Organic Chemistry

Identification of Unknown Organic Compounds by Melting Point, Boiling Point, and Infrared Spectroscopy

Methods and Background The goal of this laboratory experiment was to determine the structure and identity of an unknown solid and an unknown liquid by using elemental analysis, index of hydrogen deficiency, infrared spectroscopy, melting point (for solid), and boiling point (for liquid). In this laboratory, we were given unknown solid “C” and unknown liquid “D”. The empirical formula of the unknown was determined through the process of elemental analysis by using molecular weight and given elemental percentage composition. Next, the index of hydrogen deficiency (IHD) was used to generate a numerical value that determined various combinations of double bonds, triple bonds, and rings within a given molecule. The general formula for IHD is as follows:
IHD = 2n+2-X2 n = number of Carbon atoms
X = number of Hydrogen atoms
(Oxygen atoms are ignored) Once the formulas were obtained, the experiment was started by finding the boiling point of the unknown liquid “D” and the melting point of the unknown solid “C”. Results were used to identify the identities of the unknowns. After this, a Fourier Transform Infrared (FTIR) was used to obtain a spectrum of the unknowns. This spectrum was then used to find the structure of each unknown.
The melting point of a substance served as a reference point in determining the unknown solid. It is defined as the temperature at which the first tiny drop of liquid appears and extends to and includes the temperature at which the solid has completely melted. Likewise, the boiling point of the unknown liquid was found by examining the temperature at which the liquid bubbled rapidly. Since each solid/liquid has a unique melting and boiling point, it was useful in determining the structures of the unknowns.
Infrared spectroscopy (IR) is a tool used in order to determine the structure of an unknown. IR spectroscopy was very useful because it showed which functional groups are present. It also was used to evaluate the purity of a known compound by the absorption of the radiation; these absorptions correspond to the transitions among different vibrational-rotational levels within the same electronic state of the molecule. The stretching, twisting, and bending that atoms undergo is related to the strength of the covalent bond and is described by Hooke’s Law. When infrared light is absorbed by a bond, the atoms become more excited and cause a change in the dipole moment, which is the cause of the emission of strong infrared absorption bands. Functional groups appear at unique frequencies, so when an IR scan shows certain peaks, the functional groups present in the unknown substance are very clear and can be identified.

Experimental Procedure

One solid unknown and one liquid unknown were assigned by the TA. The mass spectrometry, elemental analysis data, and molar mass were provided in table format. By using the given data, elemental analysis was calculated to determine the molecular formula. After the molecular formula was calculated, IHD was calculated using the formula mentioned above. Next, the melting point of the unknown solid was obtained by using the melting point apparatus by placing a small sample of the unknown solid inside a capillary tube. The tube was then inverted and tapped so that the substance was evenly spread in the capillary tube before it was placed on the melting point apparatus. The apparatus was then turned on and the solid was observed as the temperature rose. Once the solid began to melt, the temperature range from when the solid began to melt until it melted completely was recorded. This procedure was performed once more for a total of two trials for better accuracy. In order to determine the boiling point of the unknown liquid, approximately 1mL of this liquid was added to a medium/large test tube and was fastened to a ring stand using clamps. As well as the 1mL of the unknown liquid, a boiling stone (KBr pellet) was placed in the long test tube and a thermometer making sure it did not touch the liquid. This was all heated by a thermal well. Boiling temperature was recorded once liquid reached boiling and bubbles were visible or when condensation could be seen on the sides of the test tube. This procedure was repeated two more times for a total of three trials. Finally, IR spectroscopies of the unknowns were obtained. First, it was made certain that the crystal surface of the IR was clean. Then, a background scan was run by selecting the icon next to “Background” and clicked “Scan”. For the solid, a small amount of the unknown was placed directly onto the crystal surface and the pressure clamp was lowered and the dial was turned until it was tight. For the liquid, one or two drops of the unknown were added directly onto the crystal surface and the top was pushed down. The scan was run again as explained before. Peak readings were added to the obtained spectrums and the graphs were printed. Finally, in order to clean the chromatograph, a Q-tip and ethyl alcohol were used to wipe down the crystal surface.

