Name: alkane halogenation: stability and reaction rates
Uploaded: 2023-04-30T11:21:58.000Z
Duration: 1 min 34 s
Channel: JoVE

Radykowa halogenacja alkanów przebiega według innej kolejności reaktywności dla różnych halogenów.
Różnicę tę można zrozumieć na podstawie termodynamiki reakcji.
W przypadku halogenowania alkanów składnik entropii jest znikomy, ponieważ liczba reagentów i cząsteczek produktu jest równa. Dlatego ΔG staje się równoważne ΔH.
Weźmy pod uwagę radykalną fluorację metanu. Entalpię reakcji szacuje się na podstawie energii dysocjacji wiązań zerwanych i powstałych wiązań.
Całkowite ΔH dla reakcji jest ujemne i duże, co sprawia, że reakcja jest termodynamicznie korzystna, ale wysoce wybuchowa i niepraktyczna.
I odwrotnie, w przypadku jodacji, dodatnia wartość ΔH sprawia, że jest ona termodynamicznie niekorzystna i niemożliwa do osiągnięcia. 
Dlatego tylko chlorowanie i bromowanie o ujemnych wartościach ΔH są termodynamicznie korzystne i praktycznie wykonalne.
Porównanie tych dwóch reakcji pokazuje, że bromowanie jest wolniejsze niż chlorowanie.
Bliższe przyjrzenie się poszczególnym etapom propagacji pokazuje, że pierwszy etap propagacji bromacji jest bardziej endotermiczny, co wpływa na ogólną szybkość reakcji.
Ponieważ reakcja netto jest nadal egzotermiczna, bromacja zachodzi, ale powoli.

halogenowanie rodnikowe: termodynamika

Radical Halogenation: Thermodynamics

Termodynamiczna korzystność reakcji zależy od zmiany energii swobodnej Gibbsa (ΔG). ΔG ma dwa składniki – entalpię (ΔH) i entropię (ΔS). Składnik entropii jest pomijalny w przypadku halogenowania alkanów, ponieważ liczba reagentów i cząsteczek produktu jest równa. W tym przypadku ΔG zależy wyłącznie od składnika entalpii. Najważniejszym czynnikiem determinującym ΔH jest siła wiązań. ΔH można wyznaczyć poprzez porównanie energii pomiędzy wiązaniami zerwanymi i wiązaniami utworzonymi.

W oparciu o termodynamikę reakcji rodnikowe halogenowanie alkanów ma inny rząd reaktywności dla fluorowania, bromowania i jodowania. Wartość ΔH dla jodowania rodnikowego jest dodatnia (+55 kJ/mol), co sugeruje, że wartość ΔG jest również dodatnia dla tej reakcji. Dlatego jodowanie jest niekorzystne termodynamicznie i reakcja nie zachodzi. Z drugiej strony, całkowite ΔH rodnikowego fluorowania metanu jest duże i ujemne (-431 kJ/mol), co sprawia, że reakcja jest korzystna termodynamicznie, ale jest wysoce egzotermiczna i nie ma żadnego praktycznego zastosowania. Wartość ΔH dla chlorowania i bromowania wynosi odpowiednio -104 kJ/mol i -33 kJ/mol, co czyni te reakcje termodynamicznie korzystnymi i praktycznie wykonywalnymi. Porównanie szybkości reakcji chlorowania i bromowania pokazuje, że bromowanie jest wolniejsze niż chlorowanie. Etapem decydującym o szybkości tej reakcji jest pierwszy etap propagacji lub etap abstrakcji wodoru. Pierwszy etap propagacji w reakcji chlorowania jest egzotermiczny, a energia aktywacji jest mała, podczas gdy w przypadku bromowania ten etap jest endotermiczny, a energia aktywacji jest duża, co wyjaśnia, dlaczego bromowanie jest wolniejsze niż chlorowanie.

