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1st Organic Chemistry Lab Report ...

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Identification of Unknown Organic Compounds by Melting Point, Boiling Point, and Infrared Spectroscopy Katie Jensen Partners: Ami Makwana and Justas Jakubonis June 16th, 2016 Section 13536

Methods and Background: The purpose of this lab is to determine the structures of one solid and one liquid compound given only their masses and the percentage (relative abundances) of carbon, oxygen, hydrogen, and nitrogen atoms in each of the compounds. The compounds’ Infrared (IR) spectra are used to elucidate the compounds’ structures; the solid compound’s melting point and the liquid compound’s boiling point are used to confirm the hypotheses. With the masses and the percentages of carbon, oxygen, hydrogen, and nitrogen atoms in each of the compounds, elemental analysis is used to establish the empirical and molecular formulas of the compounds. Assuming that there are 100 grams of each compound, the percentage of each atom directly represents the amount, in grams, of each element in the compound. Because the percent of each element equals the amount, in grams, of each element, the number of moles of each element can be calculated. Then, all the molar values are divided by the smallest number of moles to find the empirical formula. If necessary, the empirical formula is multiplied by an integer in order to determine the molecular formula. This part of the experiment is known as elemental analysis. With the molecular formulas known, the index of hydrogen deficiencies (IHD) can be determined. IHD allows for the determination of the degree of saturation by giving the sum of the number of double bonds, triple bonds, and rings. The formula to calculate IHD is IHD = (1/2)[(2C+2)(H+X)+N], where C stands for the number of carbon atoms in the molecular formula, H stands for the number of hydrogen atoms, X stands for the number of halogens, and N stands for the number of nitrogen atoms. With a general idea of what each compound looks like, based on elemental analysis and IHD, more insight can be gained through the use of IR spectroscopy. By examining the IR spectra of each compound, the presence or absence of specific functional groups can be revealed. IR spectra depend on the constant stretching, twisting, and bending of molecules along with the masses of the molecules and the chemical bonds joining them. What the IR spectrum is actually measuring is the dipole moment created when the molecule is stretching or bending. The energies of these stretches and bends are measured in frequencies and wavelengths. Modern IR instruments function by measuring the amount of light transmitted through the sample. Because of this, the spectra are plotted as percent transmissions (%T) versus wavenumber (cm-1). The technique used for obtaining IR spectra in this experiment was reflectance spectroscopy, or more specifically, attenuated total reflectance (ATR). This method monitors radiation reflected from the sample. No two compounds have the same atoms bonded the same way, therefore, the IR spectra of each compound is unique (Gilbert 228). IR spectra are useful in determining structures because very little sample preparation is required, and because the spectra can be printed in a matter of seconds. Physical properties such as melting point and boiling point are also useful in the determination of the structures of compounds. Experimentally found values can be compared to tabulated values in catalogs such as the Merck Index or Aldrich Catalog. Moreover, some compounds can have similar

physical properties, but it is extremely unlikely that these compounds will share this similarity across all of their physical properties. Other physical properties include index of refraction, density, specific rotation, and solubility. The physical properties measured in this experiment are due to ease and availability of equipment. The boiling point is determined by suspending a thermometer above the liquid sample in a test tube and reading the temperature once the liquid comes to a rapid boil. The melting point is determined by putting the solid sample into a capillary ebullition tube and using a Meltemp melting point apparatus (Gilbert 111). By putting together all of the techniques mentioned above, it is determined that the liquid sample is 3-methyl-2-butanol and the solid sample is furan-2,5-dione (see structures below). Both of these compounds have several industrial applications. Because the structures of the compounds that were used for this experiment were fairly simple, it was very easy to determine their structures through the use of only elemental analysis, the IHD, and the IR spectra. The melting point range that was determined was slightly greater than expected. The increased range may have been due to having too much of the solid in the capillary ebullition tube. The salt crystals in the center of the tube took longer to melt at the actual melting point, but the tube kept getting hotter; thus, the temperature reading was higher than was actually necessary to melt the solid compound. The melting point may have been more accurate if that part of the experiment was conducted multiple times and the results averaged. The average boiling point was found to be slightly below the tabulated boiling point range; this could be because the thermometer was more than 2cm higher than the liquid’s surface or because the temperature was read before the liquid had come to a rapid boil.

