Ch4 - 74 S olutions Chpl 1 Stereoisomerism and Chirality...

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Chapter 4: Acids an~ases Solutions 75 CHAPTER 4 Solutions to the Problems problem 4.1 For each conjugate acid-base pair, identify the first species as an acid or base and the second species. as its conjugate acid or conjugate base. In addition, draw Lewis structures for each species, showing all valence electrons and any formal charges. . (a) 'H2S04, HS04- (b) NH3' NH2- '0' '0' .. II ., .. II .. - H-N-H :N-H H-o-S-o-H H-o-S-O: II .. .. II .. I I H H :0: :0: Conjugate Acid Col\iugate base Acid base (c) CH30H, CH30- H 3 C-.C?-H Conjugate Acid base problem 4.2 Write these reactions as proton-transfer reactions. Label which reactant is the acid and ~hich is the base; which product is the conjugate base of the original acid and which is the conjugate acid of the original base. In addition, write Lewis structures for each reactant and product, and use curved arrows to showthe flow of electrons in each reaction. (a) CH 3 -S-H + OW - CH 3 -S - + H 2 0 .~ + H-o-H CH3-.~H ~: .C?-H Acid Base Conjugate Conjugate base acid --"-- ~ CI + : C1: Conjugate Conjugate Base Acid acid base Problem 4.3 Following is a structural formula for guanidine, the compound by which migratory birds excrete excess metabolic nitrogen. The hydrochloride salt of this compound is a white crystalline powder, freely soluble in water and ethanol. . . .' (a) Write a.Lewis structure for guanidine showing all valence electrofls. .. /H N H ......... II •• /H N-C-N H/ ....... H Guanidine (b) Does proton tran~fer occur preferentially on one of its -NH2 groups (cation A) or to its =NH group (cation B)? Explain .. NH+ II 2 or ~ N-C-NH 2 + Hel B Guanidine
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76 Solutions Chapter 4: Acids and Bases ~ .. li H : ~_ _ -'- \ r:. ~ N-C-NH + H-CI: - + :CI: 2 •• The proton goes on the =NH group because this is the most basic nitrogen atom, and the resulting conjugate acid has 2 H atoms on each nitrogen_ Distributing the H atoms evenly in this way is the most stable possible arrangement due to charge delocalization as indicated by the following resonance contributing structures. + \N~ N~ N~ N~ .. II,.. .. :~ I 1"'+ .. :;\ I •• •• I .. ti:!N-C-NH:1 ti:!N-C=Nti:! ti:! N=C-Nti:! ti:!~C-Nti:! + Distributing the charge over multiple atoms is always stabilizing, so the arrangement or atoms that allows the greatest possible distribution of charge will generally be the most stable one for a given ion, Problem 4.4 Write an equation to show the proton-transfer between each alkene or cycloalkene and HC!. Where-two carbocations are possible, show each. _ ( a) CH:J C~ CH=CHCH:J 2-Pentene H I + CH 3 CH 2 C ~CHC~ + :CI: H + I CH 3 CH 2 C ~CHC~ 0 Note how there are two different cations produced upon protonation - .of the alkene (b) Cyc lohexene ~ (. .. / H-C1: a d> H Problem 4.5 For each value of K". calculate the corresponding value of pK". Which compound is the stronger acid? (a) Acetic acid, K" = 1.74 x I 0- 5 (b) Chloroacetic acid K" = 1.38 x 10- 3 The pK a is equal to -logloKa. The pK a of acetic acid is 4.76 and the p~ of chloroacetic acid is 2.86, Chloroacetic acid, with the smaller pK a ,'alue, is the stronger acid. - - Problem 4 6 Predict the position of equilibrium-and calculate th'e equilibrium constant, K~q, for each acid-base reaction.
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