Question

Which substance below is resonance stabilised by delocalised pi electrons?
A. Benzene
B. Hydrochloric acid
C. Hydrogen gas
D. Potassium bromide

Answer 1

Usually in aromatic hydrocarbons, the pi-electrons are delocalized between the carbon atoms to give them stability and make the structure aromatic. So, to know the answer of this question, check each option and try to figure out which is an aromatic and aliphatic hydrocarbon.

Complete step by step answer:
So, first of all let us know what resonance actually means.
In organic chemistry, the behaviour of electrons differs when the elements other than that of the carbon atoms and hydrogen actively take part in the formation of molecular bonds. The electronic factors influencing these reactions include the electromeric effect, inductive effect, resonance effect, hyperconjugation, etc.

So, in organic chemistry, the resonance effect or “the mesomeric effect” or “mesomerism”, describes the delocalized electrons within the certain molecules where one single Lewis structure does not express the bonds. An ion or molecule with these delocalized electrons can be represented by several canonical structures and these structures are further referred to as the “resonance structures”.

Therefore, in short, the resonance effect can be defined as a chemical phenomenon which is observed in characteristic compounds having double bonds in the organic compounds. The organic compounds contain these double bonds in the structures and usually have the overlapping of the p-orbitals on the two adjacent sides of carbon atoms.
Looking at the options given in the question, the first option is an aromatic hydrocarbon and it is an organic compound having double bonds. So, it will possess a resonance effect which is stabilised by the delocalized pi-electrons.
So, the correct answer is “Option A”.

Note: Usually, the resonance effect is shown by only organic compounds that are aromatic in nature (especially aromatic hydrocarbons with double bonds), where the pi-electrons are delocalised between the carbon-carbon double bonds and in contrast, aliphatic hydrocarbon lack this delocalisation. A compound can be known whether it has resonance effect and aromatic by applying the Huckel’s rule.