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Hydrocarbons Class 11th

13.23. Out of benzene, m-dinitrobenzene and toluene which will undergo nitration most easily and why?

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Vishal Baghel

Contributor-Level 10

CH₃ group is electron-donating while -NO₂group is electron-withdrawing. Therefore, maximum electron density will be in toluene, followed by benzene and least in m-dinitrobenzene. Therefore, the ease of nitration decreases in the order: toluene > benzene > m-dinitrobenzene.

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Hydrocarbons Class 11th

13.22. Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E⁺.
(a) Chlorobenzene, 2, 4-dinitrochlorobenzene, p-nitrochlorobenzene
(b) Toluene,p—H₃C—C₆H₄—NO₂, p—O₂N—C₆H₄—NO₂.

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Vishal Baghel

Contributor-Level 10

(a) The typical reactions of benzene are electrophilic substitution reactions. Higher the electron-density in the benzene ring, more reactive is the compound towards thesereactions. Since NO₂ is a more powerful electron-withdrawing group than Cl, therefore, more the number of nitro groups, less reactive is the compound. Thus, the overall reactivity decreases in the order:Chlorobenzene > p-nitrochlorobenzene > 2, 4-dinitrochlorobenzene

(b) Here, CH₃ group is electron donating but NO₂ group is electron-withdrawing. Therefore, the maximum electron-density will be in toluene, followed by p-nitrotoluene followed by p-dinitrobenzene. Thus,

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Hydrocarbons Class 11th

13.21. Write structures of all the alkenes which on hydrogeneration give 2-methylbutane.

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Vishal Baghel

Contributor-Level 10

The basic skeleton structure of 2-methylbutane is

Placing double bonds at different positions and satisfying the tetra covalency of each carbon, the structures of various alkenes which give 2-methylbutane on hydrogenation are:

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Hydrocarbons Class 11th

13.20. How would you convert the following compounds into benzene?
(i) Ethyne (ii) Ethene (iii) Hexane.

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Hydrocarbons Class 11th

13.19. Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?

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Vishal Baghel

Contributor-Level 10

Benzene is a rich source of electrons because of the presence of an electron cloud containing 6 n-electrons above and below the plane of the ring. Consequently, it attracts the electrophiles (electron-deficient) reagents towards it and repels nucleophiles (electron- rich) reagents. As a result, benzene undergoes electrophilic substitution reactions easily and nucleophilic substitutions with difficulty.

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Hydrocarbons Class 11th

13.18. Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.

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Vishal Baghel

Contributor-Level 10

The hybridization state of carbon in these three compounds is:

Since s-electrons are closer to the nucleus, therefore, as the s-character of the orbital making the C—H bond increases, the electrons of C—H bond lie closer and closer to the carbon atom. In other words, the partial +ve charge on the H-atom and hence the acidic character increases as the s-character of the orbital increases. Thus, the acidic character decreases in the order: Ethyne > Benzene > Hexane.

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Hydrocarbons Class 11th

13.17. Write down the products of ozonolysis ofl, 2-dimethylbenzene (o-xylene). How does the result support Kekule structure of benzene?

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Vishal Baghel

Contributor-Level 10

o-Xylene may be regarded as a resonance hybrid of the following two Kekule structures. Ozonolysis of each one of these gives two products as shown below:

Thus, in all, three products are formed. Since all the three products cannot be obtained from any one of the two Kekule structures, this shows that o-xylene is a resonance hybrid of the two Kekule structures (I and II).

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Hydrocarbons Class 11th

13.16. Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.

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Vishal Baghel

Contributor-Level 10

Addition of HBr to propene is an ionic electrophilic addition reaction in which the electrophile, i.e., H⁺ first adds to give a more stable 2° carbocation. In the 2^nd step, the carbocation is rapidly attacked by the nucleophile Br~ ion to give 2-bromopropane.

In presence of benzoyl peroxide, the reaction is still electrophilic but the electrophile here is a Br free radical which is obtained by the action of benzoyl peroxide on HBr.

In the first step, Br radical adds to propene to generate the more stable 2° free radical. In the second step, the free radical thus obtained rapidly abstracts a hydrogen atom from HBr to give 1-bromopr

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