Allenes,which have cumulated double bonds are less stable than isolated double bonds H H H2,Pt C=C=C △H°=-69.8kcal H CH2CH3 Increasing Stability Order(least to most stable) Cumulated diene -69.8kcal Terminal alkyne -69.5kcal Internal alkyne -65.8kcal Isolated diene -57.4kcal Conjugated Diene -53.7kcal cumulated terminal alkyne penta-1,2-diene pent-1-yne pent-2-yne isolated diene (69.5 kcal) 275kJ penta-1.4-diene solated iene (65,8kcal) frans-hexa-14-diene conjugated 2521 diene (60.2 kcal) 2421 (57.7kcl trans-penta-1,3-diene 25 (53.7keal alkane (pentane or hexane) Ch15 Conjugated Systems (landscape) Page 3 Ch15 Conjugated Systems (landscape) Page 3 Allenes, which have cumulated double bonds are less stable than isolated double bonds. Increasing Stability Order (least to most stable) Cumulated diene -69.8kcal Terminal alkyne -69.5kcal Internal alkyne -65.8kcal Isolated diene -57.4kcal Conjugated Diene -53.7kcal C C C H CH2CH3 H H H2 , Pt H o = -69.8kcal