Alkenes (also known as olefins) are unsaturated, acyclic hydrocarbons containing a double bond between two carbon atoms having the general molecular formula CnH2n. The double bond between the two carbon atoms consists of a σ bond and a π bond. If we replace n in the general formula of alkenes, CnH2n with whole and successive values ​​starting from 2, the homologous series of alkenes is obtained (n=2,3,4...).

The simplest alkene and the first representative of this class of hydrocarbons is ethene or ethylene (C2H4), produced annually in huge quantities at the industrial level.

Structural formulas

H2C=CH2 ethene (ethylene)
H2C=CH-CH3 propene (propylene)

The equivalent unsaturation (N, N.E., samd) is 1. This denotes the presence of a double bond in the molecule.


Alkenes are named by replacing the suffix "-an" (from the name of the alkane with the same number of carbon atoms) with the suffix "-ene". Starting from the third term of the homologous series of alkenes, the name specifies the "position of the double bond". The simplest alkene is ethene C2H4, which can also be written structurally: H2C=CH2.

There are also alkenes with several double bonds, the so-called polyenes.

Their general formula is CnH2n+2-2d, where "d" represents the number of double bonds. Polyenes are named using the degree of multiplicity of the double bond. For example, a polyene with two double bonds is called a diene (d=2), with three double bonds a triene, etc.

Denumirea alchenelor cu catenă ramificată se face după regulile IUPAC, stabilite pentru alcani , la care se adaugă următoarele:

1. The base chain must contain the double bond, even if there is another chain with more carbon atoms but without the double bond.

2. The base chain is numbered so that a carbon atom in the double bond receives the lowest number.

The naming of radicals derived from alkenes is obtained using the appropriate suffix, according to the rules established for alkanes; there are also radicals that have common names, for example:

CH2=CH-ethenyl (vinyl)
CH2=CH-CH2-propenyl (allyl)

Reactions of alkenes

Addition reactions

The addition reaction results in the breaking of the π bond between the carbon atoms,

Addition of hydrogen (hydrogenation)

Alkenes add molecular hydrogen in the presence of catalysts (transitional metals: Ni, Pd, Pt), finely divided at temperatures between 80-180 degrees Celsius and pressures of up to 200 atmospheres. The alkane is obtained, which has the same chain as the alkene. R-CH=CH-R' + H-H → R-CH2-CH2-R' The hydrogenation reaction of alkenes takes place in a heterogeneous system because under the working conditions hydrogen is in a gaseous state, alkenes can be gases or in solution form, the reaction products (alkanes) are in a fluid state and the catalyst is solid.

Adiția halogenilor (halogenarea)

By adding halogens (X2 = Cl2, Br2, I2) to alkenes, dihalogenated compounds are obtained, in which the two halogen atoms are linked to two neighboring carbon atoms (vicinal dihalogenated derivatives). R-CH=CH-R' + X-X → R-CH-CH-R'

There are two properties that determine the strength with which van der Waals interactions are manifested:

Derivat halogenat vicinal

It is easiest to add chlorine, then bromine. The addition of chlorine or bromine is immediate and quantitative. The discoloration of a reddish-brown solution of bromine in carbon tetrachloride serves for the recognition and dosage of alkenes. The name "olefins", which was given to alkenes, is due to their property to transform by halogenation into saturated compounds with an oily appearance.

Addition of hydracids

Hydrazides, HX(HCl, HBr, HI) add to alkenes forming saturated monohalogenated derivatives. Hydroiodic acid is the easiest to add, then hydrobromic acid and the hardest is hydrochloric acid. In symmetrical alkenes: R-CH=CH=R + H-X → R-CH-CH-R

Derivat halogenat vicinal

The addition of water to alkenes takes place in the presence of concentrated sulfuric acid and leads to obtaining alcohols: R-CH=CH-R + H-OH → R-CH-CH-R

Derivat halogenat vicinal

For example, by adding water to ethene, ethanol is obtained. The addition of water to asymmetric alkenes is done according to Markovnikov's rule. ex: propene+water→2-propanol (isopropyl alcohol)

Polymerization reaction

The process in which several molecules of a compound, monomer (mer), bind together, forming a macromolecule (polymer), is called polymerization (gr. poly=numerous, meros=part). n - the degree of polymerization (the number of monomer molecules that form the polymer) nA → -(A)-n The degree of polymerization n and the molar mass M that characterizes a polymer are average values. The polymerization of alkenes is a polyaddition reaction that occurs with the breaking of the pi bond in each alkene molecule and the formation of new single, carbon-carbon bonds. The general equation of the chemical polymerization reaction of an alkene is: nCH2=CH → -(CH2-CH)-n

Derivat halogenat vicinal

Oxidation reactions

Mild oxidation

The oxidation reaction of alkenes with neutral/weakly basic aqueous solution of potassium permanganate (Baeyer's reagent) is called mild oxidation. Under the action of the oxidizing agent, only the pi bond from the double bond is broken and diols are formed: compounds containing two hydroxyl groups (-OH) at the two neighboring carbon atoms (vicinal diols). A stable saturated compound is obtained. 3CnH2n + 2KMnO4(purple solution) + 4H2O → 3CnH2n(OH)2 + 2KOH + 2MnO2(brown precipitate) Due to color changes, the reaction is used to identify alkenes.

Energetic (destructive) oxidation

It is done in the presence of K2Cr2O7/KMnO4 and H2SO4, obtaining carboxylic acids and/or ketones.

It can also be done with ozone, a reaction by which the position of the double bond in the alkene chain can be established.

Complete oxidation (burning)

CnH2n + 3n/2O2 → nCO2 + nH2O + Q

Physical properties

Alkenes can be gaseous, liquid or solid, depending on the number of carbon atoms in the molecule. Alkenes from ethene to pentene are gases, those from pentene to alkene 18 are liquids, and higher alkenes are solids.

Boiling and melting points increase with molecular mass, but are lower than for the corresponding alkanes. They have a lower density than water, are insoluble in water, but soluble in organic solvents. They are colorless and have a specific smell.