How can a solution's boiling point decrease during distillation?

[louise82]

I just finished a lab where I distilled a mixture of 2-methyl-2-butene and 2-methyl-1-butene in a dilute solution of sulfuric acid. The boiling points of the compounds respectively are 38.57C and 31.6C.

The bp range ended up being a constant 32C. How is this possible?

Shouldn't it be higher than either of the alkene bps?

What could cause the boiling point of a solution to decrease?

 

[chemisttree]

They have formed an azeotrope. Google 'azeotrope'.

 

[pkleinod]

I just finished a lab where I distilled a mixture of 2-methyl-2-butene and 2-methyl-1-butene in a dilute solution of sulfuric acid. The boiling points of the compounds respectively are 38.57C and 31.6C.

The bp range ended up being a constant 32C. How is this possible?

Shouldn't it be higher than either of the alkene bp?

What ended up with a boiling point of 32 C, the residue (alkenes and sulphuric acid) or the distillate (alkenes only)? What were the mole fractions of the alkenes?

The boiling point of a binary solution changes with composition. For type I solutions, the bp changes monotonically with composition; for type II there is a minimum bp (at constant pressure) at some composition; for type III there is a maximum.
Types II and III can form azeotropes, i.e. mixtures such that the composition of the liquid and the vapour at the boiling point is the same. For type II solutions, the residue ends up to be one of the pure components and the distillate has the composition corresponding to the minimum bp, which is less than the bp of either of the two components. For type III solutions, the residues tend twoard the maximum boiling mixture while the distillates tend toward the pure constituents, and the constant boiling point is higher than that of each
of the pure components.

If you are talking about a binary solution only (i.e. without the sulphuric acid), then either you have a type II azeotrope and your measurement of the constant boiling temperature is wrong (i.e. not accurate enough, because it should lie below 31.16 C) or something else is going on.

Not knowing exactly what you did, I can only speculate: perhaps there is a chemical reaction converting one alkene to the other. (Sulphuric acid can add to the double bond.) Or, if you are talking about the boiling point of the mixture including the sulphuric acid, then you would have a ternary system, which is more complicated.

 

[louise82]

The the purpose of the lab was to dehydrate 2-methyl-2-butanol with H2SO4 and heat. The reaction was taking place as the distillation was occurring.

From what the grad student in the lab told me, I was to watch the temp and stop the distillation when there was a sharp rise. It stayed at a constant 32 the whole time.


So this is a ternary system?

 

[chemisttree]

32 is not much different than 31.6 y'know.

 

[pkleinod]

The the purpose of the lab was to dehydrate 2-methyl-2-butanol with H2SO4 and heat. The reaction was taking place as the distillation was occurring.

From what the grad student in the lab told me, I was to watch the temp and stop the distillation when there was a sharp rise. It stayed at a constant 32 the whole time.


So this is a ternary system?

In the light of this information, you can ignore most of what I said in my previous post about binary and ternary mixtures: I thought you were distilling a mixture of these alkenes but I see now that you are synthesizing them. I have no explanation for the constant boiling temperature of 32 C except to say that maybe you didn't wait long enough. After all, there is water and sulphuric acid in your reaction flask (if your preparation method was similar to that in the attachment), so that eventually the temperature must rise after the organic products have come over. Did you analyse the products (alkenes) to see what did happen?