Solubility

Only one rule to know here: Like polarity dissolves like polarity

Concept: Understanding “like dissolves like”. 

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Video Transcript

All right guys, so now I want to talk about a really easy topic that has to do with electronegativity and that is solubility.
I went ahead and took the luxury of pre-drawing this little diagram for you. It's very scientific. I thought that it would help you guys understand the rule of solubility, which is just that like dissolves like.
You might have heard this in lab before. All it means is this, this has to do with polarity. Remember that we learned how to figure out if molecules had a net dipole. That's all there is to it. We're going to figure out, okay, what we're dissolving, is it going into something that has the same polarity as itself.
This is a very common scene from Friday night. Maybe this was a few nights ago for you. You're pouring a bottle and you're pouring it into a cup of water. And you've got your ethanol little molecules spilling out and they go into the water molecules which are right down here. And you're not thinking for a second that they're going to split apart and you're going to have alcohol on the top and water on the bottom. That would be really weird.
In fact, what happens is that they just mix together and you can't even tell the difference, except you can tell the difference the next morning. You know that, obviously, you weren't just drinking water. That has to do with the fact that both have the same polarity or similar polarity.
Think about it, water. Let's just expand this a little bit. Water has a net dipole, right? We said that the net dipole was pretty strong. Well, in the same way, ethanol also has a net dipole. In this case, I kind of drew it so that the net dipole would face the other direction, but it doesn't matter. It doesn't really matter what direction it's facing because obviously, I could rotate that alcohol. The important thing is that they both have a strong net dipole. Since they're both polar, they're going to dissolve into each other.
That's the entire concept between like dissolves like. 

Want a vodka tonic right about now?  The reason they mix together is due to similar polarity between molecules. Not that you should know about this if you are under the age of 21! Duh.  

Concept: Introducing common solvents and other molecules in organic chemistry. 

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Video Transcript

I'm going to use this opportunity to teach you about some really weird molecules that you're going to see the rest of orgo and maybe this will give you an opportunity to get a little bit more familiar with them.
Let me go ahead and introduce these molecules first.
We've got pyridine over here. He's like an all-star. Pyridine looks like a benzene ring. I know we haven't been over that yet, but that's a benzene ring with a nitrogen. Very commonly used in reactions.
Then on top we've got DMSO. DMSO looks a lot like acetone except that it has a sulfur instead of the carbon.
Then we've got THF, it's called tetrohydrofuran. You don't need to know the full name, but it's basically just an oxygen in a five-membered ring.
Then we've got carbon tetrachloride. That one is a carbon with four chlorines all around. The polarity is going to be interesting for that one.
And then we've got basically just a sulfur compound. Basically, that's called a thiol group. And then we've got nitrogen, which has basically three carbon groups around it. For these two we're going to wind up having to draw Lewis structures to figure out what the solubility is.
So what I want you guys to do is go through all of these and tell me if you would expect them to dissolve in an aqueous or be miscible in an aqueous solution. Now, what aqueous means, this is the tricky – it's not that tricky, but aqueous just means water.
So what I want you guys to do is think about the polarity of water, think about is it polar or is it apolar and then look for dipoles on these six molecules. Once you think you've found if they have a dipole or not, a net dipole or not, then you can draw your conclusion is this soluble or is this not going to be soluble.
Now for the last two, like I said, you are going to have to draw proper Lewis structures in order to know if it has a dipole or not because it can get a little tricky if you don't draw the Lewis structure.

An aqueous solution is one with water. That’s pretty much it.  The following molecules are commonly used in organic chemistry- so it’s a good idea to know what their solubilities are.

Problem: Would you expect the following molecule to be miscible in an aqueous solution? 

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Problem: Would you expect the following molecule to be miscible in an aqueous solution? 

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Problem: Would you expect the following molecule to be miscible in an aqueous solution? 

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Problem: Would you expect the following molecule to be miscible in an aqueous solution? 

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Problem: Would you expect the following molecule to be miscible in an aqueous solution? 

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Hint: Draw the Lewis structure before deciding!

Problem: Would you expect the following molecule to be miscible in an aqueous solution? 

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Hint: Draw the Lewis structure before deciding!

Solubility Additional Practice Problems

 

(i) Which compound is less soluble in water: (a) CH 3(CH2)4CH2OH or (b) CH3(CH2)3CH2OH? (Choose one; give the letter.)

(ii) Which compound is releases less heat when it is burned: (a) CH 3(CH2)4CH3 or (b) (CH3)3CCH2CH3? (Choose one; give the letter.)

(iii) Which compound has the higher boiling point: (a) CH 3(CH2)3CH3 or (b) (CH3)2CHCH(CH3)2? (Choose one; give the letter.)

(iv) A Lewis base can also be known as (a) an electrophile, (b) a nucleophile, or (c) a spectator ion ? (Choose one; give the letter.)

(v) Which one is the most basic: (a) (CH3)3CO ; (b) CH3CH2O; or (c) (CH3)2CHO ? (Choose one; give the letter.)

(vi) Which compound is more oxidized: (a) CH3CO2H or (b) HOCH2CH2OH ? (Choose one; give the letter. If they are the same, give both letters.)

(vii) Which one is the molecular formula of a neopentyl group: (a) C5H9 ; (b) C5H10; (c) C5H11 ; or (d) C5H12 ? (Choose one; give the letter.)

(viii) Which compound is an alcohol that is not miscible with water: (Choose one; give the letter.) (a) CH3CH2OCH3; (b) CH3CH2CH2OH; or (c) CH3CH2CH2CH2OH ?

(ix) What type of alkyl group is a 1-methylcyclohexyl group: (a) primary ; (b) secondary ; (c) tertiary ; or (d) quaternary (Choose one; give the letter.) 

(x) Which compound is more soluble in water: (i) CH3CH2CH2OH or (ii) CH3CH2CH2Cl? (Choose one; give the letter.)

 

Watch Solution

Which compound is more soluble in water?

a) CH3(CH2)2CH2OH

or

b) CH3(CH2)2CH3

Watch Solution

Select the compound that is soluble in water and explain why.

 

Watch Solution