Data Acquisition

Table 1: Unknown C and D Elemental Analysis and Molar Mass Unknowns | Carbon (C) | Hydrogen (H) | Oxygen (O) | Molar Mass (g/mol) | C (Solid) | 79.58% | 4.11% | 16.31% | 196.052 | D (Liquid) | 66.63% | 11.18% | 22.19% | 72.058 |
This information was used to determine the molecular formulas of the unknowns.

Elemental Analysis for Unknown Solid “C”

C: 79.58%, (79.58g)/(12g/mol) = 6.63 mol
H: 4.11%, (4.11g)/(1g/mol) = 4.11 mol
O: 16.31%, (16.31g)/(16g/mol) = 1.02 mol

6.63/1.02 = 6.5 x 2 = 13 mol C
4.11/1.02 = 4.029 x 2 = 8 mol H
1.02/1.02 = 1 x 2 = 2 mol O

Molar Mass of Unknown “C” = 196.052g
Molar Mass of Empirical Formula = 196 g
Molecular Formula of Unknown “C” = C13H8O2

IHD calculation for Unknown Solid “C”

IHD = 2n+2-X2 n = number of Carbon atoms
X = number of Hydrogen atoms
(Oxygen atoms are ignored)

IHD = 2(13)+2-82 = 10
(Since there is a high IHD, a benzene ring is needed)

Melting Point Range for Unknown Solid “C”

Table 2:Melting Point Ranges of “C” | °C | Trial 1 | 165-167 | Trial 2 | 167-173 | Average | 166-170 |
This table shows the temperature range from when the unknown solid began to melt until it had completely melted.

Elemental Analysis for Unknown Liquid “D”

C: 66.63%, (66.63g)/(12g/mol) = 5.55 mol
H: 11.1%, (11.18g)/(1g/mol) = 11.18 mol
O: 22.19%, (22.19g)/(16g/mol) = 1.39 mol

5.55/1.39 = 3.99 ~ 4 mol C
11.18/1.39 = 8.04 ~ 8 mol H
1.39/1.39 = 1.0 = 1 mol O

Molar Mass of Unknown “D” = 72.058g
Molar Mass of Empirical Formula = 72 g
Molecular Formula of Unknown “D” = C4H8O

IHD calculation for Unknown Liquid “D”

IHD = 2n+2-X2 n = number of Carbon atoms
X = number of Hydrogen atoms
(Oxygen atoms are ignored)

IHD = 2(4)+2-82 = 1
(IHD indicates there is 1 double bond, or 1 carbonyl group, or 1 ring)

Boiling Point for Unknown Liquid “D”

Table 3:Boiling Point of “D” | °C | Trial 1 | 65 | Trial 2 | 63 | Trial 3 | 67 | Average | 65 |
This table shows the temperature at which the unknown liquid began to either rapidly bubble, or the point at which condensate could be seen on the sides of the test tube containing the unknown liquid.

Table 4: Molecular Formulas and IHD’s Unknowns | Empirical Formula | Molecular Formula | IHD | C (Solid) | C13H8O2 | C13H8O2 | 10 | D (Liquid) | C4H8O | C4H8O | 1 |
The value of IHD seen above in the Table 4 helped determine the number of double bonds, triple bonds, or ring in the structures of the unknown compounds.

Table 5: Melting and Boiling Points of the Unknown Substances Unknowns | Melting Point | Boiling Point | C (Solid) | 168°C | - | D (Liquid) | - | 65°C |
The melting and boiling points allow the structures of the unknowns to be narrowed down since each solid and liquid has a unique melting or boiling point respectively.

