Chemistry Class 11 NCERT Solutions: Chapter 13 Hydrocarbons Part 10

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Q: 17. Write down the products of ozonolysis of 1, 2-dimethylbenzene (o-xylene). How does the result support Kekulé structure for benzene?


o-xylene has two resonance structures:

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) Ethyne

(II) Ethene

(III) Hexane


(I) Benzene from Ethyne:

(II) Benzene from Ethene:

(III) Hexane to Benzene: