KVPY Interview Important Questions Chemistry Part 3
Q: 16. Write Mechanism. Draw its energy profile
Sol. Bimolecular nucleophilic substitution reaction
reaction is also a characteristic reaction of alkyl halide, alcohol ethers.
Nucleophilic substitution in which incoming group replaces leaving group in one step only is known as reaction
The overall reaction mechanism can be visualized with stereochemistry as followed:
1. It is bimolecular, one step concerted process
2. It is second order reaction because in the rds both species are involved
3. Kinetics of the reaction:
rate∝ [substrate] [nucleophile]
If the concentration of substrate in the reaction mixture is doubled, the rate of the nucleophilic substitution reaction is doubled. If the concentration of nucleophile is doubled, the rate of reaction is also doubled. If the concentration of both are doubled then the rate of the reaction quadruples.
4. Energetics of the reaction:
Figure: A free energy diagrams for reaction
5. No intermediates are formed in the reaction, the reaction proceeds through the formation of an unstable arrangement of atoms or group called transition state
6. The stereochemistry of reactions → As we seen earlier, in an mechanism the nucleophile attacks from the back side, that is from the side directly opposite to the leaving group. This mode of attack causes an inversion of configuration at the carbon atom that is the target of nucleophilic attack. This inversion is also known as Walden inversion.
7. Factor’s affecting the rate of reaction: Number of factors affect the relative rate of reaction, the most important factors are
(i) Effect of the structure of the substrate:
The important reason behind this order of reactivity is a steric effect. Very large and bulky groups can often hinder the formation of the required transition state and crowding raises the energy of the transition state and slows down reaction.
(ii) Concentration and reactivity of the nucleophile
As nucleophilicity of nucleophile increases rate of increases
Anionic nucleophiles mostly give reaction
A stronger nucleophile attacks upon with faster rate than the rate of departing of leaving group
(iii) The nature of the leaving group: Weaker bases are good leaving groups. A good leaving group always stabilize the transition state and lowers its free energy of activation and there by increases the rate of the reaction Order of leaving ability of halide ion
Reagents for alkyl halide are: …….(Strong anionic nucleophile)
Q: 17. In reaction if concentration of reactant and the nucleophile is doubled then what will be effect on rate of Reaction
Sol. Rate of reaction increase four fold
Q: 18. ZnO is an acidic or basic oxide?
Sol. ZnO is an amphoteric oxide.
Q: 19. Which compound is more covalent in
Sol. is more covalent then . According to Fazan’s rule, due to has Pseudo inert gas configuration. Its polarization power is more then so is more covalent.
Q: 20. Why are standard reduction potentials of elements above Hydrogen in the Electrochemical series negative and why has IUPAC given them such a convention?
Sol: In ECS more reactive metal placed at above Hydrogen they are reducing agent and get oxidizes with in galvanic cell they behave as a Anode, so their standard oxidation reduction potential is positive and SRP value is negative.
Q: 21. What would you define as the standard electrode potential?
Sol. Electrode potential of any electrode at standard condition is known as standard electrode potential. It is measured with the S H E by formation of a galvanic cell.
Q: 22. What do you understand by hybridisation?
Sol. Mixing of one s-orbital and 2 p-orbital of same orbit of any central atom formed new hybrid orbitals. Shape or geometry of these hybrid orbital is triangular planner.
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