Table 6: IR Spectrum and Corresponding Functional Groups Peak in cm-1 for Solid “C” | Range Peak Falls under in cm-1 | Functional Group | Name of Group | 3080.73 | 3010-3095 | C-H | Alkene | 1699.94 | 1675-1760 | C=O | Ketone | 1655.59 | 1610-1680 | C=C | Alkene | 1601.59 | 1600-1660 | C=C | Aromatic Rings | 1479.13 | ~1340-1470 | C-H | Alkene | 1455.03 | 1340-1470 | C-H | Alkene | Peak in cm-1 for Liquid “D” | Range Peak Falls under in cm-1 | Functional Group | Name of Group | 2982.37 | 2500-3200 | O-H | Hydrogen-bonded carboxylic acid | 2940.91 | 2500-3200 | O-H | Hydrogen-bonded carboxylic acid | 1713.44 | 1705-1725 | C=O | Aldehyde or Ketone | 1461.78 and 1363.43 | ~1450 and 1375 | CH3 | CH3 bending | 1414.53 | ~1410 | CH2 | CH2 bending of carbonyl |

Hypothetical Structures of the Unknowns “C” and “D” 1st Hypothetical Structure for Unknown “C” (Solid)Fluorone (C13H8O2)Molecular Mass: 196.2g/molMelting Point:207°C | 1st Hypothetical Structure for Unknown “D” (Liquid)Butanone (C4H8O)Molecular Mass: 72.11g/molMelting Point: 79.64°C | | | 2nd Hypothetical Structure for Unknown “C” (Solid)Xanthone (C13H8O2)Molecular Mass: 196.19g/molMelting Point: 174°C | 2nd Hypothetical Structure for Unknown “D” (Liquid)Butyraldehyde (C4H8O)Molecular Mass: 72.11g/molMelting Point: 74.8°C | | |

Conclusion

The goal of this laboratory experiment was to determine the identities of unknown solid “C” and unknown liquid “D” using the following methods: elemental analysis, index of hydrogen deficiency, infrared spectroscopy, melting point (for solid), and boiling point (for liquid). After the completion and analysis of this lab, it was found that unknown solid “C” is either Fluorone or Xanthone and the unknown liquid “D” is either Butanone or Butyraldehyde. By using elemental analysis calculations, the molecular formula was obtained, the index of hydrogen deficiency was found, and an IR spectrum was obtained for each of the unknown substances. For the unknown solid “C”, the molecular formula was determined to be C13H8O2 with an index of hydrogen deficiency of 10. This high IHD indicated that a benzene ring was present in the structure. After closely analyzing the IR spectrum, alkene, ketone and aromatic rings were found to be present in the structure as well. For the unknown liquid “D”, the molecular formula was determined to be C4H8O with an index of hydrogen deficiency of 1. This suggests that there was 1 double bond, or 1 carbonyl group, or 1 ring present in the structure of the unknown liquid. After closely analyzing the IR spectrum of this unknown liquid, carboxylic acid, ketone or aldehyde were identified. Aldrich’s Manual of organic compounds was used to match the compounds to our formulas of our unknowns. The melting and boiling point of the solid and liquid respectively were useful in helping narrow down the precise structure of the unknowns. The melting point of the unknown was found to be 168°C. The melting point for xanthone is 174°C and for fluorone 207°C. So, by comparing melting, it was found that xanthone was a better hypothetical structure than fluorone. The boiling point of the unknown was found to be 65°C. The boiling point for butanone 79.64°C and for butyraldehyde 74.8°C. In this case, the boiling points of the hypothetical structures are very close, so the closest to the unknown’s boiling point was found to be the butyraldehyde. The molecular weights were so close, that it was not a limiting factor when trying to identify and narrow down the structures of the unknowns. There were a few possible errors that may have occurred during this laboratory experiment. First, since the experiment was performed on a Friday afternoon and it was the very last class of the week, impurities could have mixed with the unknown substances as well as with the KBr pellet and caused an error in melting and boiling points readings. There could have also been an error with the IR spectroscopy since it was supposed to be properly sanitized before and after each use which could have caused an error in the spectrum if contaminated by other unknowns. Also, there could have been human error when taking readings on the thermometer which could have slightly skewed the results if taken slightly off.

References

Gilbert, John C. and Martin, Stephen F. Experimental Organic Chemistry: A Miniscale and Microscale Approach. Thomson Brooks/Cole, Pacific Grove, CA, 2006, 4th Edition.

References: Gilbert, John C. and Martin, Stephen F. Experimental Organic Chemistry: A Miniscale and Microscale Approach. Thomson Brooks/Cole, Pacific Grove, CA, 2006, 4th Edition.