Figuring out the order of boiling points is all about understanding trends. The key thing to consider here is that boiling points reflect the strength of forces between molecules. The more they stick together, the more energy it will take to blast them into the atmosphere as gases.

There are 3 important trends to consider.

  1. The relative strength of the four intermolecular forces is: Ionic > Hydrogen bonding > dipole dipole > Van der Waals dispersion forces. The influence of each of these attractive forces will depend on the functional groups present.
  2. Boiling points increase as the number of carbons is increased.
  3. Branching decreases boiling point.

Let’s have a closer look:.

Trend #1: The relative strength of the four intermolecular forces .

Compare the different butane alcohol derivatives shown below. Molecules of diethyl ether, C4H10 O, are held together by dipole-dipole interactions which arise due to the polarized C-O bonds. Compare its boiling point of (35 °C)with that of Its isomer butanol (117 °C). The greatly increased boiling point is due to the fact that butanol contains a hydroxyl group, which is capable of hydrogen bonding. Still, the attractive forces in butanol pale in comparison to those of the salt sodium butoxide, which melts at an extremely high temperature (well above 260 °C) and actually decomposes before it can turn into a liquid.

Then think about butane, C4H10, which contains no polar functional groups. The only attractive forces between individual butane molecules are the relatively weak Van der Waals dispersion forces. The result is that butane boils at the temperature at which water freezes (0° C), far below even that of diethyl ether.

Moral of the story: among molecules with roughly similar molecular weights, the boiling points will be determined by the functional groups present.

You could tell a similar tale for the similar amine and carboxylic acid isomers shown below.

For a previous discussion of the 4 intermolecular forces, see here. For the reference in Reusch’s textbook, see here.

Trend #2 – For molecules with a given functional group, boiling point increases with molecular weight.

Look at the dramatic increases in boiling points as you increase molecular weight in all of these series:

Here’s the question: How, exactly do intermolecular forces increase as molecular weight increases?

Well, the key force that is acting here are Van der Waals dispersion forces, which are proportional to surface area. So as you increase the length of the chain, you also increase the surface area, which means that you increase the ability of individual molecules to attract each other.

On an intuitive level, you could compare these long molecules to strands of spaghetti –  the longer the noodles, the more work it takes to pull them apart. As the chain length increases, there will be regions where they can line up next to each other extremely well.

Individually, each interaction might not be worth very much, but when you add them all up over the length of a chain, Van der Waals dispersion forces can exert tremendous effects.

3. Symmetry (or lack thereof).

This is another byproduct of the surface-area dependence of Van der Waals dispersion forces – the more rod-like the molecules are, the better able they will be to line up and bond. To take another intuitive pasta example, what sticks together more: spaghetti or macaroni? The more spherelike the molecule, the lower its surface area will be and the fewer intermolecular Van der Waals interactions will operate. Compare the boiling points of pentane (36°C) and 2,2-dimethyl propane (9 °C).

It can also apply to hydrogen bonding molecules like alcohols – compare the boiling points of 1-pentanol to 2-pentanol and 3-pentanol, for instance. The hydroxyl group of 1-pentanol is more “exposed” than it is in 3-pentanol (which is flanked by two bulky alkyl groups), so it will be better able to hydrogen bond with its fellows.

In summary, there are three main factors you need to think about when confronted with a question about boiling points. 1) what intermolecular forces will be present in the molecules? 2) how do the molecular weights compare? 3) how do the symmetries compare?

One last quick question for the road (see comments for answer).

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{ 46 comments… read them below or add one }

Stacey October 15, 2011 at 5:45 am

Thank you! I am studying Chemistry at university, this has helped a lot!!

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james October 15, 2011 at 12:57 pm

Thanks, glad you found it useful.

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Nikola January 14, 2012 at 3:19 pm

Can somebody help me about this? Compare boilnig points of
a) NH3 b)H2O2 c)H20
a)CF4 b)CCL4 c)CH4
a)H2O b)H2S c)SIO2

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shubhamkothari August 19, 2014 at 10:54 am

b>a>c
b>a>c
c>b>a

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Sean June 11, 2014 at 10:21 pm

Thank you! This helped me a lot too!

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sophie emma October 30, 2011 at 11:38 am

1.ionic bonding is not an intermolecular force; it is an intramolecular force? network covalent, ionic, metallic… -inorganic compounds!
2. there is something called optical isomerism. R and S enantiomers’ mp and racemate’s mp are different; important stuff if you’re mastering organic chem.

