Let me introduce the properties of olefins in person!

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This post was last edited by jordan569 on 2013-1-6 at 22:31. Everyone is quite familiar with methanol, but less so with olefins; let me introduce olefins to you! Alkenes are hydrocarbons that contain a C=C bond (carbon-carbon double bond) (alkene bond). They belong to unsaturated hydrocarbons and are divided into alkenes and cycloalkenes. They are named monoenes, dienes, etc., according to the number of double bonds they contain. One of the double bonds is weak and easy to break, so an addition reaction occurs. 　　The general formula for chain monoolefins is CnH2n; at room temperature, those with 2–4 carbon atoms are gases. They are non-polar molecules and are insoluble or slightly soluble in water. Double bond groups are functional groups in alkene molecules that are reactive; they can undergo addition reactions such as hydrogenation, halogenation, hydration, halohydrination, hypohalogenation, sulfation, epoxidation, and polymerization. They can also be oxidized, resulting in the breakage of the double bond and the formation of aldehydes, carboxylic acids, and other compounds. 　　It can be prepared by the reaction of an alkyl halide with sodium hydroxide: RCH2CH2X + NaOH —— RHC=CH2 + NaX + H2O (where X is chlorine, bromine, or iodine). It can also be obtained by the dehydration of alcohols or through the reaction of vicinal dialkyl halides with zinc. Small molecular olefins mainly come from petroleum cracking gas. Cycloolefins are abundant in plant essential oils, and many of them can be used as fragrances. Alkenes are important basic raw materials in organic synthesis, used to produce polyolefins and synthetic rubbers. 　　Physical properties: C1–C4 alkenes are gases ； C5～C18 are liquids ； Solids above C19. In normal alkenes, the boiling point increases as the relative molecular mass increases. Normal alkenes with the same carbon count have higher boiling points than branched alkenes. For olefins with the same carbon skeleton, as the double bond moves from the end of the chain toward the middle, both the boiling point and the melting point increase. 　　Trans olefins have lower boiling points than cis olefins, but higher melting points, due to their lower polarity and greater symmetry. Compared to the corresponding alkanes, the boiling point, refractive index, solubility in water, and relative density of alkenes are slightly higher than those of the alkanes. 　　Chemical properties The characteristic reactions of alkenes occur at the functional groups C=C and α-H. 　　⒈Catalytic hydrogenation reaction: CH2=CH2 + H2 → CH3—CH3. The reaction in which an alkene reacts with hydrogen to form an alkane is called a hydrogenation reaction, or hydrogenization reaction. 　　The activation energy for the hydrogenation reaction is very high, making it difficult to occur even under heating conditions; however, the reaction can proceed smoothly with the help of a catalyst, which is why it is called catalytic hydrogenation. 　　In organic chemistry, hydrogenation reactions are also known as reduction reactions. 　　This reaction has the following characteristics: ①. The conversion rate is close to 100%, and the product can be easily purified; (it is commonly used in the laboratory to synthesize small amounts of alkanes) ； Olefins can absorb hydrogen quantitatively; this reaction is used to determine the number of double bonds in a molecule). 　　②Most catalysts for hydrogenation reactions are transition metals, and these catalyst powders are often impregnated onto activated carbon and alumina particles ； Different catalysts require different reaction conditions; some can react at atmospheric pressure, while others need to be carried out under pressure. Porous skeletal nickel (also known as Raney nickel) is commonly used as a catalyst in industry. 　　③The ease of hydrogenation reaction is related to the structure of the olefin. Under normal circumstances, alkenes with multiple substituents on the double-bonded carbon atoms do not readily undergo addition reactions. 　　④Under normal circumstances, the products of hydrogenation reactions are mainly cis products; hence it is called cis-hydrogenation. 　　