[cck05]

I wanna ask why benzoic acid is less soluble in cold water than ethanoic acid?
Thx everyone

[cth]

Because of the benzene moiety from benzoic acid that is not so soluble in water. It is more soluble in less polar solvents.

As an illustration, benzene (which is liquid at room temperature) is not miscible with water either.

[cck05]

Because of the benzene moiety from benzoic acid that is not so soluble in water. It is more soluble in less polar solvents.

As an illustration, benzene (which is liquid at room temperature) is not miscible with water either.

that means it is because of the number of C atoms?

[cth]

Not directly, it is more a polarity problem.

Water is a polar solvent, so it will dissolve well salts and polar molecules. But it will hardly dissolve non-polar ones. The C-C and C-H bonds are not very polar, so they don't dissolve so well in water. For example, water and oil (made of long CH chains) don't mix.
Now, if you add an electric charge (by somehow adding or extracting an electron) onto an insoluble molecule, it will become much more soluble.

In the case of benzene, you only have C-H and C=C bonds. There is no permanent electrical dipole.
Pyridine (http://en.wikipedia.org/wiki/Pyridine) is structurally similar to benzene, with only one CH replaced by N. Yet, pyridine is miscible with water, while benzene is not. The nitrogen atom in pyridine attracts a bit the electrons of the ring  electrical charges appear (negative onto the nitrogen, positive onto the carbons next to it)  it s more water soluble. And as well, pyridine can be protonated by H⁺, which gives it a positive charge and makes it even more water soluble.

[cck05]

Not directly, it is more a polarity problem.

Water is a polar solvent, so it will dissolve well salts and polar molecules. But it will hardly dissolve non-polar ones. The C-C and C-H bonds are not very polar, so they don't dissolve so well in water. For example, water and oil (made of long CH chains) don't mix.
Now, if you add an electric charge (by somehow adding or extracting an electron) onto an insoluble molecule, it will become much more soluble.

In the case of benzene, you only have C-H and C=C bonds. There is no permanent electrical dipole.
Pyridine (http://en.wikipedia.org/wiki/Pyridine) is structurally similar to benzene, with only one CH replaced by N. Yet, pyridine is miscible with water, while benzene is not. The nitrogen atom in pyridine attracts a bit the electrons of the ring  electrical charges appear (negative onto the nitrogen, positive onto the carbons next to it)  it s more water soluble. And as well, pyridine can be protonated by H⁺, which gives it a positive charge and makes it even more water soluble.

but isn't benzoic acid has COOH?
i know that benzoic acid is more acidic than ethanoic acid
that means benzoic acid is more likely to give out H+ ions
if the H atom releases, there exists anion COO- , so why benzoic acid can't dissolve in water?
thank you

[cth]

Yes, benzoic acid does have R-COOH that can be deprotonated to form R-COO^-.

The solubility of benzoic acid in water depends on the pH as well. When it is deprotonated, it has R-COO^- that makes it much more water soluble.
Benzoic acid is not a strong acid: in water, it exists as an equilibrium between R-COOH and R-COO^- forms. That equilibrium depends on the pH. Both protonated and deprotonated forms are found in solution.


You can look at it from another point of view:
* Under the R-COOH form, benzoic acid molecules can easily stack on top of one another to form a crystalline structure through π-π interactions. Water molecules will have difficulty to separate them. It is less water soluble.
* Under the R-COO^- form, benzoic acid molecules can't stack so efficiently and the negative electrical charges they carry tend to repel each other. Water molecule can more easily separate them. It is more water soluble.

[cck05]

Yes, benzoic acid does have R-COOH that can be deprotonated to form R-COO^-.

The solubility of benzoic acid in water depends on the pH as well. When it is deprotonated, it has R-COO^- that makes it much more water soluble.
Benzoic acid is not a strong acid: in water, it exists as an equilibrium between R-COOH and R-COO^- forms. That equilibrium depends on the pH. Both protonated and deprotonated forms are found in solution.


You can look at it from another point of view:
* Under the R-COOH form, benzoic acid molecules can easily stack on top of one another to form a crystalline structure through π-π interactions. Water molecules will have difficulty to separate them. It is less water soluble.
* Under the R-COO^- form, benzoic acid molecules can't stack so efficiently and the negative electrical charges they carry tend to repel each other. Water molecule can more easily separate them. It is more water soluble.

sorry, but another question springs up in my mind
for the equilibrium part, i understand
but the part below, u said the benzoic acid molecules have π-π interactions between each other. What is π-π interaction? Is it H-bond?
and as i know, there is H-bond between the O- atom in the R-COO- anion and the H atom in COOH, so extra energy is needed to break the bond and my quetion is why it is more water soluble?
Thank You

[cth]

π-π interactions are different from hydrogen bonds. They occur between aromatic rings http://en.wikipedia.org/wiki/Pi-pi_interaction.

Explained simply:
In a benzene ring, there are 6 σ-bonds and 3 π-bonds between the carbon atoms. Those 3 π-bonds are delocalised over the six carbon atoms  it forms two rings over and below the plane where π-electrons can circulate (visualise the structure in http://en.wikipedia.org/wiki/Benzene).

π-π interactions consist in the delocalised π system from one benzene interacting with delocalised π system from another benzene. This results in an attraction between two adjacent aromatic rings. This interaction is weaker than a hydrogen bond, but stronger than Van der Waals interactions. Naturally, for extended aromatic systems, the π-π interactions get stronger.