e. N/A; all of the above will act as Lewis acids

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1 Chemistry 261 Exam 2 Practice Fall 2017 The following practice examination contains 30 questions valued at 3 point/question unless otherwise noted. Wednesday s exam will also contain 30 questions, with an additional 9 points available as bonus/challenge questions Name: KEY ELECTRON DEFICIENT COMPOUNDS AND LEWIS ACID-BASE DEFINITIONS 1. Which of the following is not a Lewis acid? a. AlCl 3 b. 3 O + c. FeCl 3 d. SO 3 e. N/A; all of the above will act as Lewis acids By definition, a Lewis acid is an electron pair acceptor. G.N. Lewis s development in 1906 (the same year ønsted and Lowry developed the proton transfer concept) focuses on acceptors of an electron pair, is the broadest definition of acidity, and as such onsted acids are a subset of Lewis acids 1. Notice that SO 3 reacts with 2 O to generate 2 SO 4, a principal component of acid rain 1 For the interested student, the Lewis acid Wiki is pretty good

2 ELECTRON DEFICIENT COMPOUNDS AND LEWIS ACID-BASE REACTIONS 2. The amino group on the weak base aniline (anilinium ion pk a = 4.87) is strongly ortho and para directing, activating the benzene ring towards electrophilic aromatic substitution (EAS, chapter 16). owever, when in the presence of the common catalyst for electrophilic aromatic bromination of the benzene ring, Fe 3, the reaction is strongly inhibited, with complex I being formed. The aniline amino group has acted as a. A Lewis acid b. A Lewis base c. A ønsted acid d. A ønsted acid e. An electrophile Complex I Sorry for the wordiness on this one, but it shows that identifying the potential for Lewis adduct formation can be critical to assessing reaction potential. One way to make better sense of Fe 3 as a Lewis acid is to name it Iron(III) bromide iron is in the +3 oxidation state and has available d orbitals for coordinate covalent bond formation NUCLEOPILES, ELECTROPILES, AND LEAVING GROUPS 3. In the reaction between NaCN and propyl chloride to generate cyanopropane (formally butanenitrile) the chloro group acts as a. A nucleophile b. An electrophile c. A Lewis acid d. A Lewis base e. A leaving group Nucleophiles, electrophiles, and leaving groups, oh my! Most of the reactions you will see in this class are 2 electron processes involving a nucleophilic attacking species (I suppose it attacks based on curved arrow conventions), an electrophilic species being attacked, and a leaving group to maintain the requisite number of bonds for a noble gas electronic configuration

3 NUCLEOPILES, ELECTROPILES, AND LEAVING GROUPS 4. Which of the following is the strongest electrophile? a. C 3 CO 2 C 3 b. C 3 CO 2 c. C 3 OC 3 d. C 3 COC 3 e. C 3 C C Na + Clearly, you must be able to translate condensed structural formulas into full Lewis structures: (a) is an ester (b) is a carboxylic acid (c) is an ether, and (d) is a ketone. As a [poorly] stabilized carbanion, (e) is a strong base and nucleophile. The carbonyl is the most polarizing, but the extra oxygen in options (a) and (b) make them more electron rich BRØNSTED-LOWRY ACIDS AND BASES CONJUGATE ACID AND BASE DEFINITION 5. Which of the following is the conjugate base of ammonia? + a. N 4 b. N 3 c. N 2 d. N 2 4 e. N/A; none of the above is the conjugate base of ammonia Critical to understand this, and to understand the weaker the acid (in this case the weak base ammonia has somehow been forced to act as an acid) the stronger the conjugate base the amide ion is a very strong base BRØNSTED-LOWRY ACIDS AND BASES CONJUGATE ACID AND BASE DEFINITION 6. Which of the following are ønsted bases in the following equilibrium? 2O + 2O O + 3O + a. 2 O and O b. 3 O + and O c. 2 O and 2 O d. 3 O + and 2 O e. Only O as water cannot simultaneously be an acid and a base Water in the forward direction, hydroxide in the reverse direction. Of course water is amphiprotic, accepting or donating + depending on circumstance this is fundamentally the basis for the p scale

