General chemistry covers the composition of a solution, which consists of a solvent and a solute. The solvent is the primary component in the solution, while the solute is dissolved within it. Solubility indicates how much solute (in grams) can dissolve in 100 g of solvent until saturation occurs. Conversely, molar solubility measures the quantity of solute (in moles) that can be dissolved in one liter of saturated solution. The capacity for a compound to dissolve in another depends on the type and strength of intermolecular forces between these two constituents.
Solubility depends on molecular shape, electronegativity differences, and the presence of functional groups. Functional groups are specific arrangements of bonded atoms that have consistent chemical and physical behaviors. If solute molecules strongly attract solvent molecules, they dissolve easily. On the other hand, if solute molecules prefer bonding with each other rather than with solvent molecules, their solubility is low. The general principle “like dissolves like” helps us predict how well a substance will dissolve in a specific solvent. Polar compounds tend to dissolve other polar compounds, while non-polar compounds dissolve other non-polar compounds. It’s important to note that polar molecules have a permanent electric dipole moment, whereas nonpolar molecules lack a net molecular dipole moment.
Sucrose, also called table sugar, is a polar compound with multiple hydroxyl groups. As a result of its polarity, it readily dissolves in water, another polar substance. In contrast, it does not dissolve in hexane, a nonpolar hydrocarbon composed of six carbon atoms. Figure 1 illustrates that the presence of various functional groups denotes significant differences in electronegativity and thus the likelihood for an overall molecular dipole. Conversely, molecules containing solely hydrogen and carbon lack a permanent dipole due to their low electronegativity values and symmetrical molecular structure.
The following is a brief explanation of the intermolecular forces between neutral molecules:
Dipole-dipole interactions happen when two polar molecules interact, creating strong attractions. The strength of these attractions depends on both the size of the dipole moments and how close the molecules are to each other. Please see Figure 2 for a visual representation. Molecules with permanent electric dipole moments align themselves to maximize attractions, which in turn lowers the potential energy of the system.
The alignment of molecules causes the positive pole of one molecule to approach the negative pole of another. If there is an approach between positive-positive or negative-negative, it would result in repulsions and an increase in potential energy for the two-molecule system. Figure 2(b) demonstrates hydrogen bonding, which is a specific type of dipole-dipole attraction. For hydrogen bonding to occur, one molecule needs to have at least one nonbonding lone pair of electrons (e.g., O H H H H O), while the other atom must have a hydrogen atom covalently bonded to N, O, or F. In situations where only one of the two molecules has a dipole (called dipole-induced dipole), it can still induce a temporary dipole with an opposite orientation in a nearby non-polar molecule. The magnitude of this induced dipole can be significant if the non-polar molecule loosely holds its outer electrons in orbitals distant from its nuclei. Together, both the permanent dipole and induced dipole contribute to intermolecular attraction.
Induced-dipole-induced-dipole (also known as London forces or dispersion forces) can cause attractions between non-polar molecules. These forces were first proposed by Fritz London, a chemical physicist, in the mid-twentieth century. Non-polar molecules have external electrons that undergo random movements momentarily, leading to the formation of an instantaneous dipole. This transient dipole can induce a corresponding transient dipole in neighboring molecules, resulting in a temporary attraction.
In larger molecules or highly polarizable small molecules, the temporary forces contribute significantly to attractions. A case in point is the liquids octane (C8H18) and bromine (Br2) at room temperature, despite their non-polarity. Non-polar molecules are only influenced by dispersion forces as intermolecular forces. While the like dissolves like rule is a useful starting point for solubility predictions, one should consider the entire structure of each molecule and not just isolated segments. A compound may possess a functional group that allows it to exhibit a strong attraction to a solvent. However, if this group constitutes only a small portion of the molecule, it may not facilitate dissolution in a particular substance. Examples include molecules with polar functional groups like the hydroxyl group (?OH) or carbonyl group (C=O), but also encompassing sizable non-polar regions such as benzene rings or extended carbon chains. When comparing the behavior of a small alcohol like methanol (CH3OH) and an alcohol with a longer carbon chain like octanol (CH3(CH2)7OH), remember this distinction.
