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Aromatic Substitution Reactions

Chapter 18. Aromatic Substitution Reactions 18.1 Electrophilic Aromatic Substitution 18.2 Monosubstituted Benzene Rings 18.3 Disubstituted Benzene Rings 18.4 Nucleophilic Aromatic Substitution 18.5 Elimination and Addition18.1 Electrophilic Aromatic Substitution Stability of benzene Under same condition, Alkene  addition / Benzene  NO addition Fe(iron) into mixture e.g., Bromination, Chlorination, Nitration, Sulfonation, Friedel-Crafts Alkylation, Friedel-Crafts Acylation18.1 Electrophilic Aromatic Substitution - Bromination Electrophile(Br 2 ) approaches to π electron cloud Br 2 to generate iron tribromide (FeBr 3 ) 2 Fe + 3 Br 2 → 2 FeBr 3 Sigma complex or Arenium ion: positively charged intermediate Sigma complex is not aromatic Alternative to FeBr 3 : Aluminum tribromide (AlBr 3 )18.1 Electrophilic Aromatic Substitution - Bromination Addition is not observed Permanent loss of aromaticity  thermodynamically unfavorable Substitution  E xergonic process Addition  Endergonic process Only substitution is observed18.1 Electrophilic Aromatic Substitution - Chlorination [1 st Step] aromatic ring functions as a nucleophile and attacks the electrophilic agent Generating a sigma complex [2 nd Step] the sigma complex is deprotonated Restoring aromaticity and regenerating the Lewis acid (AlCl 3 )18.1 Electrophilic Aromatic Substitution - Sulfonation Fuming sulfuric acid : mixture of H 2 SO 4 and SO 3 gas Sulfur trioxide (SO 3 ) : very powerful electrophile S=O double bond : overlap of p orbitals p orbitals are different sizes  overlap is less efficient S−O single bond in sulfur trioxide  charge separation (S + and O − )18.1 Electrophilic Aromatic Substitution – Nitration Mixture of nitric acid and sulfuric acid  Nitrobenzene is formed Electrophile: Nitronium ion (NO 2 + ) EAS (2 steps) : Nucleophilic attacks  [ Sigma complex ]  Deprotonation Aniline: 1) Metal (Fe or Zn) and HCl, the nitro group is reduced under acidic conditions, the resulting amino group is protonated to give an ammonium ion (R-NH 3 + ) 2) Sodium hydroxide (NaOH), to remove the extra proton and regenerate the amino group18.1 Electrophilic Aromatic Substitution Friedel-Crafts Alkylation Installation of an alkyl group on an aromatic ring Friedel–Crafts alkylation : benzene is treated with alkyl chloride in the presence of AlCl 3 Primary carbocations are extremely high in energy  Hydride shift Ratio of products depends on the conditions chosen (concentrations, temperature, etc.), but a mixture of products is unavoidable  Limitation18.1 Electrophilic Aromatic Substitution Friedel-Crafts Alkylation Limitations 1) Vinyl carbocations and aryl carbocations are not stable to be formed under Friedel–Crafts conditions 2) Polyalkylations often occur 3) Certain groups, such as a nitro group, that are incompatible with a Friedel–Crafts reaction -NH 2 , -NHR , -NR 2  activate with Lewis acid18.1 Electrophilic Aromatic Substitution Friedel-Crafts Acylation Installation of acylium ion on an aromatic ring Friedel–Crafts acylation : benzene is treated with acyl chloride in the presence of AlCl 3 Second resonance structure  full octets Clemmensen reduction Polyacylation is not observed18.2 Monosubstituted Benzene Rings Strong activators Moderate activators Weak activators  Electron Donating Group, EDG18.2 Monosubstituted Benzene Rings Strong deactivators Moderate deactivators Weak deactivators  Electron Withdrawing Group, EWG18.2 Monosubstituted Benzene Rings – Activation Group Nitration of Anisole18.2 Monosubstituted Benzene Rings – Deactivation Group Nitrobenzene  Electron Withdrawing Group18.2 Monosubstituted Benzene Rings – Halogens: the Exception Activator  ortho-para director Deactivator  usually meta director Exceptionally Halogens ( weak EWG)  ortho-para director18.3 Disubstituted Benzene Rings Steric Effects Directing Effects18.4 Nucleophilic Aromatic Substitution S N Ar : Nucleophilic Aromatic Substitution 1) Aromatic ring must contain a powerful electron-withdrawing group (typically a nitro group). 2) Aromatic ring must contain a leaving group (usually a halide). 3) Leaving group must be either ortho or para to the electron-withdrawing group. If the leaving group is meta to the nitro group, the reaction is not observed18.4 Nucleophilic Aromatic Substitution [ 1 st Step ] Aromatic ring is attacked by a nucleophile  F orm the intermediate Meisenheimer complex [ 2 nd Step ] Leaving group is expelled by ring attacking E +  R estore aromaticity18.5 Elimination and Addition Absence of powerful EWG  RXN cannot occur But in condition of high temperature and pressure, RXN can occur RXN also can at lower temperature using the amide ion (H 2 N − ) as a nucleophileThank You Q A{nameOfApplication=Show}

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Acids and Bases

Ch 3. Acids and Bases 3.1 Brønsted -Lowry Acids and Bases 3.2 Brønsted -Lowry Acidity : pK a 3.3 Brønsted -Lowry Acidity : Base Stability 3.4 Leveling Effects 3.5 Solvating Effects 3.6 Lewis Acids and Bases 3.1 Brønsted -Lowry Acids and Bases Acid : defined as proton donor Base : defined as proton acceptor Conjugate base : accept proton from Acid Conjugate acid : donate proton to Base Flow of Electron Density: Curved-Arrow Notation 3.2 Brønsted -Lowry Acidity : pK a

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Infrared Spectroscopy and Mass Spectrometry

Chapter 14. Infrared Spectroscopy and Mass Spectrometry 14.1 Light and Matter 14.2 Vibration Excitation and Energy Gap 14.3 Signal Characteristics 14.4 Analyzing IR Spectrum 14.5 Mass Spectrometry 14.1 Light and Matter Electromagnetic radiation : Both wave-like and particle-like Electromagnetic spectrum Wavelength : distance between adjacent peaks Frequency : number of wavelengths per unit time

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Chemical Reactivity and Mechanisms

- Kinetics vs. ThermodynamicsCase 1. C+D: Both thermodynamically and kinetically favoredCase 2. C+D: Thermodynamically favored E+F: Kinetically favored- Transition States vs. Intermediates Energy diagram of a multistep process→ Intermediates: local minima (valleys)→ Transition States: local maxima (peaks)

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Aromatic Compounds

Chapter 17. Aromatic Compounds 17.1 Benzene 17.2 Aromatic Compounds 17.3 Reactions at the Benzylic Position 17.4 Reduction of Benzene and Its Derivatives 17.5 Spectroscopy of Aromatic Compounds 17.1 Benzene Structure of Benzene Resonance C−C bond order of 1.5 (between a single bond and a double bond) Stability of Benzene 17.1 Benzene Source of Stability 6 overlapping p orbitals 3 bonding MOs / 3 antibonding MOs Each MO: two electrons Occupy bonding MOs All six

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