Name: steric shielding in stable radicals
Uploaded: 2023-04-30T11:20:59.000Z
Duration: 1 min 10 s
Channel: JoVE

라디칼 중심에 인접한 전자 공여기, 전자 인출 및 접합 그룹의 존재는 전자적으로 안정화된 라디칼을 생성한다는 것을 기억하십시오.
이러한 전자적으로 안정화된 라디칼 중 일부는 지속성 라디칼로 알려져 있습니다. 이 급진파는 놀라울 정도로 안정적입니다. 일부는 분리하여 정제할 수도 있습니다.
전자 요인 외에도 입체 요인은 이러한 지속적인 라디칼의 안정성에 기여합니다.
예를 들어, 트리페닐메틸 라디칼을 생각해 보십시오. 이 라디칼의 매우 높은 안정성은 3 개의 페닐 고리의 존재 때문입니다.
주변의 3 개의 고리는 프로펠러와 같은 형태로 평면에서 30 ° 비틀어져 있습니다.
결과적으로, 라디칼 특성을 지닌 중앙 탄소는 꼬인 페닐 고리에 의해 입체적으로 차폐되어 라디칼을 매우 안정적이며 반응에 사용할 수 없게 만듭니다.
전반적으로 입체 효과는 라디칼을 더 안정적이고 덜 반응적으로 만듭니다.

라디칼 반응성: 입체 효과

Radical Reactivity: Steric Effects

라디칼 중심에 인접한 전자 공여성, 전자 구인성 또는 공액 그룹의 존재는 라디칼에 전자 안정화를 부여합니다. 이렇게 전자적으로 안정화된 라디칼의 예로는 트리페닐메틸, 테트라메틸피페리딘-N-산화물 및 2,2-디페닐-1-피크릴히드라질이 있습니다. 이러한 라디칼은 매우 안정적이며 지속성 라디칼로 알려져 있습니다. 일부 지속성 라디칼은 분리 및 정제될 수도 있습니다.

전자적 요인과 함께 입체적 요인도 이러한 지속적인 라디칼의 안정성을 설명합니다. 예를 들어, 트리페닐메틸 라디칼의 예외적으로 높은 안정성은 프로펠러 구조에서 평면 밖으로 30° 비틀어져 있는 세 개의 주변 페닐 고리가 존재하기 때문입니다. 이 꼬인 페닐 고리는 대부분의 라디칼 특성을 지닌 중앙 탄소를 입체적으로 보호합니다. 결과적으로 라디칼은 매우 안정해지고 분자가 반응할 수 없게 됩니다. 따라서 입체 장해는 라디칼에 안정성을 부여하여 반응성을 줄입니다.

라디칼 중심에 인접한 전자 공여, 전자 인출 또는 접합 그룹의 존재는 전자 안정화를 부여합니다.

Education

교육

JoVE Core

Organic Chemistry

유기 화학

Unit 1

제1단원

Radical Chemistry

라디컬 화학

KEY TERMS AND CONCEPTS

주요 용어 및 개념

steric shielding in stable radicals

Steric shielding helps make some radicals more stable and less reactive. A radical is a species with an unpaired electron at its center. In many cases, nearby groups can also give electronic stabilization, including electron-donating, electron-withdrawing, or conjugating groups.

Several radicals are known as persistent radicals because they are remarkably stable. Examples include triphenylmethyl, tetramethylpiperidine-N-oxide, and 2,2-diphenyl-1-picrylhydrazyl. Some of these persistent radicals can even be isolated and purified.

Steric factors also play a major role in that stability. The triphenylmethyl radical is especially stable because three phenyl rings surround the radical center. These rings are twisted out of the plane by 30° in a propeller shape.

This twisted shape shields the central carbon, which holds most of the radical character. As a result, the radical is harder for other molecules to reach and react with. Steric hindrance therefore makes radicals more stable and less reactive.

Steric Shielding in Stable Radicals

radicals: electronic structure and geometry

radicals-electronic-structure-and-geometry

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

electron-paramagnetic-resonance-epr-spectroscopy-organic-radicals

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...

radical formation: addition

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...

radical reactivity: steric effects

Steric shielding helps make some radicals more stable and less reactive. A radical is a species with an unpaired electron at its center. In many cases, nearby groups can also give electronic stabilization, including electron-donating, electron-withdrawing, or conjugating groups. Several radicals are known as persistent radicals because they are remarkably stable. Examples include triphenylmethyl, tetramethylpiperidine-N-oxide, and 2,2-diphenyl-1-picrylhydrazyl. Some of these persistent...

radical reactivity: concentration effects

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

radical-reactivity-electrophilic-radicals

Electrophilic Radicals and Alkene Reactions

Electrophilic radicals are radicals next to electron-withdrawing groups. These radicals react readily with nucleophilic alkenes. A key example is the malonate radical, where the radical center is flanked by two electron-withdrawing groups. It reacts easily with butyl vinyl ether, which has an electron-donating oxygen substituent. This reaction is favored because the malonate radical has a low-energy SOMO, or singly occupied molecular orbital. That low-energy orbital matches best with the...

radical reactivity: nucleophilic radicals

Nucleophilic Radicals and Alkene Traps

Nucleophilic radicals are electron-rich radicals that form next to electron-donating groups. They react readily with electrophilic alkenes, which are electron-poor double bonds. The key driving force is a SOMO–LUMO interaction. The high-energy SOMO, or singly occupied molecular orbital, of the radical overlaps with the low-energy LUMO of the alkene. These SOMO–LUMO interactions also explain radical traps and their effect on reaction selectivity. In radical chemistry, a trap is a molecule that...

radical reactivity: intramolecular vs intermolecular

radical-reactivity-intramolecular-vs-intermolecular

Radical Cyclization in One Molecule

Radical reactions can happen either between two separate molecules or within one molecule. In an intermolecular radical reaction, a nucleophilic radical adds to an electrophilic alkene, or an electrophilic radical adds to a nucleophilic alkene. The alkene is the radical trap, and it is a different molecule from the radical. Intermolecular reactions need the radical trap to be active and present in excess. They also usually require the radical starting material to contain a weak carbon-halogen...

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...

radical oxidation of allylic and benzylic alcohols

radical-oxidation-of-allylic-and-benzylic-alcohols

Selective MnO2 Oxidation of Alcohols

Activated manganese(IV) oxide, or MnO2, is a selective oxidizing agent for allylic and benzylic alcohols. It changes primary allylic alcohols into aldehydes and secondary allylic alcohols into ketones. The reaction happens by a radical pathway, so the carbon next to the alcohol group plays a key role. The oxidizing agent is made from the redox reaction of potassium permanganate with an Mn(II) salt such as manganese sulfate. This can be done under alkaline or acidic conditions. After that, the...

radical substitution: halogenation of alkanes and alkyl substituents

radical-substitution-halogenation-of-alkanes-and-alkyl-substituents

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: 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...

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

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

radical-substitution-hydrogenolysis-alkyl-halides-with-tributyltin

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

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

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

vicinal-diols-via-reductive-coupling-aldehydes-or-ketones-pinacol

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

alkenes-via-reductive-coupling-aldehydes-or-ketones-mcmurry

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

hydroxy-ketones-via-reductive-coupling-esters-acyloin-condensation

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 steric hindrance and surrounding phenyl rings stabilize persistent radicals such as triphenylmethyl, TMNO, and DPPH.

Steric Shielding in Stable Radicals | 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.

Steric Shielding in Stable Radicals

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