Alkene polymerization
Alkenes are capable of combining into giant macromolecules known as polymers. A classic example is the production of polyethylene from ethylene. In terms of its chemical structure, finished polyethylene is already a saturated hydrocarbon. The material's properties depend fundamentally on the reaction conditions:
- Radical polymerization (high pressure). The process proceeds at a pressure of about 1500 atm, initiated by a small amount of oxygen. The result is a white, flexible mass that is transparent in thin films. The molecular weight of the macromolecules ranges from 20,000 to 40,000 Da. Due to the chain termination mechanism, oxygen atoms may be present at the ends of the molecules, but the proportion of end groups is negligible due to the enormous length of the chain itself.
- Catalytic polymerization (low pressure). Carried out in the presence of specific Ziegler–Natta catalysts (a mixture of titanium(IV) chloride and organoaluminum compounds). This polyethylene possesses superior mechanical properties compared to its high-pressure counterpart. The main drawback is that it "ages" faster, meaning it degrades under the influence of light and environmental factors. Industrial production and mass adoption of packaging bags made from this material began around 1955.
The second most important polymer is polypropylene. During its synthesis, varying degrees of chain stereoregularity can form. When Ziegler–Natta catalysts are used, an isotactic structure is produced with a regular, ordered arrangement of methyl groups. Conversely, if polymerization proceeds under high pressure, an atactic structure is formed with a random arrangement of radicals (such polypropylene begins to soften at temperatures as low as 30 °C).
Oxidation reactions
The double bond makes alkenes extremely sensitive to oxidizing agents. The nature of the products depends entirely on the harshness of the conditions and the reaction medium:
- Mild oxidation (mildly alkaline medium). Treatment with an aqueous potassium permanganate solution cleaves only the $\pi$-bond. Hydroxyl groups attach to the carbon atoms, yielding dihydric alcohols (glycols).
- Harsh oxidation (acidic medium + heating). A complete, destructive cleavage of the alkene molecule occurs across the double bond. Depending on the structure of the starting hydrocarbon, the products can be carboxylic acids, ketones, or carbon dioxide.
- Catalytic oxidation with oxygen. This is a specific industrial reaction for ethylene. Passing a mixture of ethylene and oxygen over a catalyst (silver on aluminum oxide) yields a cyclic compound—oxirane (ethylene oxide). This is a crucial feedstock produced annually in millions of tons and used to synthesize detergents and other polymers.
Mechanism of electrophilic addition
Alkenes actively add halogens, hydrogen halides, and water across the double bond. The mechanism of this process can be examined using the classic example of bromination:
- A halogen molecule approaches the carbon atom that forms the unsaturated center.
- Under the influence of the attacking molecule, the $\pi$-bond shifts toward this carbon atom and subsequently toward the bromine. In this case, bromine acts as a typical electrophilic reagent.
- A chemical bond begins to form between the carbon and bromine, accompanied by the simultaneous cleavage of the bond within the halogen molecule.
- In the final stage, the departing bromide ion adds to the molecule, donating its unshared electron pair to the vacant orbital of the second carbon atom, thus completing the process.
Briefly: answers to topical questions
According to which rule does the addition of hydrogen halides to unsymmetrical alkenes occur?
The addition of hydrogen halides to unsymmetrical alkenes proceeds according to Markovnikov's rule.
- Markovnikov's rule states that in electrophilic addition reactions, hydrogen preferentially forms a bond with the most hydrogenated carbon atom (the one with more H atoms).
- Exceptions: In functional derivatives of alkenes containing electron-withdrawing groups, substitution may proceed counter to this rule due to a different redistribution of electron density.
What conditions and products are characteristic of alkene hydration reactions?
The hydration reaction of alkenes leads to the formation of alcohols and requires catalysts and high pressure.
- Reaction conditions: high pressure (e.g., 60–70 atm), high temperature (approx. 300 °C), an excess of water vapor, and the presence of a strong acid catalyst (e.g., phosphoric acid on aluminum oxide).
- Products: alcohols (e.g., the hydration of ethylene yields ethanol, and from but-1-ene yields butanol).
The addition of water to unsymmetrical alkenes proceeds in accordance with Markovnikov's rule.
Which reactions are used for the qualitative detection of the double bond in alkenes?
The reaction with bromine water is used for the qualitative detection of the double bond in alkenes.
- Reagent: bromine water (a yellow-colored halogen solution).
- Result: discoloration (decolorization) of the solution occurs.
Unlike alkenes, saturated hydrocarbons (gaseous alkanes) do not react with bromine water, and the initial color of the solution is preserved.
Mild oxidation yields alcohols, harsh oxidation yields fragments. In a mildly alkaline medium (mild), the bond breaks halfway (forming glycols); in an acidic medium upon heating (harsh), the molecule is cleaved in half down to acids, ketones, or gas.
Class and exam questions
What is the fundamental difference between high- and low-pressure polyethylene?
High-pressure polyethylene is produced via radical polymerization: it is more flexible, but its structure is less ordered. Low-pressure polyethylene is synthesized using Ziegler–Natta catalysts: it is mechanically stronger, but prone to faster aging when exposed to light.
What are isotactic and atactic polymer structures?
An isotactic structure features a strictly ordered arrangement of side groups (formed using catalysts). An atactic structure is characterized by a random arrangement of radicals (formed during high-pressure polymerization) and lower thermal stability.
How does the oxidation of ethylene proceed over a silver catalyst?
Ethylene is oxidized by gaseous oxygen in the presence of silver on aluminum oxide. Complete combustion does not occur; instead, oxirane (ethylene oxide) is formed, a valuable cyclic oxygen-containing compound.
How to calculate the degree of polymerization?
The degree of polymerization is calculated by dividing the molecular weight of the finished polymer by the molecular weight of the monomer. For example, for polyethylene with a mass of 28,000 Da and a monomer mass of 28, the degree is 1000 molecules.
What else to study on this topic
- Equations for the harsh oxidation of propene and butane isomers in an acidic medium
- Calculation of the number of monomer molecules (degree of polymerization) from molecular weight
- Detailed composition and mechanism of action of Ziegler–Natta catalysts
- Thermal decomposition of alkenes into simple substances at high temperatures
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