Solubility and Functional Groups You will recall from general chemistry that a solution has two components: the solvent, which is the substance present in greater amount, and the solute, which is dissolved in the solvent. Solubility is defined as the mass (in grams) of solute dissolved in 100 g of solute at saturation. Molar solubility is defined as the amount (in moles) of solute per liter of saturated solution. The solubility of one compound in another is related to the strength and type of intermolecular forces that exist between the two components.
These forces arise from factors of molecular shape and electronegativity difference (??), and are influenced by the specific functional groups contained within the molecule. A functional group is a group of atoms bonded in a particular way, with a predictable chemical and physical behavior. Solute molecules that experience strong intermolecular attractions to solvent molecules will be more likely to dissolve. On the other hand, if the solute molecules experience more attraction to each other than to the solvent molecules, then it is more energetically favorable for them to remain together and for the solvent particles to remain together, so e would observe little solubility.
Applying the broad generalization like dissolves like, we can make reasoned predictions about the solubility of a chosen substance in a given solvent. For example, polar compounds tend to dissolve other polar compounds, while non-polar compounds tend to dissolve other non-polar compounds. Recall that a polar molecule has a permanent electric dipole moment, but a nonpolar molecule has no net molecular dipole moment.
Sucrose (table sugar) a polar compound with many hydroxyl functional groups, dissolves readily in water (another polar compound), but does not dissolve in hexane (a nonpolar six-carbon hydrocarbon). As you can see in Figure 1, the presence of most functional groups suggests significant electronegativity differences, and thus the potential for an overall molecular dipole. On the other hand, molecules containing only hydrogen and carbon–through a combination of small ?? values and symmetric molecular structure–are precluded from having a permanent dipole.
A brief review of the intermolecular forces between neutral molecules follows: Dipole-dipole: When two different molecules are both polar, strong dipole-dipole attractions may result. The strength of these attractions will increase with the magnitudes of the dipole moments and as the molecules approach each other more closely. See Figure 2. Molecules with permanent electric dipole moments orient in such a way as to maximize attractions, which will lower the potential energy of the system.
The molecules align so that the positive pole of one molecule approaches the negative pole of a neighboring molecule. Positive-positive or negative-negative approach would cause repulsions, raising the potential energy of the two-molecule system. We can consider hydrogen bonding as a special case of dipole-dipole attraction. See Figure 2(b). For hydrogen bonding to occur, one molecule must O have at least one lone pair O H H H H O of nonbonding electrons, O O and the other atom must H H have a hydrogen atom a b bound covalently to a small and highly Figure 2. a) Dipole-dipole interaction in acetone. electronegative atom– (b) Hydrogen bonding between water molecules. either N, O, or F. Dipole-induced dipole: If only one molecule of the two you are considering has a dipole, it may still induce a temporary dipole of opposite orientation in a neighboring non-polar molecule. This induced dipole can be sizable if the non-polar molecule holds its outer electrons loosely, in orbitals far from the nuclei. The permanent dipole and the induced dipole interact to create the possibility of intermolecular attraction.
Induced-dipole-induced-dipole (aka London forces, dispersion forces): Even if both molecules are non-polar, they may experience attractions through London forces (named for Fritz London, a chemical physicist of the mid-twentieth century who first imagined them). The distribution of outer electrons in non-polar molecules undergoes small random shifts over short times, which can create an instantaneous dipole. Such a transient dipole in one molecule can induce a similar transient dipole in an adjacent molecule, resulting in a temporary attraction.
In larger molecules, or highly polarizable small molecules, these temporary forces can add up to significant attractions. For example, both octane (C8H18) and bromine (Br2) are liquids at room temperature, although both are non-polar. Dispersion forces are the only intermolecular forces experienced by samples of non-polar molecules. While the like dissolves like rule is a good place to start in predicting solubility, remember to look at the entire structure of each molecule, not just individual groups in isolation. A compound might ave a particular functional group that allows it to experience a strong attraction to solvent, but that group might be such a small part of the molecule that it is not enough to allow the compound to dissolve in a particular substance. Examples include molecules that have polar functional groups, such as the hydroxyl group (?OH) or carbonyl group (C=O) but that also have large non-polar regions, such as benzene rings or extended carbon chains. Keep this in mind when you investigate the difference in behavior between a small alcohol like methanol, CH3OH, and an alcohol with a longer carbon chain, like octanol, CH3(CH2)7OH.
Solubility and miscibility. In the typical organic chemistry lab activity, the solvent is a liquid and the solute is a solid or liquid (sometimes described as an oil) at room temperature. After mixing two liquids, you can expect one or the other of two distinct behaviors as a result. If the liquids mix completely in all proportions, we say they are miscible. Alternatively, if the two liquids do not mix completely in all proportions, they are immiscible: the mixture will separate into two layers, with the less dense liquid on top.
When two liquids are immiscible, this does not necessarily mean that one liquid is completely insoluble in the other. In some situations where two liquids do not mix, two layers are observed but a small amount of each liquid is dissolved in the other. Solubility can be qualitatively expressed in gradations: completely soluble, partially soluble, insoluble; but miscibility is all-or-nothing: either two liquids are miscible, or they are not. As organic chemists, how will we make use of our understanding of solubility? To separate one thing from another.
