Olefins are hydrocarbons whose molecule has one double bond. Sometimes the compounds presented are called ethylene hydrocarbons. A double bond in a molecule is easily identified by spectral analysis.
Physical properties of alkenes
The first three representatives of the homologous series of alkenes are gases. Fifth to sixteenth are liquids, high molecular weight alkenes are solids. With an increase in the length of the hydrocarbon chain, the boiling point of the substances also increases. Under laboratory conditions, these substances can be obtained in several ways: dehydrogenation of alkenes, cracking of oil, dehydrogenation of alcohols.
Oil cracking is an industrial method for producing unsaturated hydrocarbons. As a result of thermal cracking, oil products are heated to 750 degrees, the hydrocarbon skeleton of alkanes is broken:
3062 → 1530 + 1532
Dehydrogenation of alkanes . This process is carried out at a temperature of about 600 degrees. Under these conditions, H atoms are split off from the saturated hydrocarbon molecule with the formation of alkenes, for example: 512 → 510 + 2.
Dehydration of alcohols . Monohydric alcohols interacting with sulfate acid form alkenes, for example: 49 → 48 + 2.
The chemical properties of alkenes are associated with the presence of a double bond in their molecule. The electron density between C atoms bound by a double bond is higher than between C atoms bound by simple bonds. The main type of reaction that alkenes enter is the addition, accompanied by a rupture of the pi bond to form two new sigma bonds. Olefins enter into the polymerization reaction, which is also accompanied by the breaking of double bonds.
Chemical properties of alkenes: addition of hydrogen halides
Olefins easily attach hydrogen halide molecules to themselves, thus forming monohalogen derivatives (for example, 1020 + 1 → 10211). When 2 is attached to alkenes, monoatomic saturated alcohols are formed (1020 + 2 → 1021).
Alken Burning
At high temperatures, under the influence of O2, olefins are easily oxidized (burned) to H20 and CO2. Alkenes, interacting with potassium permanganate, form diatomic alcohols (glycols).
Chemical properties of alkenes: polymerization of olefins
Presented hydrocarbons are more prone to polymerization reactions. Typically, a polymer is a long chain with repeating structural units (monomers). This process can be initiated in several ways, but in most cases it is chain and refers to free radical, cationic or anionic processes.
Chemical Properties of Alkenes: Cationic Polymerization
This type of reaction is catalyzed by acids. The proton is attached to the olefin, thus forming a carbcation. Then the attack of the latter on the system of another alkene molecule follows and a longer chain carbation is formed, which attacks the next olefin molecule, etc. As a result of the emission of a proton or some other process that "quenches" the charge of the carbocation, an open circuit occurs. During anionic polymerization, the starting anion is formed as a result of a nucleophilic attack of an olefin by anion X-. When a proton is attached, the chain breaks. It should be noted that a large part of unsaturated hydrocarbons is polymerized by the free radical mechanism. This process can be initiated by H202, organic peroxides, hydroperoxides, etc. Chain termination occurs due to recombination of radicals. In some cases, a copolymerization reaction is used to produce alkenyl polymers. As a result of the copolymerization, a polymer is obtained containing various units alternating in a macromolecule in a specific way. The reaction produces a copolymer product. The physicomechanical properties of the compounds presented are determined primarily by the order of alternation of elementary units, as well as by the type and number of monomers included in the polymer chain.