Name: how conjugated dienes gain stability
Uploaded: 2023-04-30T10:40:51.000Z
Duration: 1 min 28 s
Channel: JoVE

通过比较非共轭二烯和共轭二烯的氢化热，可以评估它们的相对稳定性。
例如，1-丁烯的氢化热为 -127 kJ/mol。人们会预期该值在1,3-丁二烯中增加一倍；然而，实际值比预期低15 kJ/mol。
那么，是什么使得共轭二烯更加稳定？
在共轭体系中，碳-碳单键由sp2–sp2轨道重叠形成，而在孤立体系中则为sp3–sp2轨道重叠。由于sp2轨道的s成分更高，共轭二烯中的σ键更短且更强。
此外，π电子的离域为共轭体系提供了额外的稳定性。
为了实现电子离域效应，p 轨道必须相互平行，这使得分子几何结构被限制为平面构型。
1,3-丁二烯的s-顺式和s-反式构象满足平面性条件，其中s-反式更为稳定。顺式-反式转化的活化能约为15 kJ/mol，相当于共轭二烯的稳定化能。

共轭二烯的稳定性

Stability of Conjugated Dienes

介绍


 对二烯氢化焓的比较表明，共轭二烯在氢化时释放较少的热量，使它们比非共轭二烯更稳定。


<img alt="Figure1" src="/files/ftp_upload/12544/12544_PT_Figure_1.svg" style="text-align: center; height: 315px;">



 有助于增强共轭体系稳定性的两个主要因素是π电子的离域和形成单键的碳的sp2杂化。


共轭二烯的平面构象


 共轭二烯采用两种平面构型:s-cis式和s-trans，其中未杂化的p轨道彼此平行排列，有利于π电子在分子中的离域。 1,3-丁二烯的 s-cis式和 s-trans构象异构体如下所示。


<img alt="Figure2" src="/files/ftp_upload/12544/12544_PT_Figure_2.svg" style="text-align: center; height: 150px;">


 前缀"s"描述了 C-C 单键周围双键的方向。 构象异构体在室温下通过围绕中心单键的旋转进行快速内部转化。 在s-cis式形式中，两个双键位于单键的同一侧，并且二面角为0°。 在 s-trans中，它们位于相对的两侧，二面角为 180°。 由于两个氢之间的空间相互作用，s-cis式构象异构体比 s-trans构象异构体不稳定，如下所示。


<img alt="Figure3" src="/files/ftp_upload/12544/12544_PT_Figure_3.svg" style="text-align: center; height: 90px;">

引言  
对二烯烃氢化焓的比较表明，共轭二烯烃在氢化过程中释放的热量较少，使其……

Education

教育

JoVE Core

Organic Chemistry

有机化学

Unit 1

第一单元

Dienes, Conjugated Pi Systems, and Pericyclic Reactions

二烯烃、共轭π体系与周环反应

dienes-conjugated-pi-systems-and-pericyclic-reactions

KEY TERMS AND CONCEPTS

关键术语与概念

how conjugated dienes gain stability

Conjugated dienes gain extra stability because their electrons are spread out over the molecule. When enthalpies of hydrogenation are compared, these compounds release less heat than nonconjugated dienes. That smaller heat release shows that conjugated dienes are more stable than their nonconjugated analogs.

Two factors help explain this stability. First, the π electrons are delocalized, which means they are shared across more than one bond. Second, the carbons in the single bonds are sp2 hybridized, which supports the conjugated structure.

Conjugated dienes also form two planar conformers: s-cis and s-trans. In both forms, the unhybridized p orbitals stay parallel, so π electrons can delocalize across the molecule. The prefix “s” tells you how the double bonds are arranged around the C–C single bond.

In 1,3-butadiene, the s-cis and s-trans conformers change into each other quickly at room temperature by rotation around the central single bond. In the s-cis form, the double bonds are on the same side and the dihedral angle is 0°. In the s-trans form, they are on opposite sides and the dihedral angle is 180°.

