# C-H bond energy in substituted methanes

What happens to the $\ce{C-H}$ bond strength in methane as the degree of halogenation is increased (e.g. from $\ce{CH4}$ to $\ce{CH3X}$ to $\ce{CH2X2}$...)

Also, what is the effect of varying the halogens from fluorine to iodine?

According to David R. Lide, ed., CRC Handbook of Chemistry and Physics, 90th Edition (CD-ROM Version 2010), CRC Press/Taylor and Francis, Boca Raton, FL, the C-H bond dissociation energies for increasing chlorination of methane are as follows:

$D\left(\ce{H-CH3}\right) = (439.3 \pm 0.4) \; \text{kJ/mol}$

$D\left(\ce{H-CH2Cl}\right) = (419.0 \pm 2.3) \; \text{kJ/mol}$

$D\left(\ce{H-CHCl2}\right) = (400.6 \pm 2.0) \; \text{kJ/mol}$

$D\left(\ce{H-CCl3}\right) = (392.5 \pm 2.5) \; \text{kJ/mol}$

The effect of different halogens in H-CX3 is shown below:

$D\left(\ce{H-CF3}\right) = (445.2 \pm 2.9) \; \text{kJ/mol}$

$D\left(\ce{H-CCl3}\right) = (392.5 \pm 2.5) \; \text{kJ/mol}$

$D\left(\ce{H-CBr3}\right) = (399.2 \pm 8.4) \; \text{kJ/mol}$

$D\left(\ce{H-CI3}\right) = (423 \pm 29) \; \text{kJ/mol}$

• Is this purely experimental or Is there a reason for the lowering of BE after each chlorine is substituted ? – Del Pate Mar 1 '15 at 13:40
• Sir, are these values experimental? – Walden Mar 2 '15 at 12:42
• @MarckTagapan The values are taken from the CRC Handbook, which is not the original source but a huge data collection. The reference given in the CRC Handbook is the Comprehensive Handbook of Chemical Bond Energies, by Yu-Ran Luo, CRC Press, 2007, which again appears to be a data collection. Thus, I do not know the original experimental data and derived calculations. – Faded Giant Mar 2 '15 at 13:06
• A remark in the CRC Handbook reads: ‘The D values in polyatomic molecules are notoriously difficult to measure accurately since the mechanism of the kinetic systems involved in many of the measurements are seldom straightforward. Thus, much lively controversy has taken place in the literature and is likely to continue for some time to come. We will continue updating and presenting our assessment of the most reliable BDE data every year.’ – Faded Giant Mar 2 '15 at 13:06