Boiling Point Comparison

[sfoksn]

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2 methyl 3 pentanol vs. 3-pentanol.

Which one has higher boiling point?

I thought it woudl be 2 methyl 3 pentanol because it has hydrogen bonding and 1 more methyl group leading to greater dispersion force than just 3-pentanol?

 

[dentalpath]

Higher BP will be 3 pentanol, increased branching in 2 methyl 3 pentanol decreases BP...but increases melting point (due to better chances of forming crystals) so if the molecule has same number of carbon atoms, look for branching. Branching lowers BP while straight chain carbons have higher BP...

 

[ECUpirate19]

My vote would be for the 3-pentanol because branching lowers the bp I do believe.

 

[herkulease]

Its untrue though. I looked up the data.

3-pentanol boiling point is 115 degrees C

2-methyl-3-pentanol is 128

 

[UndergradGuy7]

Its untrue though. I looked up the data.

3-pentanol boiling point is 115 degrees C

2-methyl-3-pentanol is 128

The trend is a generalization? Not all data will be correct, but it will be correct in most cases?

I think the boiling point in the real data does not follow the trend because maybe the extra methyl group adds to the weight of the molecule. Higher weight means higher boiling point?

 

[DATkiller]

When comparing boilong points and melting points, consider branching in isomers only. In this case 2methyl-3-pentanol has a higher boiling point.

 

[Cool Beans]

Only resort to using branching as the basis of comparison for isomers (i.e. same number of carbons). Otherwise the compound with more carbons (everything else being equal) typically has the higher boiling point as is the case here.

 

[dentalpath]

I agree...pentanol only has 5 carbons but 2 methyl pentanol has 6...hence higher boiling point...sorry for the confusion before..