Organic Chemistry Chem 231 Lab Report 1 PDF

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Measuring the Melting Points of Compounds and Mixtures By: Group1 - Rashida Thompson, Samantha Manno, Carly Lefar

Organic Chemistry I (Chem-231) May 20, 2020

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Introduction In this experiment, the melting points of multiple compounds are going to be explored. The melting point of a substance is the temperature that causes the crystals to shift from solid to liquid form, and the melting point range starts when the crystals first begin to become liquid, and ends when the sample has completely liquified. A pure substance will almost always have a higher melting point than an impure, or mixture of that same substance with another. Given two pure substances Compound X and Compound Y, there are many different ratios of the two compounds that can be mixed together. It can be 80% Compound X and 20% Compound Y, 60% Compound X and 40% Compound Y, 40% Compound X and 60% Compound Y, or any other combination that adds up to 100%. The melting point of the mixture varies based on the ratio used. The eutectic composition is the ratio of Compound X to Compound Y that results in the eutectic temperature, or the lowest possible melting point for a Compound X and Compound Y mixture. At any temperature lower than the eutectic temperature, the two compounds will exist as solids together. Figure 1 in LeFevre’s Lab Manual shows how the different mass percentages for each compound of the mixture affects the melting temperature of the substance. In general, the more equal the mass percentage between the two substances, the lower the melting point and the closer the mixture falls to the eutectic temperature. The compound that is the impurity, or makes up less than half of the mass percentage, will melt more quickly than the compound that makes up more than half of the mass percentage. Melting points have a few different uses in determining the identity of compounds. First, every pure compound has a known melting point as a physical constant, so experimentally determining the melting point of an unknown compound can be used to determine its identity. Second, known compounds with known melting points can be used in mixture melting points by mixing a small quantity of the known compound with the unknown compound. If the mixture melting point is lower than that of the known compound, they are different compounds, but if the mixture melting point is the same as the known compound, then the known and unknown compounds are probably the same compound. Melting points can also determine whether a compound is pure or not; if the melting point range is greater than 5°C, then the compound is not pure. If the melting point range is 1-2°C, then the compound is considered pure. The purpose of this experiment is to determine the melting point of benzoic acid (C6H5CO2H) and mandelic acid (C₆H₅CHCO₂H), as well as the eutectic temperature and composition of a benzoic and mandelic acid mixture. Additionally, the purpose is to determine the identity of an unknown compound using a mixture melting point and a variety of known compounds to compare it to, listed in Table 1 of the LeFevre Lab Manual.

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Procedure Note that throughout this experiment, no individual sample was heated more than once; each sample that was heated was fresh from the container. In order to measure the melting points of benzoic acid and mandelic acid, two to three milligrams of powdery benzoic acid were placed on a watch glass. A melting point capillary tube was loaded with the benzoic acid by pressing the open end of the tube into the powder and tapping the closed end against the bench top to result in a total of one to two millimeters of benzoic acid into the tube. The capillary tube was dropped with the open end up through a one-meter long piece of glass tube onto the benchtop multiple times in order to pack the powder well. A melting point apparatus was used to heat the capillary tube to 110°C quickly. There are multiple methods for heating the capillary tube, including the use of the Thiele tube apparatus, the Thomas-Hoover Uni-Melt device, and the Mel-Temp apparatus, all of which are explained in further detail in the Lefevre Lab Manual. The heating rate was slowed to 1-2°C per minute. The temperature point at which liquid becomes present as well as the temperature at which all of the benzoic acid becomes liquefied were recorded. The known melting point of benzoic acid is 122-123°C. This process of measuring melting points was repeated for mandelic acid, although the known melting point for mandelic acid is 120-122°C. In order to determine the eutectic temperature and composition of benzoic acid and mandelic acid mixture, four mixture samples were obtained. Mixture 1 was 80% benzoic acid and 20% mandelic acid, Mixture 2 was 60% benzoic acid and 40% mandelic acid, Mixture 3 was 40% benzoic acid and 60% mandelic acid, and Mixture 3 was 20% benzoic acid and 80% mandelic acid. Each mixture was put in a separate capillary tube using the same technique as was used for the pure compounds, and the tubes were marked according to which Mixture was in it. Two at a time, the mixtures were heated quickly to 80°C using a melting point apparatus. The rate of warming was slowed to 1-2°C per minute. The temperature point at which liquid becomes present as well as the temperature at which all of the benzoic acid becomes liquefied were recorded. In order to identify an unknown compound by mixture melting point, ten milligrams of an unknown compound was obtained. The unknown was made into a powder, and loaded into a capillary tube using the same methods as was used for the pure known compounds. An orientation melting point was obtained, meaning that the powder was heated somewhat quickly to find an approximate melting point. Then, the apparatus was cooled to 15°C below the orientation melting point. A new sample of the same unknown compound was prepared in a capillary tube, and a more accurate measure of the melting point was obtained by increasing the temperature more slowly. Then, Table 1 from Joseph LeFevre’s “Measuring the Melting Points of Compounds and Mixtures” Lab Manual was

