Name: hbr selectivity in alkene addition thermodynamics
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 32 s
Channel: JoVE

反マルコフニコフの添加はHBrでのみ観察されることを思い出してください。HClとHIによるラジカル付加は、熱力学的に好ましくありません。
でもどうして。
ラジカルメカニズムに関与する各伝播ステップの熱力学を調べてみましょう。
反応が自発的であるためには、ギブスの自由エネルギー∆Gの変化が負でなければなりません。
最初の伝播ステップでは、∆G の符号は、エンタルピー項とエントロピー項の間の競合によって決定されます。
HBrとHClの場合、低温ではエンタルピー項が支配的であり、∆Gは負になり、熱力学的に有利な反応を意味します。
しかし、HIの場合、エンタルピー項とエントロピー項の両方が正であり、温度に関係なく∆Gが正であることを意味し、熱力学的に好ましくない反応を示しています。
2 番目の伝播ステップでは、反応物と生成物分子の数が等しいため、エントロピー項は 0 に近くなります。したがって、エンタルピー項は∆Gの符号を決定します。
明らかなように、∆GはHClに対して陽性であり、反応を熱力学的に好ましくありません。
全体として、2つの伝播ステップはHBrの場合にのみ熱力学的に有利です。

アルケンへのラジカル反マルコフニコフ付加: 熱力学

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

アルケンへのハロゲン化水素の反マルコフニコフ付加は、HBr を使用した場合にのみ熱力学的に実現可能です。 HCl や HI などの他のハロゲン化水素とのラジカル付加反応は、熱力学的に好ましくありません。



<img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;">



熱力学的要因

温度は反応の自発性に影響を与えます。これは、ギブスの自由エネルギー ΔG の変化に基づいて評価できます。 ギブスの自由エネルギーの変化 ΔG が負の場合、反応は自発的に発生します。 以下に示すように、ΔG は 2 つの項、ΔH と -TΔS の値から直接評価できます。



<img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;">



HBr による反マルコフニコフ付加の熱力学的実現可能性は、ラジカル機構に関与する各伝播ステップから予測できます。

ラジカル反応の最初の伝播ステップでは、ΔG 符号はエンタルピー項とエントロピー項の間の競合によって決定されます。 低温では、HBr と HCl によるラジカル付加がエンタルピー項を支配します。 したがって、ΔG は負となり、反応が熱力学的に有利であることを示します。 しかし、高温では反応は吸熱的であり、熱力学的に好ましくありません。 対照的に、HI では、温度に関係なく、エンタルピー項とエントロピー項の両方が正になります。 その結果、ΔG はプラスになり、反応が熱力学的に不利であることを示します。

2 番目の伝播ステップでは、反応物質と生成物分子の数が等しいため、エントロピー項はゼロに近くなります。これは、ΔG 符号がエンタルピー項に依存することを意味します。 HBr と HI ではエンタルピー項が支配的であるため、反応は発熱性であり、熱力学的に有利です。 ただし、HCl の場合、ΔG は正であり、反応が吸熱的であり、熱力学的に不利であることを意味します。

要約すると、反応における 2 つの伝播ステップは、HBr の場合にのみ熱力学的に有利です。

アルケンへのハロゲン化水素の反マルコフニコフ付加は、HBrでのみ熱力学的に実現可能です。他のラジカル付加反応...

radical-anti-markovnikov-addition-to-alkenes-thermodynamics

Education

JoVE Core

Organic Chemistry

有機化学

Unit 1

ユニット1

Radical Chemistry

ラジカル化学

KEY TERMS AND CONCEPTS

主要用語と概念

hbr selectivity in alkene addition thermodynamics

Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable.

Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term.

The first propagation step depends on the balance between enthalpy and entropy. At lower temperatures, radical addition with HBr and HCl is driven by the enthalpy term, so ΔG is negative and the step is favorable. At higher temperatures, the reaction becomes endothermic and thermodynamically unfavorable. With HI, both the enthalpy and entropy terms are positive at any temperature, so ΔG stays positive.

The second propagation step is different because the number of reactant and product molecules is equal. That makes the entropy term close to zero, so ΔG depends mainly on enthalpy. For HBr and HI, the enthalpy term makes the step exothermic and favorable. For HCl, ΔG is positive, so the step is endothermic and unfavorable. Taken together, both propagation steps are thermodynamically favorable only with HBr.

