1 CHM 301 Fall, 2018 Preliminary Problem Set 4 Due: in precept KEY I. Draw in all of the individual hydrogens on the structure A. A. Identify the most acidic proton and label it (1). B. Identify the second most acidic proton and label it (2). C. Identify the third most acidic proton and label it (3). D. Explain carefully why (1) is more acidic than (2). The carboxylic acid group has a conjugate base that has the extra electrons spread over two O atoms, as shown in the resonance structures, a and b. Equally good. The amide group has a conjugate base that has the extra electrons spread over one O and one N atoms, as shown in the structures c and d. N is less electronegative than O, so structure c is relatively less stable, and the amide conjugate base is less stable, the amide is less acidic. E. Explain carefully why (2) is more acidic than (3). Simple secondary amine leads to a conjugate base in which the anion is stabilized by the electronegative N, but no additional resonance delocalization. The other N, in the amide group, above part D, has additional stabilization due to the C=O group. Therefore, the simple amine is less acidic. Continued… F. Identify the most basic atom and draw an arrow to it. Explain your choice. The simple amine N has no special stabilization for its non-bonding electrons. The other N is in an amide group, and the non-bonding electrons are delocalized, stabilizing the electron pair and making it less basic. H2CCH2CH2HNCH3H3COOH3COH2NH (1)(2)(3)CH3most basicROOHROOROORONH2RONHRONHabcdNHN2 The non-bonding electrons on the O atoms are held tightly by the electronegative O and are very feeble bases. OH2NOH2N3 II. Consider the following set of three molecules. A. Which is most basic? Explain in comparison with the second most basic. Draw the conjugate acids. The inductive effect of the CF3 group stabilizes the electron pair in A and makes it less basic than B. Compound C has an even more stable electron pair on N due to the additional inductive and resonance withdrawing effect of the C=O; it is least basic. B. Which is most acidic? Explain in comparison with the second most acidic. Draw the conjugate bases. (Hint: the acidic protons here are bonded to carbon) NOTE: this answer is a little ambiguous. The choice that C is more acidic than A depends on the idea that the delocalization of the negative charge in the conj. base for C is more stabilizing than the inductive effect of the CF3 group in A for a quite different type of anion. We do not know quantitatively the effect of the CF3 group so it is hard to be sure it is less. The most complete answer is that we cannot tell for sure that C is more acidic than A. For sure, both are more acidic than B. N NCF3CH3NCF3OABCNCF3ANCF3HNCH3BNCH3HNCF3CNCF3HOONCF3CONCF3OHfavoredNCF3OH(resonance stabilized)NCF3ANCF3NCH3BNCH3NCF3CNCF3OO-H+-H+-H+NCF3OCpn C has a conjugate base which is stabilized by resonance delocalization into theC=O; the negative charge can reside on the electronegative O. The conjugate base ofA is localized on carbon (not electronegative) and somewhat stabilized by the adjacent N, which itself is made more electon withdrawing by the inductive effect of the attached CF3 group. It is not a very favorable conjugate base, but it is slightly better than the conjugate base of B, which is missing the stabilizing effect of the CF3 group.C > A >