Substrate + Reagent ⟶ [Intermediate state (and/or) I Transition State] ⟶ Product
Many chemical reactions are depicted in one or I more simple steps. Each step passes through an energy barrier, leading to the formation of short lived intermediates or transition states. The series of simple steps which collectively represent the chemical. change, from substrate to product is called as the mechanism of the reaction. The slowest step in the mechanism determines the overall rate of the reaction.

Resonating structures of A and B are as follows

(i) Structure II is more stable than structure I, because later carries separation of positive and negative charges.
(ii) The contribution of structure II is less than that of I.
(iii) On the contrary, structure III and IV are of equal energy and hence contribute equally towards the resonance hybrid of compound B.
(iv) ∴ Structure (B) is more stable than (A).

(i) \({ O }_{ 2 }NC{ H }_{ 2 }C{ H }_{ 2 }\overset { - }{ O } \) is expected to be more stable than \(C{ H }_{ 3 }-C{ H }_{ 2 }-\overset { - }{ O } \)
(ii) -NO₂ has -I effect, this leads to the dispersal of negative charge.
(iii) On the other hand -CH₃ group has +I effect, which intensity the negative charge
(iv) ∴ Dispersal of charge leads to the stability of ion and intensification of negative charge leads to unstability of ion.
\({ O }_{ 2 }N\longleftarrow C{ H }_{ 2 }\longleftarrow C{ H }_{ 2 }-\overset { - }{ O } \) More stable
\(C{ H }_{ 3 }\longrightarrow C{ H }_{ 3 }\longrightarrow \overset { - }{ O } \) Less stable

CH₃ - CH = O, CH₃ -C = N and CH₃-I, the underlined carbon atoms are electrophilic centres as they will have partial positive charge due to polarity of the C-O or C-N or C-I bond.