higher bp

[aviary17]

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Hey guys,

I had a Kapl questions asking which is the higher bp, heptanoic or octanoic acid? I thought it should be heptanoic because there are less carbon atoms in the way to screw up the H-bonding of the carboxylate group, so more H-bonding to go around. While the octanoic, on the other hand, has more carbons in the way which don't contribute anything significant to raising the bp (not compared to the carboxylate anyway). However, Kaplan says the octanoic wins because it has a higher MW so more Van der Waals... Does anyone see what I'm saying because I still think I'm right. I remember we had questions like this in orgo and the correct answer was usually the shorter molecule for the reasons mentioned above. What do you think?

Thanks!

 

[spyderracing32]

The long chain is not sterically hindering. If it were say 2,2-dimethylhexanoic acid rather than just octanoic acid you would be right. The chains, however, aren't really blocking the the h-bonding sites.

 

[amar21]

longer aliphatic chain = higher bp

 

[cornpops21]

The 2 molecules are roughly identical in their structure, so the factor distinguishing their boiling points would be their MW. So octanoic acid has the higher bp. This is because as you increase the number of carbons in the chain, you also increase the amount of Van der Waals forces and London dispersion forces between atoms, thereby increasing the bp.

Hope that helps.