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

Radical halogenation of alkanes follows a different order of reactivity for different halogens.
This difference can be understood from the thermodynamics of the reaction.
For alkane halogenation, the entropy term is negligible as the number of reactants and product molecules are equal. Therefore, &#916;G becomes equivalent to &#916;H.
Consider the radical fluorination of methane. The enthalpy of the reaction is estimated from the bond dissociation energies of the bonds broken and the bonds formed.
The overall &#916;H for the reaction is negative and large, making the reaction thermodynamically favorable but highly explosive and impractical.
Conversely, for iodination, the positive &#916;H value makes it thermodynamically unfavorable and impossible to achieve.&#160;
Therefore, only chlorination and bromination with negative &#916;H values are thermodynamically favorable and practically feasible.
A comparison of the two reactions reveals that bromination is slower than chlorination.
A closer look at the individual propagation steps shows that the first propagation step of bromination is more endothermic, which affects the overall reaction rate.
Since the net reaction is still exothermic, bromination occurs, but slowly.

radicale halogenering: thermodynamica

Radical Halogenation: Thermodynamics

De thermodynamische voorkeur van een reactie wordt bepaald door de verandering in de vrije energie van Gibbs (ΔG). ΔG heeft twee componenten: enthalpie (ΔH) en entropie (ΔS). De entropiecomponent is verwaarloosbaar voor alkaanhalogenering, omdat het aantal reactanten en productmoleculen gelijk is. In dit geval wordt de ΔG alleen bepaald door de enthalpiecomponent. De belangrijkste factor die ΔH bepaalt, is de sterkte van de bindingen. ΔH kan worden bepaald door de energie tussen verbroken en gevormde bindingen te vergelijken.


 Op basis van de thermodynamica van de reactie heeft radicaalhalogenering van alkanen een andere volgorde van reactiviteit voor fluorering, bromering en jodering. De ΔH voor radicaaljodering is positief (+55 kJ/mol), wat suggereert dat de ΔG waarde ook positief is voor deze reactie. Daarom is jodering thermodynamisch ongunstig en vindt de reactie niet plaats. Aan de andere kant is de totale ΔH voor de radicale fluorering van methaan groot en negatief (-431 kJ/mol), waardoor de reactie thermodynamisch gunstig maar zeer exotherm is en geen enkel praktisch nut heeft. De ΔH-waarde voor chlorering en bromering bedraagt respectievelijk -104 kJ/mol en -33 kJ/mol, waardoor deze reacties thermodynamisch gunstig en praktisch haalbaar zijn. De vergelijking van de reactiesnelheid tussen chlorering en bromering laat zien dat bromering langzamer is dan chlorering. De snelheidsbepalende stap voor deze reactie is de eerste propagatiestap of de waterstofabstractiestap. De eerste vermenigvuldigingsstap voor de chloreringsreactie is exotherm en de activeringsenergie is klein, terwijl voor bromering deze stap endotherm is en de activeringsenergie groot is, wat verklaart waarom bromering langzamer is dan chlorering.

Radicale halogenering: thermodynamica

The thermodynamic favorability of a reaction is determined by the change in Gibbs free energy (&#916;G). &#916;G has two components- enthalpy (&#916;H) ...

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Organic Chemistry

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Onderdeel 1

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,...

radical formation: overview

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...

radical formation: homolysis

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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Steric Shielding in Stable Radicals

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How Concentration Steers Radical Additions

Radical reaction selectivity can change when the concentrations of the reactants change. In this chemistry example, an alkyl halide reacts with an alkene in the presence of tin hydride and AIBN, an initiator that helps start the radical process. The reaction begins when a tin radical is formed. The tin radical removes the halogen from the alkyl halide. This step creates an alkyl radical. That radical then has two possible paths. It can react with tin hydride to form an alkane, or it can add to...

radical reactivity: electrophilic radicals

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Electrophilic Radicals and Alkene Reactions

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Nucleophilic Radicals and Alkene Traps

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radical reactivity: intramolecular vs intermolecular

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Radical Cyclization in One Molecule

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radical autoxidation

Autoxidation in Oils and Ethers

Autoxidation is the oxidation of an organic compound in the presence of air or oxygen. In this process, compounds such as cumene can react with oxygen to form hydroperoxide. The reaction follows three main stages: initiation, propagation, and termination. Many organic compounds can undergo autoxidation. Ethers are especially vulnerable when oxygen is present, and they can form hydroperoxides. Even though this reaction is slow, old ether bottles may contain small amounts of peroxide. That is...

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Selective MnO2 Oxidation of Alcohols

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radical substitution: halogenation of alkanes and alkyl substituents

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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...

radical halogenation: stereochemistry

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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