Thermochemical properties of benzoic acid derivatives XI. Vapour pressures and enthalpies of sublimation and formation of the six dimethylbenzoic acids

The energies of combustion of 3,4- and 3,5-dimethylbenzoic acids have been revised by combustion calorimetry. Vapour pressures of very pure samples of the six dimethylbenzoic acids have been determined over a range of temperatures near 298 K by the Knudsen-effusion technique. From the experimental results and our previously published thermochemical quantities the following results for the six C₆H₃(CH₃)₂CO₂H isomers at 298.15 K have been derived.

Isomer		$\text{Δ}{}_{\mathrm{<mtext>f</mtext>}}\text{H}{}_{\mathrm{m}}{}^{\mathrm{o}}\text{(cr)}$	$\text{Δ}{}_{\mathrm{<mtext>sub</mtext>}}\text{H}{}_{\mathrm{m}}{}^{\mathrm{o}}$	$\text{Δ}{}_{\mathrm{<mtext>f</mtext>}}\text{H}{}_{\mathrm{m}}{}^{\mathrm{o}}\text{(g)}$
	kJ·mol?1	kJ·mol?1	kJ·mol?1
2,6-	?440.7 ± 1.7	99.1 ± 0.2	?341.6 ± 1.7
2,3-	?450.4 ± 1.7	104.6 ± 0.4	?345.8 ± 1.7
2,5-	?456.1 ± 1.6	105.0 ± 0.6	?351.1 ± 1.7
2,4-	?458.5 ± 1.7	103.5 ± 0.3	?355.0 ± 1.7
3,4-	?468.8 ± 1.9	106.4 ± 0.3	?362.4 ± 1.9
3,5-	?466.8 ± 1.7	102.3 ± 0.3	?364.5 ± 1.7
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