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.

1-boilingpoint

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.

2-boilingpoints

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:

3-boiling points 3

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).

5-boiling points

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{ 65 comments… read them below or add one }

Stacey

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

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james

Thanks, glad you found it useful.

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Nikola

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

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

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Apurv

Shubham, I guess h20 has higher bp than nh3. Rest are correct.

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Melebe

H2S has a much lower boiling point than water as well. H2S cannot form hydrogen bonds.

utkarsh kulshrestha

it depends on the extent of h-bonding not on strength nh3 can form 3 h bonds while h2o can only form 2

Sean

Thank you! This helped me a lot too!

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sophie emma

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

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

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

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

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

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Aayushi

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

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

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

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sumaiya

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

Glad you found it useful sumaiya.

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james

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

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

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

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

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

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

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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Apoorva

Ch3ch2ch2ch3 >ch2f2>ch3oh>ch3nh2

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Aayushi

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

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Emily Schubkegel

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

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max

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

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

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Kedii Rose

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

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

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

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

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james

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

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

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james

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

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

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

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

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

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

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

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

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jane

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

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JAYANTILAL

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

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James

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

Why does branching in alkanes lows the boiling point …?

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Kin

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

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Jeris Swiney

This does not explain boiling point.

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Bella

Determine the order with lowest boiling point on the left:

A = (CH3)3CCH2CH2CH2CH2CH2C(CH3)3
B = CH3CH2C(CH3)2C(CH3)2C(CH3)2CH2CH3
C = (CH3)2CHCH2C(CH3)2C(CH3)2CH2CH2CH3

All three have the same number of carbons and the same number of branch points. Help!

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Joseph

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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roopal

Please tell me why is the boiling point of Butan-1-ol greater than that of butan-2-ol?

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jackie r

What a great summary, and really reafable. I’ve been struggling to get to grips with Chemistry as part of a course I’m doing. This is the best site I’ve found, thank you for putting things in terms I truly understand!

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James

thanks Jackie!

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jackie r

Readable. Hurumph!

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shrividhya

Thanks James! extremely helpful. Now iam clear about the concepts. Please do keep posting such concepts.

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Belle

I am also taking Orgo I and came across a problem I couldn’t figure out. Given 3 alcohols : tert-butyl, sec-butyl, and n-butyl with their respective boiling point measurements at : 82C, 100C, and 117C. 4 carbons, same molecular weight for all them, same intermolecular forces? What creates the differences in their boiling point values? Is it the location of the hydroxyl groups?

Thanks for your help!

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James

The key factor will be surface area. The linear chains have a higher surface area than the branched alkyl groups (which are more “spherical”) and since van der waals forces are proportional to boiling point, the linear alkyl groups should have higher boiling point.

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MAYA

Thanks for this!

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john

I am having a little problem in understanding the melting point trends……why the following melting point increase is uneven……….formic acid(m.p 8.4) then ethanoic acid(m.p 16)…but then propanoic acid (m.p -21)…..then pentanoic acid (m.p -35)…………………It seems appropriate to understand boiling point trends which increase with molecular size as van der waals forces incrase but how does melting point trend works?

Thanks in advance!

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Jillian

Thank you! I’m studying chemistry at my college and this was very helpful! I normally hate and can barely understand chemistry but this made it very clear and easy to understand so thank you :)

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James

Glad to hear it Jillian! Thank you for your comment!

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Pablo

I understand branching increases M.P but I don’t totally understand d reason..

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Melebe

Why does NO2 have a greater boiling point than SO2? Both are polar, dipole-dipole interactions, but SO2 has a greater dipole moment, greater molar mass, and greater surface area. I know NO2 is a radical, does that affect boiling point?

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