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Chapter 8
Reactions of Dienes

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08-00CO
Title
Protonated 1,3-Butadiene
Caption
Electrostatic potential map for carbocation formed by protonating 1,3-butadiene.
Notes
1,3-Butadiene protonated at carbon 1 is an allylic cation. Protonation removes carbon 1 from the pi system. Note that the positive charge is delocalized between carbons 2 and 4. Carbons 1 and 3 are more greenish in color than carbons 2 and 4, indicating that little positive charge resides on carbons 1 and 3. Nucleophiles are attracted to positive charge on either carbon 2 or carbon 4, depending on reaction conditions.
Keywords
protonated, butadiene, potential, map
08-00-04UN
Title
Naming Alkenynes
Caption
Three named examples of alkenynes.
Notes
When naming a compound with both a double bond and a triple bond, the longest chain has to flow through both pi systems, and the carbons have to be numbered so that either the alkene or the alkyne has the lowest possible number regardless of which functional group gets the lower number. The alkene function precedes the alkyne function in the name.
Keywords
naming, alkenynes, examples
08-00-07T01
Title
Table 8.1 Priorities of Functional Group Suffixes
Caption
Notes
Keywords
08-00-20T02
Title
Table 8.2 Carbon-carbon single bond on hybridization of orbitals
Caption
Notes
Keywords
08-01
Title
Figure 8.1
Caption
Bonding MOs in allene and structures of the two enantiomers of 2,3-pentadiene (separated by mirror), which is an allenic system.
Notes
The pi MOs in allenic systems are perpendicular to one another. They therefore do not overlap one another, and allenic systems have MOs like two isolated double bonds. Since the MOs are perpendicular to one another, allenic systems are not planar, and functional groups attached to their outer carbons do not lie cis and trans to one another, but are oriented in perpendicular planes. This situation gives rise to enantiomers in molecules like 2,3-pentadiene.
Keywords
allene, allenic, pentadiene, enantiomers, figure, 8.1
08-01-11UN
Title
Protonated 1,3-Butadiene
Caption
Electrostatic potential map for carbocation formed by protonating 1,3-butadiene.
Notes
1,3-Butadiene protonated at carbon 1 is an allylic cation. Protonation removes carbon 1 from the pi system. Note that the positive charge is delocalized between carbons 2 and 4. Carbons 1 and 3 are more greenish in color than carbons 2 and 4, indicating that little positive charge resides on carbons 1 and 3. Nucleophiles are attracted to positive charge on either carbon 2 or carbon 4, depending on reaction conditions.
Keywords
protonated, butadiene, potential, map
08-02
Title
Figure 8.2
Caption
Reaction coordinate diagram for the addition of HBr to 1,3-butadiene.
Notes
Addition in a 1,4 manner is thermodynamically controlled, since the 1,4-addition product is more stable than the 1,2-addition product. Internal alkenes are more stable than terminal alkenes. Addition in a 1,2 manner is kinetically controlled, since the 1,2-addition product is formed from the carbocation intermediate faster than the 1,4-addition product. Reaction conditions which allow the reverse reaction to occur allow the system to come to equilibrium, favoring 1,4 addition. Conditions which inhibit the reverse reaction favor 1,2 addition because this product is initially formed in greater amounts.
Keywords
reaction coordinate, diagram, HBr, butadiene, thermodynamically controlled, kinetically controlled, figure, 8.2
08-03
Title
Figure 8.3
Caption
Structures and electrostatic potential maps of ethene, propenal, and propenenitrile.
Notes
Electron-withdrawing groups attached to double bonds decrease the electron density in the double bond, shifting its color in a potential map from yellowish to greenish to bluish.
Keywords
electron-withdrawing, potential, map, ethene, propenal, propenenitrile, figure, 8.3
08-04
Title
Figure 8.4
Caption
The Diels–Alder reaction results from in-phase overlap of the HOMO of the diene with the LUMO of the diene or vice-versa. Initially, electrons are absorbed from the HOMO of one partner into the LUMO of the other.
Notes
In Diels–Alder reactions of dienes and dienophiles which do not have strongly electron-withdrawing or electron-donating groups attached, the Diels–Alder reaction prefers to transfer electrons from the HOMO of the diene to the LUMO of the dienophile, since these are closer in energy to one another than the LUMO of the diene and the HOMO of the dienophile. Substituting strongly electron-withdrawing groups on the diene lowers the energy of its HOMO, and substituting strongly electron-donating groups on the dienophile raises the energy of its LUMO. Doing both of these substitutions increases the energy difference between the HOMO of the diene and the LUMO of the dienophile, resulting in reactions involving the LUMO of the diene and the HOMO of the dienophile.
Keywords
Diels-Alder, HOMO, LUMO, in-phase, overlap, figure, 8.4
08-05
Title
Figure 8.5
Caption
Relative energies of bonding and antibonding sigma and pi orbitals, and nonbonding (n) orbitals.
Notes
Electronic transitions are named according to the kinds of orbitals involved in these transitions (e.g., n --> p*, or p --> p*).
Keywords
bonding, antibonding, sigma, pi, nonbonding, figure, 8.5
08-06
Title
Figure 8.6
Caption
The UV spectrum of acetone.
Notes
The pi bond between carbon and oxygen in acetone and the presence of nonbonding electrons on the oxygen atom of this species makes n --> p* (274 nm) and p --> p* (195 nm) electronic transitions observable in the UV spectrum.
Keywords
UV, spectrum, acetone, figure, 8.6
08-07-01T03
Title
Table 8.3 Ethylene and conjugated dienes
Caption
Notes
Keywords
08-08
Title
Figure 8.8
Caption
Nitroethane anion formation can be monitored by UV at 240 nm.
Notes
The rate of proton removal from nitroethane can be measured by monitoring the nitroethane anion formed by the reaction.
Keywords
nitroethane, anion, figure, 8.8
08-09
Title
Figure 8.9
Caption
Pyruvate concentration can be monitored by UV at 340 nm.
Notes
The rate of reduction of pyruvate by NADH can be measured by monitoring pyruvate disappearance at 340 nm.
Keywords
pyruvate, reduction, NADH, figure, 8.9
08-10
Title
Figure 8.10
Caption
The absorbance of an aqueous solution of phenol at 287 nm as a function of pH.
Notes
The absorption at 287 nm is due to the presence of phenoxide ion (the conjugate base of phenol). When half of the phenol has been converted into phenoxide, the concentrations of phenol and phenoxide are equal. At this point, pH = pKa.
Keywords
absorbance, phenol, phenoxide, pH, figure, 8.10
08-11
Title
Figure 8.11
Caption
The absorbance of a solution of DNA at 260 nm as a function of the temperature of the solution.
Notes
Single-stranded DNA absorbs at 260 nm because nonbonding electrons on nitrogen atoms, which are normally tied up in hydrogen bonding together complementary strands of DNA, are free to undergo n --> p* transitions in single-stranded DNA. When the temperature has risen to the point that half of the DNA is single-stranded, DNA is said to be at its "melting" temperature, Tm.
Keywords
DNA, absorbance, UV, figure, 8.11

Title
Table 8.4 Dependence of the Color Observed on the Wavelength of Light Absorbed
Caption
Notes
Keywords
08-00-07T01
Title
Table 8.1 Priorities of Functional Group Suffixes
Caption
Notes
Keywords
08-00-20T02
Title
Table 8.2 Carbon-carbon single bond on hybridization of orbitals
Caption
Notes
Keywords
08-07-01T03
Title
Table 8.3 Ethylene and conjugated dienes
Caption
Notes
Keywords

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