Endo effect

Figure 3.14 shows a DSC trace obtained at the scan rate of 4°C/min from the mixtures milled for 5, 10, and 40 h. A very small thermal flow effect of either exothermic or endothermic nature is seen around 140°C at the DSC traces of mixtures milled for 5 and 10 h only. At temperatures >180°C, mixtures milled for 5 and 10 h exhibit three strong endothermic effects centered at around 271, 315 and 452°C but the one milled for 40 h shows only two endo effects at around 292 and 452°C. 

Fig. 7 a and b. Scheme of the thermomechanical behaviour of a well phase-separated thermoelasto-plastic. Stress-strain (or time) curves. Plots of heat effects versus time. First loading (ABC) and unloading (CD) cycle. Second loading (AC) and unloading (CD) cycle. The yielding point occurs at B. AD indicates the residual deformation after the first cycle. AB on the dQ/dT-time curve is the endo-effect resulting from the initial small-strain deformation AB U9)... 

In addition to the representative [3 + 2] cycloaddition reactions shown in Table I, the delocalized singlet vinyl carbenes have been shown to participate as it2a components of non-Hnear cheletropic [tt 2s + tt 2a] cycloadditions to provided cyclopropanes with an observable endo effect, and as7r2s components of [tt4s + tt 2S] cycloadditions with selected dienes to provide cyclo-... 

Let us now consider Scheppele s discussion of the solvolysis of 2-d-exo- and -endo-bicyclo[2.2.1] hept-2-yl p-nitrobenzoates and p-toluenesulfonates. A maximum a isotope effect of 1 22 is suggested for kH/kD for the limiting solvolysis of simple secondary sulphonates (solvolyses where the rate is not affected by the nucleophilicity of the medium). The endo-esters yield a similar value which might be taken to imply the absence of steric hindrance in their reaction. However, whereas the maximum isotope effect for the simple sulphonates is believed to arise partly from an activated complex in the conversion of the internal ion pair into a solvent-separated ion pair, B - C, the effect in the endo-norbomyl compounds is suggested to arise during the transition state involved in the formation of the intimate ion pair, A - B. The latter is deduced from the fact that polarimetric and titrimetric rate constants are the same in acetic acid, aqueous acetone and ethanol (Winstein and Trifan, 1949 and 1952). If this is the case, the endo-effect of 1-20 should be... 

These conclusions are in agreement with DTA and TG data. Endo-effect at 365°C, which is due to dehydration Mg(OH)2, is observed for activated sample, too. Vice versa, it is absent for the activated mixture of Mg(OH>2 with Si02, starting from 1 h. This means that dehydration occurs as a result of chemical interaction. The enthalpy of dissolution of this mixture passes through a maximum versus activation time, as in the case of activated talc, Mg3Si70,o(OH)2. 

According to DTA and TG data, endo-effect at 365°C, caused by Mg(OH)2 decomposition, remains constant in the first case, becomes very small in the second case and is practically absent in the third case. This means that the interaction of initial components occurs at the stage of mechanical activation. DTA curves also exhibit endo-effect at 865°C, being maximal for the second mixture. It is due to the formation of forsterite, Mg2Si04, in the first mixture and due to formation of a mixture of forsterite with enstatite in the second and the third mixtures. 

When oxygenated diene (175 X = OR) was utilized, a single isomer of pyrrolizidine (177) was formed on thermolysis.(The stereochemistry of the carboxylate was rationalized by the endo effect.) Thus a highly selective procedure for the exhaustive synthesis of pyrrolizidine alkadoids was developed (see Sections 8.1.8 and 8.1.9). ... 

But we can t use diagrams like that for mechanisms The cycloaddition uses maleic anhydride as s two-electron component with a low LUMO. We could use one of the dienes in the ring to provide four electrons but their ends are far apart and the electron-deficient ring is a poor HOMO. If we include the sulfur atom we can provide eight electrons and an atom (S) with a large HOMC coefficient. The tricyclic product is clearly folded back on itself so that the triene in the sever.-membered ring and the carbonyl groups in the anhydride are close. There must be an endo effect i.-[8-1-2] cycloadditions. 

Now there is an intramolecular Diels-Alder reaction requiring a high temperature because the dienophUe is not activated. The stereochemistry is not obvious but there is no endo effect so the molecule folds to give the new five-membered ring a ds ring junction with the old. 

If the carbanionic carbon is trigonal, such as with enolates, the preference is to occupy the concave face, as shown in Figure 6.5c. This effect is reminiscent of the endo effect in Diels-AIder reactions (Section 6.6), and is also consistent with... 

Either of the two starting materials could absorb the light to provide the SOMO for the cycloaddition. This does not affect the stereochemistry of the reaction. There is no endo effect in [2 + 2] photocycloadditions so the molecules simply come together with the rings arranged in an exo fashion to... 

T. Mori, H. Nishida, Y. Shirai, T. Endo, Effects of chain end structures on pyrolysis of poly(l-lactic acid) containing tin atoms. Polym. Degrad. Stab. 84(2), 243-251 (2004)... 

EKmination, after intramolecular Diels-Alder of CO2 15,19 —, after intramolecular Diels-Alder of other small molecules 28 Endo effect, in Diels-Alder reaction 46 Enones — see Ketones, a, P unsaturated Equilibration, thermal 3, 6, 12, 13, 43, 45, 64, 65... 

Another common feature of Diels-Alder stereochemistry is the so-called endo effect. Phenomenologically, this is a stereochemical effect whereby an acceptor substituent on the dienophile ends up in the endo position of the product, as shown in Eqs. 15.9 and 15.10. This is a useful and fairly general feature of the Diels-Alder reaction. 

I. V. Singh, M. Tanaka, and M. Endo, "Effect of interface on the thermal conductivity of carbon nanotube composites," InternationalJournal of Thermal Sciences, vol. 46, pp. 842-847, 2007. 

The effect of organic solvents Ci-C HgOH, 2-C HgOH and i-CgH OH) on surface morfology of the precursor has been studied earlier tSJ. In the present study more solvents have been tested (Table 3). For all the solvents a product was VOHPO. 0.5H2O. In the thermogram of this compound I5I two endo-effects are present. The second one corresponds to the formation of X-ray-aunorphous... 

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