5.1.5: Polarity of organic solvents

Lab #5 CHEM 0103 Chemistry of the life sciences

Oil and water don’t mix: polarity of organic solvents

Inside our cells, biomolecules are dissolved in water. Cells are surrounded by a cell membrane consisting of nonpolar lipids, preventing the polar molecules inside the cell from spilling out. Today, we will explore how polar organic solvents are, and will test what mixes with what. Some compounds are clearly very polar (e.g. water) or quite nonpolar (e.g. oil), but many compounds are somewhere in between, and mix with both. Many drugs given orally belong into this category, and that allows them to travel through polar and nonpolar regions of the cell to reach their target. 

One important aspect for today’s experiment is to correlate the structure of a given compound with its properties. You should therefore look up the structure of the two solvent (A and B, see table) you will be working with (each group will study two different organic solvents, i.e. liquids). 

Group	Solvent A	Solvent B
Hydrogen	Methanol CH3OH	Iso-Pentyl alcohol (isoamyl alcohol)
Carbon	Anisole C6H5OCH3	Isopropanol (2-propanol)
Nitrogen	Acetone (CH3)2CO	n-Butanol
Oxygen	Iso-Pentyl alcohol (isoamyl alcohol)	Methanol CH3OH
Sulfur	Isopropanol (2-propanol)	Anisole C6H5OCH3
Phophorus	n-Butanol	Acetone (CH3)2CO

Another important aspect is safety and correct treatment of wastes. Most of the solvents we are working with are highly flammable, and some are fairly toxic. You should look up the safety data sheet (SDS) online before you start to work. To minimize the risks involved with handling flammable substances, we will work with very small volumes (you will only receive 300 µl each of the two solvents). All tubes should be labeled. Solvents should not be discarded into the sink, but collected without opening the containers

Beginning question

The question you are addressing in your experiment is how polar or nonpolar your solvents are, and which substances mix with them. 
Before you start, you should make a plan and your group leader should talk to your lab instructor. You will also gather data on the polarity of carbohydrates and lipids.

Structure of your solvents 

Draw the structures of solvent A and B and indicate polar bonds by adding partial charges. Build a model of solvent A, mark atoms with negative partial charges with masking tape, and place it on the desk in front of the lab so that the negative partial charges point toward the board and the positive partial charges point toward the other direction. 

Compare solvent A and solvent B 

Using the models and your drawings, predict whether solvent A or solvent B is more polar. More polar solvents are more likely to mix with water (a very polar solvent), and less likely to mix with oil (a non-polar solvent). Also, polar solvents are more likely to dissolve polar solids (such as sucrose) and less likely to dissolve non-polar solids (such as chocolate). Make a table to summarize your predictions (will/will not mix with water, will/will not mix with oil, will/will not dissolve sugar, will/will not dissolve butter) for each solvent. 

What do your solvents mix with? 

Under the chemical hood, fill two separate labeled tubes with 300 µL of solvent A and B, respectively, and close the lid to avoid evaporation. At your bench, using four separate tubes, test whether your solvents mix with water and whether they mix with oil (i.e. A+oil, A+water, B+oil, B+water). Use 100 µL of your solvent and 50 µL of water or oil so you know which is which in case they don't mix. If they don't seem to mix, try inverting the closed tube or vortexing it, check for separation, centrifuge, and check for separation again. If you see turbidity or "bubbles" that turn into two layers upon centrifugation, this is good evidence that they really don't mix. Record which liquid is on the bottom (has higher density). If you are unsure which is which, use water and oil with color added, or add another drop of your solvent and observe which phase it combines with (and which phase increases in volume).

Does sugar (sucrose) or butter dissolve in the solvent? 

In a fresh tube, add a couple of crystals of sugar to 100 µL of solvent A. In another fresh tube, add a a tiny speck of butter to 100 µL of solvent B. Mix by inverting observing whether the solids dissolve in your solvent. Sucrose is an example of a carbohydrate, and butter is an example of a lipid, more specifically a triglyceride made of 3 fatty acids combined with glycerol.

How volatile are your solvents?

Pipette a drop of 2 µl of solvent A onto a watch glass in the hood, and next to it a drop of solvent B. Observe whether they evaporate, and how long it takes if they do.

“Publish” your observations

Into the table on the blackboard, enter your observations about solvent A and solvent B. Once the other groups have published their observations, check whether your observations match the results of the other group. It they don't, directly compare the evidence (the respective tubes).