1. Preparation of Alkenes Elimination
Chapter 19

2. Addition of Electrophiles to Alkene

3. Electrophilic Addition of HXδ+ 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
curved arrows do not indicate which carbon is protonated
fastest protonation leads to more stable (more substituted) carbocation
more substituted carbocation = more substituted alkyl halide

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 Dehydration Mirrors Ease of Carbocation Formation
rate of dehydration = 3º > 2º > 1º alcohol
tertiary
alcohol (3º)
tertiary
cation (3º)
Reactivity
Stability
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?B. C. D. E.

17. Types of Selectivity in Organic Chemistry
There are three forms of selectivity to consider
Chemoselectivity: which functional group will react
Regioselectivity: where it will react
Stereoselectivity: how it 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 Regioslectivty The 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 most substituted 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
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. Separation of MixturesandIdentification of Components
Analytical Chemistry
Separation of MixturesandIdentification of Components

35. High Performance Liquid Chromatography

36. Gas Chromatography

37. Structural Determination

38. Spectroscopy vs. Spectrometry
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
Measurement/Application
Infrared
Spectroscopy
vibrational states: stretching and bending frequencies of covalent bonds that contain a dipole moment
functional group determination
Ultraviolet-Visible (UV-vis)
electronic states: energy associated with promotion of an electron in a ground state to an exited state
chromophore determination
Mass
Spectrometry
molecular weight: of parent molecule and fragments produced by bombardment with “free” electrons
fragment and isotope determination
Nuclear Magnetic Resonance
nuclear spin states: energy associated with spin states of nuclei in the presence of a magnetic field
determine structural groups and connectivity

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
Types of States
Energy
Range (λ)
Spectroscopic Method
nuclear spin
radiofrequency
1-10 m
NMR
rotational
microwave cm
Microwave
vibrational
infrared μm IR
electronic
ultraviolet nm
UV-vis
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) ~ ν = k
f *
Trends:
↑ bond strength =
↑ frequency
↑ mass =
↓ frequency
(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. stronger spring (bond) =
Spring Analogy
smaller mass =
higher frequency =
higher energy
stronger spring (bond) =
higher frequency =
higher energy

49. Wavenumber (ῡ) and Infrared Scale1
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 % Transmission
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

51. Characteristic Stretches - Alkanes
2 = sp3 C-H bond stretching motion; general absorb around cm-1
1 = C-H rocking motion when C atom is part of a methyl group (-CH3); cm-1
3 = scissor motion of -CH3 hydrogen atoms; cm-1
cm-1 = fingerprint region for organic molecules; typically complex and unhelpful

52. Characteristic Stretches - Alkenes
5: notice sp2 C-H (~3100 cm-1) at higher frequency than sp3 C-H (~2950 cm-1)
more s-character = stronger bond = higher frequency
4: also, C=C bond at higher frequency than C-C bond; ~1600 cm-1

53. Characteristic Stretches - Alkynes6 7
7: notice sp C-H (~3300 cm- 1) at higher frequency than sp2 C-H (~3100 cm-1), which was higher than sp3 C-H (~2950 cm-1)
6: C≡C stretch is very weak because carbons have almost identical electronegativities = small dipole moment

54. Characteristic Stretches - Alcohols
9: hydroxyl groups (-OH) exhibit strong broad bands; ~3300 cm-1
broad peak is a result of hydrogen bonding; width depends on solution concentration
lower concentration = less hydrogen bonding = more narrow -OH band

55. Characteristic Stretches - NItriles
8: nitriles ~2200 cm-1
nitriles (C≡N) absorb a greater magnitude of energy than alkynes (C≡C) because they have a larger dipole moment
larger dipole moment = more intense peak
size of the dipole does NOT affect frequency of vibration

56. Example: Ester, Amine, Benzene
10: strong carbonyl (C=O) band ~1700 cm-1
11: amines; secondary amines (-NH) give one band; primary amines (- NH2) gives two bands
4: several alkene bands ~1600 cm-1 for benzene ring C=C double bonds

57. Characteristic Stretches - Carboxylic Acids
10: strong carbonyl (C=O) band ~1700 cm-1
9: hydroxyl band (-OH) can be less intense and sharper in carboxylic acids
4: weak alkene band (C=C) since small dipole moment

58. Characteristic Stretches - Aldehydes
12: usually two bands for C- H of aldehydes; may overlap with sp3 C-H bands

59. Self Test Questions a b c d
Which molecule is represented by the IR below? a. b. c. d. e. a b c d

60. Self Test Questions a b c d
Which molecule is represented by the IR below? a. b. c. d. a b c d

61. Self Test Questions a b c d
Which molecule is represented by the IR below? a. b. c. d. e. a b c d

62. Self Test Questions a b c d
Which molecule is represented by the IR below? a. b. c. d. e. a b c d

63. Self Test Questions a b c d
Which molecule is represented by the IR below? a. b. c. d. e. a b c d

64. Self Test Questions a b c d
Which molecule is represented by the IR below? a. b. c. d. e. a b c d

65. Greater s character = stronger, shorter bonds = higher frequency
Example a. b. c. d. e. a b c d
Greater s character = stronger, shorter bonds = higher frequency

