Fun and Miscellaneous
By James Ashenhurst
Why Do Organic Chemists Use Kilocalories?
Last updated: March 27th, 2019
What’s the deal with organic chemists using kilocalories (kcal) instead of kilojoules (kJ)?
The metric system, of System Internationale (SI) was first adopted in France in 1791 soon after the French Revolution (one of their other innovations adopted by the revolutionaries – the ten-day week – did not survive). I’m Canadian, so I grew up entirely within the metric system, and since SI units are the de rigueur of measurement in science, I just assumed that all scientists used it.
When I started hanging out with organic chemists it was like finding acoven of shameless heretics operating right under the noses of the Inquisition. All the values of the bond strengths and energies that I’d learned and memorized in kilojoules/mol were being thrown back at me in kilocalories. Being Canadian (and a new grad student) I politely held back my criticism of this barbaric practice until a more opportune time, while simultaneously maintaining my smugness that SI was the more correct system.
[Just to jog your memory, a “calorie” is a unit of energy measurement. It’s the energy required to raise the temperature of 1 gram of water by 1 degree Celsius. Although it’s still used as a unit of food energy, the calorie has been replaced by the Systeme Internationale unit the joule (J). A calorie is equal to 4.184 J. ]
After a few weeks of hearing my colleagues banter in kilocalories, it started to dawn on me that it wasn’t just my own research group that did this. It was the whole community of organic chemists – in journals, at conferences, in casual conversation. And little by little, I started using it too. It felt kind of naughty at first to get away from my Systeme Internationale upbringing. Month by month, however, my resistance to using kcals started crumbling – the more I used it, the more I realized it was actually a more practical unit of measurement for organic chemistry.
In Gen chem we learn how to look up the heats of formation of whole molecules, which can be pretty big numbers. In contrast, when organic chemists talk about energies, they are usually talking about bond dissociation energies and conformations. Since the variety of molecules in organic chemistry is so huge, we often don’t have the luxury of being able to work with values from tables. Therefore, when thinking about the energy changes in a reaction, we’re often forced to base our reasoning on estimating the value of the parts rather than the whole. “What’s that C-O bond worth? What does it cost to replace it with bromide? What’s the energy for that ring flip? What’s the contribution of that hydrogen bond?”.
It’s kind of like going from working at a car dealership to a chop shop and one day finding yourself looking at a car and thinking “what’s that bumper worth?”
So why do organic chemists use kcals? I can think of 4 reasons.
1. The C-H bond strength in alkanes of 100 kcal/mol is a convenient mental anchor.
Organic chemists think a lot in terms of bond strengths. The most common bond you’ll find in organic chemistry is the C-H bond, which has a bond strength of about 100 kcal/mol (note: this value can vary considerably depending on the substitution at carbon, but for simple alkanes like methane and ethane, the C-H bond is very close to this value). What’s that value in kJ/mol? 472 kJ/mol. Guess which number is easier to remember?
There’s just something instinctive about working on a 0-100 scale.
2. Low-energy phenomena also fit well on the kcal scale too.
For organic chemistry, kcal/mol is kind of a “goldilocks” metric in that even 1 kcal/mol is significant, but not too significant. For instance, 1 kcal/mol is not even close to being enough energy to break a bond, but it’s still enough to cause an 83:17 ratio of products at equilibrium. Going a little bit higher, hydrogen bonds have strengths of anywhere from 2-7 kcal/mol.
Along the same lines, when you learn about conformations you are told that the barrier to rotation of ethane (from the “eclipsed” to the “staggered” form)is about 10 kJ/mol. Converted into kcal, that’s about 2.8 kcal/mol.
So on one end you have the strength of the C-H bond at 100 and on the other end you have hydrogen bonds and conformational changes from around 2-7 kcal/mol. The scale incorporates them both very well.
3. In general, smaller numbers are easier to deal with.
One thing about bond strength values that you’ll see is that they vary a lot. Depending on where you look you might see values of 78-82 kcal/mol for the C-C bond strength. Dealing with an uncertainty of 4 seems a lot easier than the equivalent with kJ, which would be 320-340 kJ/mol. Maybe it’s just me, but I like dealing with smaller numbers. As one commenter said when I asked Chemistry Reddit about this a few days ago, “kcals make my errors look smaller.”
4. When they can, organic chemists like to stick it to the French.
Just kidding. Actually, according to Wikipedia, the metric system was developed in England and introduced to France by Benjamin Franklin. Is there anything Ben Franklin wasn’t involved in? That guy was amazing.
Bottom line: Organic chemists find kcal/mol to be much more convenient to use as energy units. Personally, I think and talk in kcal/mol but because I don’t want to increase the confusion that people already feel about organic chemistry, so I try to keep things in kJ/mol here, but if I lapse back into the energy measurement I find most comfortable to work with, this is why.