Chromatography retention time question

[TT0]

Homework Statement

For this chromatography graph there are 2 peaks:

One of them is propanoic acid and another one is pentanoic acid. The more polar compound will have a longer retention time. Which peak belongs to which compound.

Homework Equations

The Attempt at a Solution

The answers says that the first peak is pentanoic acid and the second peak is propanoic acid. But I thought pentanoic acid would be the second peak as it has stronger intermolecular forces because it has a larger electron cloud. Both have the carboxylic acid functional group so they should be the same. Could someone tell me if I am right? And if not, why am I wrong.

Cheers!

 

[DrClaude]

The answer is in the question:

The more polar compound will have a longer retention time.

So, what can you say of the dipole moment of each molecule?

 

[TT0]

Im not sure what you mean but I think that pentanoic acid has a greater dipole moment than propanoic acid.

Thanks

 

[DrClaude]

Im not sure what you mean but I think that pentanoic acid has a greater dipole moment than propanoic acid.

It does not. A simple hand-waving explanation goes like this: longer carbon chains are "softer" with respect to the distribution of the electrons. In other words, the longer the chain, the smaller value of the partial charge δ⁺ or δ^-, leading to a smaller dipole moment.

 

[epenguin]

Is that al the information you were given?

I find it unreal, unhelpful to a student, and almost misleading and unscientific that you are supposed to reason on just the properties of the substances being separated with no consideration of the nature of the solid substrate nor the eluant solution. (It might reasonably be a hydrophilic substrate and more hydrophobic eluant, I presume acid.)

I question whether you can say the distribution of charge is significantly different in between propanoic and pentanoic acid. It is generally held that inductive effects if this is the same thing Dr. Claude talking about, do not extend significantly beyond two C atoms. At any rate if you try to explain the acid pK_as by the inductive effect, the difference between propanoic and pentanoic is small. And in the wrong direction!

So I would guess a more likely explanation (assuming the chromatography is as I mentioned above) is that pentanoic simply has a larger nonpolar group than propanoic that can interact more favourably with a nonpolar mobile phase i.e. eluant..

eluant? eluent?