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UCSB CHEM 124 - The UV spectrum

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Exam 1, Chem 124/224, winter quarter 20041. [a] The UV spectrum of acetone (0.05 M in cyclohexane, 1 cm cell) shows λmax at 279 nm with an absorbance of0.74. What is the extinction coefficient of acetone at this wavelength? Show your work.[b] What is the transition, n-π* or π - π *, that is responsible for this absorbance?2. Examine the UV spectrum of the enone illustrated below. Assign the major absorption bands using theclassifications n-π* and π - π *, and determine an approximate (to the nearest power of ten) extinction coefficient foreach.3. [a] Account for the shift that is observed as n increases from 3, to 4, to 5 in R(CH=CH)nR. [b] Which of thefollowing terms – bathochromic, hypsochromic, hyperchromic, hypochromic, blue shift, red shift – describe the shift?4. The following diagram illustrates a superposition of the infrared spectrum of two alcohols, one recorded using adilute solution, the other concentrated. Which spectrum corresponds to which set of conditions? How do you know?5. Examine the mass spectrum shown below. Account, mechanistically, for the formation of the peaks at m/z 110, 95,81, 54, and 56, providing an acceptable structure for each. Which is the base peak?6. The enone (1, below) whose UV spectrum is illustrated on the 1st page is a versatile, and often used substance. I’veillustrated three conversions. As you can see, compounds A and B are regioisomers.[a] Use the appended tables to show how one could easily differentiate between the structures using UV spectroscopy.[b] Define, explicitly, how infrared spectroscopy can be used to follow the conversion of 1 to C.OH3COOH3COABC17. What energy differences, in kcal/mol (or kJ/mol), does one associate with [a] UV spectroscopy? [b] with IRspectroscopy?8. Introductory texts often illustrate ring expansion reactions of the variety shown below. Indicate, using specificwavenumbers, how IR spectroscopy can be used to differentiate between the two possible products, D and E.H3CORCH3CH3CH2DE9. Provide specific wavenumber values corresponding to each of the functional group interchanges illustrated below.CHOCCHOH10. A natural product shows a π- π* absorption at 281 nm with an extinction coefficient of 14,000. Which structure, Aor B, best fits this data? Be certain to calculate λmax for each structure, and show your work. Choose your base

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