Chemistry Class 11 NCERT Solutions: Chapter 13 Hydrocarbons Part 10 (For CBSE, ICSE, IAS, NET, NRA 2022)

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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?

Answer

o-xylene has two resonance structures:

Q_17_Ans_1_Structure of O-Xylene Has Teo Resonance

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.

Answer:

Acidic character of a species is defined on the basis of ease with which it can lose its H – atoms.

Structure of Arrange Benzene, N-Hexane, and Ethyne in Decrea …

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?

Answer

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

Answer

(I) Benzene from Ethyne:

Q_20_Ans_I_Structure of Benzene from Ethyne

(II) Benzene from Ethene:

Q_20_Ans_II_Structure of Benzene from Ethene

(III) Hexane to Benzene:

Q_20_Ans_III_Structure of Hexane to Benzene

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