Why does sugar dissolve in Cyclohexane (1 Viewer)

[Pedro123]

Hi,

I am in year 11, and recently we had to do a practical where we determined the type of bonds 6 UNKNOWN substances made. (Metallic bonds, ionic bonds, covalent molecular, covalent lattice). The tests were:

Solubility in cyclohexane

Solubility in Water

Conductivity as a solid

Conductivity as a solution

One substance we tested gave the results of Yes, Yes, No, No (Soluble in non-polar cyclohexane and polar water, non-conductive). After the experiment, it was revealed to us to be sugar.
Keeping in mind that I am in year 11, based on those results, what would the most appropriate classification of the bonds of sugar be? (Metallic, ionic, covalent molecular, covalent lattice). Also, assume that I just had those results, and didn't know it to be sugar.
Then exiting the year 11 sphere, if sugar is polar, why does it dissolve in non-polar cyclohexane? (I am curious, so if the answer involves non-year 11 chemistry, please post anyway).

Thanks!

 

[jazz519]

The way solubility is taught at school is very over generalised. What I mean is things are not always soluble just because they are both polar or both non-polar. We have to think more deeply about interactions are present (dispersion forces, dipole-dipole or hydrogen bonding). It is a good starting point to know if something is polar or non-polar and using that to predict solubility but it doesn't always work.

Cyclohexane is non-polar and therefore this means the main intermolecular force is dispersion forces. Sugar although it is polar and has dipole-dipole and hydrogen bonding, if you look up the structure of glucose you will see it has a large molecular mass. Things that have a large molecular mass also have a considerable amount of dispersion forces. So therefore the sugar can dissolve in the cyclohexane through the dispersion forces

 

[Pedro123]

The way solubility is taught at school is very over generalised. What I mean is things are not always soluble just because they are both polar or both non-polar. We have to think more deeply about interactions are present (dispersion forces, dipole-dipole or hydrogen bonding). It is a good starting point to know if something is polar or non-polar and using that to predict solubility but it doesn't always work.

Cyclohexane is non-polar and therefore this means the main intermolecular force is dispersion forces. Sugar although it is polar and has dipole-dipole and hydrogen bonding, if you look up the structure of glucose you will see it has a large molecular mass. Things that have a large molecular mass also have a considerable amount of dispersion forces. So therefore the sugar can dissolve in the cyclohexane through the dispersion forces

Stupid question, but why do stronger bonds/intramolecular forces/intermolecular forces mean that sugar dissolves in cyclohexane?

 

[jazz519]

Stupid question, but why do stronger bonds/intramolecular forces/intermolecular forces mean that sugar dissolves in cyclohexane?

You're sort of confusing the meaning of the terms. Intramolecular forces are chemical bonds which are inside a molecule (so we are looking one molecule), they have no effect on the intermolecular forces which are interactions that happen between molecules (so you need at least 2 molecules).

When you want to discuss like chemical properties you have to use intramolecular forces

When you want to discuss like physical properties you have to use intermolecular forces. Since dissolving something is just a physical change we use intermolecular forces only. It's all about do the intermolecular forces match up (and then this is generalised to being about polar-polar dissolves and non polar-non polar dissolves). So since they both have a large amount of dispersion forces it means that the cyclohexane and sugar are able to attract each other enough to dissolve. If that interaction is not strong enough that is what leads to things being insoluble

 

[Pedro123]

You're sort of confusing the meaning of the terms. Intramolecular forces are chemical bonds which are inside a molecule (so we are looking one molecule), they have no effect on the intermolecular forces which are interactions that happen between molecules (so you need at least 2 molecules).

When you want to discuss like chemical properties you have to use intramolecular forces

When you want to discuss like physical properties you have to use intermolecular forces. Since dissolving something is just a physical change we use intermolecular forces only. It's all about do the intermolecular forces match up (and then this is generalised to being about polar-polar dissolves and non polar-non polar dissolves). So since they both have a large amount of dispersion forces it means that the cyclohexane and sugar are able to attract each other enough to dissolve. If that interaction is not strong enough that is what leads to things being insoluble

So would this mean that say HF in solid form would dissolve in H2O since they both have hydrogen bonds?

 

[jazz519]

So would this mean that say HF in solid form would dissolve in H2O since they both have hydrogen bonds?

Yep that's correct

 

[Pedro123]

Cool, thanks!

 

[Pedro123]

Wait sorry - another question - does the solubility depend on the correlation of the force, i.e. all substances have dispersion forces, so is it only the substances with very big dispersion forces dissolve in substances that work in dispersion force? Is it that a substance, say cyclohexane, because it primarily bonds due to dispersion forces, it only dissolves other primarily dispersion force-based compounds, so not H2O or HF per se?

 

[jazz519]

Wait sorry - another question - does the solubility depend on the correlation of the force, i.e. all substances have dispersion forces, so is it only the substances with very big dispersion forces dissolve in substances that work in dispersion force? Is it that a substance, say cyclohexane, because it primarily bonds due to dispersion forces, it only dissolves other primarily dispersion force-based compounds, so not H2O or HF per se?

Yep that's exactly right. The reason why sugar could dissolve in water and cyclohexane is that, sugar has both hydrogen bonding and a large amount of dispersion forces

So with the water it dissolves through hydrogen bonding

With the cyclohexane through dispersion forces.

And yes as you said because cyclohexane only has dispersion forces it can't dissolve with water or HF because they have very low dispersion forces due to the small molecular mass and the hydrogen bonding dominates very strongly for water and HF

 

[Pedro123]

Perfect, thank you so much!