Delocalized π Systems and Dienes | Organic Chemistry 2

The delocalized π systems and dienes are studied in this chapter: conjugation and resonance stabilization, allyl system and allylic reactions, conjugated dienes and their reactions, kinetic and thermodynamic control, Diels-Alder cycloadditions

Conjugation and Resonance Stabilization


Alternating arrangement of single and multiple bonds in a molecule, leading to a delocalized system of π electrons. 

This arrangement enables resonance stabilization, where the π electrons are spread over adjacent atoms with overlapped p orbitals, resulting in a larger volume of electron density ⇒ lower energy and increased stability.


Resonance structures:

A group of Lewis structures with the same placement of the atoms but a different placement of their π or nonbonded electrons ⇒ their single bonds remain the same but the position of their multiple bonds and nonbonded electrons differ. Resonance structures must be valid Lewis structures

The different resonance forms of a substance are not all equal: the form with the most bonds and fewer charges has a higher contribution to the resonance hybrid.


Evaluate the relative stability of resonance structures:

  • Structures with more bonds and fewer charges are more stable.
  • Structures in which each atom follow the octet rule are more stable.
  • Structures placing negative charges on more electronegative atoms are more stable.

Allyl System

Allyl system:

A molecular structure with a propene backbone, where a methylene group is adjacent to a carbon-carbon double bond. The three-atom allyl system:


Enhanced stability of the allyl system:

Allyl carbocation, radical, or anion is more stable than its primary counterparts due to resonance stabilization from the adjacent double bond. This stabilization can also be described in terms of molecular orbitals (MO):

The 3 p orbitals of the allyl group overlap: symmetrical structure with delocalized electrons

The 3 π MO of allyl, obtained by combining the 3 adjacent atomic p orbitals

Allylic Reactions

Radical halogenation:


  1. Initiation: radical formation, often via heat or light.

  2. Propagation: allylic hydrogen abstraction by a halogen radical, leading to an allylic radical.

  3. ​​​​​​​Termination: Combination of radicals to form products.


Allylic SN reactions:

Nucleophilic Substitution at the allylic position. Allylic halides undergo both SN1 and SN2 reactions:

  • SN1:

  • SN2:



  1. Nucleophile attacks the allylic carbon, forming a new bond.
  2. In the case of SN1, allylic rearrangement may occur for more stable products.


Allylic organometallic reagents:



Allylic organometallic reagents are highly nucleophilic and react with electrophiles. This results in the formation of a carbon-carbon bond at the allylic position.

Conjugated Dienes

Properties of conjugated dienes: 

  • 2 double bonds separated by a single bond ⇒ 4 contiguous p orbitals that overlap and allow the distribution of π electrons across 4 carbon centers.

  • The central C-C bond is shorter than normal C-C because it has a partial double bond character.
  • Conjugated dienes absorb UV light in the 200-400 nm range. As the number of conjugated π bonds increases, the absorption shifts to longer wavelengths ⇒ the extended conjugation allows for more delocalization of π electrons and results in absorption at lower energy levels.
  • π-electronic structure: 4 π MO obtained by combining the 4 adjacent atomic p orbitals:


Conformations of dienes:

Conjugated dienes, exemplified by 1,3-butadiene, can exist in two main conformations:

  • s-cis conformation: substituents on adjacent carbon atoms are on the same side, causing steric hindrance and reduced stability.
  • s-trans conformation: substituents are on opposite sides, allowing for greater delocalization of π electrons ⇒ more stable conformation.


Stability and reactivity of dienes:

  • Conjugated dienes are more stable than isolated alkenes and alkanes due to enhanced resonance stabilization ⇒ π electron delocalization over adjacent double bonds.
  • Despite their stability, dienes maintain higher reactivity than alkanes, owing to the presence of π bonds, making them susceptible to electrophilic additions.


Hydrogenation of Conjugated Dienes:

Addition of hydrogen across the double bonds to form saturated hydrocarbons (alkanes). This reaction is carried out with a metal catalyst like palladium or platinum.

Electrophilic Reactions of Dienes

1,2- vs. 1,4-Addition:

Electrophilic addition in conjugated dienes gives a mixture of products:

  • 1,2-addition product (addition to one of the double bonds): occurs when the electrophile adds to one of the two adjacent carbons of the diene.

  • 1,4-addition product (conjugated addition): occurs when the electrophile adds to the carbon atoms that are separated by one carbon-carbon double bond.



