Identification of Unknown Compound using 13C NMR

Identification of Unknown Compound #15 using ¹³C NMR and IR Spectra Paired with Melting Point Analysis

Abstract

Given an unknown compound, infrared (IR) spectroscopy can be used to determine the functional groups that make up the compound. This can be supplemented with ¹³C NMR spectroscopy and melting point analysis to determine the identity of the unknown compound. After completion of the analysis methods, it was determined that the identity of the unknown compound (unknown #15) is p-Methylaniline (p-Toluidine), C₇H₉N (Figure 1).

Introduction

          Infrared (IR) spectroscopy is extensively used instrumental technique in organic chemistry.

It is the measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample. Mid-infrared is energetic enough to excite molecular vibrations to higher energy levels. The wavelength of infrared absorption bands is characteristic of specific types of chemical bonds, and infrared spectroscopy finds its greatest utility for identification of functional groups within organic molecules (1). An infrared spectrophotometer is an instrument that passes infrared light through an organic molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. In infrared spectra the absorption peaks point downward because the vertical axis is the percentage transmittance of the radiation through the sample (2).

Nuclear magnetic resonance spectroscopy (NMR) can be used to complement IR spectroscopy. While IR can determine functional groups, NMR can determine the molecular structure. Finding the melting point of the unknown can help to determine the purity of the compound.

Results

IR Data:

Functional Group	Molecular Motion	Observed Wavenumber (cm-1)	Literature Value Range1-3 (cm-1)	Peak Intensity	Peak Shape
N-H	Stretch	3333.85	3335	Weak	Sharp
C-H	Stretch	2914.73	2780	Weak	Sharp
C=C	Stretch	1621.28	1600-1430	Medium	Sharp
C-H	Bending(in plane)	1274.33	1275-1000	Medium	Narrow

¹³C NMR Data:

Atom	Atom Group	Peak Observed (ppm)	Peak Calculated (ppm)
1	C-N	144	144
2 and 6	CH	115	115.2
3 and 5	CH	130	129.60
4	C=C	127	126.7
7	CH3	21	21.26

Melting Point Data:

Observed Melting Point Range (°C)	Literature Melting Point Range4 (°C)
Trial 1	44-47	44-45
Trial 2	44-45

Discussion

The IR analysis of the unknown compound 15 determined that it consists of a benzene ring, amino and methyl groups. According to the referred literature (3) values of 3335-3400 range for stretches are evident which indicates primary amines. The spectrum shows primary amines stretch at 3333.85 Next can be noticed that CH stretches from the benzene ring occur at 3014.65 and 2914.73. C=C stretches are known to occur at the 1600-1430 according to literature, and in this spectra are illustrated at 1621.28 At 1274.33C-H bend is evident. This spectrum correlates well with literature values which confirms successful identification of p-methylaniline.

The melting point analysis was used to determine the purity of the unknown sample. The literature values for p-methylaniline range from 44-45°C (4) and the observed melting point values were 44-47°C for trial 1 and 44-45°C for trial 2. The trial 1 has a higher melting point compare to the literature values. This could be due to the amount of sample placed in the capillary tubes as well as the purity of the sample (4). Because trial 1 has such a large range, the sample may not be pure.

The results from the ¹³C NMR showed that compound contained summetry. Two peaks of the same height were observed at about 115 and 130ppm and two other peaks at 144 and 127ppm. According to the literature values aromatic carbons appear between 120-160ppm (3), which helped to determine that this compound contains a benzene ring. It was noted as well that one other peak appeared much further downfield at 21ppm. Using an NMR predictor, the peaks for p-methylaniline were calculated to be 144, 115.2, 129.60, 126.7 and 21.26 ppm, which match the observed peaks.

Conclusion

After performing IR spectroscopy, melting point and ¹³C NMR analysis, it was found that unknown compound 15 is p-methylaniline. P-methylaniline contains a benzene ring, methyl and amine groups in a p- position. Based on slight deviations in the melting point, the sample could have some impurities. An error could be improper loading of the sample in the capillary tube. So to avoid this chemical error, a better laboratory technique could be used.

References

1. IR Spectroscopy:Mohrig J.R. et al. Techniques in Organic Chemistry 2nd Ed.

2.    Skoog, D. A., Principles of Instrumental Analysis, 3rd Ed

3.  Stawikowski, M. Experiment 2: Identification of unknown compound based on IR spectroscopy, melting point analysis and supplemental 13C NMR spectroscopy data; BlackBoard

4. Smiley RA (2000). Ullmann’s Encyclopedia of Industrial Chemistry. John Wiley and Sons.