The chemical properties of alkenes (olefins)

Olefins are hydrocarbons, the molecule of which has one double bond. Sometimes these compounds are called ethylene hydrocarbons. The double bond in a molecule is easily identified by means of spectral analysis.

Physical properties of alkenes

The first three representatives of the homologous series of alkenes are gases. From the fifth to the sixteenth - liquids, high-molecular alkenes - solids. As the length of the hydrocarbon chain increases, so does the boiling point of the substances. Under laboratory conditions, these substances can be obtained in several ways: dehydrogenation of alkenes, cracking of oil, dehydrogenation of alcohols.

Cracking oil is an industrial method for obtaining unsaturated hydrocarbons. As a result of thermal cracking, oil products are heated to 750 degrees, the hydrocarbon skeleton of alkanes is broken:

C30H62 → C15H30 + C15H32

Dehydrogenation of alkanes . This process is carried out at a temperature of about 600 degrees. Under these conditions, the H atoms are split off from the saturated hydrocarbon molecule to form alkenes, for example: C5H12 → C5H10 + H2.

Dehydration of alcohols . Monohydric alcohols, interacting with sulfate acid form alkenes, for example: C4H9OH → C4H8 + H2O.

The chemical properties of alkenes are due to the presence of a double bond in their molecule. The electron density between C-atoms bonded by a double bond is higher than that between C atoms bound by simple bonds. The main type of reaction, into which alkenes enter is the addition, accompanied by the rupture of the p-bond with the formation of two new sigma bonds. Olefins enter a polymerization reaction, which is also accompanied by the breaking of double bonds.

Chemical properties of alkenes: addition of halogenated hydrogen

Olefins easily attach molecules of halogenated hydrogen, forming monohalogen derivatives (for example, C10H20 + HC1 → C10H21Cl). When H2O is added to the alkenes, monohydric saturated alcohols are formed (C10H20 + H2O → C10H21OH).

Burning of alkenes

At high temperatures, under the influence of O2, the olefins readily oxidize (combust) to H20 and CO2. Alkenes, interacting with manganese, form dihydric alcohols (glycols).

Chemical properties of alkenes: polymerization of olefins

The present hydrocarbons exhibit a great propensity for a polymerization reaction. Typically, a polymer is a long chain with repeating units (monomers). This process can be initiated in several ways, but in most cases it is a chain process 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 joins the olefin, forming a carbocation. Then follows the attack of the latter on the py-sieve of another molecule of alkene and a longer-chain carbocation forms, which attacks the next olefin molecule, etc. As a result of the release of a proton or some other process, which "extinguishes" the charge of carbocation, the chain is broken. In anionic polymerization, the initiating anion is formed as a result of the nucleophilic attack of the olefin by the anion X-. When the proton is attached, the chain terminates. It should be noted that most of the unsaturated hydrocarbons are polymerized by the free-radical mechanism. This process can be initiated by H202, organic peroxides, hydroperoxides, etc. The chain is broken due to the recombination of radicals. In some cases, a copolymerization reaction is used to prepare the alkene polymers. Copolymerization results in the preparation of a polymer containing various units alternating in a macromolecule in a specific manner. The reaction produces a product copolymer. The physical and mechanical properties of the compounds are determined primarily by the order of alternation of the elementary units, and also by the type and amount of monomers included in the polymer chain.