J. Ind. Chem. Soc.:Decarboxylation of pyridine
Sorry for taking so much time,I've been busy,but I really
don't have an excuse,here it is:
A Study on Solid State Thermal Decomposition Characteristics of Some Metallo-organic Compounds.
Part-I: Dehydration and Decarboxylation of Hydrated Calcium
Salts of Pyridine Monocarboxylic Acids
Solid state dehydration of hydrated calcium salts of picolinic acid, nicotinic acid and isonicotinic acid and
subsequent decarboxylation of the corresponding anhydrous salts have been studied by simultaneous TG, DTA
and DTG techniques. From the analysis of the TG, DTA and DTG traces for the dehydration of the hydrated salts, the thermal
stability order of the hydrates has been found to be Ca(pic)2 . H2O > Ca(isoNic)2 .
4H2O > Ca(Nic)2 . 3H2O. But the trend observed in the decarboxylation process is
Ca(Nic)2 > Ca(isoNic)2 > Ca(pic)2. Thermal parameters like activation energy,
enthalpy change and order of reaction for each process have been computed by standard methods. An attempt has been made to
correlate the trend in the thermal stability of the anhydrous salts towards decarboxylation with their molecular
structure.
The calcium salts were prepared by the reaction of a slight excess of CaCO3 (G.R., E. Merck) with the appropriate
acids in hot aqueous solutions followed by filtration and subsequent crystallisation.
Simultaneous DTA, TG and DTG determination of the salts were carried out with a Paulik-Paulik-Erdey type MOM Derivatograph
with dry air as the atmospheric gas. The particle size of the samples was in the 100-150 mesh range. The heating reate was
about 4.25° per minute and sample size of 180-210 mg was used to make the volume nearly the same in each case. The referenc
material was aluminium oxide previously heated to 1600°. The sample holder and the reference holder were made of platinum. TG
curves were utilised for calculating the activation energies of the processes involved, whereas DTA curves were used to
evaluate the enthalpy changes accompanying the reactions. The inital, peak and final temperatures for the dehyration and the
decarboxylation processes were noted from the corresponding DTG curves. The hydrated calcium salts and their dehydrated
varieties were characterised by recording their ir spectra in halocarbon mull on a Beckman IR 20A model spectrophotometer.
All the hydrated and the anhydrous compounds were analaysed for calcium by titration with a standard EDTA solution. Carbon,
hydrogen and nitrogen were determined by microanalytical techniques.
Results and Discussions
Dehydration process: On gradual heating from room temperature, the hydrated salts were completely dehydrated within
the temperature range 39°-240°. From the TG, DTG and DTA traces of the dehydration stage it was found that all the
dehydrations occurred in one step. All these processes might be represented by the general equation:
Ca(C6H4NO2)2 . x H2O =
Ca(C6H4NO2)2 + x H2O ; when X = 1 ,
C6H4NO-2 = picolinate ion ; X = 3 , C6H4NO-2 = nicotinate
ion ; x = 4 , C6H4NO-2 = isonicotinate ion.
Initial, peak and final temperatures of the dehydration of each species, as obtained from the relevant DTG curves along with
the corresponding weight loss are given in Table 1. Enthalpy changes accompanying dehydration of each species were determined
by standard methods from the peak area of the corresponding DTA curves using CuSO4 . 5H2O as the
standard1. Activation energies for each dehydration process were computed from an analysis of the corresponding TG
curves using the method of Horowitz and Metzger2 and the
ln ln W°-Wft vs ø plots are presented in Fig 2.
--------
W-Wft
Note:substitue 'ø' for the greek letter theta,I didn't understand how to type here since it isn't an ANSI
character.
The order of reaction was determined by standard methods2,3 and was found to be unity. The results obtained are
presented in Table 1 and the corresponding curves presented in Fig. 1. IR spectra of the hydrated and the anhydrous varieties
were recorded and compared to ascertain the completion of the dehydration process.
Decarboxylation processes: All the anhydrous salts exhibit considerable thermal stability and undergo decarboxylation
within the temperature range 340°-600°. Initial, peak and final temperatures for the decarboxylation processes of the
anhydrous salts along with the corresponding weight losses are given in Table 2. The final product left in the crucible was
found to be CaCO3. This has been confirmed by comparing the X-ray diffraction pattern of the end product with that
of a pure CaCO3 sample. Applying the same methods used in the dehydration processes, activation energies of the
decarboxylation processes were evaluated from the
ln ln W°-Wft vs ø plots, given in Fig 3.
--------
W-Wft
Enthalpy changes accompanying each decarboxylation process were measured from the DTA cruves and are presented in Table 2.
the order for the decarboxylation reaction was found to be unity in all the three cases. The corresponding curves are
presented in Fig. 1.
I'm supposed to put two tables here,but I'm having some trouble,will post them later..
