Combined Experimental and Computational Study of the Thermochemistry of Methylpiperidines

Abstract

To understand the influence of the methyl group in the stability and conformational behavior of the piperidine ring, the standard (p 0 ) 0.1 MPa) molar enthalpies of formation of 1-methylpiperidine, 3-methylpiperidine, 4-methylpiperidine, 2,6-dimethylpiperidine, and 3,5-dimethylpiperidine, both in the liquid and in the gaseous states, were determined at the temperature of 298.15 K. The numerical values of the enthalpies of formation in the liquid and in the gaseous state are, respectively, -(95.9 ( 1.6) and -(59.1 ( 1.7) kJ‚mol -1 for 1-methylpiperidine; -(123.6 ( 1.4) and -(79.2 ( 1.6) kJ‚mol -1 for 3-methylpiperidine; -(123.5 ( 1.5) and -(82.9 ( 1.7) kJ‚mol -1 for 4-methylpiperidine; -(153.6 ( 2.1) and -(111.2 ( 2.2) kJ‚mol -1 for 2,6-dimethylpiperidine; and -(155.0 ( 1.7) and -(105.9 ( 1.8) kJ‚mol -1 for 3,5-dimethylpiperidine. In addition, and to be compared with the experimental results, theoretical calculations were carried out considering different ab initio and density functional theory based methods. The standard molar enthalpies of formation of the four isomers of methylpiperidine and of the 12 isomers of dimethylpiperidine have been computed. The G3MP2B3-derived numbers are in excellent agreement with experimental data, except in the case of 2,6-dimethylpiperidine for which a deviation of 9 kJ‚mol -1 was found. Surprisingly, the DFT methods fail in the prediction of these properties with the exception of the most approximated SVWN functional.

Thermochemical studies of 3-methylpyrazole and 1,3,5-trimethylpyrazole

“…A similar effect can be seen from the experimental enthalpies of formation of piperidine Àð47:15 AE 0:63Þ kJ Á mol À1 and of 2-methylpiperidine Àð84:5 AE 1:1Þ k J Á mol À1 , yielding an enthalpic increment f of ð37:4 AE 1:3Þk J Á mol À1 [36] for the insertion of a methyl group in the ring.…”

Standard molar enthalpies of formation of 1-methyl-2-piperidinemethanol, 1-piperidineethanol, and 2-piperidineethanol

“…The enthalpic increments verified for the formation of 1-methyl-2-piperidinemethanol and 2-piperidineethanol, from 1,2-dimethylpiperidine and 2-ethylpiperidine, respectively, are the same in both cases, %À147kJ AE mol À1 . A similar strategy to the one in the previous scheme was applied to the piperidinemethanols, yielding similar increments, within experimental uncertainties, for the introduction of an -OH group on alkylpiperidines, as shown below: [11] − (227.8 ± 2.0) [9] H N …”

“…Considering the standard molar enthalpies of formation, in the gas-phase, for 1,2-dimethylpiperidine, D f H m ð1; 2-dimethylpiperidine; gÞ ¼ À85:3 kJ Á mol À1 [11], for 1-ethylpiperidine, D f H m ð1-ethylpiperidine; gÞ ¼ Àð82:1 AE 2:0Þ kJÁ mol À1 [10], and for 2-ethylpiperidine, D f H m ð2-ethylpiperidine; gÞ ¼ Àð109:8 AE 1:9Þ kJ Á mol À1 [10], the scheme below shows the enthalpic increment due to introduction of a -OH group into the substitute group of the alkylpiperidines: From the increments above, it can be seen that the -OH introduction into the ethyl group of 1-ethylpiperidine provides more stabilization than similar introduction on the other compounds studied. In this case, the -OH group introduction could be followed by formation of an intramolecular hydrogen bond, as had been verified for other compounds like 1-methyl-3-piperinol and 1-methyl-4-piperidinol [31].…”

“…Recently, we have started the thermochemical study of several piperidine derivatives [9][10][11][12]. As part of this vast investigation, this work is devoted to the experimental determination of the standard molar enthalpies of formation in liquid, and gaseous phases of 1-methyl-2-piperidinemethanol [CAS 20845-34-5] The standard (p = 0.1 MPa) molar enthalpies of formation in the liquid state, at T = 298.15 K, were determined from the values of the standard molar energies of combustion.…”

Thermochemical study of some chloro and bromo alkyl substituted phthalimides: Structural–energetic correlations

“…The systematic experimental and theoretical study on the energetics of nitrogen aromatic heterocycles has been, for some years, one of the main interests of our research group, as being partially presented in two review papers [1,2] and lately extended to other aromatic nitrogen heterocycles like azols [3][4][5][6][7][8][9], phenylpyridine isomers [10], chloropyridines [11], pyrimidines [12], quinoxalines [13] and quinolines [14,15]. More recently our research interested have focused on non-aromatic nitrogen heterocycles with the piperidine ring [16][17][18][19][20][21][22] and with the phthalimide structure [23,24].…”