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Stability of Conjugated Dienes, Diel-Alder, Electrophilic Addition

A conjugate diene is characterized by alternating double bonds of carbon and carbon, separated by single bonds of carbon and carbon.

Stability of conjugated dienes

A conjugate diene is characterized by alternating double bonds of carbon and carbon, separated by single bonds of carbon and carbon. Dienes containing adjacent carbon-carbon double bonds are regarded as cumulated dienes. They are more stable from an energy perspective than isolated double bonds. Dual bonds associated with conjugated dienes are unstable. Advanced organic chemistry courses can explore the chemistry of cumulated double bonds.



Dienes with conjugated energies (whether isolated or cumulated) are more stable than non conjugated ones due to factors such as charge delocalization through resonance and hybridization energies. Hydrogenation energies of isolated and conjugated alkenes differ, indicating this stability. The high temperature of hydrogenation lowers the stability of conjugated dienes (~ 54 kcal) compared to their isolated (~ 60 kcal) and cumulated (~ 70 kcal) counterparts. Comparing different types of bonds based on their heats of hydrogenation is shown in this energy diagram. It illustrates the relative stability of each molecule:

The Resonance Theory of How Conjugated Dienes Are Stable

In conjugated dienes, a single bond separates the double bonds. A conjugated system is an excellent example of a 1,3-diene. Carbons in 1,3 dienes are sp2 hybridized, meaning that they have one p orbital each. 1,3-butadiene has four p orbitals that overlap, which results in a conjugated system.





Based on the resonance structure, it is easy to understand how the electrons in this conjugated diene are distributed between its four carbon atoms. The conjugated diene is stabilized through this delocalization of electrons:

A Molecular Orbital Explanation for the Stability of Conjugated Dienes

Below is an illustration of 1,3-butadiene's molecular orbital model. A plus or a minus sign appears in each pi-orbital lobe for each of the four p-orbital components. Different phases of this orbital are described by mathematical waves equations. Nodes are regions that undergo a phase change between adjacent orbital lobes. Such regions have no orbital electron density. Likewise, the atoms of the diene define nodal planes for all of the pi-orbitals shown here. By extension, each p-orbital has a nodal point at the nucleus. Only one nodal surface is found in the lowest energy pi-orbital, π1. As pi-orbital energy increases, the number of nodes increases. The pi electrons in 1,3-butadiene are four. For conjugated double bonds to be stable, the two bonding molecular orbitals must be filled.



Diel-alder

A four electron and a two electron system interact suprafacially (together with the same-face system) in the Diels-Alder reaction mechanism. In a Diels-Alder reaction, a new ring forms as a result of cycloaddition. An electron system with four electrons represents a diene, whereas an electron system with two electrons represents a dienophile. Even though orbital symmetry is imposed, this interaction does not create any extra energy barrier. In Diels-Alder, a substituted alkene and a substituted diene are used to act as reactants in an important organic chemical reaction. A dienophile is an alkene, which has been substituted with a diene. This reaction produces cyclohexene derivatives with substituted functional groups. Diels-Alder reactions are good examples of integrated pericyclic reactions (all bonds are broken at the same time and all bonds are formed at the same time). Diels-Alder reactions form six-membered rings when two carbon-carbon bonds form simultaneously.



Two pi bonds can be converted to two sigma bonds using the diagram above. Two pi-electron systems are concertedly bonded together in this manner. During the Diels-Alder reaction, two pi electrons from the dienophile and four pi electrons from the diene are shifted. A Vitamin B-6 is produced as a result of this reaction. With the reverse reaction (also known as the retro-Diels-Alder reaction), cyclopentadiene is produced at an industrial scale.

Mechanism

The reaction is thermodynamically preferable since pi bonds are converted into weaker sigma bonds. Electrophilic dienophiles attached to electron-withdrawing groups are more prone to producing the Diels-Alder reaction. This is also favored by nucleophilic dienes that contain electron-donating groups. In the following paragraphs, we will discuss some of the best dienes and dienophiles that can be used for the Diels-Alder reaction.




The Diels-Alder reaction proceeds in a single step because it uses a concerted mechanism. When two unsaturated molecules combine, they form a cyclic adduct. There has been a reduction in bonds. There is a simultaneous formation and destruction of bonds.
A cyclohexene derivative is generated when the diene reacts with the dienophile. From the mechanism illustration, one can observe that three carbon-carbon p bonds break, but only one carbon-carbon p bond forms, with two sigma bonds forming.

Electrophilic addition

One of the most important reactions of alkenes is electrophilic addition. There are some aspects that are common to most electrophilic additions. Hence, you will gain a better understanding of future alkene reactions by learning about electrophilic addition reactions. The pi electrons are also located above and below the double bond, which enables them to be accessed more easily during reactions. As a Lewis acid (which likes nuclear material and has a lot of electrons), a double bond can readily donate lone electron pairs to act as a nucleophile. For electrophilic addition reactions, Lewis bases donate lone pairs of electrons (electron-loving, electron-poor, organic molecules). In this section, we will focus on hydroxides (HX), which are electrophilic additions. In this section, we will focus on hydroxides (HX), which are electrophilic additions. The subsequent electrophilic addition reactions will be applicable in addition to the basic concepts discussed. During this reaction, alkenes are broken into two single sigma bonds, which are said to be pi bonds. Two of the sigma bonds in the reaction mechanism are connected to hydrogen halides' components H and X. During this reaction, H between the two halides acts as a catalyst. By adding phosphoric acid (H3PO4) and potassium iodide, you can make iodide, but you must first produce HI during the reaction.

Mechanism

Step 1 – electrophilic attack

The first step in the mechanism involves the attack, by the H in the electrophile, of two pi electrons from the double bond, as shown by the curved arrow. A C-H sigma bond is formed between HBr and a carbon in the double bond formed by the two pi electrons. In a matter of seconds, the electrons from the hydrogen-x bond are transferred to the halogen to form a halide anion. A carbocation intermediate becomes an electron deficient compound when one of the carbons is stripped of its pi electrons from the double bond. There is a positive charge in an unhybridized p orbital on this carbon, which has been sp2 hybridized.

Step 2 – nucleophilic attack by halide anion

With the carbocation, electrons can be picked up from the nucleophilic halide anion and become electrophilic. X-C sigma bonds are formed through electron pair addition to produce the neutral alkyl halide product.

All the halides can contribute energy to the reaction, such as HBr, HCl, HI, and HF. As X becomes larger, the bonding between H and X weakens (insufficient overlap of orbitals). Different types of halides, therefore, react differently.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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