Problems. 8. Stereochemical Analysis Physical Properties, Forces of Interaction Resonance, Curved Arrows, Formal Charge, Polarity 15

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1 hem 0 Midterm Exam Winter, 07 eauchamp Name Problems Points. Functional Group Nomenclature ( large structure) 0. Degrees of Unsaturation & Functional Groups (many different functional groups) or Various Nomenclature Terms 0. D Structure (long string of atoms) 0. Properties of Atoms, Logic Arguments of rganic hemistry (ionization potential, Z eff, radii, electronegativity, etc.). D Structure, ybridization, esonance, Angles, Shapes 0. yclohexane onformations ( substituents), Newman Projections, K eq calculation, Energy Diagram 0 7. hain onformations, 0 o rotations, Newman Projections, K eq calculation, Energy Diagram 0 8. Stereochemical Analysis 0 9. ysical Properties, Forces of Interaction 0. esonance, urved Arrows, Formal harge, Polarity redit Total This is a long exam. It has been designed so that no one question will make or break you. The best strategy is to work steadily, starting with those problems you understand best. Make sure you show all of your work. Draw in any lone pairs of electrons, formal charge and curved arrows to show electron movement where appropriate. Do your best to show me what you know in the time available. Students should be taught how to think, not what to think. Margaret ad

2 . Provide an acceptable name for the following molecule. Absolute configurations of any stereogenic centers (shown with dashes and wedges) count point each. (0 pts) 8 0Z 8 S Z N 0 S N N (S,8,0Z) -ethyl--butyl--cyclopentylhex-z-enyl -amino--benzyl--phenyl-7-(-amido--pentyl-7,7- dimethylcyclohept--enyl)-8-mercapto-9-hydroxy-0-(-methylpropoxy)--cyano--oxododec-0-en--ynoate. Draw an example molecule that has the indicated functional groups and label each group by its name. alculate the degree of unsaturation for a molecule with a formula of 9 ln S. Show your work. (0 pts) functional groups: carboxylic acid, anhydride, ester, acid chloride, amide, nitrile, aldehyde, ketone, alchohol, thiol, alkene, alkyne, ring, bromo, aromatic carboxylic acid anhydride ester 9 ln S amide N ketone l acod chloride N aldehyde nistrile thiol S alcohol allene maximum saturation = ()() + + = 7 -(9 + + ) = - degrees of unsaturation = / = 7 o unsaturation alkyne ring aromatic bromo

3 . Draw an acceptable Lewis structure (D) for the following formula. Show all single, double and triple bonds with one, two or three lines. Include all lone pairs of electrons as two dots. Include formal charge, if present at the atom where present. Identify any functional groups by name (i.e. ketone, amide, etc.) (0 pts). a. Ionization energy is the amount of energy required to steal an electron away from a substance (atom or compound). Provide an explanation differences in atomic ionization energy in a row ( vs. ) and in a column ( vs. S). ( pts) + + S +9 It takes more energy to ionize and electron from oxygen than carbon becaue oxygen has a higher Z eff (=+) than carbon (=+). It takes more energy to ionize and electron from oxygen than sulfur, even though they have the same Z eff because the electron lost from sulfur is in the n= shell (farther away from the nucleus) where the attraction is weaker. b. Define electronegativity and provide an explanation for the differences in electronegativity in a row ( vs F) and in a column (F vs l). ( pts). F +.0 l +. Electronegativity measure the attraction for electrons in bonds. A more electronegative element pulls the electrons closer to it and polarized the bond with a partial negative charge on it and a partial positive charge on the other atom (like an - bond). A higher number means a stronger pull for the electrons. The reasons are the same as in the above explanation (Z eff and distance from the nucleus). c. All anions below have the same atomic configuration. Write that out. Match each anion structure with its radius and give a very brief explanation for your order. ( pts)

4 . First, draw two other better resonance structures than the one shown as two-dimensional representations. Include proper arrow conventions. ank your structures from best (= ) to poorest (= ) with a very brief explanation. Draw a three-dimensional Lewis structure for the given representation and the best other resonance structure. Show sigma bonds as lines, wedges and dashes and the p orbitals in pi bonds as well as any orbitals holding lone pairs. Indicate any formal charge present and give the hybridization, bond angles and shape of each labeled nonhydrogen atom in the table below. Assume that all nonhydrogens atoms have full octets, unless a carbocation is written. (0 pts) A = = = best = (carbon is negative) (oxygen is negative) (nitrogen is negative) a b e N c d N N f (based on having the negative charge on the most electronegative atom(s) [ > N > ] D structure A and D best other N N A D (A) D (best other) Use structure A to fill in the following table. ybridization Angles Shape #σ bonds #π bonds lone pairs a sp 80 o linear b sp 0 o trigonal planar 0 c sp 0 o trigonal planar 0 d sp 0 o trigonal planar 0 e sp 09 o tetrahedral 0 0 f sp 0 o trigonal planar

