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Alkenes - Organic Chemistry Tutor

Acyclic hydrocarbons containing a double bound within two carbon atoms in the molecule are named alkenes. They are also called olefins, because of the fact that the chlorate and bromate derivates are oleaginous liquids.


The simplest alkene is ethylene (C2H4), which has the International Union of Pure and Applied Chemistry name ethene. This substance contains two carbon atoms connected through a double bound. Superior alkenes can be considered as the result of the substitution of a hydrogen atom with alkyl radicals, same as the case of saturated acyclic hydrocarbons. Hence, the superior homologue of ethylene is propene, CH2=CH-CH3. The homologue of propene is buthene, C4H8. After the position of the double bound in the molecule at buthene, you can distingiush three position isomers.


By replacing the ending “ane” from the alkane’s name with the same number of carbon atoms in the molecule with the ending “ene”, you obtain the alkenes’ name.

For example:
CH3-CH2-CH2-CH=CH-CH3 - 2 hexene

So, the three position isomers of buthene are: 1-buthene, 2-buthene and 2-methylpropene. Sometimes there can be used the position of the double bound after the name of the hydrocarbon. For example: buthene-1. Alkenes mono-valence derivates have their names ending in “enyl”, the position of the double bound being indicated, if necessary, by a number.

Vinyl for ethenyl: CH2=CH-
Alyl for 2-propenyl: CH2-CH-CH2-
Isopropenyl for 1 methyl vinyl: CH2-C-CH3

General formula

Because of the double bound in the molecule, alkenes have two hydrogen atoms less then other saturated hydrocarbons; therefore, the general formula for these hydrocarbons is CnH2n. You can observe the homologous series of the cycloalkanes. They are, consequently, raw formula isomers.

Physical properties

C2-C4 alkenes are gases, C5-C18 are liquids and the ones with more than 18 carbon atoms are solids. The boiling points are a little bit lower than the corresponding alkanes. Their density is higher than the corresponding alkenes, but all alkenes have the density smaller than 1. Alkenes are insoluble in water, but mixable with organic solvents.


Alkenes present three types of isomerism: chain isomers (the isomers differ by chain), position isomers (the chain is identical but the position of the double bound differs) and function isomers. Geometrical isomerism appears at alkenes because the double bound is stiff and doesn’t allow the rotation of the carbon atoms around it. Geometrical isomers are different from one another through the spatial position of the substitutes around the double bound. The condition of isomerism is that for each carbon atom from the double bound to exist a different substitute.

The nomenclature of geometrical isomers: it is established by the priorities of the substitutes bind with each carbon atom from the double bound: you will compare the Z of the atoms directly connected with the carbon atom out of the double bound. The one with the biggest Z will have highest priority and it will be note with 1 and the other one, with 2. If the Z’s are the same, the next pair will be compared. The comparison will be made until there appears a difference. This procedure will be applied at each carbon atom in the double bound. After establishing the priorities, the denomination will be made like this: the isomer with the substitutes on the same side of the double bound will be noted with Z (CIS) and the other, with the substitutes on one side and another of the double bound, will be noted with E (TRANS).

For example: C4H8 has 6 isomers: 1-buthene, CIS 2-buthene, TRANS 2-buthene, isobutene, cyclo butene, methylcyclopropene.

Obtaining methods

1. Thermic decomposing

CnH2n+2 CmH2m+2 + CpH2p - cracking; n = m + p, p >= 2

CnH2n+2 CnH2n + H2 - dehydrogenating

2. Alkynes hydrogenating

R-CC-R' + H2 R-CH=CH-R (alkyne) – the reaction takes place in the presence of Pd poisoned with Pb salt

CHCH + H2 CH2=CH2 (ethane)
CH3-CCH + H2 CH3-CH=CH2 (propene)

3. From halogenated derivates

Halogenated derivate + Zn ZnX2 + alkene (X=Cl,Br,I)

Example: Cl-CH2-CH2-Cl + Zn ZnCl2 + CH2=CH2 (ethene)

4. From mono-halogenated derivates through dehydrogenating (-HX)

mono-halogenated derivate alkene + HX (X=Cl,Br,I)

CH3-CH2-Cl CH2=CH2 + HCl (ethene)
Br-CH2-CH2-CH2-CH3 CH2=CH-CH2-CH3 (1 butene)

5. By dehydrating the alcohols (-H2O)

alcohol alkene + H2O

CH3-OH CH2=CH2 + H2O (ethene)
CH3-CH2-OH CH2=CH-CH3 + H2O (1-butene)

Chemical reactions

1. Hydrogen addition (hydrogenating)

alkene + H2 alkane

CH2=CH-CH3 + H2 CH3-CH2-CH3 (propane)
CH3-CH=CH-CH3 + H2 CH3-CH2-CH2-CH3 (n butane)

2. X2 addition (halogenations) Cl2, Br2, I2

alkene + X2 halogenated derivate vicinal
CH2=CH-CH2-CH3 + Cl2 CH2Cl-CHCl-CH2-CH3 (1,2-dichlorbhutan)
CH2=CH2 + Br2 CH2Br-CH2Br (1,2-dibromethan)

3. HX addition (HCl, HBr, HI)

alkene + HX mono-halogenated derivate

CH2=CH2 + HBr CH3-CH2-Br (ethyl bromide)
CH3-CH2=CH-CH3 + HCl CH3-CH2-CHCl-CH3 (s butyl chloride)

4. H2O addition (hydration)

alken + H2O alcohol

CH2=CH2 + H2O CH3-CH2-OH (ethylic alcohol)
CH2=CH-CH3 + H2O CH3-CHOH-CH3 (isopropyl alcohol)

5. HBr addition in the presence of peroxides

R-CH=CH2 + HBr R-CH2-CH2-Br

6. Hipologenos acid addition (HOX)

CH2=CH2 + HOCl CH2-OH-CH2-Cl (2-clor, 1-hidroxoethane)
CH2=CH-CH2-CH3 CH2-OH-CH-Br-CH3 (1-brom, 2-hidroxopropane)

7. Polymerization reaction

CH2=CH2 (-CH2-CH2-)(n) – polyethene / polyethylene - polymer; n = polymerization grade

At the polymerization, the chains don’t usually have the same size, hence, n is a medium value’ it is called the medium grade of polymerization.

8. Oxidation

a) Complete (combustion): CnH2n + O2 nCO2 + nH2O + Q

b) With O2 in the presence of Ag / 250 grades: alkene + 1/2 O2 ethane oxide (instable) +H2O diol

c) Mild oxidation - Bayer: KMnO4, H2O or Na2SO3: alkene + [O] + H2O diol


» ethene and propene are the organic compounds in the biggest quantity on world-wide plan. Ethene occupies the fourth place in the production of all chemical compounds (it is overtaken only by sulphuric acid, nitrogen and oxygen). The most important chemical transformations of ethene, used at a large scale, are: polyethene, acetic acid, PVC.
» ethene is produced by plants and it stimulates the growth of fruits, the sprout of seeds and the ripe of flowers

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