Diene

he Diene are alkenes containing two carbon-carbon double bonds, so they have essentially the same properties as these hydrocarbons.

Summary

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• 1 Physical properties
• 2 Chemical properties
• 3 Obtaining
• 4 Nomenclature
• 5 Isomerism
  • 1 Classification
  • 2 Direct and Conjugate Addition
  • 3 Diels-Alder reaction
• 6 uses
• 7 External Links
• 8 Sources

Physical properties

Isolated dienes do not have different binding properties than alkenes. Accumulated and conjugated dienes do have special properties.

1-.Accumulated dienes (alenes)

The central carbon of the accumulated dienes (allenes) is in hybridization sp.

The alleles can be chiral: Although the alleles do not have any stereocenter, those that have two different substituents at the ends have a chiral axis that causes the object and image to not be superimposable. Therefore 2,3-pentadiene has two enantiomeric structures.

To determine the configuration of an allene, the substituents are numbered, first those of the carbon in front and then those of the rear, following the usual rules of priority. A tetrahedron is drawn as shown in the figure, the substituent with the lowest priority is placed back and the direction of rotation necessary to go from 1 to 2 and to 3 is observed.

2-. Conjugated dienes

The central bond of conjugated dienes have a partial double character. The length of the central bond is intermediate between that of a single and a double one.

The lateral overlap of the four p atomic orbitals produces four molecular orbitals.

The p1 orbital delocalises the electron density along the four atoms involved in a conjugated diene.

This explains the partial double character of the central bond. The small contribution of charge separation resonant forms also explains why the central bond has a small double character.

• Example of physical properties of dienes.

1,3-Butadiene or Divinyl is an easily liquefiable gas (at -5 ° C).

– 2-Methyl-1,3-butadiene, or isoprene is an easily boiling liquid.

Chemical properties

Diene hydrocarbons, having double bonds in the molecules, cause current addition reactions, for example, they discolor the bromine water , they add the hydrogen halides, etc. However, addition reactions have their peculiarities. Conjugated dienes differ from simple alkenes in three ways:

1.They are more stable.

2. Suffer 1.4 addition.

3.They are more reactive in adding free radicals.

1. Combustion

C ₄ H ₈ + O ₂ → 4CO ₂ + 4H ₂ O

2. Hydrogenation CH ₂= CH-CH = CH ₂+ 2H ₂ = CH ₃ -CH ₂ -CH ₂ -CH ₃ 2.1 Isolated dienes:

The double bonds react independently, as if they were on different molecules.

Obtaining

1.By dehydrogenation of alkanes.

2. By means of hydrocarbon cracking.

3.By the process of dehydration and dehydrogenation of ethyl alcohol.

Nomenclature

Main article: Acyclic hydrocarbon nomenclature Dienes are named through the IUPAC system in the same way as alkenes, except that the ending “diene” with two numbers is used to indicate the positions of both double bonds.

Examples:

• CH ₂= CH – CH = CH ₂ → 1,3 – Butadiene or divinyl
• CH ₂= CH – CH ₂ – CH = CH ₂ → 1,4 – Pentadiene

Isomerism

Isomerism can occur:

• By the branching of the skeleton.
• By the position of two links.
• Between dienes and alkynes.

Classification

Dienes are divided into three important classes, according to the distribution of their double bonds: Dienes are di-alkenes. These can be described according to the relationship that exists between the two double bonds. Its classification is as follows:

1-Isolated dienes:

The double bonds are separated by one or more carbon atoms with sp ³ hybridization (there are two to more single bonds between the double bonds).

Example: H ₂ C = CH – CH ₂ -CH = CH ₂

2-Accumulated dienes:

The double bonds share a common atom. They are also known as alenes.

Example: H ₂ C = C = CH ₂

3-Conjugated dienes:

The double bonds are linked by a single bond, forming a continuous chain of orbitals. (a single bond separates both double bonds).

Example: H ₂ C = CH – CH = CH ₂

Conjugated dienes are more stable than isolates by 3 to 4 kcal / mol. This stability is primarily due to the delocalization of pi electrons: The conformation that a conjugated diene acquires is also related to its stability. The s-trans is said to be more stable than the s-cis (the letter “s” refers to the single bond that separates the double bonds; if the double bonds are in opposite positions to this bond, it is called trans, if they are on the same side, cis): The allylic position is occupied by the carbon adjacent to a double bond

Carbocations and allylic free radicals are two important intermediaries due to the degree of stability they reflect. In both cases, the stability is due to the delocalization of electrons along the pi bond that is generated by the overlap of the “p” orbitals:

This delocalization allows resonance; hence its stability:

Carbocations and allylic radicals are more stable than carbocations and alkyl free radicals.

Direct and Conjugate Addition

Two forms of reaction are possible when conjugated dienes undergo addition reactions: direct addition or addition1,2 and conjugate addition or addition 1,4:

At low temperatures, the reaction of electrophilic addition to a conjugated diene is subject to kinetic control, in which the proportion of the products of the reaction is governed by the relative speed of their formation: the product generated by the intermediate is formed first more stable. Once the products are formed, they do not balance or interconvert.

In the case of reaction at room temperature or higher, the addition is subject to equilibrium or thermodynamic control. Here, the distribution of the products is governed by the relative stability of the products. The products are in balance with each other, but the most stable predominates, regardless of which is formed faster.

Thus, for example, in the reaction of 1,3 butadiene, H ₂ C = CH-CH = CH ₂ , with HBr the addition product 1,4 is generated primarily at a temperature of -80 ° C, while the product of 1,2 addition is the main product at 40 ° C:

Diels-Alder reaction

Pericyclic reaction, that is, it occurs in a single step, without intermediaries and involves the cyclic redistribution of six bonding electrons. It requires a conjugated diene and a dienophile, composed of a double or triple bond. The product, known as an adduct, is a cyclohexene. The diene must have an s-cis conformation; If the dienophile has groups that attract electrons, the reaction is favored.

The reaction is stereoselective, which implies that the adduct retains the stereochemistry that the dienophile possesses.

Applications

Rubber is one of the most widely used dienes:

• Natural rubber is obtained from latex, that is, from the milky sap of some hevea plants native to Brazil .
• The most important property of rubber is its elasticity, that is, the property of experiencing considerable elastic deformations by acting a relatively small force, for example, lengthening, contracting and then restoring the initial shape after the action of the force has ceased.
• An important property of rubber is also its impermeability to water and gases.