C-H bond enthelpy of halogenated hydrocarbons

[arrowhead1232]

Homework Statement:

Hi, so based on data collected from databases, I found a trend in the C-H bond enthalpy of halogenated hydrocarbons such as those with the formula CH3X



In this case, the C-H bond enthalpy increases following this order: CH3I (greatest C-H bond enthalpy)> CH3F > CH3BR >CH3CL (lowest C-H bond enthalpy)



It can be seen that the CH bond enthalpy generally increases further down the halogens group, but fluorine is an exception to this trend, does anyone know why?

Relevant Equations:

N/A

In this case, the C-H bond enthalpy increases following this order: CH3I (greatest C-H bond enthalpy)> CH3F > CH3BR >CH3CL (lowest C-H bond enthalpy)

 

[docnet]

I don't know the definitive answer, but there are some properties that may help you explain the case with F. The following data taken from Inorganic Chemistry by Miessler.


Halogen, electron affinity, $\Delta$ H of dissociation
F, 328, 158
Cl, 349, 242
Br, 325, 192
I, 295, 151
At, 270


A few notes about F from text:

it is likely that the weakness of the F-F bond is largely a consequence of the repulsions between the nonbonding electron pairs. the small size of the fluorine atom brings these pairs into close proximity when F-F bonds are formed. Electrostatic repulsions between these pairs on neighboring atoms result in weaker bonding and an equilibrium bond distance significantly greater than would be expected in the absence of such repulsions.

 

[Borek]

In general fluorine is notorious for being "different" from its heavier cousins.