Addition of hydrogen halide:


Formation of 1,2- and 1,4-products due to the resonance-stabilized allylic carbocation intermediate.

  1. Addition of the electrophile (H+) to the π bond, forming a resonance-stabilized π complex
  2. Rearrangement to a more stable intermediate if possible
  3. Nucleophilic attack of Br-




Formation of 1,2- and 1,4-products due to the resonance-stabilized allylic carbocation intermediate.

  1. Addition of the electrophile (Br+) to the π bond, forming a resonance-stabilized π complex
  2. Rearrangement to a more stable intermediate if possible
  3. Nucleophilic attack of Br-

Kinetic versus Thermodynamic Control

Kinetic control:

  • Kinetic Conditions: low temperatures, short reaction times, or the presence of a specific catalyst that promotes the formation of the kinetic product.
  • Kinetic product: product from the fastest reaction ⇒ typically less stable and formed from reaction intermediate. 


Thermodynamic control:

  • Thermodynamic Conditions: higher temperatures, longer reaction times, or conditions that allow the reaction to reach equilibrium.
  • Thermodynamic product: more stable and often the major product at equilibrium.


1,2- vs. 1,4-Product in electrophilic addition reactions: 

The regioselectivity of an electrophilic addition reaction is influenced by kinetic and thermodynamic factors:

  • 1,2-product is kinetically favored due to faster formation of the intermediate ⇒ 1,2-product is the kinetic product.
  • 1,4-product  is thermodynamically favored due to the greater stability of the conjugate addition product ⇒ 1,4-product is the thermodynamic product.


Hydrobromination of Dienes:

The kinetic product is the product of a 1,2-addition


The thermodynamic product is the product of a 1,4-addition

Diels-Alder Cycloaddition

Diels-Alder reaction:


  • Mechanism: [4+2] cycloaddition that forms 2 σ and 1 π bond in a six-membered-ring. The mechanism is concerted: all bonds, both in the diene and dienophile, are broken and formed in a single step.
  • Initiation: typically initiated by heat, which provides the energy needed for the reaction to proceed.
  • Stereochemistry: The reaction is stereospecific, meaning the stereochemistry of the dienophile directly determines the stereochemistry of the product.


Rules governing the Diels-Alder reaction:

  • The diene can react only when it adopts the s-cis conformation.


  • Electron-donating substituents (EDG) in the diene increase the reaction rate ⇒ electronically richer.


  • Electron-withdrawing substituents (EWG) in the dienophile increase the reaction rate ⇒ electronically poorer.


  • Diels-Alder reaction forms the endo product.

Check your knowledge about this Chapter

Conjugation involves the overlap of p-orbitals across adjacent single and multiple bonds, allowing for the delocalization of π electrons across several atoms. This delocalization results in increased stability because the electrons are spread over a larger volume of space, lowering the overall energy of the molecule. Moreover, conjugated systems often exhibit enhanced reactivity, particularly in reactions such as electrophilic additions and cycloadditions, due to the presence of extended π-electron clouds that can interact with electrophiles or engage in pericyclic reactions more readily than isolated double bonds.

The allylic position in a molecule is defined as the next-adjacent carbon atom to a double bond, namely the sp3-hybridized carbon atom that is one bond away from the sp2-hybridized carbons of the alkene. This position is chemically significant because it possesses a higher reactivity due to the resonance stabilization that can occur when the π electrons in the double bond delocalize, which in turn creates a more stabilized allylic cation or radical. This stabilization often leads to increased reaction rates and unique reaction pathways in organic synthesis, making allylic positions favorable sites for chemical reactions.

Allylic systems participate in chemical reactions distinctly from non-allylic systems mainly due to the presence of resonance stabilization. The allylic position, which is the carbon atom adjacent to a double bond, has a p-orbital that overlaps with the π-orbitals of the double bond, allowing delocalization of electrons. This delocalization can stabilize carbocation, radical, or anion intermediates formed during a reaction, often leading to increased reactivity and selectivity at the allylic position.

Allylic systems are prone to nucleophilic substitution reactions and unique reactions such as allylic oxidations, which do not occur as readily with saturated carbons.