HBr Selectivity in Alkene Addition Thermodynamics

hbr-selectivity-in-alkene-addition-thermodynamics

radicals: electronic structure and geometry

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Radical Geometry: Planarity and Inversion

Radical geometry depends on the electronic structure around the radical center. An unpaired electron, like a lone pair, affects the shape of the molecule. For a trivalent radical, the structure falls between the geometries of carbocations and carbanions. A carbocation with sp2 hybridization is trigonal planar. A carbanion with sp3 hybridization is trigonal pyramidal. The difference comes from the number of nonbonding electrons. The carbocation has none, while the carbanion has two. A carbon...

electron paramagnetic resonance (epr) spectroscopy: organic radicals

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EPR Hyperfine Splitting in Organic Radicals

Electron paramagnetic resonance (EPR) spectroscopy can show how an unpaired electron behaves in an organic radical. A simple radical may give a single peak because the electron moves between two spin energy states. In many radicals, the unpaired electron also interacts with nearby spin-active nuclei. This interaction is called hyperfine coupling. It causes hyperfine splitting, which divides the EPR signal into multiplets, or groups of peaks. The number of peaks follows the pattern 2nI + 1,...

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How Radicals Are Generated

Radicals are generated when bonds break in specific ways or when one radical leads to another. In homolytic bond cleavage, a bond splits evenly and forms radicals. In heterolytic bond cleavage, the bond breaks to form ions instead. A fishhook arrow shows the movement of a single electron during homolytic cleavage. Radicals can come from spin-paired molecules by homolysis or by electron transfer. Homolysis forms two radicals. Electron transfer adds an electron to the molecule, and this process...

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Homolysis and Radical Bonds

Homolysis is a bond-breaking process that splits a shared electron pair evenly. When a bond is broken with enough energy, the two electrons can also move to one atom in heterolysis, which forms ions. Homolysis gives one electron to each atom and creates radicals, which are highly reactive particles with an unpaired electron. HCl shows the difference between these bond cleavages. In solution, HCl can break into H+ and Cl− ions, and chlorine takes both bonding electrons, leaving a naked proton.

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Radical Abstraction in Chain Reactions

Radical abstraction helps drive radical chain reactions. In this process, a relatively unstable radical removes an atom from a compound and forms a new radical that is more stable. The atom taken in this step can be hydrogen or a halogen. A radical initiator can first form radicals by homolysis, which is the breaking of a spin-paired molecule into two radicals. That is different from abstraction. In abstraction, a radical reacts with a spin-paired molecule and produces one new radical plus one...

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Radical Addition and Spin Conservation

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Radicals from Elimination Reactions

Radicals can form through elimination reactions. This route is the opposite of the addition route. It is driven by the instability of the radical. Dibenzoyl peroxide is one example of this process. It first undergoes homolysis, which is an even split of a bond, to give a pair of unstable radicals. Because the radical is unstable, it then undergoes elimination on its own. During elimination, a C-C bond breaks. The cleavage occurs between the alpha and beta positions relative to the radical...

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Radical Reaction Pathways and Stability

Radical reactions follow a small set of pathways that help highly reactive species become more stable. A radical can react with another radical, with a spin-paired molecule, or through unimolecular decomposition, which means one radical species breaks down on its own. In radical-radical coupling, two radicals join to form a spin-paired molecule. In a radical reaction with a spin-paired molecule, a new radical and a new spin-paired molecule are formed. In radical decomposition, one radical...

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Steric Shielding in Stable Radicals

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Electrophilic Radicals and Alkene Reactions

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Nucleophilic Radicals and Alkene Traps

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Radical Cyclization in One Molecule

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Autoxidation in Oils and Ethers

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Selective MnO2 Oxidation of Alcohols

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How Alkanes Form Alkyl Halides by Halogenation

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Alkane Halogenation: Stability and Reaction Rates

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Radical Halogenation: Chiral Products

Radical halogenation can change the chirality of a molecule, which is the three-dimensional arrangement of its atoms. The products depend on whether the reaction creates a new chiral center, reacts at an existing chiral center, or adds a second chiral center. When radical halogenation forms a new chiral center, the product can be a racemic mixture. For example, halogenation of butane gives 2-chlorobutane and 1-chlorobutane. The 2-chlorobutane product has a chiral center, but it is formed as a...

