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Chapter 19

Preparation of Alkenes Elimination. Chapter 19. Addition of Electrophiles to Alkene. Electrophilic Addition of HX. δ +. δ –. electrophile. nucleophile. Reaction Conditions.

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Chapter 19

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  1. Preparation of Alkenes Elimination Chapter 19

  2. Addition of Electrophiles to Alkene

  3. Electrophilic Addition of HX δ+ δ– electrophile nucleophile

  4. Reaction Conditions • hydrogen halide: HXcommon solvents: chloroform (CHCl3) ,dichloromethane (CH2Cl2), pentane, acetic acidgenerally performed at low temperature (below 0 °C)generally a fast reaction

  5. Electrophilic Addition (AdE) Mechanism • electrophilic addition: AdERDS = protonation of carbonrate = k[alkene][hydrogen halide]unlike oxygen and nitrogen, protonation of carbon is slowproceeds through carbocation intermediate

  6. HX Addition is Regioselective Regioselectivity Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction CHEM 232 Definition, 2010

  7. Markovnikov’s Rule 2º 3º addition of HX to an unsymmetrically substituted alkene proceeds so that hydrogen (H) adds to the least substituted carbon and the halide (X) adds to the most substituted carbon atom

  8. Self Test Question Predict the product when 2,4-dimethyl-2-pentene is treated with HCl? A. 3-chloro-2,4-dimethylpentane B. 2-chloroohexane C. 2,3-dichloro-2,4-dimethylpentane D. 2-chloro-2,4-dimethylpentane E. 1-chloro-2,4-dimethylpentane

  9. Mechanistic Basis for Markovnikov’s Rule

  10. Mechanistic Basis for Markovnikov’s Rule • Hammond Postulate: • transition state structure resembles closest energy intermediate • transition state resembles carbocation for endothermic RDS (late transition state) • what stabilizes carbocation also stabilizes transition state • lowest energy transition state leads to more substituted carbocation • more substituted carbocation provides more substituted alkyl halide

  11. Rate of Alcohol DehydrationMirrors Ease of Carbocation Formation Reactivity Stability rate of dehydration = 3º > 2º > 1º alcohol tertiary cation (3º) tertiary alcohol (3º) secondary cation (2°) secondary alcohol (2º) primary alcohol (1º) primary cation (1°)

  12. Self Test Question Predict the product for the following reaction scheme. A. B. C. D. E. no reaction

  13. Self Test Question Predict the product for the following reaction scheme. A. B. C. D. E. no reaction

  14. Dehydration can be “Coupled” with Other Chemical Transformation Two-step, one-pot transformation involves a Friedel-Crafts reaction (see, Chapter 12) and dehydration of the resulting 3° alcohol

  15. Regioselectivity & Stereoselectivity of Dehydration Chapter 19

  16. Self Test Question What is the product(s) of the following reaction? A. B. C. D. E.

  17. Types of Selectivity in Organic Chemistry There are three forms of selectivity to consider . . . . • Chemoselectivity: whichfunctional group will react • Regioselectivity: where it will react • Stereoselectivity: howit will react with regards to stereochemical outcome . . . for each transformation, always question which of these are factors are at play.

  18. Regioselectivity of Elimination Regioselectivity: Where Will It React? Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction CHEM 232 Definition, 2009

  19. Rearrangement Can Precede Addition

  20. Stability of Carbocations (Lecture 8) 2. Hyperconjugation stabilizing interaction that results from the interaction of the electrons in a σ-bond (C–H or C–C bond ) with an adjacent empty (or partially filled) orbital. Leads to the formation of an extended molecular orbital that increases the stability of the system • stabilization results from σ-donation to empty p orbital of planar carbocation • electron donation through σ-bonds toward carbocation delocalizes charge (spreads out) • methyl cations cannot be stabilized by hyperconjugation since σ-bonds are perpendicular to the empty p orbital 1º cation

  21. Stability of Carbocations (Lecture 8) 2. Hyperconjugation

  22. Rearrangement Can Precede Addition

  23. Reversal of Addition RegioslectivtyThe Peroxide Effect • alkyl peroxides easily fromed from alkanes/alkenes by reaction with O2 in the air • presence of peroxides leads to anti-Markovnikov product (least substituted alkyl bromide) • peroxide effect only operates when HBr

