Durch Verbrennungskalorimetrie wird die Bildungsenthalpie des Benzocyclobutens (2)zu AH: (g) = 199.4 kJ mol-' (47.7 kcal mol-') bestimmt. Aus der Kinetik der Reaktion von 2 mit Maleinsaureanhydrid (MSA) ergeben sich fur die o-Chinodimethanbildung 2 + 1 Aktivierungsparameter E, = 166.9 kJ mol-' (39.9 kcal mol-'), A = 2.8 x IOl4 s-', Die entsprechenden Werte der Ruckreaktion 1 + 2 werden aus der Dimerisierungsgeschwindigkeit des Benzocyclobutens sowie der durch Blitzlichtphotolyse von 5,6-Dimethylenbicyclo[2.2.l]hept-2-en-7-on (5) ermittelten Geschwindigkeitskonstanten der o-Chinodimethandimerisierung zu E, = 122.7 kJ mol-' (29.3 kcal mol-'), A = 2.2 x IOl3 s-', bestimmt. Energy Profile of the o-Quinodimethane-Benzocyclobutene EquilibriumThe enthalpy of formation of benzocyclobutene (2) is measured by combustion calorimetry to be AHp(g) = 199.4 kJ mol-' (47.7 kcal mol-'). From the kinetics of the reaction of 2 with maleic anhydride the activation parameters for the o-quinodimethane formation 2 + 1 are obtained: E, = 166.9 kJ mol-' (39.9 kcal mol-'), A = 2.8 x I O l 4 s-'. The corresponding values for the back reaction 1 + 2 are derived from the dimerisation rate of o-quinodimethane, obtained by flash photolysis of 5,6-dimethylenebicyclo[2.2.l]hept-2-en-7-one (S), and from the dimerisation of benzocyclobutene yielding E, = 122.7 kJ mol-' (29.3 kcal mol-I),o-Chinodimethan (1) kann, wie seine Resonanzstruktur 1 b anzeigt, als hoch stabilisiertes Diradikal aufgefal3t werden. Im Hinblick auf die Diskussion uber die Lage von Diradikalen auf der Reaktionskoordinate ') ist es von Interesse, das Energieprofil der Cyclisierung des o-Chinodimethans (1) zu Benzocyclobuten (2) zu ermitteln. Dieses wird durch ') L. Salem und C. Rowland, Angew. Chem. 84, 86 (1972); Angew. Chem., Int. Ed. Engl. 11, 92 (1972).
The enthalpies of combustion at 298.15 K of the alkanol series from pentan-1-01 up to and including hexadecan-1-01 have been measured. The enthalpies of combustion for the liquid state of the four highest members of the series were obtained by taking into account the enthalpies of melting at 298.15 K. The densities of the crystals stable at room temperature, required as auxiliary data, were determined.This study is the last of a series 1 * concerning the enthalpies of formation of the normal primary alkanols in the liquid state. This paper reports the results obtained for pentan-1-01 (C,)up to and including hexadecan-1-01 (C, 6 ) . The data for the liquid state of the four highest members of the series, which are solids at 298.15 K, were obtained from those for the solid state by taking into account the enthalpies of melting at 298.15 K.Two studies of the enthalpies of formation of normal primary alkanols have been published by other groups while this work was in progress.3* However, our results are for a larger number of members of this series.The densities of the alcohols were required for buoyancy corrections on the weighed masses. Since no reliable data were available for the solid members, density determinations were carried out. EXPERIMENTAL MATERIALSThe substances were in general purified by fractional distillation in columns with 40 to 60 theoretical plates at reflux ratios ranging from 30 to 150, depending on the degree of difficulty of separation, and the purities of the fractions determined by g.1.c. The samples of C13, C14, C15 and c 1 6 have been described else~here.~ The starting materials for the samples of C5 up to and including Clz were commercial products of about 98 mol % purity, except c 6 . The latter was a technical grade material and it contained 2-ethylbutanol. It was used initially to test the distillation columns but during this process it was purified sufficiently.Crl contained unsaturated compounds not separable by distillation, and the sample was hydrogenated on Pd/C. Clo was subjected to inclusion in urea, and an impurity difficult to remove by distillation was included preferentially.
The vaporltatkn enthalpb AH of water, methanol, ethanol, and benzene are c.lculated, from p , t data and auxHlary data, wlth the dlfferont&l Clapeyron equation and wtth an equatkn derived from tho exactly intograted Clapeyron .quakn, and the methods are compared as regards the sendtlvtty of AH to dovlatkns In the p , t data.The Integral motlwd k found to be 1-2 ordm kr, sendtlve. I t k d.mondrated how the clrp.yron equatkn, htthwto k l k v e d to be w u o l v~ without pnlhrkrrry subrtrtutkn of abnpk e x p " for AH and Ab', can be transformed to an "oxact dffferontlal equaHon" by muM@ylng by an Integrating factor, and the rborow and general l n t~& o n k p.rlomud. The rdvantage of the Integral mothod wtth reaped to the "thkd-law nwthod" In det.ctlng dovlatlng data polnk k dkcwsed. From the Integrated Clapeyron .qwtkn, several qurtknr are derlvd and uI.k podble w . as prerwre-t.mp.rature relatlom k cUaums4. I t k ahown how Nnpblcal equations Rko that of Cryp. and Froa/K1#cwrrl can be derked ttom a bmod equation, and how new oq" can be ddgned whlch are focuud on spedal prop.rtl08 such as aaeoclatkn. IntroductlonThe present study is a corollary of the question of how to extrapolate enthatpies of vaporlzatlon, AH, as accurately as posslMe from accurate vapor p " data. Ow interest arises from our studies in combustion calorimetry of the condensed normal primary alkands ( 7 -3) and from the finding of Sunner, Wulff, and co-workers (4-6) that this group of compounds is interegting in a formulatbn for the CH2 increment in the enthalpy of formation of a-substituted n -alkanes.Usually, the Clapeyron equation is used with a dlfferentiated empirical pressure-temperature relatlon (EPTR) like that of Rankine, Cragoe, Cox, etc. The subject of EPTRs recehred and receives much attention. Partington (7) listed over 50 E m s in 195 1 (for reviews, see ref 7-7 7). Becawre EPTRs are not AH@o,ToXTo-l -T -l ) (3)An equatkn wlth a comparable objective has been given earfier (75), but it is wrong, since it has a form for AC, which would, in our notation, be ACp(po,T0) instead of AC,(p,,T).Equation 3 permits the calculation of AH at any coexistence situation bo, To) from the p , T data if the equations of state and ACp at one pressure (not necessarily p o since conversion is possible by means of the equations of state) are known (see Applications).An expression for AV@,,T0) analogous to eq 3 is yielded by eq 2b. The analogue here of the heat capacity term in eq 3The acld dlmoclatlon constant of aqueous phenylproplollc acld (3-phenyl-2-propynok acld) has been determlned between 15 and 45 OC by pH potentlometry. The standard enthalpy and entropy of dksoclatlon are calculated from the temperature varlatlon of the dksoclatlon constant. The lac NMR resonance dlsplacement of the carboxylate carbon upon acld dlsroclatlon was measured, and Its correlatlon wlth the standard entropy of dlbsoclatlon lmplles that the molecular form of aqueous phenylproplolk acld exlsts partly as an Ion palr In equlllbrlum wlth the covalently bonded structure.The liter...
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