Simple techniques to predict the melting point (T m) and boiling point (T b) of non-hydrogen bonding rigid molecules have been developed. The compounds. Pure, crystalline solids have a characteristic melting point, the temperature at temperature range, melting points are often used to help identify compounds. Simple Relationships for the Estimation of Melting Temperatures of Homologous Series. James S. Chickos and Gary Nichols. Journal of Chemical & Engineering.
Water is a terrible solvent for nonpolar hydrocarbon molecules: Next, you try a series of increasingly large alcohol compounds, starting with methanol 1 carbon and ending with octanol 8 carbons. You find that the smaller alcohols - methanol, ethanol, and propanol - dissolve easily in water. This is because the water is able to form hydrogen bonds with the hydroxyl group in these molecules, and the combined energy of formation of these water-alcohol hydrogen bonds is more than enough to make up for the energy that is lost when the alcohol-alcohol hydrogen bonds are broken up.
When you try butanol, however, you begin to notice that, as you add more and more to the water, it starts to form its own layer on top of the water.
The longer-chain alcohols - pentanol, hexanol, heptanol, and octanol - are increasingly non-soluble. What is happening here? Clearly, the same favorable water-alcohol hydrogen bonds are still possible with these larger alcohols. The difference, of course, is that the larger alcohols have larger nonpolar, hydrophobic regions in addition to their hydrophilic hydroxyl group. At about four or five carbons, the hydrophobic effect begins to overcome the hydrophilic effect, and water solubility is lost.
Now, try dissolving glucose in the water — even though it has six carbons just like hexanol, it also has five hydrogen-bonding, hydrophilic hydroxyl groups in addition to a sixth oxygen that is capable of being a hydrogen bond acceptor. We have tipped the scales to the hydrophilic side, and we find that glucose is quite soluble in water. We saw that ethanol was very water-soluble if it were not, drinking beer or vodka would be rather inconvenient!
How about dimethyl ether, which is a constitutional isomer of ethanol but with an ether rather than an alcohol functional group?
We find that diethyl ether is much less soluble in water. Is it capable of forming hydrogen bonds with water? Yes, in fact, it is —the ether oxygen can act as a hydrogen-bond acceptor. The difference between the ether group and the alcohol group, however, is that the alcohol group is both a hydrogen bond donor and acceptor.
The result is that the alcohol is able to form more energetically favorable interactions with the solvent compared to the ether, and the alcohol is therefore more soluble.
Here is another easy experiment that can be done with proper supervision in an organic laboratory. Try dissolving benzoic acid crystals in room temperature water — you'll find that it is not soluble. As we will learn when we study acid-base chemistry in a later chapter, carboxylic acids such as benzoic acid are relatively weak acids, and thus exist mostly in the acidic protonated form when added to pure water.
Acetic acid, however, is quite soluble. This is easy to explain using the small alcohol vs large alcohol argument: Now, try slowly adding some aqueous sodium hydroxide to the flask containing undissolved benzoic acid. As the solvent becomes more and more basic, the benzoic acid begins to dissolve, until it is completely in solution. What is happening here is that the benzoic acid is being converted to its conjugate base, benzoate.
2.5: Solubility, melting points and boiling points
The neutral carboxylic acid group was not hydrophilic enough to make up for the hydrophobic benzene ring, but the carboxylate group, with its full negative charge, is much more hydrophilic. Whereas, if you look at pentane, pentane has a boiling point of 36 degrees C, which is higher than room temperature.
So we haven't reached the boiling point of pentane, which means at room temperature and pressure, pentane is still a liquid. So pentane is a liquid. And let's think about the trend for branching here. So we have the same number of carbons, right? Same number of carbons, same number of hydrogens, but we have different boiling points. Neopentane has more branching and a decreased boiling point. So we can say for our trend here, as you increase the branching, right?
Solubility, melting points and boiling points - Chemistry LibreTexts
So not talk about number of carbons here. We're just talking about branching.
- Relationships between melting point and boiling point of organic compounds
As you increase the branching, you decrease the boiling points because you decrease the surface area for the attractive forces. Let's compare three more molecules here, to finish this off. Let's look at these three molecules. Let's see if we can explain these different boiling points. So once again, we've talked about hexane already, with a boiling point of 69 degrees C.
If we draw in another molecule of hexane, our only intermolecular force, our only internal molecular force is, of course, the London dispersion forces. So I'll just write "London" here. So London dispersion forces, which exist between these two non-polar hexane molecules. Next, let's look at 3-hexanone, right?
Boiling points of organic compounds
Hexane has six carbons, and so does 3-hexanone. One, two, three, four, five and six.
So don't worry about the names of these molecules at this point if you're just getting started with organic chemistry. Just try to think about what intermolecular forces are present in this video. So 3-hexanone also has six carbons.
And let me draw another molecule of 3-hexanone. So there's our other molecule. Let's think about electronegativity, and we'll compare this oxygen to this carbon right here. Oxygen is more electronegative than carbon, so oxygen withdraws some electron density and oxygen becomes partially negative. This carbon here, this carbon would therefore become partially positive.
And so this is a dipole, right? So we have a dipole for this molecule, and we have the same dipole for this molecule of 3-hexanone down here.
Melting Point, Freezing Point, Boiling Point
Partially negative oxygen, partially positive carbon. And since opposites attract, the partially negative oxygen is attracted to the partially positive carbon on the other molecule of 3-hexanone. And so, what intermolecular force is that? We have dipoles interacting with dipoles.
So this would be a dipole-dipole interaction.
So let me write that down here. So we're talk about a dipole-dipole interaction. Obviously, London dispersion forces would also be present, right? So if we think about this area over here, you could think about London dispersion forces. But dipole-dipole is a stronger intermolecular force compared to London dispersion forces.
And therefore, the two molecules here of 3-hexanone are attracted to each other more than the two molecules of hexane. And so therefore, it would take more energy for these molecules to pull apart from each other.
In a typical pressure cooker, water can remain a liquid at temperatures as high as oC, and food cooks in as little as one-third the normal time. To explain why water boils at 90oC in the mountains and oC in a pressure cooker, even though the normal boiling point of water is oC, we have to understand why a liquid boils. By definition, a liquid boils when the vapor pressure of the gas escaping from the liquid is equal to the pressure exerted on the liquid by its surroundings, as shown in the figure below.
Liquids boil when their vapor pressure is equal to the pressure exerted on the liquid by its surroundings. The normal boiling point of water is oC because this is the temperature at which the vapor pressure of water is mmHg, or 1 atm. Under normal conditions, when the pressure of the atmosphere is approximately mmHg, water boils at oC.
At 10, feet above sea level, the pressure of the atmosphere is only mmHg. At these elevations, water boils when its vapor pressure is mmHg, which occurs at a temperature of 90oC.
Pressure cookers are equipped with a valve that lets gas escape when the pressure inside the pot exceeds some fixed value. This valve is often set at 15 psi, which means that the water vapor inside the pot must reach a pressure of 2 atm before it can escape.
Because water doesn't reach a vapor pressure of 2 atm until the temperature is oC, it boils in this container at oC. Liquids often boil in an uneven fashion, or bump. They tend to bump when there aren't any scratches on the walls of the container where bubbles can form. Bumping is easily prevented by adding a few boiling chips to the liquid, which provide a rough surface upon which bubbles can form.