mp/bp of alcohols question (1 Viewer)

[mandemindiguise]

why is it that branching in alcohols result in lower bp but greater mp?
i somewhat understand that the compact structure of branched alcohols allow it to tight more packly together, where it requires greater energy to break bonds, but why doesnt this also apply with boiling point?
also this is my first message, is this the correct place to ask this

 

[wizzkids]

Well that's an interesting hypothesis. Let's see if there is any evidence from the chemical data. You would need to compare molecules with approximately the same molecular weight.
If you compare a range of primary, secondary and tertiary alcohols with 4 carbons, the trend is not clear-cut:
I have compared iso-butyl alcohols with molecular weight 74.1 and I don't think there is a clear trend in m.p. and b.p. You could also compare the difference between the m.p and b.p, and the differences are 207.5^oC, 214.2^oC , 215.9^oC and 56.9^oC. The one that definitely stands out is the tertiary alcohol.

 

[Luukas.2]

why is it that branching in alcohols result in lower bp but greater mp?
i somewhat understand that the compact structure of branched alcohols allow it to tight more packly together, where it requires greater energy to break bonds, but why doesnt this also apply with boiling point?
also this is my first message, is this the correct place to ask this

Firstly, welcome @mandemindiguise, and yes, this is a suitable place for your question.

Branching in most organic compounds leads to decreases in boiling point as the resulting molecular shapes are generally less efficient to pack together, increasing average intermolecular distances and thus weakening dispersion forces. Since the strength of intermolecular interactions dictate the energy barrier that must be overcome for vapourisation, this weakening of intermolecular interactions correlates with a decrease in boiling points.

We can see this in the data that @wizzkids posts:

View attachment 41490

• The highest BP is the primary alcohol 1-butanol, with a no branching structure.
• The next highest is 2-methyl-1-propanol, with a methyl branch but also a terminal hydroxyl. This provides better access to the hydroxyl than in 2-butanol, where the OH is itself the branch. (Note that the structure shown is 2-pentanol rather than 2-butanol).
• The tertiary alcohol, with both a non-terminal hydroxyl and a chain branch, has the lowest of the boiling points.

Trends in melting points are much more difficult to predict as the intermolecular interactions are not being completely overcome, as does occur when separating molecules in the gas phase. Melting requires sufficient energy for net motion of molecules relative to each other and so depends on steric factors like bulk and shape, as well as energetic ones, plus the extent of weakening of intermolecular interactions that is required is not constant.

In general, it is best to avoid making MP predictions in HSC chemistry, beyond the most general ones, such as:

• MP generally increases with stronger intermolecular interactions - so MP(methanol) > MP(ethane)
• MP generally increases with molar mass due to increases in dispersion forces - so MP(decane) > MP(pentane)