Name: Problem Set Chapter 3. Acids and Bases. The CurvedArrow Notation 3.1 Use the curvedarrow notation to derive a structure for the product of each of the following Lewis acid base association reactions; be sure to assign formal charges. Label the Lewis acid and the Lewis base, and identify the atom that donates electrons in each case. 3 C C 2 F F N 3 B F 3.2 For each of the following cases, give the product(s) of the transformation indicated by the curvedarrow notation. ( ) 2 C C( ) 2 Br 2 C 2 C 3.4 Using the curvedarrow notation, derive a resonance structure for the allyl anion (shown here) which shows that the two carbon carbon bonds have an identical bond order of 1.5 and that the unshared electron pair (and negative charge) is shared equally by the two terminal carbons. 2 C C C 2? allyl anion 1
3.5 In the following reactions, label the conjugate acid base pairs. Then draw the curvedarrow notation for these reactions in the lefttoright direction. N 2 N 3 2 N 3 N 3 N 2 N 4 3.6 Write a Brønsted acid base reaction in which 2 and act as conjugate / C 3 / acid base pairs. 3.8 In each of the following processes, complete the reaction using the curved arrow given; classify the process as a Brønsted acid base reaction or a Lewis acid base association/dissociation; and label each species (or part of each species) with one or more of the following terms: Brønsted base, Brønsted acid, Lewis base, Lewis acid, nucleophile, electrophile, and/or leaving group. In each part, once you complete the forward reaction, draw the curved arrow(s) for the reverse reaction and do the same exercise for it as well. 2 C C 2 Br Reverse: 2 B Reverse: 2
3.9 For each of the following reactions, give the curvedarrow notation and write the analogous Brønsted acid base reaction. S S 3 C 3 C C 2 Br C N C 3 C 2 C N Br 3.14 Using the pka values in Table 3.1, calculate the equilibrium constant for each of the following reactions. F acting as a base toward the acid CN Table 3.1 N 3 pk a = ~35 pk a = 15.7 N 4 pk a = 9.25 CN pk a = 9.40 F pk a = 3.2 3 pk a = 1.7 3.15 Write an equation for each of the following equilibria, and use Table 3.1 to identify the pk a value associated with the acidic species in each equilibrium. ammonia acting as a base toward the acid water ammonia acting as an acid toward the base water Which of these reactions has the larger Keq and therefore is more important in an aqueous solution of ammonia? 3
3.20 Rank the following four acids in order of increasing Brønsted acidity. S 2 3 C 2 F A B C D Rank the following compounds in order of increasing basicity. (int: Think about the acidity of the conjugate acids and the relationship between the acidities of acids and the basicities of their conjugate bases.) N 2 A B C 3.21 In each of the following sets, arrange the compounds in order of decreasing pka, and explain your reasoning. ClC 2 C 2 S ClC 2 C 2 C 2 A B C C 2 C 3 C C C 2 C C (c) 3 C C Cl C 2 C A B C 3 C C 3.24 Which of the following are electrondeficient compounds? Explain. (c) (e) C N 3 C 3 C 3 C N C 3 B C 3 N 3.25 Give the curvedarrow notation for, and predict the immediate product of, each of the following reactions. Each involves an electrondeficient Lewis acid and a Lewis base. 3 C C 3 BF 3 4
3 C C C 3 Cl (c) C 2 C 2 C 2 C (int: this reaction forms a ring) (e) C N 2 3.30 A scientist at an Air Force Research Laboratory in California studies highly energetic materials explosive materials) and, more significantly, lives to tell about it. In 2000, he and his equally adventurous collaborators determined the Xray crystal structure of the N 5 cation; most salts of this cation are highly explosive. This is the first species ever isolated in modern times that contains more than three contiguously bonded nitrogens. The crystal structure revealed a Vshape for the cation, as follows: Notice that the lines do not indicate the bonding pattern, but only the shape. N N Draw an acceptable Lewis structure, including unshared electron pairs, that accounts for the shape of the molecule and its overall plus charge. Explain why your molecule meets these criteria. Be sure to show in your structure the formal charge of every atom with nonzero formal charge. Using the curvedarrow notation, derive two additional resonance structures for this cation that meet the same criteria. N N N 5
