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The olefinic hydrocarbons are Hydrocarbure S unsaturates, characterized by a double Covalent bond between two atoms of Carbone. These connections are always of type covalent perfect normal.

The term olefin was the name given in the past to olefinic hydrocarbons; although still employed, this term falls more and more in disuse.

The simplest olefinic hydrocarbon is the ethylene (usual name of ethene) of which the developed structure is:

Nomenclature

Not-ramified olefinic hydrocarbons

The same name should be used as that of the Alcane increasing the same number of carbon molecules, by using the suffix “- ene” rather than “- ass” and by intercalating its index of position (see rule 2a) in the word, before the suffix, and limited by two indents of each dimensions.

Ramified olefinic hydrocarbons

Here rules to follow to name an olefinic hydrocarbon ramified :
Rule 1
In the formula of structure, to determine the principal carbonaceous chain (i.e. the longest chain of elements containing one C) comprising the double connection obligatorily. Practically, that returns to:

to choose an end of the carbonaceous chain,
to traverse the chain while passing by the greatest number of atoms C being followed.

Rule 2

in order to locate the double connection, to number the principal chain so that the number of the atom C carrying the double connection is smallest possible.
to follow same classification to locate the ramifications.

Rule 3

To quote the name of the ramification Alkyl, followed by its index of position surrounded by indents. To make follow name of olefinic hydrocarbon as if it were not ramified.

The double connection can be given according to the rules of priority of Cahn-Ingold-Prelog:

if each connection are same side, the double connection names Z (of German zusammen), example:

(Z) Pent-2-ène

if each connection are opposite, the double connection names E (of German entgegen), example:

(E) Pent-2-ène

Physical properties

The olefinic hydrocarbons have boiling points a little lower than those of corresponding alkanes because the forces of Van der Waals are weaker; indeed, a double connection takes more space than a simple connection, therefore the molecules pile up in a less compact way and the intermolecular forces are less important. It results from it that it is necessary to provide less energy to break them: the boiling points are lower. They burn with a clear flame.

The olefinic hydrocarbons are gas until the Butène, then liquid and finally solids starting from C₁₆. Their Solubility, poor in water, is good in the alcohol and the ether.

Reactivity

The double connection is made of a connection σ ( sigma ) strong (E_L = 347 kJ/mol) and of a connection π ( pi ) also called unsaturated connection, weaker (E_L = 263 kJ/mole). The force of the connection π being weaker than that of the connection σ, it yields more easily.

The principal reactions of olefinic hydrocarbons are:

reaction of addition on carbons carrying the double connection
reaction of destruction and degradation

Reaction of addition

During the reactions, the connection π can break: a reagent A-B will be added on the double connection with olefinic hydrocarbon and there is formation of a saturated product. The connection π was thus replaced by two connections σ.

¤ Action of dihydrogene:

¤ Action of a halogenous derivative

This reaction obeys the Loi of Markovnikov:

In a reaction of addition of H-X on an olefinic hydrocarbon, in the absence of peroxide, the hydrogen atom migrates towards carbon less substituted (C. - with-D. most hydrogenated).

¤ Action of a halogen:

¤ Action of water (hydration)

This reaction obeys the law of Markovnikov:

In a reaction of addition of H₂O on an olefinic hydrocarbon, in the absence of peroxide, the hydrogen atom migrates towards carbon less substituted (C. - with-D. most hydrogenated)

¤ Action of a strong oxidant solution (Ozonolyse for example):

If R₂ is a hydrogen atom, then R₁CH=O is a Aldéhyde.

If R₁ and R₂ are alkyls groups, then R₁R₂C=O is a Cétone.

Reaction of destruction by combustion

Reaction of Diels-Alder

Reaction of Diels-Alder

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