[curiouscat]

What's a good way to estimate the enthalpy change (heat of reaction) for this reaction:

  +     + H2O

I guess I could do the bond energy bookkeeping the old school way by hand & looking up bond energy tables but I'm wondering if there are any calculators / Software that facilitates this tedium.

Maybe an automated group contribution method or fast DFT estimate? I don't need it to be too accurate.

[discodermolide]

Why not just mix your reactants and the catalyst and measure the DSC?

[billnotgatez]

@disco

Are we Edison or Tesla today?

Just had a minute of whimsy 

[Enthalpy]

For the accuracy needed to compare rocket propellants, I had to abandon every estimation method and rely exclusively on measured enthalpies of formation, checking with disconfidence if a published enthalpy had resulted from some software, and whether the compounds are in the proper state at the right temperature.

Without experimental data, the least bad method is indeed hand evaluation. This does make sense with your branched compounds without short nor multiple rings - but mind the two double bonds. Hand evaluation has some value only if you can refer to very similar compounds in the same state at the same temperature and apply minor corrections. Bond energies are already far less accurate.

Additive rules, preferably improved with nearby interactions rules, have the big drawback of neglecting the state and temperature of the compound - that's a superiority of hand estimation based on similar compounds.

Molecule modeling software (I have ArgusLab on Windows, Linux runs more varied software, all rely on the same Am1/Pm3/etc models) use to be >20kcal/mol wrong on such molecules, which is useless to my needs. Software generally computes an enthalpy of formation (...from the molecular elements, check it) for the gas at zero kelvin. Some software also estimates the heat capacity of a gaseous molecule to extrapolate it to the gas at room temperature - ArgusLab doesn't. While the heat of vaporization could be somewhat estimated, the heat of fusion isn't up to now (research topic) since it depends on every detail of molecule stacking in the solid, and the heat of formation of the solid is inaccessible to software.

So if you accept 40-50kcal uncertainty, several methods are possible. If not, only experimental values work - and these are too much neglected in the computer era.

[curiouscat]

Thanks for the lead to ArgusLab. Never used that before.

50 kCal / mol does not sound terrible. If you are dealing with MW~100 species & liq. phase reactions that ought to translate to an error of approx 50 C in the adiabatic exotherm temperature.

I'm assuming non-aqueous systems (say, Cp = 2200 J/kgK ) with say 25% dilution.

If it were an aqueous phase reaction that error should be much lower. So also, in real, non adiabatic reactors the real error in temp. will be smaller.

Hope I've done my math right.

[Enthalpy]

Already some experimental enthalpies of formation in kJ/mol as liquids at 298K:
(A) -248.4 Dimethylketone
(B) -192.2 Acetaldehyde
(C) -138.7 trans-2-Butenal
(D) -285.8 Water

From B-A I claim that the methyl that formally transforms an aldehyde into a methylketone releases 56.2kJ. Admitting the same effect on your third compound, it brings
(e) -194.9 Methyl-1-propenylketone

Then I claim that an added methyl has about the same effect in Ethylmethylketone and in the third compound. Not perfect because the methyl adds near a double bond; we could improve that by comparing more molecules. Anyway, the reaction now releases the same heat as if dimethylketone made Methyl-1-propenylketone (that is, with one methyl less at each side), olé!

Bookkeeping with one methyl less at each side:
-248.4 and -192.2 become -194.9 and -285.8
the reaction releases 40.1kJ.
This neglects all mixing heat.
Software inaccuracy wouldn't have told if positive or negative.

Please double-check, since such reasoning is prone to mistakes.

[curiouscat]

Hey Enthalpy. Thanks for the number crunching. That's very helpful.

The closest hit I have is for a very similar reaction (Acetaldehyde to β-hydroxybutryaldehyde) where 40 kJ/mol (exotherm) is reported.

https://books.google.co.in/books?id=GC6LnmwfQ9EC&lpg=PA340&ots=75tOf792X3&dq=aldol%20condensation%20exotherm&pg=PA339#v=onepage&q&f=true

So your estimate sounds very close.

[Enthalpy]

Yes, that prudent, and comparing with a known final result is safer... 

Out of curiosity, I checked what Am1 run by Arguslab tells
http://www.arguslab.com/arguslab.com/ArgusLab.html
http://www.arguslab.com/arguslab.com/ArgusLab_files/arguslab.zip

Converted to kJ/mol:
(A estim soft!) -200.3
(B estim soft!) -170.8
(C estim soft!) -107.4
(D estim soft!) -243.9

About 40kJ/mol wrong at each compound, which happen to cancel out approximately to release 20kJ/mol as estimated by Am1. Not very accurate.

[curiouscat]

20 kJ/mol per Argus means only an errors of 20 kJ/mol with respect to your earlier by-hand estimate.

20 kJ/mol sounds like a decently small error. That's only 5  kcal/mol. Even when I had used some serious DFT  work by the professionals running on massively parallel computers the promised accuracy was never better than 0.1  eV ( 9 kJ/mol)