4 FUN WIT pk a s 7. What is the approximate pk a of hexanoic acid (commonly caproic acid)? a. 2 b. 3 c. 4 d. 5 e. 6 Actual pk a = Acetic acid pk a = Without anything to distort the electron distribution in relation to acetic acid, the pk a will not change much FUN WIT pk a s 8. Which of the following compounds would be deprotonated by potassium tert-butoxide? a. exane b. 1-exene c. 1-exyne d. (b) & (c) e. None of the above This question really isn t that difficult given tbuok is the conjugate base of an alcohol (an alkoxide, with the negative charge on an electronegative oxygen) while the answer options would leave a carbon anion. Options (1) or (2) sinply are not happening regardless of the base employed, while (c) has an approximate pk a = 25 while tbuo has a pk a = 18. A 7 unit difference is too large to affect any significant deprotonation FUN WIT pk a s 9. Which of the following compounds would be deprotonated by sodium hydride? a. exane b. 1-exene c. 1-exyne d. (b) & (c) e. None of the above With a pka 35, Na easily deprotonates terminal alkynes of pk a = 25. I suppose by this point you have rightly concluded the free energy of activation for proton transfer reactions is sufficiently low that if there is a favorable pk a difference, reaction will occur. In fact, some of the very strong base reactions must be run at reduced temperature so that things don t get out of hand

5 FREE ENERGY AND CEMICAL EQUILIBRIUM 10. Which of the following correctly expresses the relationship between pk a and the standard free energy of dissociation? a. G o a = 2.3RTpK a b. G o a = -2.3RTpK a c. G o a = 10 pk a/2.3rt d. G o a = 10 pk a/2.3rt o e. pk a = 2.3RT G a Math! Note the change from the more usual K a form given the definition of p as log; i.e. G o a = -2.3RTlogK a. Rearranging and taking the antilog places - G o a /2.3RT as an exponent. Since 2.3RT translates to 1.4 kcal/mol under standard conditions, slight changes in G o a correspond to large scale changes in equilibrium ratios. Spending some time with table 3.2 gives you a very nice feel for this fact STRUCTURE-ACIDITY RELATIONSIPS 11. Rank the bold-faced hydrogens in the following compounds from most acidic to least acidic. COO CF 3 O 3 C 3 C C 3 O I II III IV V a. I > II > III > IV > V b. III > V > II > I > IV c. V > II > IV > III > I d. III > I > V > II > IV e. V > III > I > II > IV Protonated ether (-2.5) benzoic acid with electron withdrawing group ( 3) phenol (10) vinylic (44) and straight up alkane (50) STRUCTURE-ACIDITY RELATIONSIPS

6 12. Rank the identified hydrogens from the most acidic to least acidic in the compound shown below a. a > b > c > d b. a > c > d > b c. c > d > a > b d. d > a > c > b e. c > a > d > b I really like this question imagine you were starting at p = -1 and then the p was slowly raised can you see the protons being sequentially removed? The ring structure corresponds to the non-nuclephilic (too bulky and inductive removal of lone pair electron density) weak base pyridine (pyridinium pk a = 5.2) which is a very useful proton sink for reactions that generate acid by-products STRUCTURE-ACIDITY RELATIONSIPS 13. For the simple hydrides listed below, which is the correct order of decreasing pk a values? a. C 4 > N 3 > 2 O > 2 S > b. > 2 S > 2 O > N 3 > C 4 c. > 2 O > N 3 > 2 S > C 4 d. N 3 > 2 S > C 4 > 2 O > e. 2 S > 2 O > > N 3 > C 4 Decreasing? With methane on the list? Game over! Notice 2 S has a pk a = 7 (thiols similar to 10-12), while water pk a = 15.7 just as Cl is a much stronger acid than F