Halogenowanie rodnikowe: termodynamika

Termodynamiczna korzyść reakcji zależy od zmiany energii swobodnej Gibbsa (ΔG). ΔG ma dwie składowe- entalpię (ΔH) ...

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Organic Chemistry

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Radical Chemistry

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alkane halogenation: stability and reaction rates

Alkane halogenation is shaped by thermodynamic stability and reaction rate. In radical halogenation, the key question is whether the reaction is favorable and how fast it happens. Gibbs free energy, or ΔG, helps answer the first question.

ΔG has two parts: enthalpy, or ΔH, and entropy, or ΔS. For alkane halogenation, the entropy term is negligible because the number of reactant and product molecules is the same. That means ΔG is controlled mainly by enthalpy.

Bond strength is the most important factor in ΔH. Scientists estimate ΔH by comparing the energy needed to break bonds with the energy released when new bonds form. This comparison explains why fluorination, chlorination, bromination, and iodination do not all behave the same way.

Radical iodination has a positive ΔH of +55 kJ/mol. That makes ΔG positive as well, so the reaction is thermodynamically unfavorable and does not take place. Radical fluorination of methane has a very large negative ΔH of -431 kJ/mol, so it is favorable but highly exothermic and not practical.

Chlorination and bromination are both thermodynamically favorable. Their ΔH values are -104 kJ/mol for chlorination and -33 kJ/mol for bromination. This makes both reactions feasible, but bromination happens more slowly than chlorination.

The rate-determining step is the first propagation step, also called hydrogen abstraction. For chlorination, this step is exothermic and has a small activation energy. For bromination, the same step is endothermic and has a large activation energy, which explains the slower rate.

Alkane Halogenation: Stability and Reaction Rates

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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.

radical formation: abstraction

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

Radical addition can form a new radical when an unpaired electron adds to a spin-paired molecule. This is often seen with unsaturated species. In these reactions, the radical adds across a pi bond and breaks that bond to make a new radical. A bromine radical adding to an alkene is a clear example. The product is a carbon-centered radical. Radical reactions also follow spin conservation, much like chemical reactions follow charge conservation. If the starting material has an unpaired electron,...

radical formation: elimination

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...

radical reactivity: overview

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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Electrophilic Radicals and Alkene Reactions

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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

Alkanes can react with halogens to form alkyl halides when heat or light is present. This reaction is called radical halogenation. It is a substitution reaction, which means a hydrogen atom on the alkane is replaced by a halogen atom. Radical chlorination of methane shows the process clearly. The reaction begins with initiation, when a chlorine molecule splits evenly under light or heat. This homolytic cleavage forms two very reactive chlorine radicals. The next stage is propagation. In the...

radical halogenation: thermodynamics

Alkane halogenation is shaped by thermodynamic stability and reaction rate. In radical halogenation, the key question is whether the reaction is favorable and how fast it happens. Gibbs free energy, or ΔG, helps answer the first question. ΔG has two parts: enthalpy, or ΔH, and entropy, or ΔS. For alkane halogenation, the entropy term is negligible because the number of reactant and product molecules is the same. That means ΔG is controlled mainly by enthalpy. Bond strength is the most...

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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

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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

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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

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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

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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...

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...

alkenes via reductive coupling of aldehydes or ketones: mcmurry reaction

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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

Acyloin condensation is a reaction that joins two ester molecules to form an alpha-hydroxy ketone. The product is also called an acyloin. In this reaction, sodium metal is used in an aprotic solvent, which supports the reductive coupling step. The reaction is related to the pinacol and McMurry reactions, which also involve reductive coupling of ketones or aldehydes. Here, the ester first becomes a ketyl. These ketyl species undergo radical dimerization and form an unstable tetrahedral...

key terms and concepts

Explore how bond strength, ΔH, and ΔG affect radical halogenation of alkanes, and why chlorination, bromination, fluorination, and iodination differ.

Alkane Halogenation: Stability and Reaction Rates | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Alkane Halogenation: Stability and Reaction Rates

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