alkene

Alkenes
In organic chemistry, an alkene is an unsaturated hydrocarbon that contains at least one carbon–carbon double bond . Molecules with multiple double bonds are called polyunsaturated The words alkene and olefin are often used interchangeably .
Acyclic alkenes, with only one double bond and no other functional groups, known as mono-enes, form a homologous series of hydrocarbons with the general formula( CnH2n ).
Alkenes have two hydrogen atoms fewer than the corresponding alkane (with the same number of carbon atoms).
The simplest alkene, ethylene (C2H4), with the International Union of Pure and Applied Chemistry (IUPAC) name ethane, is the organic compound produced on the largest scale industrially.
Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes.

IUPAC Rules for Alkene and Cycloalkene Nomenclature
1. The ene suffix (ending) indicates an alkene or cycloalkene.
2. The longest chain chosen for the root name must include both carbon atoms of the double bond.
3. The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
4. The smaller of the two numbers designating the carbon atoms of the double bond is used as the double bond locator. If more than one double bond is present the compound is named as a diene, triene or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.
5. In cycloalkenes the double bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.
Geometric Isomers
Double bonds can exist as geometric isomers and these isomers are designated by using either the cis / trans designation or the modern E / Z designation.
cis Isomers
The two largest groups are on the same side of the double bond.

trans Isomers
The two largest groups are on opposite sides of the double bond.

examples :
1-ethenylcyclohexene
1-methylcyclobutene OR 1-methylcyclobut-1-ene.





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Alkenes properties
1.Ethene, Propene, and Butene exists as colorless gases. Members of the 5 or more carbons such as Pentene, Hexene, and Heptene are liquid, and members of the 15 carbons or more are solids.
2.Alkenes are lighter than water and are insoluble in water due to their non-polar characteristics. Alkenes are only soluble in nonpolar solvents.
3.Alkenes are virtually insoluble in water, but dissolve in organic solvents.
4.The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperature. All the rest that you are likely to come across are liquids. Boiling points of alkenes depends on more molecular mass (chain length). The more intermolecular mass is added, the higher the boiling point. Intermolecular forces of alkenes gets stronger with increase in the size of the molecules. In each case, the alkene has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains.
5.Melting points of alkenes depends on the packaging of the molecules. Alkenes have similar melting points to that of alkanes, however, in cis isomers molecules are package in a U-bending shape, therefore, will display a lower melting points to that of the trans isomers.
6.Chemical structure and fuctional groups can affect the polarity of alkenes compounds. The two individual dipoles together form a net molecular dipole. In trans-subsituted alkenes, the dipole cancel each other out. In cis-subsituted alkenes there is a net dipole, therefore contributing to higher boiling in cis-isomers than trans-isomers.
Reactions
Alkenes are relatively stable compounds, but are more reactive than alkanes, either because of the reactivity of the carbon–carbon pi-bond or the presence of allylic CH centers. Most reactions of alkenes involve additions to this pi bond, forming new single bonds. Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions, prominently polymerization and alkylation
Addition reactions
Alkenes react in many addition reactions, which occur by opening up the double-bond. Most of these addition reactions follow the mechanism of electrophilic addition. Examples are hydrohalogenation, halogenation, halohydrin formation, oxymercuration, hydroboration, dichlorocarbene addition, Simmons–Smith reaction, catalytic hydrogenation, epoxidation, radical polymerization and hydroxylation.




*Hydrogenation
Hydrogenation of alkenes produces the corresponding alkanes in presence of a metallic catalyst platinum, nickel or palladium.
CH2=CH2 + H2 Ni? CH3–CH3
* Hydration
Hydration, the addition of water across the double bond of alkenes, yields alcohols. The reaction is catalyzed by strong acids such as sulfuric acid. This reaction is carried out on an industrial scale to produce ethanol.
CH2=CH2 + H2O H2SO4? CH3–CH2OH

*Halogenation
In electrophilic halogenation the addition of elemental bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes (1,2 dihalides or ethylene dihalides), respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:
CH2=CH2 + Br2 ? BrCH2–CH2Br
*Hydrohalogenation
Hydrohalogenation is the addition of hydrogen halides such as HCl or HI to alkenes to yield the corresponding haloalkanes:
CH3–CH=CH2 + HI ? CH3–CHI-CH2–H
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. This patterns is known as Markovnikov s rule. The use of radical initiators or other compounds can lead to the opposite product result. Hydrobromic acid in particular is prone to forming radicals in the presence of various impurities or even atmospheric oxygen, leading to the reversal of the Markovnikov result
CH3–CH=CH2 + HBr ? CH3–CHH–CH2–Br
*Halohydrin formation
Alkenes react with water and halogens to form halohydrins by an addition reaction. Markovnikov regiochemistry and anti stereochemistry occur.
CH2=CH2 + X2 + H2O ? XCH2–CH2OH + HX
*Oxidation
Alkenes are oxidized with a large number of oxidizing agents. In the presence of oxygen, alkenes burn with a bright flame to produce carbon dioxide and water. Catalytic oxidation with oxygen or the reaction with percarboxylic acids yields epoxides. Reaction with ozone in ozonolysis leads to the breaking of the double bond, yielding two aldehydes or ketones. Reaction with concentrated, hot KMnO4 (or other oxidizing salts) in an acidic solution will yield ketones or carboxylic acids.
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.
Synthesis of alkenes
Industrial methods
Alkenes are produced by hydrocarbon cracking. Raw materials are mostly natural gas condensate components (principally ethane and propane) Alkanes are broken apart at high temperatures, often in the presence of a zeolite catalyst, to produce a mixture of primarily aliphatic alkenes and lower molecular weight alkanes. This is mainly used for the manufacture of small alkenes (up to six carbons).




Elimination Reactions
Alkenes are commonly made by
• Elimination of HX from alkyl halide (dehydrohalogenation), Uses heat and KOH.


• Elimination of H-OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 ?C).
CH3CH2OH + H2SO4 ? H2C=CH2 + H3O+ + HSO4
From alkynes
Reduction of alkynes is a useful method for the stereoselective synthesis of disubstituted alkenes. If the cis-alkene is desired, hydrogenation in the presence of Lindlar s catalyst (a heterogeneous catalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead) is commonly used Reduction of the alkyne by sodium metal in liquid ammonia gives the trans-alkene.