3.32 Draw the products of each of the following reactions indicated by the curvedarrow notation. MgBr C 3 C (c) 3 C C C Si( ) 3 F 3 C C C 2 5 N 2 3.33 Use the curvedarrow notation to indicate the flow of electrons in each of the transformations: ( ) 2 N 2 C C C C 2 (C 5 3 ) 2 N C 2 C C C 2 5 Br C 2 C 2 C Br 2 C C 2 C 2 C C 2 _ CN _ C 2 C 2 CN 3.34 Predict the products of each of the following reactions, and explain your reasoning. Use the curvedarrow notation to help you, and show the notation. S (int: Remember that Brønsted acid base reactions are in most cases much faster than nucleophilic reactions.) 6
C 3 C AlCl 3 N 3 C2 3.35 In each of the following processes, give the products and classify each of the groups indicated by a colored label with one or more of the following terms: Brønsted base, Lewis base, Brønsted acid, Lewis acid, nucleophile, electrophile, and/or leaving group. You may have to be a bit creative in assigning labels to some groups. If you have a problem, try to state what the difficulty is. A C C 2 Br B Ph C D S 2 C C C A B C D 3.36 The examples of incorrect curvedarrow notation in Fig. P3.36 were found in the notebooks of Barney Bottlebrusher, a student who was known to have difficulty with organic chemistry. Explain what is wrong in each case. 3 C C 3 C C C 3 Br 3 C Br 7
3.37 Use the curvedarrow notation to derive three other resonance structures for anthracene. three additional structures anthracene 3.39 Arrange the compounds in each of the following sets in order of decreasing pk a, highest first. Explain your reasoning. C 2 Cl 2 CC 2 ClC 2 C 2 ClC 2 C 2 S C 2 C 2 S (c) As ( ) 2 As( ) 2 N( ) 2 P ( ) 2 C 2 ( ) 2 N C 2 C 2 ( ) 2 N 3.43 Malonic acid has two carboxylic acid groups and consequently undergoes two ionization reactions. The pk a for the first ionization of malonic acid is 2.86; the pk a for the second is 5.70. The pk a of acetic acid is 4.76. C C 2 C C Write out the equations for the first and second ionizations of malonic acid, and label each with the appropriate pk a value. Why is the first pk a of malonic acid much lower than the pk a of acetic acid, but the second pk a of malonic acid is much higher than the pk a of acetic acid? 8
(c) Malonic acid is one member of a homologous series of unbranched dicarboxylic acids, socalled because they have two carboxylic acid groups. Compounds in this series have the following general structure. C (C 2 )n C ow would you expect the difference between the first and second pk a values to change as n increases? q1q2 Explain. (int: Look at the denominator of the electrostatic law, Eq. 3.40, p. 113: E = k ) ε r 3.45 From Fig. 3.2, p. 113, how would a structural effect that destabilizes the acid component of a conjugate acid base pair affect its acidity? Use your analysis to predict which of the following two compounds is more basic. C 2 N 2 Cl C 2 C 2 N 2 A B 3.48 The acid I is considerably stronger than Cl (see Table 3.1, p. 103). Why, then, does a 10 3 M aqueous solution of either acid in water give the same p reading of 3? From Table 3.1: pk a of I = 9.5 to 10; pk a of Cl = 6 to 7 N 2 The amide ion,, whose conjugate base ammonia has a pk a = 35, is a much stronger base than hydroxide. Yet a 10 3 M solution of either base in water has p = 11. Explain why the solution of the stronger base doesn t have a higher p. 9
3.49 The borohydride anion reacts with water in the following way: B 4 2 B 3 2 This overall transformation occurs in a sequence of two reactions, the first of which is shown in Eq. 3.6 on p. 91. Write out both steps and give the curvedarrow notation for each. B B 10