The molecular orbitals of conjugated dienes, such as 1,3-butadiene, show a continuous overlap of p-orbitals along the chain of alternating single and double bonds, which allows for delocalization of π electrons across all the adjacent atoms involved. This conjugation results in a stabilization of the molecule due to the lower energy of the delocalized π electrons compared to localized π electrons in non-conjugated dienes. In contrast, non-conjugated dienes have isolated double bonds where p orbitals are not aligned in a continuous system, and thus their π electrons are localized within the individual double bonds, leading to higher energy and less stabilization.

The s-cis and s-trans conformations of 1,3-butadiene are critical to its reactivity, particularly in Diels-Alder reactions. The s-cis conformation allows the π electrons in the conjugated diene to align properly with the π electrons of the dienophile, facilitating the formation of the cyclic adduct. In contrast, the s-trans conformation is not reactive in the Diels-Alder reaction because the π systems cannot overlap effectively due to the extended configuration, which places the reactive π bonds too far apart. As a result, only the s-cis conformation can participate in the Diels-Alder reaction, illustrating the importance of molecular geometry in chemical reactivity.

Electrophilic addition reactions with conjugated dienes can proceed through two possible pathways, leading to 1,2-addition or 1,4-addition products, depending on the conditions.

In the reaction of 1,3-butadiene with HBr, the reaction can give a 1,2-adduct (where the electrophile adds to the first carbon and the nucleophile (Br-) adds to the second carbon) or a 1,4-adduct (where the electrophile adds to the first carbon and Br- adds to the fourth carbon).

The regioselectivity is influenced by the stability of the intermediate carbocation. Under kinetic control (low temperatures, fast reaction), the 1,2-adduct is favored due to the faster formation of the more stable allylic cation. Under thermodynamic control (high temperatures, slower reaction), the 1,4-adduct is favored because it is the more stable product due to the delocalization of charges over the conjugated system in the final product.

  • Kinetic control in reactions of conjugated dienes refers to conditions under which the reaction products form faster, typically at lower temperatures, and are governed by the activation energy barriers of the reactions. The products formed under kinetic control are not necessarily the most stable, but they are the ones that form the fastest.
  • Thermodynamic control, on the other hand, is observed at higher temperatures, which allow the reaction to reach equilibrium. Under thermodynamic control, the reaction yields the most stable products, regardless of the rate at which they are formed. This is because higher temperatures provide the energy necessary for the system to overcome activation energy barriers and allow for the interconversion between less stable and more stable products.

Temperature and reaction conditions can dramatically affect the outcome of electrophilic reactions of dienes, especially in terms of kinetic versus thermodynamic control: 

  • At low temperatures, the reaction is likely to be under kinetic control, favoring the formation of the product that forms at the fastest rate, which is typically the less stable, less substituted alkene (the kinetic product).
  • However, as the reaction temperature is increased, the system shifts towards thermodynamic control, where the more stable, more substituted alkene (the thermodynamic product) predominates because products can interconvert and the equilibrium favors the more stable product.


The Diels-Alder reaction is an addition reaction between a conjugated diene and a substituted alkene, known as the dienophile, to form a cyclohexene system. It is widely used in organic synthesis because it forms cyclic compounds in a stereoselective, regioselective, and often stereospecific manner, which allows for the construction of complex molecular architecture from simpler substances. This reaction occurs under thermal conditions, generally without the need for a catalyst, and the resulting adducts have a high degree of stereochemical control, providing an efficient method for creating six-membered rings, which are common structural motifs in natural products and pharmaceuticals.

Orbital symmetry plays a crucial role in Diels-Alder cycloadditions by dictating the interaction between the Highest Occupied Molecular Orbital (HOMO) of the diene and the Lowest Unoccupied Molecular Orbital (LUMO) of the dienophile. This pericyclic reaction requires that the orbitals overlap in a constructive manner, which is only possible when the symmetries of the interacting orbitals are compatible. The HOMO of the diene must have a p-orbital symmetry that allows "side-on" overlap with the p-orbital symmetry of the LUMO of the dienophile. This symmetry-matching sets the stage for a concerted mechanism that leads to the formation of a new six-membered ring with conservation of orbital symmetry throughout the reaction.

The regio- and stereoselectivity of the Diels-Alder reaction can be influenced by the electronic and steric properties of the dienes and dienophiles involved:

  • Electron-donating groups on the diene and electron-withdrawing groups on the dienophile enhance the reactivity (increase the reaction rate) and direct the reaction toward the most stable, electron-rich double bond in conjugated dienes.
  • Steric effects can influence the approach of the dienophile, resulting in the preferential formation of endo over exo products.