!!1!CHM 301 2018 Preliminary Precept Problem Set 6. Due in precept KEY Name_________________________________________________________________________________ I. Using the empirical formula indicated at each part, propose one structure that best fits the description for each of the following statements. No formal charge nor unpaired electrons allowed. In each case, explain your thinking in words and pictures. !A. C5H6O (no rings), a carbonyl compound with C=O stretch at lowest wavenumber in the IR spectrum. Calculate the degrees of unsaturation for the molecule. Lowest wavenumber means lowest energy, more single bond character in the C=O. More resonance structures with single bonds for C-O. (2x5 + 2 + 0 – 0 -6)/2 = 3 B. C5H6O that shows p-p* absorption at longest wavelength in the UV spectrum. Same answer! Maximum conjugation leads to smallest HOMO-LUMO gap, lowest energy jump, longest wavelength. OHOHOHOHcontribute to single bond character, lower the stretching frequencyO OO OOOH6 p orbitals in conjugation.!!2! II. Consider the following pairs of molecules. Think about whether IR, UV, or mass spec would show a clear and predictable difference for one member of each pair from the other member. Circle a form of spectroscopy that would clearly differentiate each, and explain what that difference is. ! cis trans These are isomers, so the mass spec is the same for both. No conjugated pi bonds, so the UV is the same for both—uninformative. Basic unobservable, except for possible weak n to pi* absorption. In the IR, the C=C stretch for the cis alkene is observable as a weak peak at 1680 cm-1, while the trans C=C shows no peak for the C=C stretch due to no dipole moment change on stretching. ! conjugated unconjugated The conjugated diene will show an observable UV peak, presumably around 220 nm, while the unconjugated diene will not give an observable peak above 180 nm. IR C=C stretch will be different, but not predictably so. Isomers, so mass spec does not differentiate. The aldehyde C-H stretch in (a) is at a distinctive position, about 2700 cm-1 (also second peak at 2800 cm-1, but that peak is often obscured by the sp3 C-H stretches), while the ketone (b) does not show this peak. The C=O stretches will also be at predictably different positions (1740-1720 for the aldehyde and 1725-1705 cm-1 for the ketone). Some overlap, so not conclusive). Isomers, so the mass spec does not differentiate. UV similar for both and not predictably different. (a)(b)HOOHOOClClA.B.(a)(b)C.OHMe(a)OMe(b)IR Mass Spec UV IR Mass Spec UV IR Mass Spec