Boiling point order

[joonkimdds]

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n-butane
isopantane
ethanol

The reason I get confused is because higher BP means less branch, longer carbon, and capability of forming H bond.

n-butane is shorter than isopantane but isopentane has branch. In this case, this is about branching vs. number of carbons.

And ethanol can form H bond so its BP must be high but it's very short so I don't know the order of BP among three of these.

Help~! 🙂

 

[tdkyun]

well.. think about it. Soju is ethanol and it is liquid at room temp. Butane is gas at room temp (you know those portable stoves that use butane gas).

 

[joonkimdds]

well.. think about it. Soju is ethanol and it is liquid at room temp. Butane is gas at room temp (you know those portable stoves that use butane gas).

so I guess that means H bond is the priority before when it comes down to branching and length.

How about n-butane and isopantane?

 

[hunterpostbacst]

the longer one beats the branched one because the size is the most affecting factor for this kind of questions.

 

[tdkyun]

General rule of thumb
C1-4 = gas
C5-17= liquid
C18- = solid
Even though branching reduces boiling pt, isopentane will have higher boiling pt than n-butane (I think).

 

[MTD52]

I thought that branching was more important than size in boiling point determination? Obviously the alcohol is the highest, but I think butane (no branching) would have the lowest.

Correct me if wrong

 

[bbwreckerz04]

General rule of thumb
C1-4 = gas
C5-17= liquid
C18- = solid
Even though branching reduces boiling pt, isopentane will have higher boiling pt than n-butane (I think).

Yea thats right butane will have a lower BP due to molecular mass. When mass is similar than you consider branching.

 

[MTD52]

So for BP determination the order of importance is:

1) H-bonds
2) molecular weight
3) branching


Is that right? I thought the 2nd and 3rd were the other way around.

 

[fifi1314]

Yes.
Therefore the boiling point from low to high is:
isopantane (branched) < n-butane < ethanol (H-bond)

 

[tdkyun]

Yes.
Therefore the boiling point from low to high is:
isopantane (branched) < n-butane < ethanol (H-bond)

Like I said isopentane will have a higher boiling point.

Ethanol — Boiling Point: 78.4 °C
Isopentane — Boiling Point: 28 °C
Butane — Boiling Point: -0.5 °C