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Solutions to Problems 219 11-11. Degree of Unsaturation When you are faced with the problem of coming up with a reasonable structure to match some spectroscopic or chemical data. it is possible to waste a lot of time writing answers that are incompatible with the molecu- lar formula of the molecule. Determining the number of rings + TT bonds ahead of time (the degree of unsatu- ration) can make solving these problems go much more smoothly: You automatically know whether or not you need to consider these structural elements as possible parts of an unknown molecule. In practice, you should try to reconcile your IR and NMR data with the degree of unsaturation before you start writing down possible structures. For example, if the IR and NMR indicate the absence of bonds (no IR bands around 1650 cm⁻¹; no NMR signals downfield of about δ = 5) but the formula indicates one degree of unsaturation, then your answer must contain one ring. On the other hand, if the IR and NMR do show such signals in a molecule with exactly one degree of then the unsaturation must be a bond and a ring cannot be present. Thus by the process of elimination, you get closer to the right answer more quickly. Try it yourself, beginning with Problem 34. In some cases you will be given an additional piece of data: the result of hydrogenation. Use it to distinguish TT bonds from rings, in after hydrogenation the bonds will be gone, but the rings will still be there. The degree of unsaturation will be given in the answers in this study guide. Solutions to Problems I Br 29. (a) (b) HO OCH₃ (c) (d) (e) HO 30. (a) cis- or Z-2-Pentene (b) 3-Ethyl-1-pentene (c) trans- or (d) (Priorities are 1> Br on and > F on C2.) (e) (f) 1,1-Dichloro-1-butene (g) Z-1,2-Dimethoxypropene (h) (i) I-Ethyl-6-methylcyclohexene. This name is better than 2-ethyl-3-methylcyclohexene, because the first number is smaller. 31. (a) (E because the highest-priority groups on the two alkene carbon atoms are and CH₃, respectively, and they are trans to each other; note that cis/trans may not be used in the name be- cause the alkene carbons collectively have more than two in this case); (b) E- or trans- 1-chloro-2-butene; (c) 32. In each case both the higher dipole moment and the higher boiling point are found in the compound that has the fluorine atoms cis to each other. (a) (b) Z-1,2-difluoropropene; (c) Z-2,3-difluoro-2-butene.