1-Butene (A) and 1,3-Butadiene (B) differ not only in the number of \( \pi \) bonds, but (B) also has \( \sigma \) and \( \pi \) bonds at alternate positions. This type of the system is called conjugate system. Following are some of the conjugate system. acrolein acrylic acid benzene
in such systems, \( \pi \) electron shifting takes place consecutively giving permanent polarity on the chain. This - type of \( \pi \)-electron shift in the conjugate systems is called mesomeric effect or resonance.
Rules for resonance forms :
(i) Individual resonance forms are imaginary, not real
(ii) Resonance forms differ only in the placement of their \( \pi \) electrons or nonbonding electrons.
(iii) Different resonance forms of a substance dont have to be equivalent.
(iv) Resonance forms must be valid Lewis structures and obey normal rules of valencey.
(v) The resonace hybrid is more stable than any individual resonance.
Rules for stability :
(i) Structures with more covalent bonds are more stable than other structures.
(ii) Structures in which all of the atoms have a complete valence shell of electrons (i.e. the noble gas structure) are especially stable and make large contribution to the hybrid.
(iii) Stability is decreased by an increase in charge separation.
(iv) Structure that carry negative charge on a more electronegative atom and positive charge on less electronegative atom are comparatively more stable.
- Which of the following statement is incorrect -
(A) In a resonance hybrid all the molecules are the same. A resonance hybrid cannot be expressed by any single structure.
(B) The value of the resonance energy of any resonance hybrid is not an absolute value
(C) The energy of hybrid structures is always less than that of any resonating structure
(D) Only one individual resonating structure explains all characteristics of the molecule
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