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james October 30, 2011 at 5:39 pm

why can’t it be both? I’m thinking mostly of salts of organic compounds like Me4N+ Cl- which have very high melting points as opposed to network solids like NaCl or other inorganic compounds which are not really “molecules” per se.
thanks for mentioning the R/S as well, that’s an important point!

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Ash February 7, 2012 at 5:11 am

I’m having a problem with the branching rule you pointed out in this article. Wouldn’t branching increase the boiling point as it leads to the molecule’s shape being more spherical and tightly packed. So with a tightly packed molecule, it takes more energy to take it apart thus, branching should increase b.p.

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james February 7, 2012 at 5:38 am

More tightly packed would give it a higher melting point. But the lessened surface area would result in a lower boiling point due to lessened Van der Waals interactions.

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Aayushi July 29, 2012 at 12:11 pm

how can u compare , mp and bp , on same molecule,?????? mp is given 4 solids , and bp 4 liquids …..

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Aayushi July 29, 2012 at 12:10 pm

i think there is a conceptual problem ,,,, b.p doesn’t means the attraction ,,, or packing of a molecule,,, its a physical property,,, hence depends on the interaction b/w the MOLECULES OF A COMPOUND ,,,now,,, as it is spherical in nature ,, hence the area of contact decreases ,,,,,and therefore,,,, interaction becomes weaker ,, and hence b.p decreases ………………
for instance —– we have a glass of water,, in which there r many H2O,, molecules,,, here we r not talking abt ,,,, h-o bond,, its abt the interaction b/w 2 H2O molecules ,,,,,,,wat we generally says , intermolecular forces ……..

hope u r now, clear with ur concept …………

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TV February 12, 2012 at 12:27 am

How does branching affect melting point?
I get the fact that branching decreases bp because of the decrease in surface area, hence less opportunities for Van de Waals interactions. However, how does it work for melting point? When it is branched it is harder to stack, right? Then, a lower melting point?
Thanks! :)

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james February 15, 2012 at 1:59 am

I would think that branching leads to lower melting points, for exactly the reason you mentioned.

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sumaiya February 15, 2012 at 4:41 pm

this helped me a lot to understand.i have visited many sites before but it is the most useful 1….thanks

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james March 12, 2012 at 2:45 pm

Glad you found it useful sumaiya.

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james March 12, 2012 at 2:44 pm

Answer to question in post: the amine with the NH2 will have highest boiling point (most hydrogen bonds) followed by NH and then the molecule on the farthest right.

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keith May 5, 2012 at 8:49 pm

gen chem 1 sin city. this site is really helpful. it’s like playing tennis with a much better player; you can only learn and get better from a pro if ya want too! Thanks! Great stuff

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red May 6, 2012 at 10:06 am

thanks i found answers to my questions.. one thing ,, how does functional group affects boiling point in most understandable way ? thanks

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james May 6, 2012 at 6:35 pm

If you had to focus on one thing, I’d look for hydrogen bonding groups such as OH and NH. This will increase the boiling point significantly.

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Abiodun June 10, 2012 at 7:03 am

Thanks. This piece is very educative.
(1) Why does butanol (bp- 117) have a higher boiling point than butanamine (bp-77.8)
(2) How does intramolecular Hydrogen bond affect the boiling point of an organic molecule

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james June 15, 2012 at 4:16 pm

Good question. It is likely due to the greater electronegativity of oxygen (3.4) versus nitrogen (3.0) leading to a larger dipole, which means that the molecules will have a greater force holding them together.

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Aram June 30, 2012 at 6:34 am

Can somebody help me with boiling points of these substances, (just comparison)
CH3 NH2, CH2 F2, CH3 OH, CH3 CH2 CH2 CH3

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Aayushi July 29, 2012 at 11:55 am

n-Butylamine>Diethylamine>N,N-Dimethyl ethylamine

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Emily Schubkegel August 28, 2012 at 11:58 pm

Post more examples! Students would refer to them for practice. Good site!

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max September 15, 2012 at 10:41 am

This is an amazing reference that simplifies properties affecting boiling point, which can be confusing when it comes to the many factors that can come into play!! It does a much better job than my professor, who’s really good, or the book were able to do in demystifying all of the above. Thank you to whoever made it!!

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Becky Hdz October 24, 2012 at 12:58 pm

I’m cramming for the OAT and this table helped a LOT! Thanks!

