CHEM 51A Chapter Notes - Chapter 14: Ethyl Group, Benzene, Chemical Shift

A Portion of the 'H NMR spectrum of 4-tert-butylcydohexanol is shown below, along with a peak list of chemical shifts, multiplicities, and numbers of protons (from integration). the alcohol O-H proton is NOT observed in this NMR spectrum. the spectrum is consistent with a single isomer of 4-tert-butylcyclohexanol. Which one is it, as or trans? Copy into your lab report the picture shown below the spectrum. Circle the structure of the correct isomer identified in (a). Within the circled structure, circle the most important proton or protons that informed your answer to part (a). What peak or peaks in the spectrum peak list cones pond to the circled proton or protons in part (c)? in this molecule, what conformational relationship between two coupled protons determines their coupling constant to each other (J values)? For the selected peak(s) in (d), explain the multiplicity Be as specific as possible by referring to the labeled protons in your circled structure and highlighting relevant coupling constants.

F g 2. Esamine the spectrum. Determine the sumber of oquivalene types of protons in the compound 3. Istegrate the spectrum. Determine the ratios of protons 4 Determise the types of protos preseat Examination of the number of signals and 60.9-1.5 82-2.5 Methyl, methylene, and methine protons Protoes adjacent to a carbomyl group, benrylie hydrogens, or Protots on a carbon adjacent to an oxygen or halogen Alkeme protoes: coupling cosstant ef 0-2 Hz-pesinal alkene, coupling constant of S-12 constact of 12-23 Hr-trans alkene, or proson on carbon coupling 57-8 89-10 810-13 Aromatic ring procons Prubiem 24 109 CH Aldehyde proo Carboxylic acid proton further enhance the information obtained from the chemical shifts Characteristic a An ethyl group occurs as a triplet (which integrates to 3H) and a quartet(c b. An isopropyl group eccurs as a doublet (which integrates to 6H) and a multiplet (a ic splitting patterss. Splitting patterss and coupling constant 5, Look or c patterns are listed below 3.5 CH) ietegrates to 2H) The triplet is always furtber upfield beptet, achually, which iste grates to IH) Often, the outer peaks of maltiplets can't be seen c. A tert-butyl group appears as a singlet (which istegrates to 9H1 d. Barring overlapping signals, a singlet that integrates to 3H is usually one methry robilen 2.B a signal that integrates to 6H is usually two methyl groups, and a signal that integrates to 9H is usually three methyl groups. 82.9(IM) 5. Put it all together. Draw a structure that fits the elements of unsaturation, the chemical shifts and splitting patterns 1. A compound with the Sormula CHn0 gives the 'H NMR spectrum shows in Figure 1 Determine the structure of the compound consistent with the data for 2A through 20

Interpreting Your NMR Spectrum 1. Read the chemical shift of each signal directly from the NMR spectrum by looking at the numbers at the top of the peaks. If the signal is not a singlet, then choose the number from a middle peak 2. To find the number of protons represented by each signal, use the integration numbers that appear below the baseline. You must calculate the ratio of these numbers to get the ratio of numbers of hydrogens represented by each signal 3. Fill in the "splitting" column by looking at each signal and deciding if it is a singlet, doublet, triplet, quartet, or multiplet. You can calculate the J value for a coupling constant by subtracting the chemical shift of one peak from the chemical shift of another peak within the same signal and multiplying by 80, since these spectra were taken on a 80 MHz NMR The J value may be helpful in assigning NMR signals to specific protons 4. To fill in the "assignment" column, assign each signal in the NMR to a set of chemically equivalent protons on your labeled structure. The proton(s) you assign should make sense Esterification, IR, and NMR CHEM 344 Modern Organic Chemistry Laboratory for each of the column headings: the chemical shift will be affected by heteroatoms or aromatic rings near the proton; the integration should match the correct number of equivalent protons for the assignment you are making; the splitting pattern may not be clear, but protons on neighboring carbons generally give reliable splitting patterns (protons on oxygen or nitrogen might not split your assigned proton). You may also need to consider whether you have two overlapping signals: if you have identified four different types of hydrogens, but only see three signals, two signals may be overlapping. The integrations should help you determine if that is the case H NMR Analysis Table: draw the structure with labeled protons in the lab report Coupling Constant (J-value) Assignment ChemicalIntegration Splitting Shift (ppm)