°. However, in ethylene oxide, due to the small size of the ring, the bond angles between C-O-C atoms are forced to be smaller, close to 60°. This results in considerable ring strain, which makes the molecule more reactive. It also leads to addition reactions similar to ethylene (\(\mathrm{C}_{2}\mathrm{H}_{4}\)), with reactions occurring across the C-O bond.

b. Structure of Ethylene Oxide Polymer

Ethylene oxide undergoes addition polymerization, during which the three-membered ring in each molecule opens up and links to other ethylene oxide molecules, forming a long polymer chain. The structure of the ethylene oxide polymer (poly(ethylene oxide) or PEO) can be represented as follows: \[ - \underset{n}{\underbrace{[-\mathrm{C}_{2}\mathrm{H}_{4}-\mathrm{O}-]}} - \] In this structure, "n" represents the number of repeating ethylene oxide units in the polymer chain, which can vary. So, the structure of the ethylene oxide polymer is a chain of connected ethylene oxide molecules with the C-O bond opened, creating a long polymer suitable for various applications requiring nonionic surfactants.

Key concepts

VSEPR Model

The VSEPR (Valence Shell Electron Pair Repulsion) model is a fundamental theory in chemistry that helps predict the geometry of molecules. It is based on the idea that electron pairs arrange themselves around a central atom to minimize repulsion, resulting in specific bond angles. For a molecule with two bonding pairs, like ethylene oxide, the model suggests a bond angle of approximately 109.5°, characteristic of a tetrahedral geometry.
However, ethylene oxide doesn't follow this ideal angle because of its unique structure. It is a three-membered ring compound, which forces the bond angles to much smaller values, close to 60°. This deviation from the ideal angle creates a significant amount of strain within the molecule, known as ring strain. This is because the atoms are forced into uncomfortable positions, and the electron clouds are closer together than they would typically be.
Understanding this concept through the VSEPR model helps explain why ethylene oxide is so reactive compared to other larger cyclic ethers.

Reactivity of Epoxides

Epoxides, like ethylene oxide, are known for their heightened reactivity compared to other types of ethers. The primary reason for this lies in their strained three-membered ring, which creates a situation of high energy and instability. The ring strain arises from the less than ideal bond angles and torsional strain due to eclipsed interactions.
The strain makes the molecular structure eager to react in order to relieve this energy. When ethylene oxide undergoes reactions, it often involves opening up the ring structure. These reactions typically occur through addition reactions, where other chemical entities attack the strained ring, focusing on the easily breakable C-O bond.
This remarkable reactivity is harnessed in numerous chemical processes and applications, making epoxides useful in the production of alcohols, glycols, and as intermediates in organic synthesis.

Addition Polymerization

Addition polymerization is a chain reaction that results in the formation of polymers. In this process, monomers add to each other without the loss of any small molecules, as is the case with condensation polymerization. For ethylene oxide, addition polymerization occurs as the ring structure opens and links with other ethylene oxide molecules.
This is facilitated by the reactive C-O bond in the ethylene oxide. As a polymer forms, the opened epoxide units connect through their ends to form long repeating chains. These chains are known as poly(ethylene oxide) (PEO) and have extensive applications due to their unique properties.
Polymer chemists can influence the length and properties of the polymer chain by controlling the reaction conditions, such as temperature and the presence of catalysts.

Polyethylene Oxide

Polyethylene oxide (PEO) is a polymer that forms through the polymerization of ethylene oxide. It consists of repeating units of ethylene and oxygen, forming a long chain structure. PEO is notable for its solubility in water and various organic solvents, making it highly versatile.
Due to its nonionic nature and ability to modify surface interactions, PEO is widely utilized in the production of nonionic surfactants. It finds applications in products ranging from pharmaceuticals to cosmetics and detergents.
Moreover, PEO is used as a thickening agent and in the formation of hydrogels, which are substances capable of holding significant amounts of water, making it useful in medical applications like wound dressings and contact lenses.

Ring Strain

Ring strain is a concept in chemistry that describes the instability in cyclic compounds that happens due to deviations from ideal bond angles. In a stable molecule, atoms are arranged in such a way that the angles between bonds minimize repulsion and strain. However, in a strained ring such as the three-membered ring in ethylene oxide, the bond angles are forced to be much smaller, close to 60°.
This geometric constraint causes increased energy compared to more relaxed structures or larger rings. The strain manifests itself in terms of torsional strain, which occurs due to eclipsed bond interactions, and angle strain, due to compressed bond angles.
Ring strain is not just a point of academic interest; it has practical implications. This built-up tension makes molecules like ethylene oxide highly reactive, as they undergo chemical reactions more readily to release the strain, often breaking open the ring as a way to achieve a more stable configuration.