P3_Reaction Energies

Two molecules of methane need to be added to the left

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Unformatted text preview: e set of “simplest molecules” that contain the same bonds. For example, the bond separation reaction for acrolein, relates it to ethane, ethylene and formaldehyde, the simplest molecules with carbon-carbon single and double bonds and a carbon-oxygen double bond. Two molecules of methane need to be added to the left hand side to achieve stoichiometric balance. H2C=C-C(H)=O + 2CH4 → H2C=CH2 + H3C-CH3 + H2C=O Aside from the conformation of the reactant, the energy of a bond separation reaction is well-defined and unique as long as the Lewis structure is well defined and unique. Because it maintains both total and individual bond counts, it might be expected to be reasonably well described even by models that provide poor account of electron correlation. Bond separation reactions may be used to determine whether two substituents bonded to carbon interact constructively (bond separation reaction is endothermic), destructively (bond separation reaction is exothermic) or not at all. XCH2Y + CH4 → CH3X + CH3Y Use the HF/6-31G* model to obtain equilibrium geometries and energies for all molecules involved in bond separation reactions of molecules, CH2X2 (X=CH3, CMe3, CN, F, SiH3). Interpret the calculated reaction energies both in terms of steric interactions and the σ and π donor/acceptor properties of the individual substituents: σ donor, π donor σ acceptor, π acceptor σ acceptor, π donor σ donor, π acceptor Me, CMe3 CN F SiH3 Which of the substituents interact favorably? Which interact unfavorably? Provide a rationale for each. Compare bond distances in CH3X and CH2X2 compounds. Do the changes parallel the interaction energies? Elaborate. 37 Bond Stengths vs. Bond Separation Energies in Fluoromethanes: In an earlier problem (see Chapter P2), you found that CF bond energies in fluoromethanes, CFnH4-n (n=1-4), increase with increasing number of fluorines. Is this trend reflected in the bond separation energies? Use the HF/6-31G* model to calculate energies for all reactants and products for the three bond separation reactions below. CH2F2 + CH4 → 2 CH3F CF3H +2CH4 → 3CH3F CF4 + 3CH4 → 4CH3F Do the bond energies of these reactions (normalized for the number of CF bonds) parallel the bond dissociation energies calculated previously? Repeat your calculations and analysis for bond separation reactions of fluorosilanes, SiFnH4-n (n=1-4). Repeat your calculations and analysis for bond separation reactions on both carbon and silicon compounds substituted by chlorine instead of fluorine, CClnH4-n and SiClnH4-n, (n=1-4). 38 Ion Molecule Reactions Quantum chemical calculations are not restricted to uncharged molecules, but may also be applied to cations and anions. While there are virtually no experimental data relating to the geometries of in the gas phase, there is a wealth of data relating to their energies. The two most common sources are proton transfer reactions, that is, acidities and basicities, and electron transfer reactions, that is, ionization potentials and electron affinities. There are an enormous number of X-ray crystal structures of ions (together with their counterions). Discussion has already been provided in Chapter P2. As discussed earlier in this chapter, relative acidities and basicities are most commonly determined by ion-cyclotron-resonance (ICR) spectroscopy. What is actually measured is the equilibrium abundance of the ions involved in a proton transfer reactions between two bases (or two acids) of similar strength, for example, that between methylamine and ethylamine (bases) or between acetic acid and propanoic acid (acids). The neutral molecules are not seen. MeNH2 + EtNH3+ MeNH3+ + EtNH2 MeCO2H + EtCO2- MeCO2- + EtCO2H Measurements are all finally related to a single standard base (ammonia) and standard acid (???), giving rise to absolute basicities and acidities. Relative Acidities of Propene and Propyne: Is propene or propyne the stronger acid in the gas phase? Use the B3LYP/6-311+G** model to calculate the geometries for the reactants and product of the reaction. Be certain to consider all possible anions resulting from deprotonation of propene and propyne. propene + propyne-H+ propene-H+ + propyne Rationalize your result. Is it anticipated by comparison of electrostatic potential maps for propene and propyne? Elaborate. Why is Cyclopentadiene a Strong Acid? Cyclopentadiene is a much stronger acid than 1,3-pentadiene in the gas phase, that is, the following reaction is highly exothermic. cyclopentadiene + 1,3-pentadiene - H+ 39 cyclopentadiene-H+ + 1,3-pentadiene Use the B3LYP/6-31G* model to obtain equilibrium geometries for cyclopentadiene and its deprotonated form (cyclopentadienyl anion). Note any structural changes that have occurred to cyclopentadiene as a result of deprotonation, and use these to rationalize its high acidity. Gas and Aqueous-Phase Basicities of Amines: The relative base strengths (proton affinities) of free amines are known to differ significantly from those in water. For example, in the gas-phase,...
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