radical substitution: allylic chlorination

radical-substitution-allylic-chlorination

Allylic Chlorination by Radical Chain Reaction

Allylic chlorination is a radical chain reaction in alkenes. It happens when halogen reactions move away from simple addition and toward substitution at the allylic position, which is the carbon next to a double bond. At low temperatures, alkenes usually react with halogens by addition. When the temperature increases, or when the reaction conditions create radicals, allylic substitution becomes favored. This works because allylic hydrogens are very reactive and the intermediate formed is...

radical substitution: allylic bromination

Allylic Bromination with NBS

Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form. The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene...

radical substitution: hydrogenolysis of alkyl halides with tributyltin hydride

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Alkyl Halide Reduction with Tributyltin Hydride

Alkyl halide reduction with tributyltin hydride removes a halogen and replaces it with hydrogen. This reaction turns alkyl halides into alkanes. The reagent Bu3SnH has a weak Sn–H bond, and the reaction forms tributyltin halide as a byproduct. Because the new bonds are stronger than the bonds that are broken, the reaction is energetically favorable. The reaction proceeds by a radical chain mechanism, which is similar to radical halogenation. Light causes Bu3SnH to split homolytically, meaning...

radical anti-markovnikov addition to alkenes: overview

radical-anti-markovnikov-addition-to-alkenes-overview

Peroxide Effect in HBr Addition to Alkenes

Hydrogen bromide can add to alkenes through a peroxide-driven radical pathway. In this reaction, hydroperoxides or peroxides change the usual addition pattern and lead to alkyl bromides with anti-Markovnikov regiochemistry, which means bromine ends up on the less substituted carbon. This regioselectivity can be explained by radical stability. When the bromine radical adds to the alkene at the less substituted carbon, the reaction forms a more stable tertiary radical intermediate. The same...

radical anti-markovnikov addition to alkenes: mechanism

radical-anti-markovnikov-addition-to-alkenes-mechanism

Hydrogen Bromide Addition to Alkenes by Radicals

Hydrogen bromide can add to alkenes through a radical chain reaction when hydroperoxides or peroxides are present. This pathway gives anti-Markovnikov addition, which means the bromine ends up on the less substituted side of the double bond. The reaction begins with chain initiation. First, mild heating breaks the weak peroxide bond by homolytic cleavage, which means each atom keeps one electron and two alkoxy radicals form. Next, an alkoxy radical abstracts a hydrogen atom and forms a bromine...

radical anti-markovnikov addition to alkenes: thermodynamics

Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable. Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term. The...

vicinal diols via reductive coupling of aldehydes or ketones: pinacol coupling overview

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Pinacol Coupling and Solvent Effects

Pinacol coupling is a radical dimerization reaction that joins aldehydes or ketones to form vicinal diols. Wilhelm Rudolph Fittig first discovered this reaction in 1859. It is a reductive coupling, which means the molecules are joined after they gain electrons. The reaction begins with a single electron transfer from metals such as sodium or magnesium to an aldehyde or ketone. This forms a ketyl, a radical anion. In a ketyl, the carbon atom can carry radical character while the oxygen atom...

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Titanium-Coupled Alkenes from Carbonyls

The McMurry reaction turns aldehydes or ketones into alkenes with titanium metal. It first forms a diol-like intermediate through a coupling step, and then removes oxygen to make the double bond. In some cases, this process can be compared to pinacol coupling, which also joins carbonyl compounds. Titanium acts as a source of electrons in the first step. It transfers a single electron to the carbonyl group and forms a ketyl intermediate. That intermediate can lead to a vicinal diol, which is a...

&alpha;-hydroxy ketones via reductive coupling of esters: acyloin condensation overview

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Acyloin Condensation: Ester Coupling Pathway

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key terms and concepts

See why anti-Markovnikov addition to alkenes is thermodynamically favorable only with HBr, and how ΔG, enthalpy, entropy, and temperature affect each step.

HBr Selectivity in Alkene Addition Thermodynamics | JoVE K12

organic chemistry

JoVE Core 有機化学では、立体化学、反応、メカニズムの基本概念を、簡潔でわかりやすいアニメーション ビデオ レッスンを使用して説明します。さらに、科学者の実際のビデオでは、世界中の今日の研究室で行われている古典的および独自の研究実験における関連概念の適用を示しています。

HBr Selectivity in Alkene Addition Thermodynamics

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