The molecular structural formula of monoolefin is CnH2n, C2-C4 is a gas at room temperature, it is a non-polar molecule, insoluble or slightly soluble in water. Hydrocarbyl functional groups are functional functional groups in the molecular structure of olefins, which have reaction specificity and can produce addition reactions such as esterification, halogenation, hydration, haloesterification, hypohalogenation, hydrochloride esterification, ring-air oxidation, and convergence. It can also be oxidized to generate cracking of hydrocarbon groups to form aldehydes, carboxyl groups, etc.
It can be prepared by the reaction of halogenated alkyl and sodium hydroxide solution:
RCH2CH2X+NaOH——RHC=CH2+NaX+H2O (X is chlorine, bromine, iodine).
It can also be obtained by the lack of water in alcohol or by the reaction of ortho-dihaloalkane and zinc. Small molecule water olefins mainly come from petroleum cracking gas. Cyclic olefins are abundant in plant essential oils, many of which can be used as spices. Alkenes are key basic raw materials in organic chemistry for the production of polyolefins and styrene-butadiene rubber.
The characteristic reactions of alkenes are produced on the functional group isomerism C=C and α-H.
1. Catalytic reaction hydrocracking reaction
The reaction of olefins with hydrogen to form alkanes is called hydrocracking, also known as hydrogenation.
The activation energy of the hydrocracking reaction is very large, and it is difficult to generate even under the heating standard, but the reaction can be carried out smoothly under the effect of metal catalysts, hence the name catalytic hydrocracking.
In analytical chemistry, hydrocracking is also known as oxidation.
This reaction has the following properties:
①The conversion rate is close to 100%, the substance is very easy to purify, and it is often used to generate a small amount of alkanes in the laboratory; alkenes can be quantitatively analyzed to digest and absorb hydrogen, and this reaction is used to measure the number of hydrocarbon groups in the molecular structure.
②Most of the metal catalysts in the hydrocracking reaction are transition elements, and this metal catalyst powder is often pre-impregnated on activated carbon and aluminum oxide particles; different metal catalysts have different reaction standards, and some superheated steam can Reactions, and some need to be carried out under work pressure. The framework nickel (also known as Raney nickel) with a common porous structure in industrial production is a metal catalyst.
③ The difficulty coefficient of hydrocracking reaction is related to the structure of olefin. In general, olefins with many functional groups on the hydrocarbon oxygen atom are not easy to carry out addition reactions.
④ Under normal circumstances, the hydrocracking reaction material is dominated by cis substances, so it is called taking the opportunity to hydrocracking.
⑤ The effect of metal catalysts is to change the reaction mode and reduce the activation energy of the reaction. It is generally believed that the hydrocracking reaction is that H2 and olefins are additionally adsorbed on the surface of the metal catalyst, and the metal catalyst promotes the cracking of the σ bond of H2 to generate 2 M-Hσ bonds, and then react with the olefins coordinated on the metal surface.
⑥ Hydrocracking has a key application in industrial production. The crude motor gasoline obtained from petroleum processing is usually hydrocracking to remove olefins to obtain hydrocracking motor gasoline and improve the quality of refined oil. For another example, unsaturated fats are often esterified to produce unsalted butter, which increases the value of consumption.
⑦ The hydrocracking reaction is a chemical reaction, and the chemical equilibrium constant is called the enthalpy of esterification, and the enthalpy of esterification of olefins with different structures is different.
2. Add halogen bulb reaction:
Olefins are very easy to react with halogen bulbs and are the key way to prepare ortho-dihaloalkanes:
① This reaction can react rapidly at room temperature, and the laboratory uses it to identify the presence of olefins (the aqueous solution of bromine in carbon tetrachloride is brown-red, and it becomes colorless after the bromine is exhausted).
②Different halogen bulb response specificity regularity:
The reaction of fluorine is fierce and difficult to control; iodine is a reversible reaction, and the balance is biased towards the olefin side; the common halogens are Cl2 and Br2, and the reactivity is Cl2>Br2.
③ The reaction of alkene and bromine is a trans-addition substance, and the substance is a racemate.