Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond. The simplest acyclic alkenes, with only one double bond and no other functional groups, form an homologous series of hydrocarbons with the general formula CnH2n.
The simplest alkene is ethylene (C2H4). Alkenes are also called olefins (an archaic synonym, widely used in the petrochemical industry). Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes. 
Physical properties
The physical properties of alkenes are comparable with those of alkanes. The physical state depends on molecular mass (gases from ethene to butene - liquids from pentene onwards). 
Chemical properties
Alkenes are relatively stable compounds, but are more reactive than alkanes due to the presence of a carbon-carbon double bond (pi-bond). It is also attributed to the presence of pi-electrons in the molecule. The majority of the reactions of alkenes involve the rupture of this double bond (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.
Addition reactions
Alkenes react in many addition reactions, which occur by opening up the double-bond. Most addition reactions to alkenes follow the mechanism of electrophilic addition

(a)	Hydrogenation
Hydrogenation of alkenes produces the corresponding alkanes. The reaction is carried out under pressure at a temperature of 200°c in the presence of a metallic catalyst. Common industrial catalysts are based on platinum, nickel or palladium. For laboratory syntheses, Raney nickel (an alloy of nickel and aluminium) is often employed. The simplest example of this reaction is the catalytic hydrogenation of ethylene to yield ethane:
CH2=CH2 + H2 → CH3-CH3

(b)	Hydration reaction
Hydration reaction is a chemical reaction in which a hydroxyl group (OH-) and a hydrogen cation (an acidic proton) are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous solution. Hydration differs from hydrolysis in that hydrolysis cleaves the non-water component in two. Hydration leaves the non-water component intact.
The general chemical equation of the reaction is the following:
RRC=CH2 in H2O/acid → RRC(-OH)-CH3
In the first step, the acidic proton bonds to the less substituted carbon of the double bond following Markovnikov's rule. In the second step an H2O molecule bonds to the other, more highly substituted carbon. The oxygen atom at this point has three bonds and carries a positive charge. Another water molecule comes along and takes up the extra proton.
(c)	Halogenation
In electrophilic halogenation the addition of elemental bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes, 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 is the addition of hydrohalic acids such as HCl or HBr to alkenes to yield the corresponding haloalkanes.
CH3-CH=CH2 + HBr → CH3-CHBr-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 (Markovnikov's rule).

(d)	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. 
•	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
Cis-Trans notation
In the specific case of disubstituted alkenes where the two carbons have one substituent each, Cis-trans notation may be used. If both substituents are on the same side of the bond, it is defined as (cis-). If the substituents are on either side of the bond, it is defined as (trans-).

 E,Z notation
When an alkene has more than one substituent (especially necessary with 3 or 4 substituents), the double bond geometry is described using the labels E and Z. These labels come from the German words "entgegen," meaning "opposite," and "zusammen," meaning "together." Alkenes with the higher priority groups (as determined by CIP rules) on the same side of the double bond have these groups together and are designated Z. Alkenes with the higher priority groups on opposite sides are designated E. A mnemonic to remember this: Z notation has the higher priority groups on "ze zame zide."