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(i) What are the properties of resonance hybrid?

(ii) (CH3)3C+ (tert-Butyl carbocation) is more stable than (CH3)2C+H (sec-Propyl carbocation). Why?

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(i) There are following properties of resonance hybrid:

(a) Stability: A resonance hybrid is more stable than any of the contributing structures. Greater the number of equivalent contributing structures of a molecule, greater will be the stability of the resonance hybrid.

(b) Resonance energy: It is equal to the difference between the energy of the resonance hybrid and of the most stable resonating structure. More the resonance energy of a molecule, greater is the stability due to resonance, e.g., the resonance energy of benzene is 151 kJ mol-1. It means benzene is more stable than any of its contributing structures by 151 kJ mol-1.

(c) Bond lengths: The bond lengths of resonance hybrid are always different from that of any of the contributing structures e.g., in benzene.

(ii) Tert-Butyl carbocation is more stable than sec-propyl carbocation due to hyperconjugation. Similarly, hydrogen of other two methyl groups can undergo hyperconjugation resulting in 10-resonance structures. However, in case of sec-propyl carbocation, only 7-resonance structures are possible.

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