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Chapter 13 Hydrocarbons Solutions

Question - 21 : - Write structures of all the alkenes which on hydrogeneration give 2-methylbutane.

Answer - 21 : -


Question - 22 : - Arrange the following set of compounds in order oftheir decreasing relative reactivity with an electrophile, E+.
(a) Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
(b) Toluene,p—H3C—C6H4—NO2,p—O2N—C6H4—NO2.

Answer - 22 : -

(a) The typical reactions of benzeneare electrophilic substitution reactions. Higher the electron-density in thebenzene ring, more reactive is the compound towards these reactions. Since N02is a more powerful electron-withdrawing group than Cl, therefore, more thenumber of nitro groups, less reactive is the compound. Thus, the overallreactivity decreases in the order:
Chlorobenzene > p-nitrochlorobenzene > 2, 4-dinitrochlorobenzene
(b) Here, CH3 group is electron donating but N02 group is electron-withdrawing.Therefore, the maximum electron-density will be in toluene, followed byp-nitrotoluene followed by p-dinitrobenzene. Thus, the overall reactivitydecreases in the order:
Toluene >p—H3C—C6H4—NO2, p—O2N—C6H4—NO2.

Question - 23 : - Out of benzene, m-dinitrobenzene and toluene whichwill undergo nitration most easily and why?

Answer - 23 : - CH3 groupis electron-donating while—NO2 group is electron-withdrawing.Therefore, maximum electron density will be in toluene, followed by benzene andleast in m-dinitrobenzene. Therefore, the ease of nitration decreases in theorder: toluene > benzene > m-dinitrobenzene.

Question - 24 : - Suggest the name of another Lewis acid instead ofanhydrous aluminium chloride which can be used during ethylation of benzene.

Answer - 24 : -

Anhydrous Ferric Chloride (FeCl3)is another Lewis acid which can be used.

Question - 25 : - Why is Wurtz reaction not preferred for preparationof alkanes containing odd number of carbon atoms? Illustrate your answer bytaking one example.

Answer - 25 : - Forpreparation of alkanes containing odd number of carbon atoms, a mixture of twoalkyl halides has to be used. Since two alkyl halides can react in threedifferent ways, therefore, a mixture of three alkanes instead of the desiredalkane would be formed. For example, Wurtz reaction between ‘1-bromopropane and1-bromobutane gives a mixture of three alkanes i.e., hexane, heptane and octaneas shown below:

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