  24. Mechanistic Rationale for Peroxide Effect peroxides are radical initiators: they undergo homolysis to generate alkoxy radicals, which begin the chain mechanism

  25. Mechanistic Rationale for Peroxide Effect • bromine radical adds to the least substituted carbon of alkene • this generates the mostsubstituted and most stable alkyl radical • alkyl radical undergoes hydrogen abstraction from HBr to generate a new bromine radical (chain mechanism)

  26. Addition of Sulfuric Acid to Alkenes alkene compare to: alcohol elimination alkene(s)

  27. Sulfuric Acid Addition: AdE Mechanism • Markovnikov’s rule appliesprotonation occurs to provide most stable (most substituted) carbocationleads to formation of most substituted alkyl hydrogen sulfate

  28. Hydrolysis of Alkyl Hydrogen Sulfates • don’t worry about mechanism for hydrolysisonly requires hot watercleavage of the O-S bondsubstitution of S with H

  29. Examples of Alkene Hydration

  30. Hydration of Alkenes (Addition of Water) compare to: alcohol elimination alkene(s)

  31. Hydration: AdE Mechanism Principle of Microscopic Reversibility in an equilibrium, the forward mechanism is identical to the reverse mechanism

  32. Hydration: AdE Mechanism Principle of Microscopic Reversibility in an equilibrium, the forward mechanism is identical to the reverse mechanism

  33. Spectroscopy &Spectrometry

  34. Analytical Chemistry Separation of MixturesandIdentification of Components

  35. High Performance Liquid Chromatography

  36. Gas Chromatography

  37. Structural Determination

  38. Spectroscopy vs. Spectrometry Spectroscopy study of the interaction of electromagnetic radiation with matter; typically involves the absorption of electromagnetic radiation Spectrometry evaluation of molecular identity and/or properties that does not involve interaction with electromagnetic radiation

  39. Spectroscopic Methods

  40. Absorption/Transmission Spectroscopy: Simplified Principles • sample absorbs different frequencies of light corresponding to molecular vibrations (IR) or electronic transitions (UV-vis) • detector determines what frequencies of light passed through (transmittance) and what frequencies of light were absorbed (absorbance)

  41. Electromagnetic Spectrum shorter wavelength (λ) higher frequency (ν) higher energy (E) longer wavelength (λ) lower frequency (ν) lower energy (E) Electromagnetic Radiation • propagated at the speed of light (3 x108 m/s) • has properties of particles and waves • energy is directly proportional to frequency • energy is indirectly proportional to wavelength E = hν c = νλ

  42. Quantized Energy States Increasing Energy

  43. Infrared Spectroscopy

  44. Principles of Infrared Spectroscopy IR: Measures the vibrational energy associated with stretching or bending bonds that contain a dipole moment (µ). Stretching Bending

  45. Stretching & Bending Vibrations

  46. Dipole Moment In order to measure the stretching or bending frequency of a covalent bond, it must have a dipole moment (μ).

  47. Hooke’s Law: Bonds are Like Springs Vibrational Energy Depends both on bond strength (spring force constant) and the mass of atoms (objects) attached √ (m1 + m2) ~ f * Trends: ↑ bond strength = ↑ frequency ↑ mass = ↓ frequency ν = k (m1 * m2) ~ ν = vibrational “frequency” in wavenumbers (cm-1) k = constant (1/2πc) f = force constant; strength of bond (spring) m1, m2 = masses (not molecular weights) of attached atoms

  48. Spring Analogy smaller mass = higher frequency = higher energy stronger spring (bond) = higher frequency = higher energy

  49. Wavenumber (ῡ) and Infrared Scale 1 ῡ (cm-1) = λ (cm) higher wavenumber (ῡ) = higher frequency (υ) = lower wavelength (λ) = higher energy (E) lower wavenumber (ῡ) = lower frequency (υ) = longer wavelength (λ) = lower energy (E) wavenumber = reciprocal of the wavelength measured in centimeters (cm); directly proportional to frequency

  50. Infrared Spectrum Transmittance: amount of light that passes through sample; not absorbed by molecular vibrations % Transmission Frequency: typically measured in wavenumbers; higher wavenumber = higher frequency = higher energy vibration Bands: frequency of vibration absorbed by molecules; can be broad or narrow; number of bands does not equal number of bonds

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