P3_Reaction Energies

The latter reflects the fact that all other things

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Unformatted text preview: interaction involving hydrogens on adjacent centers. This combination of factors is commonly referred to as ring strain, and the associated “cost” in energy is commonly referred to as the strain energy. For example, cyclopropane, is known to be ~60 kJ/mol less stable than its isomer, propene, while cyclohexane is ~80 kJ/mol more stable than 1-hexene. The latter reflects the fact that, all other things being equal, σ bonds are stronger than π bonds. The difference (~140 kJ/mol) might be considered as the strain energy of cyclopropane. cyclopropane propene cyclohexane 1-hexene -60 kJ/mol 80 kJ/mol Isomerization energy is but one way to assess strain energy. Hydrogenation reactions may also be used to distinguish strained from unstrained molecules. For example, hydrogenation of cyclopropane to propane is exothermic by ~144 kJ/mol, while hydrogenation of cyclohexane to hexane is exothermic by only ~44 kJ/mol, yielding a strain energy of ~100 kJ/mol. 46 Ring Strain in Oxirane and Aziridine: Which molecule is more strained oxirane or cyclopropane? Use the HF/6-31G* model to evaluate the hydrogenation energy of oxirane (leading to dimethyl ether) relative to that of cyclopropane (leading to propane). (Hydrogen molecule appears on both sides of the equation and its energy is not needed.) Offer an explanation for your result. O H2 C CH2 + CH2 H3 C O CH3 H3 C + CH3 CH2 H2 C CH2 Repeat the calculation for hydrogenation of aziridine (leading to dimethylamine) relative to hydrogenation of cyclopropane. NH H2 C CH2 + CH2 H3 C NH CH3 H3 C CH3 + CH2 H2 C CH2 Which is more strained, aziridine or oxirane? Suggest an explanation for your result. Strain Energies of Small-Ring Cycloalkanes: As discussed above, cyclopropane is a highly strained molecule whereas cyclohexane is not. Is the strain energy of cyclobutane closer to that of cyclopropane or cyclohexane? Of cyclopentane? Is cycloheptane more strained than cyclohexane? To assess the strain in cyclopropane, cyclobutane, cyclopentane and cycloheptane relative to that in cyclohexane, obtain energies of reactions with n = 1, 2, 3 and 5. Use the HF/6-31G* model. (Energies for cyclopropane and propane are available if you completed the previous problem.) H2C CH2 CH3 CH3 CH3 H2C CH3 + CH2 + (CH2 )4 (CH2 )n (CH2 )n (CH2 )4 Cycloalkynes: Because alkynes exhibit a linear or nearly linear C-C≡C-C unit, it is unlikely that they will be able to incorporate into small rings. Identify the smallest cycloalkyne where both C-C≡C bond angles are within 10° of being linear (Start with cyclohexyne and minimize with molecular mechanics. Use the B3LYP/6-31G* model to calculate the equilibrium geometry of this cycloalkyne and cycloalkynes with one and two fewer carbons. Also obtain geometries of the corresponding cycloalkenes as well as hydrogen molecule. Calculate energies of the hydrogenation reaction. (CH2 )n C (CH2 )n H2 C C H n = 4–9 C H Is there a relationship between C-C≡C bond angle and hydrogenation energy for the three cycloalkynes? Strain Energies in Polycyclic Alkanes: Polycyclic cycloalkanes are likely to be even more strained than cycloalkanes. An extreme example of a strained molecule is tetrahedrane. While the parent compound, C4H4, has yet to be experimentally characterized, X-ray crystal structures exist for several simple derivatives, and appear to 47 be quite “normal”. For example, tetra-tert-butyltetrahedrane, shows carbon-carbon bond lengths of 1.49Ǻ, virtually identical with those found in cylopropane (1.50Ǻ). Use the B3LYP/6-31G* model to calculate equilibrium geometries for all molecules involved in hydrogenation of tetrahedrane, first to bicyclo[1.1.0]butane and then to cyclobutane. H2 H2 Cubane: It may come as a surprise that despite what appears to be a highly-strained carbon skeleton, derivatives of cubane are rather common (the crystal structures of well over a hundred of them have been reported). Use the B3LYP/6-31G* model to obtain equilibrium geometries for cubane and the first and second hydrogenation products (the former is unambiguous (all twelve CC bonds are the same), but the second is not. Assume the structure shown below. H2 H2 Inter and Intramolecular Hydrogen Bonding Discussion to be written The Enol of 1,3-Malonaldehyde: The enols of most carbonyl compounds are higher in energy than the keto forms. A possible exception arises in dicarbonyl compounds such as 1,3-malonaldehyde where there is the possibility of intramolecular hydrogen bonding. O O O H O H O O Use the B3LYP/6-31G* model to obtain equilibrium geometries for malonaldehyde and for cis-3-hydroxyacrolein. Start with a conformation of the latter that allows a hydrogen bond to be formed. Also obtain the geometry of trans-3-hydroxyacrolein (a molecule that cannot form an intramolecular hydrogen bond). Is cis-3-hydroxyacrolein lower in energy than malonaldehyde? If so, does this appear to be due to the hydrogen bond (compare the energies of cis and trans-3-hydroxyacrolein to tell)? Problem (using canned files) on hydrogen-bonding in base pairs and related systems. 48 H3C CH3 N O O N H H3C H N O N H O H2N N N H N N N CH3 O H N N CH3 N H N CH3 49 N N...
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