⑤The role of a catalyst is to alter the reaction pathway and reduce the reaction activation energy. It is generally believed that in the hydrogenation reaction, H2 and olefins are simultaneously adsorbed on the catalyst surface; the catalyst facilitates the breaking of the σ bond in H2, resulting in the formation of two M-H σ bonds, which then react with the olefins coordinated to the metal surface. 　　⑥Hydrogenation reactions have important applications in industry. Crude gasoline obtained from petroleum refining is commonly treated with hydrogenation to remove olefins, yielding hydrogenated gasoline and improving the quality of the fuel. For example, unsaturated fatty acid esters are often hydrogenated to produce margarine, thereby improving its edible value. 　　⑦The hydrogenation reaction is an exothermic reaction, and the heat released during this reaction is known as the hydrogenation enthalpy; the hydrogenation enthalpies of alkenes with different structures vary. 　　⒉Reaction with halogens: Olefins readily react with halogens, and this is the main method used to prepare vicinal dihalides: CH2=CH2 + X2 → CH2X–CH2X. ①. This reaction occurs rapidly at room temperature, and it is used in the laboratory to detect the presence of olefins (a bromine in carbon tetrachloride solution is red-brown; it becomes colorless once the bromine is consumed). 　　②. Different patterns of reactivity among halogens: Fluorine reacts violently and is difficult to control ； The iodination reaction is reversible, with the equilibrium leaning toward the alkene side ； The commonly used halogens are Cl2 and Br2, with Cl2 having greater reactivity than Br2. 　　③The reaction of olefins with bromine yields trans-addition products, which are racemic. 　　III. Reaction with Protonic Acids Alkenes can undergo addition reactions with protonic acids: CH2=CH2 + HX → CH3CH2X Characteristics: 1. Rule of addition for asymmetric alkenes When the alkene is asymmetric (the two carbon atoms of the double bond are substituted asymmetrically), the proton from the acid attaches mainly to the carbon atom with more hydrogen atoms, while the negative ion attaches to the carbon atom with fewer hydrogen atoms. This is known as Markovnikov’s rule, or the rule of addition for asymmetric alkenes. The greater the asymmetry of the olefin, the more pronounced the rule of asymmetric addition becomes. 　　2. The structure of alkenes affects addition reactions. Reactivity in alkene addition reactions: (CH3)2C=CH2 > CH3CH=CH2 > CH2=CH2. 3. The influence of proton acidity: The stronger the acidity, the faster the addition reaction; reactivity of hydrogen halides in addition reactions with alkenes: HI > HBr > HCl. If the acid is a weak acid such as H2O or ROH, a strong acid is required as a catalyst. 　　The addition of olefins to sulfuric acid yields bisulfates, which upon hydrolysis give alcohols; this is an indirect method for synthesizing alcohols:
CH3CH=CH2 + H2SO4 → H3CCHCH3 ----(H2O)---- CH3CHCH3 + H2SO4
│ │
OSO3H OH

IV. Reaction with hypohalic acids
Olefins react with aqueous solutions of halogens to produce β-haloalcohols:
CH2=CH2 + HOX → CH3CH2OX

Halogens, protonic acids, and hypohalic acids are all electrophilic reagents, and the addition reaction of olefins is an electrophilic addition reaction. The reaction can proceed because the electrons in the alkene’s π bond are mobile and, under the influence of the environment (reagents), shift to one of the carbon atoms of the double bond. In the case of asymmetric alkenes such as propylene, the electron-donating nature of the alkyl groups causes an uneven distribution of electrons in the π bond; the carbon atom closest to the methyl group carries a slight positive charge, while the carbon atom farther away from the methyl group has a slight negative charge. In the presence of an external electric field, this separation of positive and negative charges is further intensified, allowing electrophilic reagents to readily undergo electrophilic addition to the alkene. 　　Carbon atoms in saturated hydrocarbons cannot bond directly with other atoms or groups; only substitution reactions can occur. The carbon atoms in unsaturated hydrocarbons can directly bond with other atoms or atomic groups, undergoing addition reactions. This post was last edited by firefox1981 on 2009-4-15 09:38. Note: $ # , $ $