7 ACID-BASE REACTION PRODUCT PREDICTION CURVED ARROW APPROAC 14. What is/are the products of the following acid-base mechanism? Mg a. + Mg b. Mg + C 3 C 3 c. d. Mg + C 3 C 3 e. None of the above Pretty straightforward nowhere else for the carbanion to resonate to, and having an electron deficient C next to a carbanion simply isn t tenable (option (d))

8 ACID-BASE REACTION PRODUCT PREDICTION CURVED ARROW APPROAC (IMPLIED) 15. What is/are the products of the following acid-base reaction? a. b. c. Na N N + N + Na + Na N d. Na N + e. N/A; There is no acid-base reaction between the reactants shown Option (a) seems pretty tempting, but comparing the pk a values shows the alkyl amide ion (yes, I don t like the naming any more than you do) competes much more fiercely than the alkynide ion for the hydrogen in question

9 STRUCTURE AND BONDING IN ALKENES 16. Which of the following unknown acyclic molecules contains no rings or double bonds a. C 5 13 N b. C 5 10 O c. C 6 10 Cl 2 O d. C 7 12 I e. More than one of the above C n 2n+2, adding 1 for N, ignoring O, and counting halogens as hydrogen STRUCTURE AND BONDING IN ALKENES 17. For which of the following molecules can resonance give a completely equivalent, noncharge separated form? a. Benzene b. 1,3,5-exatriene c. 1,3-Cyclobutadiene d. 1,3-Butadiene e. More than 1 of the above Benzene is pretty obvious, 1,3-cyclobutadiene less so. If the molecule is not cyclic, then a charge separated form must be the end result. 1,3-cyclobutadiene is the poster child for anti-aromatic compounds, and has a very short half-life, since electrons need to be placed in non-bonding MO s 2 DOUBLE BOND STEREOISOMERS/CIS-TRANS & E,Z NOMENCLATURE 18. Please draw the structure for (Z)-2-vinyl-2-pentenoic acid Z for zusammen or together. Phantom rule on carbonyl oxygen gives higher priority than phantom rule on ethenyl group 2 Much more on this later the interested student should see ückel s Rule

10 DOUBLE BOND STEREOISOMERS/CIS-TRANS & E,Z NOMENCLATURE 19. Please name the following compound, adhering to IUPAC conventions Name: 2,5-cyclohexadien-1-ol The higher oxidation state of the alcohol dictates numbering. Alkenes are within a 6 membered ring on need to indicate Z/cis or E/trans. Indicating the alcohol as the 1 position is overkill in this case UNSATURATION NUMBER OR INDEX OF YDROGEN DEFICIENCY 20. What is the unsaturation number for the compound immediately below a. 2 b. 3 c. 4 d. 5 e. 6 5 bonds and 1 ring PYSICAL PROPERTIES OF ALKENES 21. Which of the following will have the highest boiling point a. 1-pentene b. 1-hexene c. cyclopentene d. cyclohexene e. All of the above boil within 5 o C of one another Some questions I like so much I may just revisit them (wink). See practice quiz 2 for further details

11 RELATIVE STABILITIES OF ALKENE ISOMERS 22. Which alkene would you expect to be most reactive toward acid-catalyzed hydration? int: resonance possibilities may surprise you a. Styrene (vinyl benzene) b. Toluene (methyl benzene) c. 1-exene d. 2-exene e. Cyclohexene The benzylic cation is highly resonance stabilized, which gives it the approximate stability of a 2 o carbocation. Notice that the cation formed has stability more like a 3 o carbocation since it 2 o to begin with (C + bonded to benzene ring as well as the terminal C) ADDITION OF YDROGEN ALIDES/CARBOCATION STABILITY/CARBOCATION REARRANGEMENTS 23. Which of the following carbocations would not undergo rearrangement? a. C 3 CCC 3 b. C 3 C 3 C 3 CCC 3 c. C 3 C 3 C 3 CC 2 C 3 d. C 3 e. C 3 CC 2 C 3 C 3 CCC 2 C 3 C 3 The one that is already a 3 o carbocation. (a) and (d) would undergo hydride shifts, while (b) and (e) undergo methide shifts

