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Question

For the following question, an assertion (A) is followed by a corresponding Reason (R). Use the following keys to choose the appropriate answer.
Assertion (A): N(SiH3)3 is a weaker base than N(CH3)3.
Reason (R):  Due to pπ-pπ back bonding in N(SiH3)3, the availability of electrons on the N atom in N(SiH3)3 decreases.
 


Your Answer
A
If both  (A) and (R) are correct and (R) is the correct explanation of (A)
Your Answer
B
If both (A) and (R) are correct but (R) is not the correct explanation of (A)
Correct Answer
C
If (A) is correct , but (R) is incorrect
Your Answer
D
If (A) is incorrect, but (R) is correct

Solution

The correct option is D If (A) is correct , but (R) is incorrect
Assertion suggests that N(CH3)3 is a stronger base while reasoning suggests that N(SiH3)3 is a stronger base. So, only one of these two statements can be right.
We need to check the availability of lone pair of N on each of the two molecules. In N(CH3)3, we see sp3 hybridization and hence tetrahedral geometry. However in N(SiH3)3, we see that there is a possibility of back bonding due to the availability of empty d-orbitals in Si. Thus, it is planar.
Now that we understand the geometry and shape, we can easily understand that the lone pair on N(SiH3)3 is less available for donation. Hence, N(CH3)3 is a stronger base. This was suggested by assertion. Hence, assertion is correct and reasoning isn't.

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