2-methyl-2-butanol

furan-2,5-dione( Maleic Anhydride)

Experimental Procedures: To determine the empirical and molecular formulas of the unknowns, it was assumed that there was 100 grams of each compound. This allows the abundances of each element to directly represent their masses in grams. Once the number of grams was determined for each element, the number of moles of each element was calculated by dividing the masses by their atomic masses. Then, the number of moles of each element was divided by the lowest molar quantity. This results in the empirical formula. In order to determine the molecular formula, the mass of the molecular formula is divided by the mass of the empirical formula. This gives the integer that the empirical formula needs to be multiplied by in order to obtain the molecular formula. If the integer is one, then the empirical formula is also the molecular formula. The IHD of each unknown was determined using the formula IHD= (1/2)[(2C+2)-(H+X)+N]. It should be noted that the presences of oxygen in the molecular formula does not affect the IHD value. The number of halogen atoms is accounted for by subtracting the number of halogen atoms from the number of hydrogen atoms and the number of nitrogen atoms is accounted for by adding the number of nitrogen atoms to the number of hydrogen and carbon atoms. With the molecular formulas and IHDs known, it was possible to hypothesize some possible structures of the unknown compounds by predicting the presence (or absence) of functional groups. The peaks on the IR spectra of the compounds were used to confirm the structures.

The boiling point of the liquid sample was determined by placing a small amount of the sample into a Pyrex test tube. The test tube was then suspended above a heating mantle with a thermometer suspended about two centimeters above the level of the liquid. The temperature was taken when the equilibrium vapor pressure of the liquid was equal to the total pressure above the liquid and it had come to a vigorous boil. The melting range of the solid was determined through the use of the Mel-temp apparatus. In order to prepare the capillary ebullition tube, a small amount of the solid sample was placed on a clean watch glass. The sample was transferred into the capillary tube by inverting the tube onto the sample and tapping it on the work surface, allowing the sample to travel to the bottom of the tube. The tube was then placed into the slit in the Mel-temp apparatus. The temperature was controlled and set at 1˚C per minute. Finally, the sample was observed to determine the melting point range. The first value of the range was recorded when the first drop of liquid was observed; the second number in the melting point range was recorded when the solid was completely melted. In order to obtain an IR spectrum for both unknowns, an ATR FT-IR spectroscopy apparatus was used. The very first step was to clean the crystal surface with a cotton swab and acetone. To prevent foreign samples from interfering with the outcome of the scans, a background scan was first performed. Then, a small amount of the solid sample was placed onto the crystal surface and another scan was performed. However, before performing the scan, the pressure clamp was lowered to ensure close contact with the crystal surface and the sample. After adjusting the baseline and marking the peaks, the spectrum was printed out. Lastly, the surface of the crystal was cleaned with a cotton swab and acetone. For the liquid sample, the same procedures were followed except that the a few drops of the sample were placed inside a liquids retainer on top of the crystal surface. The pressure clamp is not needed for liquid samples because the sample is already in close contact with the crystal surface. The spectra of both samples were printed as functions of percent transmission (%T) versus wavenumber (cm -1). By observing where the peaks occurred and where there were no peaks, the presence and absence of specific functional groups was established.

Data Acquisition: Due to the simplicity of the unknown compounds in this experiment, determination of the molecular formulas and IHDs allowed for a general idea of the identities of the compounds to be formed. Using the ATR accessory, the FT-IR spectrum made it possible to detect the presence or absence of specific functional groups. The boiling point was determined by placing a sample of the liquid in a test tube with a thermometer about 2 centimeters above the liquid. The temperature was taken when the liquid came to a rapid boil. The melting point range was determined by using a Mel-temp apparatus. By comparing the melting and boiling points to those values tabulated in the Aldrich catalog and Merck Index, the structures predicted by consideration of the IR spectra, molecular formula, and IHD were confirmed. Table 1: Summary of the Properties of the Unknown Compounds Unknowns

Empirical Formula

Molecular Formula

IHD

G (solid)

C4H2O3

C4H2O3

4

Experimental Melting Point (˚C) Trial 1 50-54 Trail 2 50-55

Theoretical Melting Point (˚C) 51-56

Experimental Boiling Point (˚C) N/A

Theoretical Boiling Point (˚C) N/A

H (liquid)

C5H12O

C5H12O

0

N/A

N/A

Trail 1 100 Trail 2 100

102

Calculations for Empirical and Molecular Formulas for Unknown Solid G: 1. Convert the relative abundances of the elements into grams by assuming that there is 100 grams of the compound; the percent of each element should equal the amount in grams. Carbon: 48.99% = 48.99 grams Hydrogen: 2.06% = 2.06 grams Oxygen: 48.95% = 48.95 grams 2. Convert the amount of each element from grams to moles. Carbon: (48.99 grams) / (12.01 grams/mole) = 4.08 mol C Hydrogen: (2.06 grams) / (1.008 grams/mole) = 2.04 mol H Oxygen: (48.95 grams) / (16.00 grams/mole) = 3.05 mol O 3. Divide by the smallest number of moles. Carbon: (4.08 moles) / (2.04 moles) = 2 Hydrogen: (2.04 moles) / (2.04 moles) = 1 Oxygen: (3.05 moles) / (2.04 moles)= 1.5 4. Multiply by smallest integer possible to create integer value. Carbon: 2 x 2 = 4 Hydrogen: 1 x 2 = 2 Oxygen: 1.5 x 2 = 3 5. Using the quantities calculated as the subscripts in the empirical formula. C4H2O3 6. Calculate the mass of the empirical formula. 4 moles C (12.01 grams/mole) + 2 moles H (1.008 grams/mole) + 3 moles O (16.00 grams/mole)= 98.06 grams 7. Divide the mass of the molecular formula by the mass of the empirical formula. Multiply this value with the subscripts of the empirical formula to attain the molecular formula. (98.00 grams / 98.06 grams) = 1 1 x [C4H2O3] = C4H2O3 Molecular Formula of Unknown Solid G: C4H2O3

