All three products, i.e., methyl glyoxal, 1, 2-demethylglyoxal, and glyoxal are obtained from two Kekule structures. Since all three products cannot be obtained from any one of the two structures, this proves that a-xylene is a resonance hybrid of two Kekule structures (I and II).
Q: 18. Arrange benzene, n-hexane, and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.
Acidic character of a species is defined on the basis of ease with which it can lose its H– atoms.
The hybridization state of carbon in the given compound is:
As the s–character increases, the electronegativity of carbon increases, and the electrons of bond pair lie closer to the carbon atom. As a result, partial positive charge of H– atom increases and ions are set free.
The s–character increases in the order:
Hence, the decreasing order of acidic behaviour is .
Q: 19. Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?
Benzene is a planar molecule having delocalized electrons above and below the plane of ring. Hence, it is electron-rich. As a result, it is highly attractive to electron deficient species i.e., electrophiles.
Therefore, it undergoes electrophilic substitution reactions very easily. Nucleophiles are electron-rich. Hence, they are repelled by benzene. Hence, benzene undergoes nucleophilic substitutions with difficulty.
Q: 20. How would you convert the following compounds into benzene?
(I) Benzene from Ethyne:
(II) Benzene from Ethene:
(III) Hexane to Benzene: