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Molecular Modeling Problems Chapter 6: Properties of Haloalkanes 1. Charge Distributions in Transition States. Organic chemists have developed a powerful nomenclature to designate chemical reactions. Here, curved or “curly” arrows are used to indicate transfer of a pair or electrons (a bond or a lone pair) from one place in a molecule to another. A particularly simple example is found for the SN2 reaction of cyanide anion and methyl iodide leading to acetonitrile and iodide anion. One arrow indicates that the lone pair on cyanide will be used to make a new carbon-carbon bond, while the second arrow indicates that the carbon-iodine bond will break with the pair of electrons going to iodine. H H N C– + C H H N C H + I– C H H Curly arrows do not say anything either about the geometry or charge distribution in the transition state, or about changes in geometry and charge distribution that occur in moving from the reactants to the transition state and onto the product. All that they do is identity the product. On the other hand, quantum chemical calculations are able to provide insight into both the geometry and charge distribution of the transition-state. Comparison with the corresponding information for the reactants, reveal any changes that occur during the course of a chemical reaction. An electrostatic potential map for “one frame” in the SN2 reaction cyanide anion and methyl iodide appears on screen. A plot of energy vs. CI distance is also provided. Step through the frames by clicking on at the bottom left of the screen, or animate the sequence by clicking on . Does “cyanide” or “iodide” appear to hold the bulk of the negative charge at the transition state? Is cyanide or iodide the better leaving group? Elaborate. Is your conclusion consistent with the fact that the reaction as written is exothermic? Explain. 2. Carbon-Fluorine Bond Lengths in Fluoromethanes. Do the lengths of bonds between sp3 carbon and attached atoms or functional groups vary significantly with the number of groups? Experimental carbon-carbon bond To accompany Organic Chemistry, Sixth Edition by Vollhardt & Schore Molecular Modeling Problems lengths for ethane (1.531Ǻ), propane (1.526Ǻ), 2-methylpropane (1.541Ǻ) and 3,3-dimethylpropane (1.540Ǻ) show only slight variation. Is this always the case? Obtain carbon-fluorine bond lengths in fluoromethanes, CFnH4-n (n=1-4). Do they increase or decrease significantly with increasing number of fluorines, or are they relatively constant? If you find that bond lengths change significantly, offer an explanation as to why. Repeat your calculations and analysis for the chloromethanes, CCl nH4-n (n=1-4). Point out any significant differences with the results for fluoromethanes. For which, if either, set of compounds is the change in bond lengths with increasing substitution most similar to what is observed for alkanes? 3. Bond Stengths in Fluoromethanes. Carbon-fluorine bond distances change significantly with increased number of fluorine atoms attached to carbon. Are these changes are mirrored by changes in carbon-fluorine bond strengths? One measure is provided by comparing the energies of the three reactions below. CF4 + CH4 CF3H + CH4 CH2F2 + CH4 CF3H + CH3F CH2F2 + CH3F 2CH3F Calculate energies for all reactants and products for (alternatively use heats of formation from the database accessible from SpartanModel), and evaluate reaction energies. Is replacement of a CF bond in carbon tetrafluoride by a CH bond more or less difficult than replacement in fluoroform (CF3H)? Is replacement of a CF bond in fluoroform more or less difficult than replacement in methylene fluoride (CF2H2)? Is there a relationship between CF bond lengths and bond energies? Elaborate. Repeat your calculations and analysis for the chloromethanes, CClnH4-n (n=1-4). Point out any significant differences with the results for fluoromethanes. Are any differences in bond energy changes between fluoromethanes and chloromethanes reflected in differences in bond length changes? To accompany Organic Chemistry, Sixth Edition by Vollhardt & Schore