Chemistry Class 11 NCERT Solutions: Chapter 13 Hydrocarbons Part 11

Q: 21. Write structures of all the alkenes which on hydrogenation give 2-methylbutane.

Answer:

The basic skeleton of 2-methylbutane is shown below:

On the basis of this structure, various alkenes that will give 2-methylbutane on hydrogenation are:

(A) Equation

(B) Equation

(C) Equation

Q: 22. Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, Equation

(A) Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene

(B) Toluene, Equation .

Answer:

Electrophiles are reagents that participate in a reaction by accepting an electron pair in order to bond to nucleophiles. The higher the electron density on a benzene ring, the more reactive is the compound towards an electrophile, Equation (Electrophilic reaction).

(A) The presence of an electron withdrawing group (i.e., Equation and Equation ) deactivates the aromatic ring by decreasing the electron density.

Since Equation –group is more electron withdrawing (due to resonance effect) than the Equation group (due to inductive effect), the decreasing order of reactivity is as follows:

Equation

(B) While Equation is an electron donating group, Equation group is electron withdrawing. Hence, toluene will have the maximum electron density and is most easily attacked by Equation .

NO2– is an electron withdrawing group. Hence, when the number of Equation substituents is greater, the order is as follows:

Equation

Q: 23. Out of benzene, m–dinitrobenzene and toluene which will undergo nitration most easily and why?

Answer

The ease of nitration depends on the presence of electron density on the compound to form nitrates. Nitration reactions are examples of electrophilic substitution reactions where an electron-rich species is attacked by a nitronium ion Equation

Now, Equation group is electron donating and Equation is electron withdrawing. Therefore, toluene will have the maximum electron density among the three compounds followed by benzene. On the other hand, m– Dinitrobenzene will have the least electron density. Hence, it will undergo nitration with difficulty. Hence, the increasing order of nitration is as follows:

Q: 24. Suggest the name of a Lewis acid other than anhydrous aluminium chloride which can be used during ethylation of benzene.

Answer

The ethylation reaction of benzene involves the addition of an ethyl group on the benzene ring. Such a reaction is called a Friedel-Craft alkylation reaction. This reaction takes place in the presence of a Lewis acid.

Any Lewis acid like anhydrous Equation , Equation , Equation etc. can be used during the ethylation of benzene.

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