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CHEM 4341/6371 Fall 2016 Homework 4 Questions Answers must be submitted on the answer form. You may collaborate on the development of strategies to answer these problems, but completion of your final answers should be done independently of others. Identify compounds A-X based on the Idata provided. A possible approach: If provided, consider the IR spectra – are oxygen and/or nitrogen-containing bonds present? And if so, what type of functional group is present? Is there evidence for a benzene ring, or an alkene? If so, what is the substitution pattern? What other common functional groups and structural features are present? If it is not provided, determine the molecular formula using clues from the IR spectrum (e.g., presence of O and N) and mass spectrum (e.g., presence of Cl, Br; molecular weight) Consider the mass spectra. Be on the lookout for peaks with even values of m/z in the mass spectra of compounds with even molecular masses – these are usually good clues. If you have determined the functional group that is present, consult the appropriate section of the textbook and the “handy dandy” appendix; consider how molecules containing that functional group undergo fragmentation. In most cases, once you have determined the molecular formula and the presence of specific functional groups, considering the fragmentation that gives the base peak is a huge clue. In other cases it will be useful to also consider: - fragmentation that lead to a peak with the highest m/z below the molecular ion (e.g., loss of a small radical or molecule , e.g., CH3, H2O). - the presence of large peaks corresponding to loss of 15, 29, 43, 57, etc (e.g., loss of .R). - fragments arising from loss of a neutral molecule (i.e., with an even m/z; e.g., loss of H2O; C2H2, etc). - McLafferty rearrangement. In some cases it might not be possible to determine the exact structure of an alkyl group in a compound. For example, you might determine that fragmentation of the molecular ion leads to loss of C4H9•, but in some cases it will not be possible to determine whether this is an n-butyl, isobutyl, sec-butyl or t-butyl chain. In these cases, it is appropriate to simply label a substituent on the molecule as C4H9–. In other cases there is evidence for a specific type of alkyl group (e.g., from the IR or NMR spectrum). Do your best to be as specific as possible, but do not “over interpret! 1. Compounds A-C, all C10H14 mono-substituted benzenes All C10H14 mono-substituted benzenes (there are only four possibilities) – draw them and predict how they might fragment (hint – consider how cleavage at the benzylic position might give a tropylium ion), then look at the spectra. You should be able to identify the alkyl groups here. 2. Compounds D-F, all C6H14O alcohols All C6H14O alcohols – consider the section in the book that deals with fragmentation of alcohols. You might not be able to identify the exact arrangement of carbon atoms, but you should be able to determine the position of the OH group in the molecule. 3. Compounds G-I, all MW = 70 Molecular weight is only 70, no oxygen atoms. The two of the mass spectra are almost identical, but their IR spectra are different. What functional group is present? What is the most common mode of fragmentation for molecules containing this functional group? Hint: Pavia. 4. Compounds J-L, all C8H19N For these compounds, SODAR = 0, so these are all saturated amines. The IR might give clues about the nature of the amine (1°, 2° or 3°). Then you need to determine the structure of the alkyl group on the nitrogen. Consider how amines fragment. 5. Compounds M-O, all MW = 101 Note molecular weight of 101 (that’s odd!) Consider the base peak in the mass spectra (note that these all have an even molecular weight, but that this compound has an odd molecular weight, so these all result from loss of alkyl radicals – but which ones?) What functional group is present? What is the most common mode of fragmentation for molecules containing this functional group? Consult the appropriate section of the textbook. 6. Compounds P-Q, both C9H13N 7. Compounds R-W, all C10H14 SODAR = 4, and the infrared spectra all suggest the presence of a benzene ring (with four additional carbon atoms; this leads to <20 possible compounds). Determine the substitution pattern of the benzene ring. Then consider how alkylbenzenes fragment (HINT: tropylium) to identify the alkyl substituents on the ring. There might be other clues in the IR spectra regarding the presence of i-Pr and t-Bu groups. In some cases, the IR spectra of spectra are nearly identical, but there are important differences in the mass spectra, and vice versa. 8. Compound X Problem 1: compounds A-C (all C10H14 mono-substituted benzenes) A B C Problem 2: compounds D-F (all C6H14O alcohols) D E F Problem 3: compounds G-I (molecular weight = 70) G Problem 3: compounds G-I (molecular weight = 70) H Problem 3: compounds G-I (molecular weight = 70) I Problem 4: compounds J-L (C8H19N) J Problem 4: compounds J-L (C8H19N) K Problem 4: compounds J-L (C8H19N) L Problem 5: compounds M-O (molecular weight = 101) M Problem 5: compounds M-O (molecular weight = 101) N Problem 5: compounds M-O (molecular weight = 101) O Problem 6: compounds P-Q (C9H13N) P Problem 6: compounds P-Q (C9H13N) Q Problem 7: compounds R-W (C10H14) R Problem 7: compounds R-W (C10H14) S Problem 7: compounds R-W (C10H14) T Problem 7: compounds R-W (C10H14) U Problem 7: compounds R-W (C10H14) V Problem 7: compounds R-W (C10H14) W Problem 8: compound X X Formula not given Chemical ionization mass spectrum gives (M+H)+ peak at145 1H NMR spectrum provided (bottom right) 1 13 6