ir-nmr-discussion

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The use of Infrared Spectroscopy and Nuclear Magnetic Resonance to determine the structure of unknown compounds were proven to be powerful tools in this lab. I used both known and unknown compounds to practice using IR and NMR tables, identify peaks and the meanings of chemical shifts, determine proton splitting, and analyze the strength of peaks. After studying the IR and NMR spectra of my unknown compound (#7), I was able to make what I believe is a reasonable identification of its structure: benzyl alcohol: Please see my data and results section for full analyses of each IR and NMR spectrum for my oxidation product, known reagent, and unknown #7.
 * student A **


 * student D **

Refer to Figure 2 of my IR data and results section. This is the IR spectrum for the oxidation product of benzyl alcohol with PDC. The desired product was benzaldehyde. The sharp peak at 1700 cm-1 indicates the presence of benzaldehyde (a carbonyl) in the oxidation product. The peak at around 3500 cm-1 confirms that there is un-reacted benzyl alcohol in the product. If the solution was refluxed for a longer period of time, then there would not be a peak at 3500 cm-1.

__ **student C** __ __ IR spectroscopy __ The IR spectrum I obtained for benzyl ether in the lab (Figure 1) was very similar to the spectrum from the SDBS database (Figure 2). The percent transmittance for each of the peaks in both spectra have nearly identical absorption frequencies however. In both figures there is the following indicated: -C-H stretch at 2800 (alkane), =C-H stretch at 3000 (alkenes, aromatics), C=C stretch-twin peaks at 1500-1600 (aromatic), R-O stretch at 1000-1100 (ether), mono-substitution indicated by peaks at 700 and 740. The IR spectrum for Unknown #20 (Figure 3) contained peaks at the following frequencies in cm-1: 2969 and 2882 (C-H stretch), 1742 (C=O stretch), 1465 (-CH2), the peaks at 1308, 1237, 1064 indicate a =C-O and R-O in an ester. I used this information and compared my spectrum to some of the known spectra obtained in the lab to try and determine its identity. My guess is that Unknown #20 is ethyl acetate. The spectrum of our oxidation product (Figure 4) confirms that we likely obtained a mixture of benzaldehyde and benzyl alcohol as the end result. The characteristic broad -O-H stretch form benzyl alcohol is present from 3100-3600. The strong peak present at 1700 in our spectrum is indicative of -C=O which shows that benzaldehyde is present. __ NMR spectroscopy __ The spectrum that was produced for benzyl ether in the lab (Figure 1) showed the presence of three different signals. The signal at 7.3 ppm corresponds to the hydrogens located on the benzene rings, while the signal at 4.5 ppm corresponds to the hydrogens bonded to the carbons on the oxygen atom. I determined that the signal that was present at 2.2 ppm most likely resulted from acetone that was present in the NMR tube. The NMR spectrum that resulted for Unknown #6 was much more complicated than previous NMR spectra I had seen. It appeared to have three signals with different splitting patterns as follows: quartet at 2.3 ppm, sextet at 1.6 ppm, and triplet at 1.0 ppm. I used this information as well as the molecular weight of the unknown (provided) to try and find its identity. I did not have much luck, but after searching the SDBS I was able to find a spectrum that was remarkably similar to the unknown spectrum. The only difference was that the signal I believed to be a quartet was a actually two signals: a triplet at 2.4 ppm and a singlet at 2.1. Once I figured this out, it was much easier to suggest an identity and analyze the spectrum. From the information I obtained through the SDBS and the lab, I believe Unknown #6 is 2-pentanone.


 * student B **

The IR spectrum data of o-xylene acquired at NEIU (Figure 2) can be compared to the standard spectra from the SDBS database (Figure 1). These two figures appear very similar, confirming that the measured substance was indeed o-xylene. On both IR plots, we can see C-H stretch peaks on both sides of the 3000 cm-1 mark, indicating the presence of alkane and alkene C-H bonds. The alkene C-H bonds confirm the presence of the aromatic ring in o-xylene. Both plots also show a peak around 1600 cm-1, indicating the presence of C-C aromatic bonds.

There is a partial match between the NMR spectra of o-xylene from the SDBS database (Figure 3) and the NMR data acquired at NEIU (Figure 4). The spectrum acquired at NEIU has strong peaks at 5.4 and 0 ppm that are absent in Figure 3. The peak at 0 ppm is the reference peak, and the peak at 5.4 ppm is an alkene, most likely from at contaminant. Aside from these peaks the plots match very well, the peak at 7 ppm corresponds to the four aromatic hydrogens and the peak at 2.4 ppm corresponds to the six isolated alkane hydrogens. The SDBS plot shows no splitting however we see some splitting in the NMR plot acquired at NEIU, this could be due to problems with the NMR machine.

The IR spectrum of unknown #3 indicates no functional groups. Peaks just below 3000 cm-1 indicate C-H stretch of alkanes. The peak at 1378 cm-1 could indicate a C-O stretch from an ether, however this absorbance is above the typical absorbance range for ethers (1300-1000 cm-1). The peak at 1466 cm-1 is within the range for aromatic C=C stretch, however we do not see an aromatic C-H stretch peak above 3000 cm-1 therefore we can disregard this. The 5.4 ppm peak corresponds to alkene hydrogens and suggests a C=C bond however the IR data does not support this. It is also possible that this peak is associated with a contaminant and is the same peak that was seen in the o-xylene NMR spectrum. This contaminant could be chloroform however its HNMR spectrum shows a peak at 7.2 ppm, far above the contaminant peak. It is possible that the HNMR machine was not calibrated correctly, which would explain this discrepancy. It is difficult to discern splitting from the NMR spectrum of the unknown; this may also be due to a problem with the spectrometer. Without splitting it is difficult to determine the structure of the carbon chain; however the 1.3 ppm peak is about four times larger than the 5.4 ppm peak, possibly indicating that there are four times more alkane hydrogens than alkene. It is difficult to discern the structure of the unknown compound without more information; however I can speculate that unknown #3 is a terminal alkene with at least four carbons. Further analysis using a mass spectrometer would provide more information about the carbon chain and potentially allow more accurate identification of the compound.

The expected oxidation product (benzaldehyde) was analyzed with HNMR to determine purity and presence of the product. Unfortunately only a single drop of product was isolated during the experiment, therefore there was not enough product for both IR and NMR analysis. It was decided that only HNMR should be performed; the result can be seen in Figure 6. The small peak around 10 ppm corresponds to the aldehyde hydrogen while the peak cluster around 7.8 ppm corresponds with the phenyl hydrogens. This pattern matches the HNMR spectrum of benzaldehyde from the SDBS database, confirming the existence of benzaldehyde in the product.