Solid state thermal decomposition of the salts of aromatic or heterocyclic acids has received little attention from the
kinetic point of view. Most of the workers, who studied the thermal decomposition of the salts of organic acids (mostly
aliphatic acids) in the solid state, used the maximum point method
4-6 and the Freeman-Carrol method
7 for the determination of the kinetic parameters. In the present work we have utilised Horowitz and Metzger's method, an
analytical technique which utilises a single TG plot to determine the pertinent Arrhenius parameters and reaction order. The
values obtained by the application of this method have been verified by the Coats and Redfern
8 method wherever
possible. We have utilised the DIA curves for the evalution of enthlpy changes for the dehydration as well as decarboxylation
processes.
The single step dehydration of calcium nicotinate trihydrate and the calcium isonicotinate tetrahydrate indicated that all
the water molecules in the nicotinate trihydrate are similarly bound and such is also the case with the water molecules in
the calcium isonicotinate tetrahydrate. A single endotherm in each of the DTA curves definitely points to the accuracy of the
above conclusion. In the single step dehydration of all the three hydrates represented by the general equation in Table 1,
the activation energies and enthalpy changes of dehydration are in order picolinate > isonicotinate > nicotinate. This
difference in thermal stabilities of the hydrates may be attributed to the differences in the mode of binding of the water
molecules in the crystals of the three different pyridine monocarboxylates. It may thus be concluded that the water molecules
in calcium picolinate are more firmly bound than those in isonicotinate and nicotinate and the water molecules in the calcium
isonicotinate are held more stronlgy in comparison to those present in the calcium nicotinate.
In the decarboxylation of the anhydrous salts represented by equations given in Table 2, the activation energy follows the
order nicotinate > isonicotinate > picolinate. The enthalpy change in the decarboxylation process also follows the same
order.
It may be concluded from the data in Table 2 that the thermal stabilities of the carboxylates follow the order nicotinate >
isonicotinate > picolinate. This trend is quite logical as the negatively charged carboxylate ion has a + I
effect
8 and hence releases electrons with a consequent increase of electron density on the ring carbon atom which,
if present in a benzene ring, would have stabilized the ring carbon-carboxyl carbon bond. But in the present case, as the
ring is a heterocyclic ring, the nitrogen atom being much more electronegative than carbon, would change the situation
significantly and we find that unlike benzene, electron density distribution in the pyridine ring
9 is as follows:
0.95 /----\ 0.85
/ \
0.82 \ / 1.58
\____/
Due to the
ortho-para orienting influence of the carboxylate ion the positions
ortho and
para to the
ring carbon containing the carboxylate ion will have greater electron density and in the case of calcium picolinate, where
the electronegative nitrogen atom is present in the position
ortho to the ring carbon containing the carboxylate ion,
it conveniently draws away this excess electron cloud towars itself and consequently reduces the electron cloud accumulated
on the adjacent ring carbon atom considerably. It thus weakens the ring carbon-carboxylate carbon bond to a significant
extent making it comparatively thermolabile.
In the case of the 4-picolinate anion, the enhanced electron density in the position
para with respect to the
carboxylate ion is accomodated on the nitrogen atom as a result of which the 4-carbon atom is relieved of some of its
electron cloud acquired from the carboxylate anion and results the weakening of the ring carbon-carboxyl carbon bond and
consequently induces thermolability. But overall destabilisation effect is greater in 2-picolinate than in 4-picolinate due
to the closeness of the ring nitrogen to the carboxylate substituet in the former and makes the 4-picolinate thermally more
stable than the 2-picolinate compound. In the case of the 3-picolinate, the
ortho-para orienting carboxyate ion would
again result in electron enrichment in the
ortho and
para positions leaving the
meta position
unaffected. Thus, the increased electron density on the ring carbon atom containing the carboxylate ion is not decreased in
this case. This leads to the stabilisation of the ring carbon-carboxyl bond when compared to the 2- and 4-picolinates and
makes the 3-picolinate thermally stabler than the other two. Thus, the thermal stability order nicotinate > isonicotinate >
picolinate, observed in this study, is quite in accordance with the theoretical principles.
References
1. K. Sano, Sci. Rep. tohuku, Imp. Univ., 1st Ser., 1936, 24, 719.
2. H. H. Horowitz and G. M. Metzger, Anal. Chem., 1963, 35, 1464.
3. A. W. Coates and J. P. Redfern, Nature, 1964, 201, 68
4. K. Akita and M. Kase, J. Polymer Sci., 1967, A1, 833.
5. J. H. Flynn and L. A. Wall, J. Res. Nat. Bur. Stand., 1966, 70A, 487
6. R. M. Fuoss, I. D. Salver and H. S. Wilson, J. Polymer Sci., 1964, A2, 3147
7. E. S. Freeman and B. Carrol, J. Phys. Chem. Ithaca, 1959, 62, 394
8. J. March, "Advanced Organic Chemistry: Reactions, Machanisms and Structure", McGraw-Hill, 1968, p. 21.
9. I. L. Finar, "Organic Chemistry", ELBS and Longmans, Green, 1964, Vol 1, p. 760.
Sorry,I'll have to post the 2 missing tables tomorrow,I won't bee posting the two Figures,doubt they are useful and
my photocopies aren't very good.BTW,SPISSHAK,you owe me $1.5 
Weedar
Me fail English?That's
unpossible!