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Kedii Rose January 27, 2013 at 11:51 am

this was quite insightful, but i have a problem i was given a question to compare pentane and diethyl ether. Based on the information i have seen it looks like pentane has a higher boiling point than di-ethyl ether, based on actual BP but your information tells me that it should be otherwise as diethyl ether has dipole-dipole interactions and pentane has van der waal forces. Could you clear up for me which should actual have the higher BP and give a reason for the answer?

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james January 27, 2013 at 10:48 pm

Excellent point! I don’t have a good answer as to why pentane’s BP (36 deg C) is higher than Et2O(34.5 deg C). Interestingly the boiling point of tetrahydrofuran (C4H8O) is 66 deg C, because the ring gives it a permanent dipole moment.

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Sharon April 7, 2014 at 7:16 am

pentane has higher bp due to more no. of C atoms.
but butane has lower bp than diethyl ether due to the reason you’ve mentioned

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Amer June 7, 2013 at 10:56 am

Why CCl4 has greater boiling point than CH4 ?? ( CH4 has more H bonds )

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james June 8, 2013 at 7:21 pm

Higher molecular weight is one thing. C-H does not count as a “hydrogen bond’ because carbon is not electronegative enough. Only O-H, N-H and F-H bonds participate in hydrogen bonding.

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Elizabeth June 7, 2013 at 8:10 pm

I don’t understand why octane has a higher BP than chloropentane.

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james June 8, 2013 at 7:20 pm

You’re comparing two different variables there; chain length (8 carbons vs. 5 carbons) and polarity (chlorine is electronegative). It’s hard to predict how boiling points are affected when you change more than one variable.

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Russ June 23, 2013 at 4:40 pm

If we have a molecule with hydrogen bond and one alkane. If alkane has greater molecular weight than hydrogen bond, which on has highest boiling point?

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Lil August 12, 2013 at 11:24 am

Hi thanks for this info helps a lot but I have a question. It concerns organic compounds. Say you have a ketone such as heptanone. So the boiling point of this compound should be relatively high because it has a large surface area and it is a polar molecule so there are dipole -dipole forces present. Now take a alcohol that is a shorter chain such as propanol. Alcohol should have a higher boiling point because it has a hygrogen bond. But the propanol molecule is a much shorter chain so it has a smaller surface area than the heptanone. So how can we work out which one has a higher boiling point? (In this case heptanone has a higher boiling point than propanol but I would like an explanation as to why.)

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James Ashenhurst August 13, 2013 at 12:11 pm

Hi Lil, it’s difficult to compare two variables at once because it’s hard to gauge the relative importance of each variable from first principles alone [this is what experiments are for]. In the example mentioned, comparing heptanone and propanol is difficult because we’re comparing two variables at once: chain length and the polarity of the functional group. The key takeaway from this article is that if all variables except one are the same, you can predict boiling points based on these concepts. You’ll find that exams will generally test one variable too.

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nick August 23, 2013 at 9:05 pm

do we only can compare the molecular size and molecular shape for the similar molecules or we can compare the size and shape for molecules with different functional groups ? for example can i compare the size and shape of pentanoic acid with heptane ?

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James Ashenhurst September 4, 2013 at 2:40 pm

It’s tricky to compare two variables (chain length and functional group) at once. Generally you just want to know the impact of each variable individually.

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Grace August 29, 2013 at 1:52 am

Thank you so much, i’ve been looking for something clear and easy to understand about trends in boiling points and this has answered so many of my questions! Keep up the fantastic work.

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Brooklyn November 5, 2013 at 3:02 pm

Does vander walls force mean the same thing as dipole induced dipole force?

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jane November 25, 2013 at 3:42 pm

which will have the higher boiling point, C6H14 or C2H5OH and why ?

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JAYANTILAL January 26, 2014 at 7:50 am

what if we have different compounds with huge difference in milecular weight

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James January 27, 2014 at 10:29 am

In this case we’d be comparing two different variables (chain length *and* polarity of functional groups) and it is difficult to predict which factor is more important. I would look for examples of compounds you are curious about and compare their known melting points/boiling points.

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Danny January 31, 2014 at 1:08 am

Why does branching in alkanes lows the boiling point …?

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Kin June 15, 2014 at 6:58 pm

Branching reduced the ability of a molecule to ‘stack,’ making its state less solid and its boiling point lower.

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Joseph October 1, 2014 at 2:25 am

I cannot not leave a comment. Thank you so much :)) I’m currently taking up Organic Chemistry and this has helped a lot!

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