Sir this is the relative stability of conjugated diene and isolated diene in terms of heat of hydrogenation plz explain sir mages aae ito 2.5. relative stabili

mageS aae ITO 2.5. RELATIVE STABILITIES OF DIENES-STRUCTURE OF 1, 3-BUTADIENE Heats of hydrogenation can be used to determine the relative stabilities of dienes. We know that alkenec with similar structures have approx. the same value of heat of hydrogenation. For mono substituted alkenes RCH CH,), it is approx. 125-4 kJ/mole, for disubstituted alkenes, (RCH = CHR or R,C = CH,), it is about 117-1 kJ/mole and for trisubstituted alkenes (R,C = CHR), it is about 112.9 kJ/mole. For a compound having more than one double bond, it is reasonable to expect a heat of hydrogenation which is the sum of the heat of hydrogenation of the individual double bonds. Let us consider, the heats of hydrogenation of 1, 3-pentadiene (a conjugated diene) and 1, 4-pentadiene (an isolated diene). Xottu 2 3 4 5 CH, = CHCH = CHCH+ 2H, 1,3-Pentadiene Ni CH CH,CH,CH,CH n-Pentane Found : AH =- 226-2 kJ mol- AM Calculated: AH = (117.1 + 125-4) = - 2425 kJ mol 2 5 CH,=CH ČH, CH=CH, + 2H Ni CH,CH,CH,CH,CH 1,4-Pentadiene n-Pentane Found: AH =-254-2 kJ mol- Calculated: AH = - 2x 125.4 =-250-8 kJ mno o-