Which would have the highest boiling point?

Question

out of:
pentane
2-methylbutane
2,2-dimethylpropane

Would it be 2-methylbutane because it is the only polar compound?

So branched means having more branches?

that makes a lot of sense, thanks.

Answer 1

Pentane

It depends on which compound has the strongest intermolecular forces. In the case of hydrocarbons the intermolecular forces that are present are dispersion forces.
In such cases:
"The more branched a molecule is, the more "spherical" it is, and the smaller surface area it will have - since dispersion forces increase with surface area, the more branched molecules will have a lower boiling point. You may notice that the trend in the melting points is rather different - this is because the longer a branch is, the more potential it has to flop around and disrupt the regular crystalline lattice of frozen pentane, giving a liquid state."

Answer 2

Ooops! None of the three isomers are polar! They are all C5H12. One is linear, one is a "Y" and one an "X". These are the only ways to connect only 5 Carbon atoms together (ignoring rings - including them would REALLY complicate things!). The answer has to do with geometry. It wasn't until I was in school (long long ago!) that we were even sure which was more electronegative: carbon or the hydrogen attached to it. What I'm saying is that the differerence in electronegativity between the C and H is so tiny and generally can be ignored, especially in alkanes. (not so much in alkenes and forget about alkynes and more exotic chemical groups!). The geometry (steric considerations) means that the "X" is most like a ball (sphere) and the linear alkane the least. So, why does that matter? Why is a liquid a liquid and why does it take energy (heat) to convert it into a gas? Because of the "secondary" forces between atoms and molecules. These are the temporary changes in the "location" of the electrons which leads to attraction between atoms, even if they have exactly the same polarity. (called in the old days dipole and van der waals forces among others) Even the nobel gases and hydrogen can be liquified So, we can pretty much ignore the difference between the CH polarity in the
-CH3, >CH2, >CH- and >C< groups, as far as molecular polarity goes (remember boiling point is not a number like pi, known to a zillion decimal places - it is an average (like atomic weight) and will vary). It even depends on where the carbon and hydrogen come from (a wee little bit different in isotope ratios if from CO2 the air or from oil in the ground!). Anyway, the most important thing is that mushing 5 balls together into an "X" shape (like 2,2-dimethylpropane) leaves less surface exposed than if you mush 5 balls together in a line (like one of those toy catepillars). How much surface of each ball is exposed and [ here's the key point] can "touch" (interact) with other molecules. Since the linear will be best able to touch (come into contact), it will take a little bit more energy to boil.
I pondered (for more than a moment) if a mixure of the three isotopes could boil at an even higher temperature. Like mixtures can have higher densities - you can often get more stuff of different sizes into a space than stuff with the same shapes and sizes. This, I think is more subtle a question. I believe the answer is "no" because of the way boiling works - one molecule at a time - and at a point where the molecules are bumping, crashing, dancing around - not the same as melting point - but it may depend on the complexity of the shapes involved. For these simple shapes, I don't think so - I'm pretty sure...
Anyway I bring this up because I didn't really answer the question! You asked which has a higher boiling point and I answered that it would take more energy to boil n-pentane. But IF it also took more energy to bring n-pentane to the temperature that dimethylpropane boils at then it still could have been that n-pentane boils at a lower temperature. Lucky for you that the pentanes have very similar heat capacities so that the difference in boiling point (n-pent is 35-36C, iso-pent is 30C, and dimethylpropane = ?) is due to the shapes of the isomers. How would you have known that? Hmm, I wouldn't be surprised if your teacher hadn't considered how hard the question really was! Heat capacity has to do with both atomic weight, bonds, and polarity. So for molecular isotopes the heat capacities are going to be very similar, don't you know, because those three things a very very similar. Man, it seemed like such a simple question....But look what it took me to answer it.

Answer 3

Pentane. They are all nonpolar compounds, but the elongated shape of pentane lets it associate with other molecules most effectively.