66. Mass Spectrometry

67. Spectroscopy vs. Spectrometry
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

68. Self Test Question a b c d e
Which molecule corresponds to the IR spectrum below? a b c d e a b c d e

69. Self Test Question
Which covalent bond, highlighted in bold (red) in the molecules below, would not be expected to exhibit an IR stretching band?
CO2 has no dipole moment! A B C D E

70. Mass Spectrometry
Section 13.24
You are responsible for section 13.25!

71. Primary Applications:
Mass Spectrometry
Primary Applications:
Determine molecular mass.
Establish fragmentation patterns, which can be indexed in a database.
Determine presence of some heteroatoms.
Determine the exact mass of molecules.

72. Mass Spectrometer Schematic
creates charged molecules called radical cations
radical cations are accelerated by negatively charged plates
magnetic fields exert forces on moving charges

73. Formation of Radical Cations
organic molecules are bombarded with 70-eV electrons
causes organic molecule to lose one electron from a covalent bond
organic molecule is then charged
the mass of charged species is determined by a mass spectrometer

74. Molecular Ion Peaks C: 6 x 12 = 72 H: 6 x 1 = 6 Total = 78
molecular ion peak = highest m/z (mass/charge) peak
since charge (z) is usually 1, molecular ion peak = molecular mass
molecular ion peak does not have to have relative intensity of 100%
most intense peak = base peak
relative intensity = height of peak ÷ base peak

75. Fragmentation
radical cation fragments (heterolysis) to give a neutral radical species and a cation
fragment contains less mass than parent ion
relative intensity of the fragment depends on its concentration (likelihood of occurring)
more stable cations are more likely; give more intense peaks
heterolysis

76. Common Fragmentation Pattern for Alkanes

77. Fragmentation Common Fragmentation Pattern for Alkyl Benzenes
benzylic carbon
benzylic carbocation
benzylic carbocation stabilized by resonance =
common fragment in MS

78. Fragmentation

79. Fragmentation
Since fragmentation patterns should be the same for identical molecules, they can be saved in a database and matched to unknowns later. CSI anyone?

80. Isotopic Clusters: Carbon and Hydrogen
M + 1
M + 1

81. Isotopic Clusters: Carbon and Hydrogen
Probability of M+1
6 x 1.1% = 6.6% of 13C
6 x 0.015% = 0.1% of 2H
Total Probablility = 6.7%
M + 1
M + 1
Natural Abundance of Isotopes
Isotope
Abundance
13C
1.10%
12C
98.90%
2H (D)
0.015% 1H
99.985%
mass spectrometry is sensitive enough to resolve exact masses of isotopes
intensity of the peaks corresponds to natural abundance of each isotope
probability = number of atoms in molecule x natural abundance

82. Isotopic Clusters: Chlorine & Bromine
Natural Abundance of Isotopes
Isotope
Abundance
35Cl
75.77%
37Cl
24.23%
79Br
50.69%
81Br
49.31%
Chlorine: M:(M+2) ~ 3:1
Bromine: M:(M+2) ~ 1:1
probability = number of atoms in molecule x natural abundance

83. Spectroscopic Methods
Measurement/Application
Infrared
Spectroscopy
vibrational states: stretching and bending frequencies of covalent bonds that contain a dipole moment
functional group determination
Ultraviolet-Visible (UV-vis )Spectroscopy
electronic states: energy associated with promotion of an electron in a ground state to an exited state
chromophore determination
Mass
Spectrometry
molecular weight: of parent molecule and fragments produced by bombardment with “free” electrons
fragment and isotope determination
Nuclear Magnetic Resonance
nuclear spin states: energy associated with spin states of nuclei in the prescence of a magnetic field
determine structural groups and connectivity

84. Spectroscopic Methods
Measurement/Application
Infrared
Spectroscopy
vibrational states: stretching and bending frequencies of covalent bonds that contain a dipole moment
functional group determination
Ultraviolet-Visible (UV-vis )Spectroscopy
electronic states: energy associated with promotion of an electron in a ground state to an exited state
chromophore determination
Mass
Spectrometry
molecular weight: of parent molecule and fragments produced by bombardment with “free” electrons
fragment and isotope determination
Nuclear Magnetic Resonance
nuclear spin states: energy associated with spin states of nuclei in the prescence of a magnetic field
determine structural groups and connectivity

85. Mass Spectrometry: Mass and Molecular Formula

86. Mass Spectrometer - General Layout

87. Fragmentation of Toluene Parent Ion

88. Different Molecules Can Have the Same Molecular Weight!

89. Accurate Mass Measurement is the Solution!

90. Parent Ions Undergo Fragmentation

91. Mass Spectrometer - Location of Fragmentation

92. The Course of Fragmentation is Directed by Daughter Ion Stability

93. Fragmentation Patterns - Elimination of Water

94. Fragmentation Patterns - McLafferty Rearrangement

95. McLafferty Rearrangement of Butyraldehyde