Calculations for the Empirical and Molecular Formulas for Unknown Liquid H: 1. Convert the relative abundances of the elements into grams by assuming that there is 100 grams of the compound; the percent of each element should equal the amount in grams. Carbon: 68.13% = 68.13 grams Hydrogen: 13.72% = 13.72 grams Oxygen: 18.15 = 18.15 grams 2. Convert the amount of each element from grams to moles. Carbon: (68.13 grams) / (12.01 grams/mole) = 5.673 mol C Hydrogen: (13.72 grams) / (1.008 grams/mole) = 13.61 mol H Oxygen: (18.15 grams) / (16.00 grams/mole) = 1.134 mol O 3. Divide by the smallest number of moles. Carbon: (5.673 moles) / (1.134 moles) = 5.003 Hydrogen: (13.61 moles) / (1.134 moles) = 12.00 Oxygen: (1.134 moles) / (1.134 moles)= 1.000 4. Using the quantities calculated as the subscripts in the empirical formula. C5H12O 5. Calculate the mass of the empirical formula. 5 moles C (12.01 grams/mole) + 12 moles H (1.008 grams/mole) + 1 moles O (16.00 grams/mole)= 88.15 grams 6. Divide the mass of the molecular formula by the mass of the empirical formula. Multiply this value with the subscripts of the empirical formula to attain the molecular formula. (88.089 grams / 88.15 grams) = 1 1 x [C5H12O] = C5H12O Molecular Formula of Unknown Liquid H: C5H12O Calculations for the Indexes of Hydrogen Deficiencies for Unknown Solid G an Unknown Liquid H: 1. Use formula: IHD = (1/2)[(2C+2)-(H+X)+N] a. Solid

C4H2O3: IHD = (1/2)[(2(4)+2-2] = 4 b. Liquid C5H12O: IHD = (1/2)[(2(5) + 2 -12] = 0

Figure 1: IR Spectrum of Unknown Solid G—see attached Characteristic Stretch (cm-1) 1752.23 1630.83

Functional Group alkene (hydrogen) ester alkene

furan-2,5-dione( Maleic Anhydride)

Figure 2: IR Spectrum of Unknown Liquid H—see attached Characteristic Stretch (cm-1) 3359.94 2968.07 2925.90

Functional Group alcohol alkane alkane

3-methyl-2-butanol

Conclusion: In this lab, physical properties like melting point and boiling point were used alongside Infrared spectroscopy to elucidate the structures of unknown compounds given only the compounds’ molar masses and the percentages of carbon, hydrogen, and oxygen atoms in them. With this information and through elemental analysis, the compounds’ molecular formulas were determined. Using Infrared spectrometry, it was possible to establish the compounds’ structures. The peaks at 3122.07cm -1, 1752.23cm-1, and 1630.83cm-1 are characteristic of furan-2,5-dione and the peaks at 3359.94cm-1, 2968.07cm-1, and 2925.90cm-1 are characteristic of 3-methyl-2-butanol. Determination of the boiling point of 3-methyl-2-butanol and the melting point of furan-2,5-dione allowed for the confirmation of the structures predicted through elemental analysis and Infrared spectroscopy.

In doing this lab, a few difficulties were encountered. The melting point range determined experimentally was a greater range than was expected. The range should have been about 1˚C, but was found to be 1.6˚C. This may have occurred because there was too much of the solid within the capillary tube; the excess of solid present would lead to the crystals at the center of the tube taking a much longer time to melt than the crystals closer to the surface of the capillary tube. This would elongate the melting process and lead to a longer melting point range. The Merck Index, however, lists the melting point range of furan-2,5-dione over a 3˚C range, so this may simply be a characteristic property of this particular compound. Another complication that was encountered was that the experimentally determined boiling point was found to be 2˚C lower than the tabulated value for 3-methyl-2-butanol. This seems noteworthy even when it is taken into consideration that the thermometer was uncorrected. The lower boiling point temperature can easily be explained by the fact that the thermometer may have been more than 2cm away from the surface of the liquid; another possibility is that the temperature was recorded before the alcohol had come to a rapid boil and the equilibrium vapor pressure had not yet equaled the total pressure. Overall, however, the experiment was very successful and the structures of both of the unknown organic compounds were easily determined.

References: Gilbert, J.C., Experimental Organic Chemistry, Saunders Publishing, New York, 2011, 5 th Ed, pp. 111-112, 228-229....