Recrystallization and chromatography are two techniques you will use this semester to separate the components of a mixture from one another. Both of these rely on differences in the solubilities of their components in a particular solvent. To help determine what solvent is best for a reaction. For a reaction between two chemicals to occur, their molecules have to encounter each other. Thus, if they are both dissolved solutes in the same solvent, they are more likely to collide with each other and react than if one is an insoluble solid at the bottom of a flask and the other one is dissolved in the solution.
Depending on the reaction, we might choose one solvent over another to increase or decrease the reaction rate, so choice of solvent is an important consideration. To take advantage of solubility in the aqueous workup. After a typical organic reaction, we separate the desired organic product from unwanted salts that have accumulated during the reaction. We take advantage of the fact that many organic solvents do not mix with water and use aqueous solutions to “wash” our reaction mixture, removing the unwanted water-soluble salts from the organic solvent that contains the product we want.
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 some predictions of the solubilities of other compounds. Finally, you will use the Merck Index to check your predictions. Procedure For these exercises, you will work with a partner (no groups of more than 2). One of you should do Part “A” while the other person does Part “B”. When you’re both finished, compare and discuss results. Both people need to record both sets of results in “Data and Observations. Most of the compounds you will use today are hazardous; e. g. , methanol, if ingested, is toxic. Keep all reagents capped when not in use. This guideline is particularly important for volatile (high vapor-pressure) compounds like diethyl ether. Conduct all experiments in the fume hood with the sash at the proper operating level. (Your instructor will explain. ) 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 for a total of nine experiments.
You will need to draw the structures of the three compounds in your notebook before coming to lab. Place approximately 40 mg (0. 040 g) of the solid compound in a 10 ? 100 test tube. Add about 1 mL of solvent. Put the flat end of your spatula into the liquid and mix by swirling the spatula for 30 seconds. (Wipe off the spatula with a paper towel between experiments to avoid cross-contamination. ) At the end of the 30 seconds of mixing, record your observations in the chart at the end of this handout. If you can see no solid particles, the compound is fully soluble. If none of the solid appears to have dissolved, it is insoluble.
If some solid dissolves, but you still see some solid remaining, the compound is partially soluble. Helpful hints: Weigh out all the samples of a particular solid at the same time, i. e. , prepare three test tubes each with 40 mg of salicylic acid all at once. (Use a Sharpie® marker or tape to keep track of which solvent goes into which tube. ) Measure out 1 mL of water using a graduated cylinder or plastic syringe, and transfer it into a test tube. Use a Sharpie® marker to mark the level of the water in the test tube. This is the approximate volume of liquid to use for all subsequent tests.
Solubility of Liquid Compounds In this part of the experiment, you will determine whether two liquids are soluble in one another, and whether they are miscible or immiscible. Solubility/Miscibility of Water With Other Liquids Place 1 mL of water in each of six 10 ? 100 test tubes labeled “1” through “6”. Then, to the indicated test tube, add 20 drops of the second liquid using a Pasteur pipet. To tube 1. 2. 3. 4. 5. 6. Add 20 drops of…. methyl alcohol (methanol), CH3OH 1-octanol, CH3(CH2)7OH diethyl ether (ether), CH3CH2OCH2CH3 methylene chloride (dichloromethane), CH2Cl2 hexane, CH3CH2CH2CH2CH2CH3 acetone, CH3COCH3
After adding the 20 drops, mix the two liquids either by twirling the flat end of your spatula in the mixture or by carefully flicking the test tube with your finger. After you have finished adding and mixing, line up all the test tubes so that you can compare results, and record your observations in the data table at the end of the handout. Solubility/Miscibility of Hexane With Other Liquids Place 1 mL of hexane in each of five 10 ? 100 test tubes labeled “7” through “11”. Then, to the indicated test tube, add 20 drops of the second liquid using a Pasteur pipet.
To tube 7. 8. 9. 10. 11. Add 20 drops of…. methyl alcohol (methanol), CH3OH 1-octanol, CH3(CH2)7OH diethyl ether (ether), CH3CH2OCH2CH3 methylene chloride (dichloromethane), CH2Cl2 acetone, CH3COCH3 After adding the 20 drops, mix the two liquids either by twirling the flat end of your spatula in the mixture or by carefully flicking the test tube with your finger. After you have finished adding and mixing, line up all the test tubes so that you can compare results, and record your observations in the data table at the end of the handout.
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 hexane
Discussion and Conclusion: Post-lab questions 1. Using your knowledge of intermolecular forces, explain your experimental results for the following: a. Salicylic acid and water b. Salicyclic acid and methanol c. Ascorbic acid and water d. Water and methanol vs. water and 1-octanol e. Acetone and water vs. acetone and hexane f. Water and methanol vs. water and diethyl ether 2. Find the chemical structure of a molecule called benzophenone. Draw this molecule in your report. (See comment at the bottom of this page. ) Would you expect 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 verify 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 expect 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. b. c. Your name; title and date of experiment Purpose of the lab 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.