CHEMISTRY Organic Chemistry Laboratory II Spring 2019 Lab #5: NMR Spectroscopy

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1 Team Members: Unknown # CHEMISTRY Organic Chemistry Laboratory II Spring 2019 Lab #5: NMR Spectroscopy Purpose: You will learn how to predict the NMR data for organic molecules, organize this data into tables, and use it to identify an unknown compound. You will be following the information in this handout and use your chemical intuition. For 13 C NMR the prediction of the number of signals and their relative chemical shifts is required. For the 1 H NMR the prediction of the number of signals, their relative chemical shifts, the integration, and splitting of the signals are required. Important Notes: You will not need to record information in your notebook. Instead, you will turn in this handout at the end of lab to serve as your lab report grade for lab # 5. The SDBS and the Sigma Aldrich websites have NMR data for some organic molecules that you can use as a resource. Although the chemical shifts should be the same, the resolution can vary. You may also want to check out the SDBS web site where you can type in a formula or name, and then view IR and NMR spectra: The link to this web site is also on Dr. Brush s lab web page. ChemDraw has a prediction tool for proton and carbon that estimates the spectra that you could attain experimentally. The software generated spectra is oversimplified and does not perfectly match experimental data. However, it does provide the basic information. There are two examples in this handout that contain ChemDraw predicted spectra for you to review. Try to be as specific as possible and follow the n+1 rule when assigning the splitting of aromatic protons for this exercise. Keep in mind that depending on the molecule, you may see the clean splitting patterns you predict or they may appear as overlapping signals (multiplets). Even the experimentally collected NMR data might NOT perfectly match the predictions you construct in your tables. Often, you are working to assign the optimal fit. Don t forget to staple your unknown NMR spectra to this report/handout Procedure for predicting the 13 C NMR data corresponding to a molecule: 13 C NMR is performed in decoupled mode which generates singlets for each chemically distinct carbon. The two goals in your strategy are to: determine the number of carbon signals and estimate the chemical shifts. Begin by drawing in any planes of symmetry. Then determine the number of chemically distinct (unique carbons) - This is called the number of carbon signals. 1) For each of these compounds, write the number of carbon signals you expect in a 13 CNMR experiment.

2 The location of the signals in the spectrum (the chemical shift) is due to an effect called shielding. Nuclei (carbons or protons) in an external magnetic field absorb at different frequencies depending on the electron density around that nucleus. High electron density shields the nucleus causing absorptions to be upfield / higher energy closer to 0 ppm. Lower electron density de-shields the nucleus causing absorptions to be downfield in the spectrum away from 0 ppm. Pi bonds and electronegative atoms nearby will de-shield nuclei by pulling electron density away from the atom. 2) Circle the carbons on each molecule that would generate the most upfield signal in the 13 C NMR. Procedure for predicting the 1 H NMR data corresponding to a molecule: In Proton NMR, each chemically distinct proton set has a signal (chemical shift) representing its chemical environment and will appear with a pattern (mulitplicity) based on the number of non-equivelant protons on the neighboring carbon(s). The relative area (integration) of the proton signals is also important. After drawing in the hydrogens and finding any planes of symmetry, you should make a table by doing the following: * Determine the number of chemically distinct hydrogens by taking into account any symmetry in the molecule. This is the number of hydrogen signals. * Understand that hydrogens with different shielding result in different chemical shifts for the protons. * Count the number of hydrogens (integration) corresponding to each signal. * Signal Splitting: A proton signal often shows up as a pattern due to a splitting based on the number of protons on adjacent carbons and follows the n + 1 rule in which n = the number of protons on the carbon adjacent to the signal of interest. 3) The following compounds generate 2 and 3 proton signals respectively. Draw in any planes of symmetry, draw in the hydrogens, label the protons from left to right alphabetically, then write the pattern & integration for each proton signal in the table started for you. The options here are: Septet (1H) Singlet (3H) Triplet (6H) Quartet (4H) Doublet (6H)

3 4) Example 1: The molecule below has a plane of symmetry which plays a role in the number of expected proton and carbon signals. Review the NMR tables and the ChemDraw predictions. Find and fix the ERRORS in the data tables for example # 1. 5) Example 2: This molecule below does not have a plane of symmetry and has more signals than the prior example. The ChemDraw predicted spectra is provided, but the tables are incomplete. Finish completing the carbon and proton tables:

4 6) For the pool of unknowns, construct a proton and carbon NMR table. Begin by drawing in any symmetry. Then predict the number of carbon signals and number of each type. For the proton NMR, draw and label the hydrogens, predict the number of signals, the integration, and the splitting pattern. (Do not draw the molecules on ChemDraw)

5 Unknown determination: You will be given a set of experimental proton and carbon NMR spectra for your unknown that was collected using the chemistry department s 400 MHz JEOL spectrometer. Use a highlighter to indicate the solvent on both spectra and the integration on only the proton spectra. 7) Evaluate your spectra and declare which of the four structures (above) best fits your set of data. Circle this molecule on the pool of unknowns page and write your unknown number here: 8) Write a paragraph that addresses how the carbon NMR spectra fits your data table (Be specific): 9) Write a paragraph that addresses how the proton NMR spectra fits your data table (Be specific):