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used to determine which two of the compounds listed in the table have a melting point closest to the unknown compound. A few milligrams of the unknown compound was added to a watch glass with a few milligrams of Benzhydrol (C13H12O), one of the known compounds. A few milligrams of the unknown compound was added to a watch glass with a few milligrams of Biphenyl ((C6H5)2), the second known compound. Each sample was mixed, and then loaded into separate capillary tubes. Another two capillary tubes were filled with pure samples of the unknown compound. A sample of one of the mixtures and a sample of one of the pure unknowns were heated quickly to 30°C below the pure compound’s melting point, then the heating rate was slowed to 1-2°C per minute. The other mixture and the other tube of pure compound were heated using the same tactic afterward. The unknown was identified by determining whether the melting point was closer to that of Benzhydrol (65-67°C) or Biphenyl (69-72°C).

Melting Point Data for Possible Unkown Identification/ Results Compound

MP ºC

Compound

MP ºC

Benzhydrol

65-67

Trans-cinnamic Acid

133-134

Biphenyl

69-72

Benzoin

135-137

Phenanthrene

99-101

Benzilic Acid

150-153

O-toluic Acid

103-105

Adipic Acid

152-154

Acetanilide

113-115

Benzanilide

164-166

Fluorene

114-116

4-bromoacetanilide

167-169

(R,S)-mandelic Acid

120-122

4-hydroxybenzoic Acid

215-217

Benzoic Acid

122-123

Anthracene

216-218

(Please see Flowchart and Diagram for Unknown Determination and Mixture Compositions)

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Part 1: Pure Compound Melting Points Assigned Compound #1: Benzoic acid (BA) Appearance: Colorless Solid with “crystal” pieces Literature Melting Point: 122-123 ℃ Experimental Melting Point: 119℃ - 121℃ Assigned Compound #2: Mandelic Acid (MA) 

Appearance: White Crystalline Solid Literature Melting Point: 120-122 ℃ Experimental Melting Point: 118℃ - 127℃

Part 2: Mixed Compound Melting Points Compounds: Benzoic Acid (BA) /Mandelic Acid (MA) 80/20 BA/MA Experimental Melting Point: 94℃-108℃ 60/40 BA/MA Experimental Melting Point: 92℃-102℃ 40/60 BA/MA Experimental Melting Point: 91℃-104℃ 20/80 BA/MA Experimental Melting Point: 95℃-111℃

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Part 3: Unknown #10 Determination

Benzhydrol + Unknown: 54-59 C Biphenyl + Unknown: 54-59 C Unknown Control: 68-70 C

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Post-Lab Questions 1.

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Create

a Melting Point Mass Percent Composition Diagram using the Data from parts 1 and 2 of the lab.

4.

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Devise

a Flowchart similar to the one in Figure 2 to show how you identified your unknown.

5.

Using

your textbook or another appropriate resource, find the structural formula for your unknown.

Make a drawing of the formula.