5 . Draw all possible chair conformations of trans--isoproppyl--phenylcyclohexane. Start with on the right side and count towards the back. Which conformation is more stable? Draw it first. Draw a Newman projection of each conformation using the and bonds to sight along. Point out any gauche interactions shown in your Newman projections. If the axial energy of an isopropyl group is. kcal/mole and.9 kcal for a phenyl group and an isopropyl/phenyl gauche interaction is.0 kcal/mole, what is the difference in energy between the chair conformations? What are the relative percents of each conformation (show work)? Sketch an energy diagram that shows how the energy changes from the conformational changes (start with low and go to high energy). (0 pts) gauche =.0 kcal/mole +. kcal/mole +.9 kcal/mole total =.0 kcal/mole most stable chair total =. +.9 =.0 kcal/mole least stable chair Newman projection i-pr gauche gauche axial i-pr gauche axial Newman projection chair K = chair = 0 G.T = 0 (000) 80 = = 07 0 PE 0 oat +.0 kcal/mole 0 Energy changes of trans--isoproppyl--phenylcyclohexane

6 7. Use a Newman projection of the bond of -phenylpentane to show the most stable conformation first. otate through all of the eclipsed and staggered conformations. Using the energy values provided in the table below, calculate the relative energies of the different conformations. Show all energy values by your structures. Plot the changes in energy in the graph diagram provided. alculate and equilibrium constant between the lowest and highest energy conformations. int: Draw a D structure first and bold the bond viewed in your Newman projection, then decide your line of sight. (0 pts) D structure Approximate Eclipsing Energy Values (kcal/mole) Approximate Gauche Energy Values (kcal/mole) Et i-pr t-u Et i-pr t-u Et G K eq = 0 -.T Et i-pr i-pr t-u t-u most stable conformation o 0. Et 0.9. Et. 0 o Et o.0.8 Et o =. o =.8 o =. o =. o =.. 0 o 0.. Et o.0.7 Et.7 K eq (lowest to highest) = conf. K = conf. = 0 G.T.8 = 0 (800) 80 = = 00 = 000.

7 8. For the following set of Fischer projections, answer each of the questions below by circling the appropriate letter(s) or letter combination(s). int: edraw the Fischer projections with the longest carbon chain in the vertical direction and having similar atoms in the top and bottom portion. lassify all chiral centers in the first structure as or S absolute configuration. (0 pts) A D E a. Which are optically active? A D E b. Which are meso? A D E c. Which is not an isomer with the others? A D E d. Which pairs are enantiomers? A A AD AE D E D E DE e. Which pairs are identical? A A AD AE D E D E DE f. Which pairs are diastereomers? A A AD AE D E D E DE g. Which pairs, when mixed in equal amounts A A AD AE D E D E DE will not plane polarized light? h. Draw any missing stereoisomers of A. D A E None are missing j. Paclitaxel (discovered in the bark of a yew tree in 97) is a potent chemotherapy medication used to treat several types of cancer, including ovarian cancer, breast cancer, lung cancer, Kaposi sarcoma, cervical cancer, and pancreatic cancer. ircle all chiral centers. ow many stereoisomers are possible from the stereogenic centers shown? Tetrahedral carbon atoms with different groups are chiral centers. N * * * * * * * * * * * Maximum possible stereoisomers = = 08 The complete formula is 8 N What is the degree of unsaturation? Show your work. ()(8) + + = 79 - = 8 o

8 9. Match the given boiling points with the structures below and give a short reason for your answers. (- o, 0 o, +8 o, +98 o, 0 o ) (0 pts) propanol MW = g/mol KF potassium fluoride MW = 8 g/mol butane MW = 8 g/mol propanal MW = g/mol -methylpropane MW = 8 g/mol 98 o 0 o 0 o 8 o - o hydrogen ionic forces dispersion, bonding strong dipole dispersion, are strongest but linear and branched b. Match the melting point with the isomers below and provide a brief explanation for the difference. ( pts) mp = - o The long flexible side chain prevents efficient packing in the lattic structure, so it does not take much thermal energy to break down the structure. T = 7 o mp = + o All of the molecules have the same very rigid shape, which allows close packing and is difficult to disrupt the lattice structure, so requires more thermal energy to melt (a higher mp). 0. a. esonance structures having fewer bonds and incomplete octets are considered of lesser importance. Yet, often they are drawn with functional groups having a carbonyl group (=). Show this extra resonance structure (include ALL lone pairs and proper resonance arrow conventions), and explain why it is considered acceptable for these groups. ( pts) A The second resonance structure is not as good as the first because it separates charge and removes a bond. owever, it supports the inherent electronegativities of oxygen and carbon and shows the oxygen as the negative end, able to donate electrons and the carbon as electron deficient center, able to accept electrons. This is the way carbonyl groups actually react, so we include such structures. b. Indicate whether the changes made on A in each structure below ( and ) increase the importance of resonance or reduce its importance. ffer a very brief explanation. (0 pts) N N N The nitrogen atom actually helps stabilize the electron deficient carbon atom through sharing its lone pair, making an extra bond and completing all the octets. This is even better resonance than the ketone. This resonance does nothing good. It destroys a bond and neither carbon atom wants to be charged. This is poorer resonance. Facts do not cease to exist because they are ignored. Aldous uxley