Q29 of 30 Page 188

(a) Explain why :

(i) BCl3 is a Lewis acid.


(ii) Boric acid is a monobasic acid.


(b) Compound ‘A’ of boron reacts with excess NH3 to give a compound ‘B’. Compound ‘B’ on heating gives cyclic compound ‘C’. Compound C is called inorganic benzene.


(i) Identify compounds ‘A’, ‘B’ and ‘C’


(ii) Give the reactions involved in these processes.


(a)


(i) As BCl3 is an electron deficient molecule with B having only 6 electrons and therefore it acts as Lewis acid and can accept a pair of electron to complete its octet. This is done on the basis of Lewis concept which states that any electron deficient or positively charged species acts as Lewis acid.


E.g. : NH3 + BCl3→ H3N : BCl3


(ii) Monobasic acids are those which donate one proton to a base in an acid base reaction. Boric acid in water, accepts OH- ion acting as lewis acid and water releases H+ ions. So instead of donating one proton, it accepts one pair of electron and thus it is a monobasic acid and the reaction is as follows:


B(OH)3 + H2O → H+ + [ B(OH)4]-


(b)


(i) A = B2H6 B = B2H6.2NH3 C=B3N3H6


B2H6 reacts with excess NH3 to give B2H6.2NH3 which is formulated as [BH2(NH3)2]+[BH4]-. This on heating gives B3N3H6 (borazine) which is called inorganic benzene.


(ii) Reactions involved in these processes are-


3B 2H6 +6NH3 → 3[BH2 (NH3 )2 ]+ [BH4 ]- → 2B3N3H6 +12H


More from this chapter

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26

Assertion (A): Nitration of benzene requires the use of concentrated sulphuric acid and nitric acid.

Reason (R): The mixture of acids produces the electrophile for the reaction.


A. A and R both are correct but R is not the explanation of A.


B. A is false but R is correct.


C. A and R both are correct and R is the correct explanation of A.


D. A and R both are incorrect.


 27

Assertion (A) : Ozone is destroyed by solar radiations in upper stratosphere.

Reason (R) : Thinning of ozone layer allows excessive UV radiations to reach the surface of earth.


A. A and R both are correct but R is not the explanation of A.


B. A is false but R is correct.


C. A and R both are correct and R is the correct explanation of A.


D. A and R both are incorrect.


 28

(a) Liquids can be considered as very dense gases. When a liquid phase changes to gas phase, the liquid and the gas phases are in equilibrium and a surface separates the two phases. This surface is visible if both phases are in equilibrium and are below critical tempertaure and pressure. However, it is possible to interconvert liquid and gas wherein two phases are never present together.

With the help of a well-labelled diagram show that CO2 gas can be liquified by changing the pressure and temperature without passing through the situation when both gaseous and liquid CO2 are at equilibrium.


(b) Arrange the following liquids in increasing order of their viscosities. Give reason for your answer.


Water, benzene, ethane-1,2-diol.


 30

(a) Write two important differences between inductive and resonance effects.

(b) Give reasons to explain the following observations:


(i) Carbon number ‘2’ in CH3CH2Cl has more positive charge than that in CH3CH2Br.


(ii) CH3–CH = CH–CH = CH2 (I) is more stable than CH3–CH = CH–CH2–CH = CH2 (II).