Biphenyl C12 H10

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DISCUSSION  Measuring the melting point can be useful when determining an unknown compound. The correlation between the experimental value of an unknown compound and the accepted value of a known compound portrays that the compounds may be the same. The data indicates that the unknown compound in the experiment may be biphenyl, due to the correlation in melting points. An error that may have occurred while observing the melting may be due to minuscule heat transfer from the source of heat to the compound. This can be caused by adding too much of the sample into the capillary tube (part of the heating apparatus). In addition, another error that may have occurred is the particles from the sample not being grounded properly. This may cause air pockets which can slow down the transfer of heat. Lastly, if a sample is not dried properly, droplets of solvent may be present on the outside of the sample known as sweating. In some cases, compounds may decompose near their melting point by displaying a dark color. The decomposition of a compound over a narrow temperature range can be used to indicate that a sample is pure. In other cases, compounds pass from a solid to vapor, skipping the liquid phase, called sublimation. When sublimation occurs, the sample may recrystallize at a higher point in the capillary tube. In future experiments, the proper use of lab equipment, and adequate measuring can limit drawbacks when properly heating substances and grounding materials. These protocols can lead to more accurate results.

Literature Cited LeFevre, Joseph W. 2009. Measuring the Melting Points of Compounds and Mixtures. Signature Labs Series. CER Labs. Cengage Learning. Mason, OH pp. 1-14.

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Post-Laboratory Questions  1.

Briefly identify or explain a.

Two useful functions served by knowing the melting point of organic compounds i. ii.

b.

To access the purity of a substance

Why a finely powdered sample should be used in a melting point measurement i.

c.

To confirm the identity of an unknown compound

Finely powdered samples are the best for even quick heat transfer.

Why is it so important to heat a sample slowly to obtain an accurate melting point i.

Heating a sample too quickly can cause the thermometer to read differently from the actual temperature of the heat source.

d.

Two reasons why is it sometimes difficult to measure the temperature at which the crystals first begin to liquefy i.

Because of sweating, when traces of the solvent are present due to ineffective drying and appear as water droplets.

ii.

Because changes in the crystalline structure cause sagging or shrinking before melting occurs.

e.

What two effects a soluble impurity usually has on the melting point of a compound i. ii.

f.

A decrease in melting point Broader range in melting point temperature

What occurred when crystals began to disappear from the bottom of the capillary tube rather than turning to a liquid i.

Sublimation occurs when the sample at the bottom of the tube vaporizes then recrystallizes farther up the tube.

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2.

A sample has an experimental melting point of 100-101 degrees celsius. Can you conclude that the sample is pure? Briefly explain your reasoning. A sample is known to be impure if it has a melting point range that is lower than the literature or experimental values or has a wider melting point temperature range than five degrees celsius. The more the impurity the larger the deviation. The range is just 1ºC. A range of 1-2º indicates a pure substance.

3.

Using figure 1, explain in detail the melting point behavior of a mixture composed of 60 mass percent X and 40 mass percent Y. The compound X melts at 150º C and the compound Y melts at 148º C. As Y is added to X, the melting point of the mixture decreases, along the diagram lines until the minimum temperature of approximately 130º C is reached. This melting point of the mixture is lower than the pure compound by about 19 degrees. At this point, both compounds will melt in the ratio of 40 parts X to 60 parts Y. At the 40:60 X/Y mix, this is the eutectic composition where both compounds are at equilibrium with the liquid, and the eutectic temperature, the lowest possible melting point of 130ºC that both X and Y mixture. For example, in a mixture of 80:20, if 20 mass percent of X was melted, 13 mass percent of Y would melt as well (20 Y * 40/60 = 13 mass percent of Y). At the point of 60:40 X/Y mix, 30 mass percent of X melts at the temperature of about 136º C. (20 X * 60/40= 30 mass percent of X). At 140º C, there is still some solid, but above 142ºC, it is all liquid.

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4.

An unknown compound melted at 131-133°C. It is thought to be one of the following compounds (mp °C): trans-cinnamic acid (134-137); benzamide(128-130); DL-malic acid(131-133); or benzoin(135-137). The mixture melting points of the unknown compound with each of the test compounds are listed below. What is the unknown compound? Briefly explain your reasoning.

-trans-cinnamic acid...........110-120 -benzamide.....................130-132 -DL-malic acid..................114-124 -benzoin.........................108-116

The unknown compound is benzamide. If the compound was different compound than the unknown the melting point would decrease and the melting point range to be large. Adding benzamide to the unknown cause a small variation in the range well within the pure substance range of 1-2 degrees.

5.

Using your textbook or another appropriate resource, find the structural formula for benzoic acid and mandelic acid. Draw the structural formulas of these compounds.

Benzoic Acid C7H6O2

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Mandelic Acid C8H8O3...