Some doubts I have while trying to compare boiling points/ solubility

[jaumzaum]

I was trying to compare some solubilities (in water) and some boiling points, and I could not explain it for some molucules. Also, I have some doubts in the theory itself.

Why is 2-pentanone more soluble than pentanoic acid?
Why is a ketone more soluble than the respective aldeyde?

When I compare boiling points, I was tauch to compare the interactions between the molecules, and the size of the molecule. How can I justify the size of a molecule interfering in solubility? I have 2 theories but I really don't know if any of them are right. First I would say a big molecule would do more interactions than sa smaller one, so the boiling point should be greater. Also I would say that at the same temperature, all molecules has the same kinetic energy, and how kinetic energy is something like mv²/2, big molecules should have less velocity, so it would be harder for them to escape the liquids. Is it right?

Another doubt I had was when I had to compare solubility.

In this case I know the number of hyfrogen bond is important. Is the size important in this case too?

In an exercise of my chemistry school book I was asked to compare the solubility of acetylsalicylic acid and salicylic acid.

http://upload.wikimedia.org/wikipedia/commons/6/67/Aspirin-skeletal.svg
http://upload.wikimedia.org/wikipedia/commons/8/8e/Salicylic-acid-skeletal.svg

The first one makes 9 hydrogen bonds with water (is it right?) and has a bigger size (more interaction) also, the second one makes only 8 hydrogen bonds and has a smaller size. The answer, though, says that salicylic acid has a greater solubility. Why is this true?

 

[chill_factor]

the first one has only 1 hydrogen bond donor (OH groups). the second has 2 hydrogen bond donors (OH groups).

Always remember: OH groups trump all in terms of hydrogen bonding.

 

[jaumzaum]

the first one has only 1 hydrogen bond donor (OH groups). the second has 2 hydrogen bond donors (OH groups).

Always remember: OH groups trump all in terms of hydrogen bonding.

But shouldn't the elektron pairs in =O be considerated too? In water these molecules would be pretty much diluted so there should be a lot of H in the H2O molecules making hidrogen bond with the pairs of electrons of the oxygens of the carbonyl. Am I wrong?

 

[chill_factor]

think about it this way:

a single O can accept hydrogen bonds. However, it cannot donate hydrogen bonds. The degree of solubility of something is determined by how much it can interact with the solvent, or put it simpler, how much it is similar to the solvent. In a polar solvent such as water, something with more things that are "like water" in terms of being able to both accept and donate hydrogen bonds will probably be soluble.

acetylsalicylic acid is less soluble in water because it is "less water-like" than salicylic acid. another easy example is diethyl ether is less soluble than ethanol, propanol (which has more carbons) or butanol.

now what I've said is generally true, not totally true, but it is a good rule of thumb. it also only applies to small molecules (molecular mass < 1000 daltons). for polymers, things get tricky because they can fold back on themselves or get tangled together and all sorts of messy stuff.

 

[LudusRex]

I understand your doubts and questions regarding the comparison of boiling points and solubility. These properties are influenced by various factors and it can be challenging to explain them for certain molecules.

First, let's address the question of why 2-pentanone is more soluble than pentanoic acid. This can be attributed to the presence of a polar carbonyl group in 2-pentanone, which allows it to form hydrogen bonds with water molecules. On the other hand, pentanoic acid has a polar carboxyl group which can form hydrogen bonds, but its longer carbon chain may interfere with the formation of these bonds, resulting in lower solubility.

The difference in solubility between a ketone and its respective aldehyde can also be explained by the presence of the polar carbonyl group in the ketone, which enhances its ability to form hydrogen bonds with water molecules.

In terms of comparing boiling points, your theories are partially correct. Larger molecules do indeed have more opportunities for intermolecular interactions, which can result in higher boiling points. However, the size of a molecule can also affect its shape and the strength of its intermolecular forces, which can also influence its boiling point.

Regarding the comparison of solubility, the number of hydrogen bonds is an important factor, but it is not the only one. The size of a molecule can also play a role in its solubility, as larger molecules may have more difficulty fitting into the solvent's structure. In the case of acetylsalicylic acid and salicylic acid, the smaller size of salicylic acid may allow it to fit more easily into the water structure, resulting in greater solubility despite having one less hydrogen bond.

In conclusion, the properties of boiling point and solubility are complex and can be influenced by various factors such as molecular size, shape, and functional groups. It is important to consider all of these factors when comparing these properties for different molecules.