Fun and Miscellaneous

By James Ashenhurst

Why Do Organic Chemists Use Kilocalories?

Last updated: May 24th, 2021 |

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 Although it’s still used as a unit of food energy, the calorie has been replaced by the Systeme Internationale unit the joule (J).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.


Comment section

6 thoughts on “Why Do Organic Chemists Use Kilocalories?

  1. Wow. You make a living by being passionate about the metric system. That’s amazing.

    If it’s any consolation the chance of organic chemists moving to barrel-oil-equivalents for bond strengths is pretty low.

  2. I suppose that you can measure energy in joules or you can use one or other of the alternatives.

    Here are some that I have collected in the last 12 months.

    Atomic energy unit, barrel oil equivalent, bboe, billion electron volts, Board of Trade unit, BOE, BOT, brake horsepower-hour, British thermal unit, British thermal unit (16 °C), British thermal unit (4 °C), British thermal unit (international), British thermal unit (ISO), British thermal unit (IT), British thermal unit (mean), British thermal unit (thermal), British thermal unit (thermochemical), British thermal unit-39, British thermal unit-59, British thermal unit-60, British thermal unit-IT, British thermal unit-mean, British thermal unit-th, BThU, BThU-39, BThU-59, BThU-60, BThU-IT, BThU-mean, BThU-th, Btu, Btu-39, Btu-59, Btu-60, Btu-IT, Btu-mean, Btu-th, cal, cal-15, cal-20, cal-mean, calorie, Calorie, calorie (16 °C), calorie (20 °C), calorie (4 °C), calorie (diet kilocalorie), calorie (int.), calorie (IT) (International Steam Table), calorie (mean), calorie (thermochemical), calorie-15, Calorie-15, calorie-20, Calorie-20, calorie-IT, Calorie-IT, calorie-mean, Calorie-mean, calorie-th, Calorie-th, cal-th, Celsius heat unit, Celsius heat unit (int.), Celsius heat unit-IT, Celsius heat unit-mean, Celsius heat unit-th, centigrade heat unit, centigrade heat unit-mean, centigrade heat unit-th, Chu, Chu-IT, Chu-mean, Chu-th, coulomb volt, cubic centimetre atmospheres, cubic foot atmospheres, cubic metre atmospheres, double Rydberg, duty, dutys, dyne centimetres, E-h, electron mass energy equivalent, electron volt, equivalent volt, erg, eV, foot grains, foot pound, foot pound force, foot poundal, ft-lb, ft-lbf, ft-pdl, gigaelectronvolt, gram calorie, gram calorie-15, gram calorie-20, gram calorie-IT, gram calorie-mean, gram calories (mean), gram calorie-th, grand calorie, grand calorie-15, grand calorie-20, grand calorie-IT, grand calorie-mean, grand calorie-th, hartree, Hartree energy, horsepower hours, horsepower hours (metric), inch pound force, Kayser, kcal, kcal-15, kcal-20, kcal-mean, kcal-th, kgfm, kilocalorie, kilocalorie (16 °C), kilocalorie (4 °C), kilocalorie (int.), kilocalorie-15, kilocalorie-20, kilocalorie-IT, kilocalorie-mean, kilocalorie-th, kiloelectronvolt, kilogram calorie, kilogram calorie-15, kilogram calorie-20, kilogram calorie-IT, kilogram calorie-mean, kilogram calories (int.), kilogram calorie-th, kilogram force metre, kiloton TNT equivalent, kilowatt hour, kilowatt minute, kilowatt second, kWh, large calorie, large calorie-15, large calorie-20, large calorie-IT, large calorie (mean), large calorie-th, Latm, latm, litre atmosphere, major calorie, major calorie-15, major calorie-20, major calorie-IT, major calorie-mean, major calorie-th, megaelectronvolt, megaton TNT equivalent, megawatt hours, metric ton oil, metric ton TNT, metric ton coal, micri-erg, natural unit of energy, newton metre, petit calorie, petit calorie-15, petit calorie-20, petit calorie-IT, petit calorie-mean, petit calorie-th, Q unit, quad, quadrillion, Rydberg, small calorie, small calorie-15, small calorie-20, small calorie-IT, small calorie-mean, small calorie-th, therm, therm (EC), therm (EU), therm (UK), therm (US), thermie (16 °C), ton coal equivalent, ton oil equivalent, ton TNT equivalent, tonne coal equivalent, tonne oil equivalent, tonne TNT equivalent, watt hour, watt minute, and watt second.

    Conversion factors needed for 199 names = 39 402

    Notice all the different BTUs, BThU, btus, calories, Calories, Cals, and cals as these all have different values according to the temperature that the observations were made.

    Also note the near impossibility of our politicians rationally discussing energy in the context of peak oil, peak energy, global warming, or climate change given the confusion and obfuscation possible with all of these units.

    On the other hand, if we use joules for energy in all contexts, there is only one value and only one definition, and the only conversion is to decife on whether to use millijoules, joules, kilojoules, and so on.


    Pat Naughtin
    Geelong, Australia

  3. I like that perspective – units are a means to an end.

    Another thing to realize is that they’re essential for communication between disciplines. There is no end to the confusion that can be created by not using units, something a lot of engineers I work around are prone to do.

  4. Thanks for explaining this. As a physicist, I too was reared with the notion that to deviate from SI was a sin. I recall being upset when people would express particle energies in MeV or keV.

    What’s interesting is that, in my current field, we talk about particle energies a lot, and I’ve finally become accustomed to using these units and see the utility in adapting the system to serve your needs. If someone walked up to me and started talking 8 femtojoule protons, I’d probably look at them as if they were from Mars….50 MeV, though, that’s alright.

    Thanks for explaining this to me – our class just got into conformations this past week and I see now why expressing energies in kcal/mol is useful, now that I’m looking at a ring or chain and trying to figure out what the change in the Gibbs energy is from one conformer to another. Thanks James.


    1. My pleasure, glad you found it useful.
      The classes I’ve seen seem to vary about 50:50 on whether kcal or kJ are used. I think sometimes we forget that the units exist to serve us, not the other way around. I’m all for consistency but if there is a significant advantage in understanding to gained by using a slightly different metric, by all means go for it.

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