The s-cis conformer is less stable than the s-trans conformer. That lower stability comes from steric interaction, which is crowding between the two hydrogens. This small difference helps show how structure affects the behavior of conjugated dienes.

How Conjugated Dienes Gain Stability

structure of conjugated dienes

Conjugated Dienes: Bonding and Isomers

Conjugated dienes are compounds with alternating double and single bonds. In a conjugated system such as 1,3-butadiene, the unhybridized 2 p orbital on each carbon overlaps continuously. This lets the pi electrons spread out, or delocalize, across the whole molecule. Cumulated and isolated dienes do not show this same overlap. In 2,3-pentadiene, a cumulated diene, and 1,4-pentadiene, an isolated diene, the pi electrons stay localized between the double bonds. The difference in overlap helps...

stability of conjugated dienes

Conjugated dienes gain extra stability because their electrons are spread out over the molecule. When enthalpies of hydrogenation are compared, these compounds release less heat than nonconjugated dienes. That smaller heat release shows that conjugated dienes are more stable than their nonconjugated analogs. Two factors help explain this stability. First, the π electrons are delocalized, which means they are shared across more than one bond. Second, the carbons in the single bonds are sp2...

&pi; molecular orbitals of 1,3-butadiene

1,3-Butadiene Orbital Energy and Nodes

1,3-Butadiene shows how conjugated dienes gain stability through π molecular orbitals. Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, so they are more stable. The simplest conjugated diene is 1,3-butadiene, a four-carbon molecule with each carbon sp2-hybridized and each carbon carrying an unhybridized p orbital with an unpaired electron. Molecular orbital theory explains this pattern by combining atomic orbitals into molecular orbitals. In...

&pi; molecular orbitals of the allyl cation and anion

molecular-orbitals-of-the-allyl-cation-and-anion

Allyl Cation and Anion Orbital Patterns

The allyl cation and allyl anion both have the same three-carbon orbital pattern. An allyl group is a conjugated system, which means its electrons are spread across connected p orbitals. In the allyl anion, propene is treated with a strong base that removes a methyl hydrogen. In the allyl cation, the species forms as an intermediate in substitution reactions that involve allylic halides. In both cases, the allylic carbon changes from sp3 to sp2 hybridization. That change leaves three...

&pi; molecular orbitals of the allyl radical

Allyl Radical SOMO and Pi Orbitals

Allyl radicals are three-carbon conjugated systems. They often form as intermediates in alkene halogenation when halogen adds to the allylic carbon instead of the double bond. Each of the three sp2-hybridized carbon atoms has an unhybridized p orbital, and these p orbitals combine to form three pi molecular orbitals. The allyl radical shares the same molecular orbital pattern as allyl cations and allyl anions. The three systems differ only in the number of pi electrons. Allyl cations have two...

electrophilic 1,2- and 1,4-addition of hx to 1,3-butadiene

electrophilic-12-and-14-addition-of-hx-to-13-butadiene

HBr Addition to 1,3-Butadiene

Hydrogen halide addition to 1,3-butadiene gives two possible products instead of one. With conjugated systems like this diene, adding one equivalent of HBr leads to a mixture of the 1,2-addition and 1,4-addition products. This is different from the single product usually seen with alkenes and nonconjugated dienes that follow Markovnikov’s rule. The reaction starts when H+ adds across one of the double bonds in 1,3-butadiene. That step forms a resonance-stabilized allyl cation, which is an ion...

electrophilic 1,2- and 1,4-addition of x2 to 1,3-butadiene

electrophilic-12-and-14-addition-of-x2-to-13-butadiene

Halogen Addition to 1,3-Butadiene

Halogen addition to 1,3-butadiene can give two different products. Because 1,3-butadiene is a conjugated diene, it reacts with halogens in a way that is similar to an alkene, but it can form both a 1,2-dihalide and a 1,4-dihalide. The reaction begins when one of the diene pi bonds attacks the electrophilic part of the polarized halogen molecule. This forms a cyclic halonium ion intermediate. The displaced halide ion then attacks the intermediate at C2 to give the 1,2-product or at C4 to give...

