[quadsofdgods]

It seems that alkyl fluorides do not follow the trend of increasing boiling points (when you increase the number of halides in the compound) that all other alkyl halides do.

CH3CH2-F  -32*C
CH3CH-F2  -25*C
CH3C-F3    -47*C
(All temps are negative)



I can understand that CF3CF3 would have the lowest B.pt of the ethane`s due to the very very low induced dipole -induced dipole forces (London forces).

It seems that it goes up then goes down again. Why is this? Is there even truth to this trend?
I couldn`t find anything on the internet and i have extracted the info from teacher created notes not from a book.

[orgopete]

It seems that alkyl fluorides do not follow the trend of increasing boiling points (when you increase the number of halides in the compound) that all other alkyl halides do.

CH3CH2-F  -32*C
CH3CH-F2  -25*C
CH3C-F3    -47*C
(All temps are negative)

I can understand that CF3CF3 would have the lowest B.pt of the ethane`s due to the very very low induced dipole -induced dipole forces (London forces).

It seems that it goes up then goes down again. Why is this? Is there even truth to this trend?
I couldn`t find anything on the internet and i have extracted the info from teacher created notes not from a book.

Let me add to the list.
CH4     -161.6°C
CH3F     -78.2°C
CH2F2      -52°C
CHF3     -82.1°C
CF4     -127.8°C
CF3CF3  -78.2°C

This is data from wikipedia. Although not helpful for the poster, it too reveals a similar trend. If the question is, "What does explanation does the poster's professor expect?", then I cannot answer. Similarly, I find London forces rather nebulous, so whether it suffices to explain this or not, I don't know.

My preferred explanation relates to the properties of fluorine (and oxygen). I don't know of a literature reference corroborating this, but simple hydrogen bonding would explain the phenomena. Although I use the word simple, I only mean that in the sense of the electrostatic interactions, not in the strength. The more acidic a hydrogen is, the more easily it can be attracted to a pair of electrons. The more the electrons are available, the more they can be attracted to protons. The more electrons are available, the less electron withdrawing they are. Water is surprisingly high boiling as it matches two protons and two electron donors, a good balance of properties. I think fluorine has similar properties and would explain the bp.