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In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond. The simplest alkenes, with only one double bond and no other functional groups, form a homologous series of hydrocarbons with the general formula CnH2n. The simplest alkene is ethylene (C2H4), which has the International Union of Pure and Applied Chemistry (IUPAC) name ethene. Alkenes are also called olefins (an archaic synonym, widely used in the petrochemical industry) or vinyl compounds.
Shape of alkenes As predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes includes bond angles about each carbon in a double bond of about 120°. The angle may vary because of steric strain introduced by nonbonded interactions created by functional groups attached to the carbons of the double bond. For example, the C-C-C bond angle in propylene is 123.9°. The alkene double bond is stronger than a single covalent bond and also shorter with an average bond length of 133 picometres. Molecular geometry
Physical properties The physical properties of alkenes are comparable with alkanes. The physical state depends on molecular mass. The simplest alkenes, ethylene, propylene and butylene are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids. Chemical properties Alkenes are relatively stable compounds, but are more reactive than alkanes. This is compatible with the idea that the carbon-carbon double bond in alkenes is stronger than the carbon-carbon single bond in alkanes, however, as the majority of the reactions of alkenes involve the rupture of this bond to form two new single bonds. Synthesis CH3CH2OH + H2SO4 → CH3CH2OSO3H + H2O → H2C=CH2 + H2SO4 + H2O Other alcohol eliminations are the Chugaev elimination and the Grieco elimination in which the alcohol group is converted to a short-lived intermediate first. Reactions Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions. Addition reactions Alkenes react in many addition reactions. CH2=CH2 + H2 → CH3-CH3 CH2=CH2 + Br2 → BrCH2-CH2Br It is also used as a quantitive test of unsaturation, expressed as the bromine number of a single compound or mixture. This is the mechanism for the reaction: The reaction works because the high electron density at the double bond causes a temporary shift of electrons in the Br-Br bond causing a temporary induced dipole. This makes the Br closest to the double bond slightly positive and therefore an electrophile. CH3-CH=CH2 + HBr → CH3-CHBr-CH3 If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents (Markovnikov's rule). This is the reaction mechanism for hydrohalogenation: Oxidation Alkenes are oxidized with a large number of oxidizing agents. R1-CH=CH-R2 + O3 → R1-CHO + R2-CHO + H2O This reaction can be used to determine the position of a double bond in an unknown alkene. Polymerization Polymerization of alkenes is an economically important reaction which yields polymers of high industrial value, such as the plastics polyethylene and polypropylene. Polymerization can either proceed via a free-radical or an ionic mechanism. IUPAC Names To form the root of the IUPAC names for alkenes, simply change the -an- infix of the parent to -en-. For example, CH3-CH3 is the alkane ethANe. The name of CH2=CH2 is therefore ethENe. In higher alkenes, where isomers exist that differ in location of the double bond, the following numbering system is used: Common Names Despite the precision and universal acceptance of the IUPAC naming system, some alkenes are known almost exclusively by their common names: See also | ||||||||||
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