electrophilic addition of hx to 1,3-butadiene: thermodynamic vs kinetic control

electrophilic-addition-hx-to-13-butadiene-thermodynamic-vs-kinetic

HX Addition to Butadiene: Product Ratios

Hydrogen halide addition to 1,3-butadiene gives two main products: the 1,2-adduct and the 1,4-adduct. These products form because the double bonds in butadiene can react at more than one position. The product mix depends strongly on temperature. At low temperature, the 1,2-adduct is favored. In this condition, the two products are not in equilibrium, so the outcome depends on how fast each product forms. This is called kinetic control, and the 1,2-adduct forms faster. At higher temperature,...

uv&ndash;vis spectroscopy of conjugated systems

Conjugation and Color in UV-Vis Spectra

Conjugated organic compounds absorb light in the UV-visible range because of π→π* electronic excitations. Their UV-vis absorption spectrum is usually shown as absorbance versus wavelength. The strongest peak in that spectrum is called the wavelength of maximum absorbance, or λmax. The extent of conjugation affects λmax. As conjugation increases, λmax shifts to longer wavelengths. When a π system is highly conjugated, it can absorb in the visible region instead of the UV. These compounds often...

uv&ndash;vis spectroscopy: woodward&ndash;fieser rules

Predicting UV-Vis Peaks with Woodward-Fieser

Woodward-Fieser rules help predict UV-Vis absorption peaks for conjugated molecules. In UV-Visible spectroscopy, the light-absorbing part of a molecule is the chromophore. Groups attached directly to the chromophore are called auxochromes. Conjugated pi systems absorb light through the lowest-energy pi to pi star transition. Their absorption maximum, or lambda max, often changes with structure. The Woodward-Fieser rules estimate lambda max by starting with a base wavelength for a diene or...

pericyclic reactions: introduction

Pericyclic Reaction Types and Features

Pericyclic reactions are organic reactions that happen through a concerted mechanism, which means they proceed in one step without intermediates. Electrons move in a closed loop to form a cyclic transition state. In that transition state, the sigma and pi bonds rearrange to give specific products. These reactions are grouped into three classes: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic rearrangements occur within...

thermal and photochemical electrocyclic reactions: overview

thermal-and-photochemical-electrocyclic-reactions-overview

Electrocyclic Reactions: Ring Opening and Closing

Electrocyclic reactions are reversible reactions that can either close a ring or open one. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. In one example, ring closure is favored. In another, ring opening is favored because forming cyclobutene would create too much ring strain. These reactions are highly stereospecific, which means the product shape depends on the starting material and the reaction conditions. For a substituted polyene, the stereochemical...

thermal electrocyclic reactions: stereochemistry

thermal-electrocyclic-reactions-stereochemistry

Thermal Ring Closure in Electrocyclic Reactions

Thermal electrocyclic reactions are controlled by the stereochemistry of ring closure. The key factor is the orbital symmetry of the polyene HOMO, or highest occupied molecular orbital. Under heat, the reaction follows the ground-state HOMO. The selection rules depend on the number of pi-electron pairs in the conjugated system. Systems with an even number of pi-electron pairs undergo conrotatory ring closure, where both ends rotate in the same direction. Systems with an odd number of...

photochemical electrocyclic reactions: stereochemistry

photochemical-electrocyclic-reactions-stereochemistry

Photochemical Ring Closures and Stereochemistry

Photochemical electrocyclic reactions form rings when conjugated systems absorb UV-visible light. The light promotes an electron from the ground state to the excited state. Because of that change, the reaction follows the excited-state HOMO instead of the ground-state HOMO. The HOMO, or highest occupied molecular orbital, has different symmetry in the excited state than it does in the ground state. That difference changes the stereochemical outcome of electrocyclic reactions. As a result, the...

