On a per gram basis are alkanes or silanes better

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Unformatted text preview: Compare silane combustion energies with those of the corresponding hydrocarbons (see previous problem; you need to perform B3LYP/6-31G* calculations on 2-methylpropane and 2,2-dimethylpropane). On a per gram basis, are alkanes or silanes better fuels? 2,4-Cyclohexadiene vs. Phenol: As a rule, enols (unsaturated alcohols) are less stable than their keto isomers (aldehydes and ketones). For example, vinyl alcohol is estimated to be 43 kJ/mol less stable than its “keto” tautomer, acetaldehyde. CH3C(H)=O H2C=C(H)OH 29 ∆E ≈ 43 kJ/mol Use the B3LYP/6-31G* model to obtain the equilibrium geometry and energy of 2,4cyclohexadienone and its enol tautomer, phenol. O OH Which is more stable? If this is an exception to the “rule”, provide a rationale as to why. What temperature would be needed in order for the higher-energy structure to be present as 10% of the equilibrium mixture? Addition vs. Substitution: Alkenes typically undergo addition reactions whereas aromatic compounds typically undergo substitution reactions. For example, reaction of bromine and cyclohexane yields trans-1,2-dibromocyclohexane not 1-bromocyclohexene, whereas bromination of benzene yields bromobenzene not trans-5,6-dibromo-1,3cyclohexadiene. Br + Br2 Br vs. + HBr Br Br + Br2 Br vs. + HBr Br What is the reason for the change in preferred reaction in moving from the alkene to the arene? Use the HF/6-31G* model to obtain equilibrium geometries and energies for reactants and the products of both addition and substitution reactions of both cyclohexene and benzene (four reactions in total). Is your result consistent with what is actually observed? Are all four reactions exothermic? If one or more are not, provide a rationale as to why. Bromonium Ions: Addition of Br2 to an alkene, for example, cyclopentene, is usually represented as a two-step process. In the first step, the electrophile Br+ adds to the double – bond giving rise to a “bromonium ion” intermediate. In the second step, Br reacts with the intermediate to give a trans brominated product. Br2 Br– Br H H Br The 13C NMR spectrum of the intermediate cyclopentenebromonium ion has been recorded and shows resonances at 19, 32 and 115 ppm (relative to tetramethylsilane). This is consistent with one of two possible structures. Either the bromine is bonded to both carbons, leaving it with the (formal) positive charge, or it bonded to only a single 30 carbon, leaving the positive charge on the other carbon. In the latter case, the bromine needs to transfer between the two carbons with a very low energy barrier. Br+ + H H H Br + H H H Br Use the B3LYP/6-31G* model to obtain equilibrium structures, relative energies and 13C NMR chemical shifts for the two alternative structures. Is the cyclic structure better represented as a three-membered ring (like cyclopropane or oxacyclopropane) or as a weak complex between cyclopentene and bromine cation? Examine the carbon-carbon bond distance to tell. Which structure is lower in energy? Which better fits the observed 13 C NMR spectrum, the bridged structure or an equal mixture of the two open structures? Would you expect the NMR spectrum to change were the measurement carried out at 160K? Elaborate. Are the energetic and NMR results consistent with each other? Hydrogenation of Acetylene: Both steps in the hydrogenation of acetylene to ethane involve formation of two new CH bonds from the loss of an HH bond and a π bond. Use the HF/6-31G* model to obtain equilibrium geometries and energies for all molecules involved in the hydrogenation reaction. Which step is the more exothermic? Rationalize your result. H C C H H2 H H C H2 C H CH3CH3 H Ketene Dimer: There are six possible structures for the dimer of ketene, H2C=C=O. CH2 CH2 C O O C CH2 O O C C C O O H2 C CH2 C CH2 O C H2 C C CH2 O O O C H2 C CH2 C C C H2 C O O CH2 Before you do any calculations, predict which of these is likely to be the most stable based on what you know about the relative stabilities of CC and CO π bonds (lost in the dimerization) and CC and CC σ bonds (gained in the dimerization). Then, obtain equilibrium geometries for all six using the HF/6-31G* model. Which structure is actually preferred. Assuming thermodynamic control, are two or more structures likely to be seen (assume 5% as the limit of detection of any structure). Is the preferred structure in line with your prediction? Is the dimerization exothermic or endothermic? Is this consistent with changes in bond strengths and the strain of the resulting four-membered ring? Elaborate. Molecular Phosphorous: The most stable arrangement of molecular phosphorus (white phosphorus) is tetrahedral P4. Dissociation to two molecules of P2 is estimated to be endothermic by >200 kJ/mol, and can only be detected upon heating to 1000 K. To the 31 contrary, the stable form of molecular nitrogen is N2, and N4 has never been detected let alone isolated. Use the B3LYP/6-31G* model to obtain equilibrium geometries and energies for tetrahedral...
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This note was uploaded on 02/22/2010 for the course CHEM N/A taught by Professor Head-gordon during the Spring '09 term at University of California, Berkeley.

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