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Unformatted text preview: P4 and N4 (as well as P2 and N2) and calculate energies for the two dissociation
reactions. Are your results consistent with what is known (or unknown) about the two
systems? Is tetrahedral N4 actually a stable molecule? Explain your reasoning.
Is tetrahedral As4 a stable molecule? Use the B3LYP/6-31G* model to obtain its
equilibrium geometry and vibrational frequencies. Is its dissociation to As2 endothermic
(like P4) or exothermic? What is the room temperature equilibrium distribution of As4 and
Tetramethylbutane: The central carbon-carbon bond in 2,2,3,3-tetramethylethane
(tetramethylbutane) is known to be significantly weaker than the CC bond in ethane. One
explanation is that bond cleavage relieves the crowding of the methyl groups. Another is
that is tert-butyl radical (the product of bond dissociation of tetramethylbutane) is much
more stable than the methyl radical (the product of dissociation of ethane). To find out
which explanation is correct or if both contribute, calculate energies for the following
four reactions. Use the B3LYP 6-31G* model. (Do not to start with a symmetrical
staggered structure for tetramethylbutane.)
Me3C-CMe3 + H3C· → Me3C· + H3C-CH3
Me3C-CMe3 + H2 → 2Me3CH
H3C-CH3 + H2 → 2CH4
Me3CH + H3C· → Me3C· + CH4 The first reaction directly compares carbon-carbon bond energies of tetramethylbutane
and ethane. The next two reactions provide the energies of hydrogenation of
tetramethylbutane and ethane (both giving rise to uncrowded products). The last reaction
compares bond energies of these (uncrowded) products (providing a measure of the
relative stabilities of the methyl and tert-butyl radicals).
Is the bond energy in tetramethylbutane less than that in ethane? If so, by how much? Is
there evidence for crowding in the geometry of tetramethylbutane? Is the central carboncarbon bond similar in length to the bond in ethane? Are all single bonds staggered? Does
the difference between the hydrogenation energies of tetramethylbutane and ethane
partially or fully account for the difference in bond energies? Does the difference in
difficulty of formation of tert-butyl and methyl radicals partially or fully account for the
difference in bond energies?
Dimerization of Alkylboranes: Borane (BH3) exists in an equilibrium with its dimer,
diborane (B2H6) that strongly favors the dimer (from B3LYP/6-31G* calculations).
ΔE = -164 kJ/mol
Trimethylborane also dimerizes but the equilibrium favors the monomer
32 ΔE = 113 kJ/mol
What is the reason for the change? Is a hydrogen bridge strongly favored over a methyl
bridge? To tell, consider the dimerization of methylborane which may lead to four
possible products: cis and trans isomers with both methyl groups terminal, with both
methyl groups bridged and with one methyl group terminal and the other bridged. Use the
B3LYP/6-31G* model to obtain equilibrium geometries for all four isomers in addition to
methylborane. Which dimer is favored? Is dimerization to one or both of the isomers with
the two methyl groups in terminal positions exothermic (as with dimerization of borane)?
Is dimerization to the isomer with both methyl groups in bridged positions endothermic
(as with dimerzation of hexamethyldiborane)? Speculate on the reason for the difference
in dimerization energies between borane and trimethylborane.
Repeat the calculations for dimerization of both fluoroborane and chloroborane, each
leading to four different products. Is bridging by hydrogen of fluorine (chlorine) favored?
Obtain dimerization energies for alane (AlH3) to dialane (Al2H6) and trimethylaluminum
(AlMe3) to hexamethyldialane (Al2Me6). Continue to use the B3LYP/6-31G* model.
Point out any significant differences from those of the corresponding boron compounds.
Hydrogen Azide: Hydrogen azide is purported to have a high energy cyclic isomer,
cyclotriazine. This is not unreasonable. While the cyclic isomer is likely to be highly
strained, its Lewis structure doe not involve separated positive and negative charges. Attempt to obtain equilibrium geometries for both using the B3LYP/6-311+G** model.
(Start from a non-planar geometry for the cyclic isomer.) Are both acyclic and cyclic
structures energy minima? Justify you answer. If both structures are minima, which is
more stable? What temperature would be required in order for both to be observed in an
equilibrium mixture? (Assume that the minor isomer must be at least 5% of the mixture
in order to be detected.)
Repeat your calculations and analysis for methyl azide and methyl substituted
cyclotriazene. Isomers of Adamantyl Cation: Tertiary carbocations, such as tert-butyl cation are more
stable than secondary carbocations, such as isopropyl cation. It is also important that the
33 carbocation center is planar or nearly planar. Which is more important when only one of
the two can be realized? The isomers of adamantly cation provide the opportunity to tell.
2-adamantyl cation incorporates a non-planar tertiary carbocation center, whereas the 1adamantyl cation incorporates a planar s...
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- Spring '09