# Why does the C−H bond dissociation energy vary in a homologous series of primary alcohols?

Specifically, for the primary carbon atom in the alcohol. Here is bond dissociation energy (BDE) data from chapter three of Luo's Comprehensive handbook of chemical bond energies [1] (boldface refers to dissociated atom):

$$\begin{array}{llc} \hline \text{Compound} & \text{The broken bonds} & \mathrm{BDE}/\pu{kJ mol-1} \\ \hline \text{Methanol} & \ce{\textbf{H}-CH2OH} & 401.92±0.63 \\ \textit{n}\text{-Ethanol} & \ce{\textbf{H}-CH2CH2OH} & 410.0±8.4 \\ \textit{n}\text{-Propanol} & \ce{\textbf{H}-CH2CH2CH2OH} & 406.3±8.4 \\ \hline \end{array}$$

Many websites like Chemistry LibreTexts describe how the average C−H BDE is different from the BDEs in specific compounds. Some explain how the hybridisation of the C−H bond has an effect or whether you're considering the primary, secondary, or tertiary carbon makes a difference.

In the case of the homologous series of primary alcohols though, the hybridisation for the C−H bond is always the same. Also, I'm only using values from the primary carbons. I checked the bond lengths for the C−H bond on the primary carbon for all the alcohols on Molview too. They're all the same.

What other factors can influence the variation in the BDEs?

### References

1. Luo, Y.-R. Comprehensive Handbook of Chemical Bond Energies; CRC press, 2007. ISBN 978-0-8493-7366-4.

An idea my chemistry teacher described:

Perhaps, the intermolecular forces could influence the bond enthalpy between C-H bonds?

For example, as the homologous series of alcohols gains more methylene groups, the London Dispersion Forces (LDF) between each alcohol molecule increase. So let's say you're heating up the alcohols to break the C-H bonds. In the process, some of the heat energy will go towards overcoming the LDF's attraction between molecules. Then, additional energy would be needed to break the C-H bonds after that.

At the same time though, you would have to consider the effect of decreasing polarity of the alcohol molecules and decreasing dipole-dipole forces. Does anyone know what kind of data could be used to quantify the strength of these different effects?

• Think about the inductive effect – Adiboy Dec 6 '20 at 3:16
• There is a big error in measurement for ethanol and propanol. – user55119 Dec 6 '20 at 3:40
• I wouldn't trust Molview data on equal bond length though. Molview only roughly predicts the values. Experimental data collected by using, say, microwave spectrometry or x-ray diffraction (or even better, neutron diffraction since we are looking for hydrogen atoms), will show some deviation. – andselisk Dec 6 '20 at 7:58
• Better think about products - what radical you get. – Mithoron Dec 7 '20 at 0:49