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Date: 28-8-2018
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Date: 27-8-2018
1266
Date: 4-10-2020
1226
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The homolytic cleavage of covalent bonds produces radicals, and since this is an endothermic process, it requires the introduction of energy from the surroundings. Heat serves this purpose by collisional interconversion of kinetic energy into vibrational energy, and the temperature required for bond homolysis will be proportional to the bond dissociation energy. Absorption of light may also lead to radical species by intra- or intermolecular conversion of the increased electronic energy into vibrational energy. As expected, weaker covalent bonds dissociate into radicals more readily than stronger covalent bonds. The following table lists standard bond energies (D) for the C–C, C–O and C–H bonds commonly found in organic compounds, together with bond energies for some weaker bonds that have been found useful for generating radicals. Approximate homolysis temperatures at which half the bonds are cleaved in one hour are also given.
Bond | D kcal/mole |
T ºC | Bond | D kcal/mole |
T ºC | Bond | D kcal/mole |
T ºC | ||
C–C | 85 | 670 | O–O | 34 | 160 | O–Cl | 49 | 280 | ||
C–H | 99 | 850 | N–N | 39 | 230 | C–I | 51 | 350 | ||
C–O | 84 | 680 | S–S | 55 | 440 | C–Br | 67 | 480 |
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علامات بسيطة في جسدك قد تنذر بمرض "قاتل"
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أول صور ثلاثية الأبعاد للغدة الزعترية البشرية
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مكتبة أمّ البنين النسويّة تصدر العدد 212 من مجلّة رياض الزهراء (عليها السلام)
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