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instructor_supplement_23 - Instructor Supplemental...

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Instructor Supplemental Solutions to Problems © 2010 Roberts and Company Publishers Chapter 23 The Chemistry of Amines Solutions to In-Text Problems 23.1 (b) (d) (f) 23.2 (b) N , N -Dimethyl- p -nitroaniline (common), or 1-dimethylamino-4-nitrobenzene (c) Dicyclohexylamine (common), or N -cyclohexylcyclohexanamine 23.3 (b) Apply the principle used in part (a). The order of increasing carbon–nitrogen bond length is A < C < B Compound B has a true carbon–nitrogen single bond, and hence has the longest carbon–nitrogen bond. Compound A has a true carbon–nitrogen double bond, and hence has the shortest carbon–nitrogen bond. The carbon–nitrogen bond of C has some double-bond character because of resonance interaction of the nitrogen unshared pair with the double bond. (Draw the appropriate resonance structure.) Another way to compare the C—N bond lengths of compounds B and C is to note that the C—N bond in B is an s p 3 sp 3 single bond, whereas the C—N bond in C is an sp 2 sp 3 single bond, and to remember that s bonds with greater s character are shorter. (See discussion on text pp. 127 and 678.) 23.5 The NMR spectrum indicates the presence of only two phenyl hydrogens, and the singlet absorptions at d 2.07 and d 2.16 indicate the presence of chemically nonequivalent methyl groups in the ratio 2:1. Only compound (2), 2,4,6- trimethylaniline, fits the data. The two resonances at d 2.07 and d 2.16 correspond to the ortho and para methyl groups, respectively, and the d 3.19 resonance to the —NH 2 protons of the amine. 23.6 (a) 2,2-Dimethyl-1-propanamine (neopentylamine, (CH 3 ) 3 CCH 2 NH 2 ) has a maximum of three resonances in its CMR spectrum. 2-Methyl-2-butanamine, CH 3 CH 2 C(CH 3 ) 2 NH 2 , should have four resonances in its CMR spectrum. 23.8 From Table 23.1, text p. 1123, we see that the basicity order is as follows: A < B < C The basicities of all these amines are reduced by resonance interaction of the nitrogen unshared electron pair with the ring. The electron-withdrawing polar effect of the nitro group lowers basicity further, and resonance interaction of the nitrogen unshared pair with the p -nitro group stabilizes p -nitroaniline and lowers its basicity even more. (Resonance structures depicting this interaction are shown in the solution to Problem 23.3(a) on p. 610 of the Study Guide and Solutions Manual.) 23.10 Mix the racemic acid with one equivalent of the enantiomerically pure amine. (The S enantiomer of the amine is utilized in the equation below, but either pure enantiomer of the amine could be used.) The carboxylic acids will react with the amine to give a mixture of diastereomeric salts.
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 23 2 Because these salts have different properties, they can be separated by differential solubility, for example, by fractional crystallization from some solvent, probably an alcohol. After separating the salts, aqueous acid is added to each. The amine resolving agent dissolves in the aqueous acid solution (from which it can be recovered), leaving the enantiomerically pure free carboxylic acid as a solid that can be isolated by filtration and recrystallized.
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