In organic chemistry lab activities, the combination of liquids involves a liquid solvent and a solid or liquid (oil) solute at room temperature. When two liquids are mixed, they can exhibit either miscible or immiscible behavior. Miscible liquids fully blend regardless of proportion, while immiscible liquids do not completely blend in all proportions. This leads to the formation of two layers, with the less dense liquid settling on top.
When two liquids do not mix together, they are considered immiscible; however, this does not mean that one liquid is completely insoluble in the other. In certain cases, although the liquids do not mix, a small amount of each liquid dissolves in the other, resulting in the formation of two distinct layers.
Solubility can be categorized into different degrees: complete solubility, partial solubility, or insolubility. Conversely, miscibility is a binary state where two liquids are either miscible or not.
Understanding solubility is essential for organic chemists as it enables effective separation of components from each other.
Recrystallization and chromatography are two techniques used this semester. Both rely on the solubilities of components in a solvent, aiding in selecting the ideal solvent for a reaction. In order for a reaction to occur between chemicals, their molecules must come into contact. When both chemicals are dissolved in the same solvent, they have a higher chance of colliding and reacting compared to when one chemical is insoluble at the bottom of a flask while the other chemical remains dissolved.
The selection of a solvent is crucial in reactions as it can impact the rate of the reaction. Solvents are chosen based on their ability to either increase or decrease the reaction rate. The solubility in aqueous workup is also important for separation purposes. Following an organic reaction, it is necessary to eliminate unwanted salts that have built up during the reaction from the desired organic product. Aqueous solutions are employed to cleanse the reaction mixture and separate the water-soluble salts from the organic solvent containing the product.
In today’s experiment, you will test a variety of compounds in different solvents to determine their relative solubilities. You will then use this information to make predictions for the solubilities of other compounds. Finally, you will use the Merck Index to verify your predictions.
Procedure:
For these exercises, you will work with a partner (no groups of more than 2). One person should complete Part “A” while the other person completes Part “B”. Compare and discuss results when both are finished. Both individuals need to record both sets of results in “Data and Observations”.
Note: Most of the compounds used today are hazardous, such as methanol which is toxic if ingested. Keep all reagents capped when not in use. This is particularly important for volatile compounds like diethyl ether. Conduct all experiments in the fume hood with the sash at the proper operating level (your instructor will provide further explanation).
Part A: Solubility of Solid Compounds
In this part of the experiment, you will determine the qualitative solubility of three organic compounds (salicylic acid, ascorbic acid, and cholesterol) in three different solvents, resulting in a total of nine experiments.
Before coming to the lab, make sure to draw the structures of the three compounds in your notebook. In a 10 ? 100 test tube, add approximately 40 mg (0. 040 g) of the solid compound and about 1 mL of solvent. Swirl the spatula with the flat end in the liquid for 30 seconds to mix it. Remember to wipe off the spatula with a paper towel between experiments to prevent cross-contamination. After mixing for 30 seconds, write down your observations in the chart provided at the end of this handout. If you cannot see any solid particles, it means that the compound is fully soluble. If the solid does not appear to have dissolved at all, it is insoluble.
If you can still see some remaining solid when a solid dissolves, it means that the compound is partially soluble. Here are some helpful hints for conducting tests:
- Weigh out all the samples of a specific solid at the same time. For example, prepare three test tubes with 40 mg of salicylic acid all at once. Use a Sharpie® marker or tape to keep track of which solvent goes into each tube.
- Using a graduated cylinder or plastic syringe, measure out 1 mL of water and transfer it into a test tube. Use a Sharpie® marker to mark the water’s level in the tube. This volume can be used as an approximation for all future tests.
