Clearly indicate true (T) or false (F) for the following statements (2 pts each): 0 1 __F__ HO– is a stronger base than –NH2 __T__ A Lewis acid is a compound that accepts a share in a pair of electrons. Multiple Choice: (2 pts each) Which of the following molecules has a conjugated pi system? Which of the following statements are correct concerning delocalized electrons? Electrons do not belong to a single atom. Electrons are not confined to a bond between two atoms Electrons are shared by three or more atoms A and B A, B and C The delocalized π system in benzene is formed by a cyclic overlap of 6 _______ orbitals. s p sp sp 2 sp 3 Which of the following statements is incorrect about benzene? All of the carbon atoms are sp hybridized It has delocalized electrons The carbon-carbon bond lengths are all the same All six atoms lie in the same plane Which of the following compounds is not a Lewis base? NH 3 BF 3 CH 3 OCH 3 CH 3 OH Which arrow is used to represent two contributors in a resonance hybrid? (6 pts) Consider the following two compounds (A and B) and circle which compound will have the highest boiling point. Briefly explain your answer, including the intermolecular forces for each. Boiling point is based on the strength of the intermolecular forces between molecules. Both molecules A and B have an electronegative oxygen, but molecule B has an alcohol (OH) group with hydrogen-bonding forces between molecules, while molecule A is an ether with only dipoledipole interactions between molecules. Hydrogen-bonding forces between molecules are stronger than dipole-dipole interactions, so molecule B has a higher boiling point. Delocalized electrons and resonance (11 pts total) Circle any of the following compounds that have a nitrogen atom with delocalized electrons. Only compounds where the nitrogen lone pair is directly attached to a pi bond!! This is like questions #1 and 18 from chapter 6. For each pair of resonance structures shown below (A and B), circle which resonance contributor makes a greater contribution to the resonance hybrid. Briefly explain your answer. Structure B is the greater contributor to the resonance hybrid because the negative charge is on the most electronegative atom (oxygen) . Structure B is the greater contributor to the resonance hybrid because it has all atoms with filled octets. (Structure A has a carbon with only 6 electrons, but structure B has all atoms with 8 electrons – draw lone pairs on oxygen to convince yourself). In this case, the electronegativity of the atom that the charge is on does not matter because the octet rule is more important. (This is just like the example on the bottom of the delocalized electrons and resonance handout) (4 pts) Which compound (A or B) is a stronger acid? Provide a brief explanation for your answer. negative charge is stabilized by electronegative oxygen and delocalization into pi bond A stronger acid is more willing to give up an acidic proton and has a more stable conjugate base. When comparing the conjugate base for molecules A and B, the conjugate base of molecule B will have the negative charge on an oxygen atom, which is more electronegative and can better stabilize a negative charge. (Both have resonance stabilization because both have an adjacent pi-bond that the lone pair (negative charge) can delocalize into.) Therefore, molecule B has a more stable conjugate base, making it a stronger acid. (Although it is not required to draw the structure of the conjugate base for this question, it is helpful to have the structure to refer to.) (4 pts) Lactic acid (also know as milk acid) has two acidic protons (A and B). Explain which proton is more acidic and draw the conjugate base that will result upon deprotonation. conjugate base with deprotonation of proton A is a carboxylate anion. This conjugate base is stabilized by resonance (the anion is delocalized in the adjacent pi bond) A stronger acid is more willing to give up an acidic proton and has a more stable conjugate base. Proton A is part of a carboxylic acid and proton B is just part an alcohol. The conjugate base with deprotonation of proton A is stabilized by resonance. The anion is delocalized/shared on two electronegative oxygen atoms. (in fact, now that we have discussed inductive effects more, you know that the second oxygen also helps to stabilize the carboxylate anion and make the carboxylic acid more acidic by withdrawing electron density through sigma bonds…) (6 pts) The following molecule has an acidic proton. Draw the conjugate base for this molecule and then show why this molecule is acidic by drawing a relevant resonance structure. The question did not ask you to show curved arrows, but I have included them so you can see the electron movement. **if a molecule is neutral, the conjugate base will always have a negative charge (when it gives up the acidic proton H + ) (See How to answer an “Explain” Question )