cycloaddition reactions: overview

Cycloaddition Reaction Rules and Examples

Cycloaddition reactions are an important way to build cyclic compounds. They are concerted pericyclic reactions, which means the bond changes happen in one step between two unsaturated compounds. The product is a ring with two new sigma bonds formed as pi bonds are used up. The [4 + 2] cycloaddition, called the Diels-Alder reaction, is the most common example. Another example is the [2 + 2] cycloaddition. These reactions help show how different unsaturated molecules can combine to make cyclic...

cycloaddition reactions: mo requirements for thermal activation

cycloaddition-reactions-mo-requirements-for-thermal-activation

Thermal Cycloadditions and Orbital Symmetry

Thermal cycloadditions are reactions that use heat as the activation energy. In these reactions, the molecular orbitals of the reacting pi systems control whether the process can happen. A common thermally allowed example is the Diels-Alder reaction. It is a [4 + 2] cycloaddition, where a 4 pi component reacts with a 2 pi component. The frontier molecular orbitals are the highest occupied molecular orbital, or HOMO, and the lowest unoccupied molecular orbital, or LUMO. Under thermal...

cycloaddition reactions: mo requirements for photochemical activation

cycloaddition-reactions-mo-requirements-for-photochemical-activation

Light-Driven [2+2] Cycloaddition Reactions

Some cycloaddition reactions need light instead of heat. A [2 + 2] cycloaddition between two ethylene molecules is one example. It is allowed under photochemical conditions but forbidden under thermal conditions. A thermal [2 + 2] cycloaddition is symmetry forbidden. The ground-state HOMO of one ethylene molecule and the LUMO of the other are out of phase. That mismatch prevents a concerted suprafacial-suprafacial overlap. UV light changes the electron arrangement in one ethylene molecule.

[4+2] cycloaddition of conjugated dienes: diels&ndash;alder reaction

4-2-cycloaddition-of-conjugated-dienes-dielsalder-reaction

Diels-Alder Ring Formation in Alkenes

The Diels-Alder reaction forms a six-membered ring from a conjugated diene and an alkene or alkyne. The alkene or alkyne is called the dienophile. This thermal pericyclic reaction is a [4+2] cycloaddition because four π electrons come from the diene and two come from the dienophile. The bond changes happen in one concerted step. Six π electrons move together, and new σ bonds form at the same time. The product is an unsaturated six-membered ring. Molecular orbital symmetry explains why the...

diels&ndash;alder vs retro-diels&ndash;alder reaction: thermodynamic factors

dielsalder-vs-retro-dielsalder-reaction-thermodynamic-factors

Temperature and Diels–Alder Reaction Reversibility

The Diels–Alder reaction is thermally reversible. At suitable temperatures, it can return to the starting diene and dienophile. The forward reaction forms a cyclohexene derivative and is favored at low to medium temperatures. The reverse process is called the retro-Diels–Alder reaction. It is a ring-opening reaction that is favored at high temperatures. This temperature shift helps determine which direction is more likely to occur. Thermodynamics explains this behavior through Gibbs free...

diels&ndash;alder reaction forming cyclic products: stereochemistry

dielsalder-reaction-forming-cyclic-products-stereochemistry

Diels–Alder Stereospecificity and s-Cis Geometry

The Diels–Alder reaction is a concerted cycloaddition that builds unsaturated six-membered rings. It uses six pi electrons in one cyclic movement. Four pi electrons come from the diene, and two come from the dienophile. The reaction depends on stereochemical and conformational requirements. The frontier molecular orbitals that meet the symmetry rules are the HOMO of the diene and the LUMO of the dienophile. These orbitals interact suprafacially, which means the addition happens on the same...

diels&ndash;alder reaction forming bridged bicyclic products: stereochemistry

dielsalder-reaction-forming-bridged-bicyclic-products-stereochemistry

Endo and Exo Products in Diels–Alder Reactions

Diels–Alder reactions can form bridged bicyclic products when a cyclic diene is locked in the s-cis configuration. This arrangement lets the diene react with a dienophile to build the bicyclic ring system. The stereochemistry of the product depends on the dienophile. When the dienophile has one or more electron-withdrawing substituents, the products can place those groups in an endo or exo position relative to the double bond. Endo means the substituents point toward the double bond, while exo...