The purpose of this experiment is to determine the solubility and miscibility of two liquids. To conduct the experiment, you will need six 10 ? 100 test tubes labeled from “1” to “6”. Each test tube should contain 1 mL of water. Using a Pasteur pipet, add 20 drops of the designated second liquid to each respective test tube. The following liquids should be added:
- methyl alcohol (methanol), CH3OH
- 1-octanol, CH3(CH2)7OH
- diethyl ether (ether), CH3CH2OCH2CH3
- methylene chloride (dichloromethane), CH2Cl2
- hexane, CH3CH2CH2CH2CH2CH3
- acetone, CH3COCH3
After adding 20 drops, thoroughly mix the two liquids by either twirling the flat end of your spatula in the mixture or carefully flicking the test tube with your finger. Once you have completed adding and mixing, arrange all the test tubes in a line to facilitate comparison of results. Record your observations in the data table provided at the end of this handout. This experiment focuses on determining solubility/miscibility of hexane with other liquids. Begin by placing 1 mL of hexane into each of five 10 – 100 test tubes labeled as “7” through “11”. Next, using a Pasteur pipet, add 20 drops of the second liquid to its corresponding test tube as indicated.
To tubes 7, 8, 9, 10, and 11, introduce the following substances: methyl alcohol (methanol), CH3OH; 1-octanol, CH3(CH2)7OH; diethyl ether (ether), CH3CH2OCH2CH3; methylene chloride (dichloromethane), CH2Cl2; acetone, CH3COCH3. After adding the drops, mix the liquids by swirling the flat end of your spatula in the mixture or gently flicking the test tube with your finger. Once finished adding and blending, align all the test tubes for easy comparison and record observations in the provided data table.
Data Summary Tables Create tables like the ones shown here in your lab notebook to record observations. Be sure to leave plenty of room. One page per table should be fine.
Part A. Solubility of Solids: water, salicylic acid, ascorbic acid, cholesterol, methanol, hexane.
Part B. Solubility of Liquids: Include the following in your observations: If you observe two layers, indicate which liquid is the top layer. Be sure to note the relative sizes of the layers among different experiments. Remember to make notes about miscibility vs. solubility. Water, methanol, 1-octanol, diethyl ether, methylene chloride, acetone, hexane.
Discussion and Conclusion: Post-lab questions
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1. Discuss your experimental findings for the following combinations of substances based on your understanding of intermolecular forces:
- Salicylic acid and water
- Salicyclic acid and methanol
- Ascorbic acid and water
- Water and methanol compared to water and 1-octanol
- Acetone and water compared to acetone and hexane
- Water and methanol compared to water and diethyl ether
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2. Locate the chemical structure of a molecule known as benzophenone, illustrate this molecule in your report (See comment at the bottom of this page). Do you anticipate this molecule to be soluble in water?
Would you expect it to be soluble in methanol? Explain your reasoning. Then, use the Merck Index (available at the stockroom for checkout) to confirm your reasoning. The lab text (Zubrick, page 31) provides an explanation for interpreting entries in the Merck Index. (You don’t need to change your prediction if the Merck Index disagrees! The point is to make an educated guess based on your results and to learn how to use the Index.)
3. Find the chemical structure of glucose. Draw this molecule in your report. Do you anticipate glucose to be soluble in water? In hexane?
Explain your reasoning, then use the Merck Index to check your prediction.
4. Find the chemical structure of tetrahydrofuran (“THF”). Draw this molecule in your report. THF is a liquid at room temperature. Do you expect THF and water to be miscible or immiscible? Do you expect that THF will be soluble in water? Explain your reasoning, then use the Merck Index to check your prediction.
Comment on drawing organic molecules: You can draw your molecules in your report by hand, or if you like using computers, try downloading a free chemical drawing program (ChemSketch) at ACD Labs website, http://www.cdlabs.com/download/
Lab Report Instructions
Since this is your first organic chemistry lab report, here is a guide to doing it well. For additional details about each section, refer to the syllabus.
1. Type pre-lab before coming to lab.
Include:
a. Your name; title and date of experiment
b. Purpose of the lab
c. A table of physical data for all the chemicals you will be using in the experiment. Include chemical structure, molecular weight, melting point, boiling point, density, and hazards. You can draw in the structure by hand.
A quick Google or Wikipedia search should easily provide all the structures, along with the other physical properties. Procedure, in list form. Summarize the procedure given above. Don’t photocopy the procedure in this printout; make your own. 2. For your data and observations, recreate the tables shown in the lab handout in your lab notebook. 3. After lab, type your conclusion. This time you will just be answering a series of numbered questions. 4. Before you turn in the lab the following week, staple the pages together in this order: pre-lab, data and observations, conclusion.
