[AdiDex]

I've been heard about the species having identical(similar) resonatic structures
Are more stable.
For example-

If we rotate any of one we get will another one.
That why benzylic carbocation have comparable energy with the allylic carbocation.
So can anybody tell me the reason...??

[zsinger]

You told yourself the reason!  They are resonance structures, which is an average between the two.

[zsinger]

Also, no rotation….hydride shift.

[Dan]

Also, no rotation….hydride shift.

Hydride shift?

[Enthalpy]

Could it be that all these refinements of resonances are requested only because one draws bonds limited to atom pairs? And that a representation as molecular orbitals would be more natural, needing no such additional consideration?

[zsinger]

As shown, the + charge moves as a result of a hydride shift I'm assuming?  Or am I brain-farting?
             -Zack

[mjc123]

No hydride shift - both structures are [CH₂CHCH₂]⁺. Only the electrons move (or, the electron structure is an average of that of the resonance structures).
Resonance structures always have the same arrangement of atoms, only differing in how we draw the electrons in terms of formal bonds. A hydride shift would give structural isomers, not resonance structures.

[zsinger]

Yep…..sorry….brainfart.
      -Z

[AdiDex]

I got the correct explanation....
Which is something like this..
Assume you have four rooms to live...if one of the them is highly comfortable to you...then you always prefer to live in that room.....you will not go in another room.....as it is not comfortable....inspite of having a big house...you will not use it economically
The situation second ...let assume that you have only two rooms which are equally comfortable to you...now you have a small house but you're using it economically..
Now you will spend your time equally in both of the room....
So you can consider situation one equivalent to benzylic carbocation
And second situation equivalent to allylic carbocation..

I hope my english is understandable...

[Enthalpy]

Don't forget that the resonance is static. The electron(s) is at both places at the same time.

[AdiDex]

Yep i know..
It was just an example....
Nothing more....