diels&ndash;alder reaction: characteristics of dienes

dielsalder-reaction-characteristics-of-dienes

Diels–Alder Dienes: Shape and Reactivity

The Diels–Alder reaction depends on the shape and reactivity of the diene. In this reaction, a diene and a dienophile join to form a six-membered ring. The diene must have the right geometry, and its orbitals must also be able to interact well with the dienophile. A simple example is 1,3-butadiene, which is an acyclic conjugated π system. At room temperature, it exists as a mix of s-cis and s-trans conformers because the carbon-carbon single bond can rotate. The s-trans form is more stable,...

diels&ndash;alder reaction: characteristics of dienophiles

dielsalder-reaction-characteristics-of-dienophiles

Diels–Alder Dienophiles and Reaction Speed

The Diels–Alder reaction depends strongly on the dienophile, which is the electron-deficient partner in the reaction. The diene is usually electron-rich and acts as a nucleophile, while the dienophile acts as an electrophile. Because of this balance, the structure of the dienophile can change how well the reaction works. The best dienophiles are often alkenes with electron-withdrawing substituents. Common examples include carbonyl, nitrile, and nitro groups. These groups help make the...

thermal sigmatropic reactions: overview

Thermal Sigmatropic Shifts and Reaction Paths

Thermal sigmatropic shifts are intramolecular pericyclic reactions. In these reactions, a sigma bond moves from one part of a pi system to another. The total number of sigma and pi bonds stays the same. These shifts are named with an order term written as [i,j]. The numbers i and j show how many atoms each end of the sigma bond travels across. A [3,3] shift in 1,5-hexadiene is called the Cope rearrangement, and a [1,7] hydride shift appears in 1,3,5-heptatriene. Heat can drive sigmatropic...

[3,3] sigmatropic rearrangement of 1,5-dienes: cope rearrangement

33-sigmatropic-rearrangement-of-15-dienes-cope-rearrangement

Cope Rearrangement and Stable Diene Isomers

The Cope rearrangement is a [3,3] sigmatropic shift in 1,5-dienes. It turns one 1,5-diene into a more stable isomeric 1,5-diene. The reaction is concerted, which means six electrons move at the same time. Two electrons come from a sigma bond, and four electrons come from two pi bonds. The change passes through an energetically favorable chair-like transition state. From a molecular orbital view, the rearrangement reflects how the ground-state frontier orbitals of the allyl anion and allyl...

[3,3] sigmatropic rearrangement of allyl vinyl ethers: claisen rearrangement

33-sigmatropic-rearrangement-allyl-vinyl-ethers-claisen

Claisen Rearrangement Products and Mechanism

The Claisen rearrangement is a [3,3] sigmatropic rearrangement that turns allyl vinyl ethers into unsaturated carbonyl compounds. It is a concerted pericyclic reaction, which means the bonds change in one linked step. The reaction also proceeds through a chair-like transition state. An aromatic Claisen rearrangement works with allyl aryl ethers. It forms an unstable ketone intermediate first. That intermediate then tautomerizes, or shifts to a more stable form, to give ortho-substituted...

woodward&ndash;hoffmann selection rules and microscopic reversibility

woodwardhoffmann-selection-rules-and-microscopic-reversibility

Orbital Symmetry in Pericyclic Reactions

Pericyclic reactions follow orbital symmetry rules that help predict how products form. These concerted reactions include electrocyclic reactions, cycloadditions, and sigmatropic rearrangements. They move through a cyclic transition state, which means the bonds change in a coordinated way. These reactions are stereospecific and regioselective. In other words, the product geometry and position of bond formation depend on the symmetry of the interacting orbitals and on the reaction conditions.

key terms and concepts

Compare conjugated and nonconjugated dienes, then see how π electron delocalization, sp2 carbons, and s-cis/s-trans forms affect stability.

How Conjugated Dienes Gain Stability | JoVE K12

organic chemistry

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How Conjugated Dienes Gain Stability

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