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Chemistry
4th Organic Edition Paula Yurkanis Bruice
Chapter 8
Reactions of Dienes Ultraviolet and Visible Spectroscopy
Irene Lee Case Western Reserve University Cleveland, OH 2004, Prentice Hall
Hydrocarbon containing two double bonds: diene three double bonds: triene four double bonds: tetraene many double bonds: polyene
Different Kinds of Dienes
Nomenclature of Alkenes with More than One Functional Group
1. Identify the longest continuous chain containing both double bonds 2. Give the double bonds the lowest possible number 3. The numbers indicating the double bonds are cited either before the name of the parent compound or before the suffix
4. Substituents are cited in alphabetical order
When the double bonds are a double bond and a triple bond
If there is a tie, the double bond gets the lowest number
A chain is numbered to give the lowest number to the functional group with the highest priority
CH3 H2C CHCH2OH CH3C CHCH2CH2OH H2C
NH2 CHCH2CH2CH2CHCH3
2-propen-1-ol
4-methyl-3-penten-1-ol
6-hepten-2-amine
CH3 OH
NH2
6-methyl-2-cyclohexanol
3-cyclohexenamine
Configurational Isomers of Dienes
The Z isomer has the high-priority groups on the same side
Why is conjugated diene more stable than isolated diene?
The closer the electrons are to the nucleus, the shorter and stronger is the bond
Electron Delocalization Stabilizes a Conjugated Diene
H2C H2C CH CH CH2 CH CH CH2 H2C CH CH CH2
resonance contributors
CH2
CH
CH
CH2
resonance hybrid
A cumulated diene is less stable than an isolated diene
Electrophilic Addition Reactions of Isolated Dienes
Addition can occur at only one of the double bonds when 1 mole of HBr is available
Electrophilic Addition Reactions of Conjugated Dienes
A conjugated diene undergoes both 1,2- and 1,4-addition
Compare the two addition product distribution at different reaction temperatures
kinetic control
thermodynamic or equilibrium control
Thermodynamic Versus Kinetic Control
The thermodynamic product is the most stable product
The thermodynamic product predominates when the reaction is reversible (thermodynamic control)
The kinetic product is the product that is formed most rapidly The kinetic product predominates when the reaction is irreversible (kinetic control)
The 1,4-addition product has the greater number of alkyl groups bonded to the sp2 carbon (more stable)
Why is the 1,2-addition product the kinetic product?
Consider the reaction coordinate diagram ...
Consider 1,3-pentadiene + H2C DCl
H2C CHCH CHCH3 + DCl D CHCH Cl CHCH3
1,2-addition product 78%
+ CH2CH D CHCHCH3 Cl
1,4-addition product 22%
The 1,2-addition is the major product because the chloride ion can stabilize the positive charge better at C-2 than at C-4 simply because it is closer to C-2 (the proximity effect)
Do not assume that the 1,4-addition product is always the thermodynamic product ...
The DielsAlder Reaction: A 1,4-Addition Reaction
The DielsAlder reaction is a pericyclic reaction; a [4+2] cycloaddition reaction
new bond
+
diene dienophile
transition state new bond
The reactivity of the dienophile is increased if one or more electron-withdrawing groups are attached to its sp2 carbons
A Molecular Orbital Description of the DielsAlder Reaction
Let's focus on the HOMO and the LUMO of the reactants
Stereochemistry of the DielsAlder Reaction
The reaction is a syn addition
The DielsAlder reaction is stereospecific The configuration of the reactants is maintained The reaction is concerted
Predicting the reaction products 1. Consider the alignment of the reactants
2. Consider the charge distribution in each of the reactants
Only the cis conformation can participate in a DielsAlder reaction
Two Possible Configurations of Bridged Bicyclic Compounds
Secondary orbital overlap favors the endo product formation
Ultraviolet and Visible Spectroscopy
Spectroscopy is the study of the interaction between matter and electromagnetic radiation
UV/Vis spectroscopy provides information about compounds with conjugated double bonds
UV and Vis light cause only two kinds of electronic transition
Only organic compounds with electrons can UV/Vis spectra A visible spectrum is obtained if visible light is absorbed A UV spectrum is obtained if UV light is absorbed
A chromophore is the part of a molecule which absorbs UV or visible light
The BeerLambert Law
A = cl
A = log(I0/I) c = concentration of substance in solution l = length of the cell in cm = molar absorptivity The molar absorptivity of a compound is a constant that is characteristic of the compound at a particular wavelength
Effect of Conjugation on
max
Both the max and increase as the number of conjugated double bonds increases
An auxochrome is a substituent in a chromphore that alters the max and the intensity of the absorption
The Visible Spectrum and Color
Uses of UV/Vis Spectroscopy
Measure the rates of a reaction
Determine the pKa of a compound
Estimate the nucleotide composition of DNA

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