Test: Reaction Mechanism Level - 3 - Chemistry MCQ

Substitution reaction:

1. A substitution reaction is a type of chemical reaction, in which one functional group is replaced by another functional group.
2. Metal complexes in which the rate of ligand displacement reaction is very fast and hence show very high reactivity is called labile complexes.
3. Metal complexes in which the rate of ligand displacement reaction is very slow and hence show less reactivity are called labile complexes.

Explanation:

1. In complexes, I, II, and III the metal ion involved is Ni, Pd, and Pt. Ni, Pd, and Pt are the elements of the 3d, 4d, and 5d series respectively. 
2. The electrons present in d orbitals cannot shield the valence electrons and hence experience a much more effective nuclear charge on it.
3. From 3d to 5d orbital, the effective nuclear charge on the valence electrons increases. Hence moving down the group from 3d to 5d series along a group, the inertness increases.
4. So, from Ni to Pt, the inertness increases, and hence the rate of substitution of chloride by pyridine decreases.

Conclusion: Hence, the order of rate of substitution of chloride by pyridine (in ethanol) in the following complexes is I > II > III.

NaOEt is a strong base and it will abstract the acidic proton from the alpha carbon. Dueterium is abstracted from alpha carbon because it is show more +I effect than hydrogen. After that then elimination will occur with bromide as leaving group.

Hence C

Singlet carbenes give stereospecific reactions. Option A is an example of a stereospecific reaction.

SN1 reaction proceeds through carbocation as intermediate. Br- removed and made up carbocation in option A, B, C then we check the stability of carbonations. C is more stable because carbocation of C is aromatic, A is non aromatic and B is anti aromatic. So order of stability is:

Aromatic > Non-Aromatic > Anti Aromatic. Then more stable carbocation higher will be the reactivity and faster will be the SN1 reaction.

Hence C is correct.