12 ADDITION OF YDROGEN ALIDES/CARBOCATION STABILITY/CARBOCATION REARRANGEMENTS 24. Treating 1-methylcyclohexene with 3 O + would produce which of the following as the principal product(s)? O O O I II III O O a. I & V b. II c. III & IV d. IV e. I, III, & V IV V Direct formation of a tertiary carbocation followed by the addition of water ADDITION OF YDROGEN ALIDES/CARBOCATION STABILITY/CARBOCATION REARRANGEMENTS 25. What is the principal product of the following reaction? 2, C 3 O a. O + enantiomer b. OC 3 + enantiomer c. + enantiomer d. 3 CO

13 OC 3 + enantiomer e. More than one of the above Variation on halohydrin formation where alcohol is the solvent, leading to an ether (as opposed to an alcohol product). Other principles apply since you are adding 2, not, do not expect a rearrangement. 2 o carbocation is more stable, so we expect alcohol addition adjacent the quaternary positon ADDITION OF YDROGEN ALIDES/CARBOCATION STABILITY/CARBOCATION REARRANGEMENTS 26. Reacting 1-methylcyclopentene with bromine monochloride (Cl) yields which of the following as principal products? Cl + enantiomer a. I b. II c. III d. IV e. V C 3 C 3 Cl + enantiomer Cl I II III C 3 + enantiomer IV C 3 Cl + enantiomer V C 2 Cl + enantiomer Too hard? Not if you apply fundamental principles! Chlorine is more electronegative than bromine, making the bromine end of the molecule more electrophilic. Thus, a bromonium ion bridges between the [formerly] alkene carbons in question, with a large degree of carbocation character at the 3 o position. Chloride attacks this site of greatest electrophilicity from below, leaving the chloro and bromo groups on opposite faces of the ring

14 REACTION RATE CONSIDERATIONS 27. Which of the following correctly expresses the relationship between rate and the standard free energy of activation? a. log(rate) = 2.3RT G o b. log(rate) = -2.3RT G o c. log(rate) = - G o /2.3RT d. log(rate) = G o /2.3RT e. ln(rate) = - G o /2.3RT Notice the negative value associated with the free energy change larger free energy barriers lead to larger negative exponents and much slower rates of reaction. To wit, just as the 1.4 kcal/mol difference changes the equilibrium concentration by a factor of 10, a 1.4 kcal/mol increase in free energy of activation slows the reaction by a factor of 10 REACTION RATE CONSIDERATIONS 28. iefly, what is the difference between the standard free energy of activation and the standard free energy of dissociation? The standard free energy of activation pertains to the relationship between free energy changes and rate of reaction, whereas the standard free energy of dissociation relates overall free energy changes to equilibrium concentrations of reactants and products. Importantly, there are only positive values for G o, since it is the difference between the transition state and reactant system free energy. For G o a, negative free energy changes (release of free energy) between product and reactant free energy levels are common, if not expected, with large negative changes driving the reaction to completion, as should be evident with an overall positive value of G o a in K a = 10^- G o a /2.3RT

15 CATALYSIS/CATALYTIC YDROGENATION OF ALKENES 29. Under usual hydrogenation conditions, how many moles of diatomic hydrogen will react with the compound immediately below a. 2 b. 3 c. 4 d. 5 e. 6 5 bonds and 1 ring, and the ring won t react CATALYSIS/CATALYTIC YDROGENATION OF ALKENES 30. Which of the following will convert cyclohexene into cyclohexane? a. 2 /Ni/high pressure b. 2 /Pd/C c. 2 O/Ni d. More than 1 of the above e. All of the above ydrogenation of an alkene requires hydrogen (and a metal catalyst). LiAl 4 and NaB 4 provide hydride, which effectively amounts to hydrogenation upon acidic workup, but only for more electrophilic systems like carbonyls, and NOT for alkenes. Similarly, carbonyls are typically resistant to conditions that hydrogenate alkenes a very useful difference