Video Transcript
The table below shows the solubility of five unknown esters. Which of the unknown esters is likely to have the longest chain length? Assume the chains are linear and there is no branching. Ester A has a solubility of 8.7 grams per 100 milliliters. Ester B has a solubility of 0.6 grams per 100 milliliters. C has a solubility of 10.5 grams per 100 milliliters, D 2.2 grams per 100 milliliters, and E 4.9 grams per 100 milliliters.
An ester has the following general displayed formula, where this R group, which represents the rest of the molecule, can be an alkyl group, an aryl group, or a hydrogen atom. And this R group can be an alkyl group or an aryl group but not a hydrogen atom. The question asks us about the solubility of esters. Hydrogen bonding is what allows some esters to be soluble. A hydrogen bond is a type of electrostatic interaction between a hydrogen atom covalently bonded to an electronegative atom, primarily fluorine, oxygen, and nitrogen, and another electronegative atom with a lone pair of electrons. Hydrogen bonding exists between water molecules. This is the hydrogen bond which occurs between a hydrogen donor, which is a hydrogen atom, bonded to an electronegative atom, which in this case is oxygen, and a hydrogen acceptor, which is an electronegative atom with a lone pair of electrons.
Now let’s look at hydrogen bonding in an ester molecule. As this R group cannot be a hydrogen atom, ester molecules don’t contain a hydrogen atom covalently bonded to an electronegative atom. So they do not contain a hydrogen donor. They do, however, contain two oxygen atoms, which can act as hydrogen acceptors. Since esters don’t contain hydrogen donors, they cannot hydrogen-bond with themselves, but they can hydrogen-bond with water molecules, as the hydrogen atom in the water molecule can act as the hydrogen donor. There are actually four lone pairs on the oxygen atoms in this ester molecule. So a maximum of four hydrogen bonds can form.
Hydrogen bonding is a type of dipole–dipole interaction. There is an attraction between the electronegative oxygen atoms, as they have a partial negative charge, and the hydrogen atoms, which have a partial positive charge. Other atoms, such as this carbon atom, will also have a partial positive charge but aren’t directly involved in the hydrogen bonding. It is this interaction between esters and water molecules that makes it possible for esters to be soluble.
Now we know what causes the solubility of esters, we need to figure out which of the esters in the table has the longest chain length. To do this, we need to know how the chain length of an ester affects its solubility. If we take an ester with a short chain, we can see that quite a large proportion of the molecule contains oxygen atoms and can, therefore, hydrogen-bond to water. However, if we increase the chain length, making sure that the chain is linear as specified in the question, then a smaller proportion of the molecule can hydrogen-bond with water.
The majority of the molecule is now made up of alkyl chains. Alkyl chains are nonpolar, and they cannot hydrogen-bond. There is little interaction between alkyl chains and the water molecules. So the presence of the alkyl chains makes the ester less soluble. If we have an ester with even more carbon atoms, the proportion of the ester that can hydrogen-bond with water is even smaller still. This means that the solubility decreases. Therefore, as we increase the chain length, a higher proportion of the molecule will be insoluble. So the overall solubility of the ester will decrease.
The question asks us, which of the esters is likely to have the longest chain length? An increase in chain length causes a decrease in solubility. So the longest chain length should have the lowest solubility. The units for solubility are grams per 100 milliliters. So it measures the maximum number of grams of a substance that can dissolve in 100 milliliters of a solvent, which is likely to be water. If a substance had low solubility, then very few grams would dissolve. If we look at the values for solubility in the table, the lowest value is 0.6 grams per 100 milliliters. This is the solubility of ester B. As ester B has the lowest solubility, it is likely to have the longest chain length.
Therefore, the answer to the question “Which of the unknown esters is likely to have the longest chain length?” is ester B.
