CBSE Class 12 Chemistry 2023 Delhi Set 1 Solved Paper

Question : 31

Total: 35

SECTION - D

The following questions are case - based questions. Read the passage carefully and answer the questions that follow:
Nucleophilic Substitution Nucleophilic Substitution reaction of haloalkane can be conducted according to both SN1 and SN2 mechanisms. SN1 is a two step reaction while SN2 is a single step reaction. For any haloalkane which mechanism is followed depends on factors such as structure of haloalkane, properties of leaving group, nucleophilic reagent and solvent.
Influences of solvent polarity: In SN1 reaction, the polarity of the system increases from the reactant to the transition state, because a polar solvent has a greater effect on the transition state than the reactant, thereby reducing activation energy and accelerating the reaction. In SN2 reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (SN1) of tertiary chlorobutane at 25°C in water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (SN2) of 2-Bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. Hence the level of solvent polarity has influence on both SN1 and SN2 reaction, but with different results. Generally speaking weak polar solvent is favourable for SN2 reaction, while strong polar solvent is favourable for SN1. Generally speaking the substitution reaction of tertiary haloalkane is based on SN1 mechanism in solvents with a strong polarity (for example ethanol containing water).
Answer the following questions:
(a) Why racemisation occurs in SN1 ?
(b) Why is ethanol less polar than water?
(c) Which one of the following in each pair is more reactive towards SN2 reaction?
(i) CH3−CH2−I or CH3CH2−Cl

OR
(c) Arrange the following in the increasing order of their reactivity towards SN1 reactions:
(i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane
(ii) 1-Bromo-3-methylbutane, 2-Bromo-2methylbutane, 2-Bromo-3-methylbutane

Solution: 👈: Video Solution

(a) In SN1 mechanism, there is intermediate carbo cation formed. Due to which recmization of the product take place or D and L form formed.
(b) In ethanol alkyl chain is present which is responsible for non-polar nature. Water has high value of dipole moment than ethanal. That's why water is more polar than ethanal.
(c) (i) CH3−CH2−I will react faster than CH3− CH2−Cl because I is bigger in size and more polarized atom. Its bond dissociation enthalpy is less So, it easily react with other substances.
(ii)

bond length is less and it is closely attached with cyelohexane while 1-methyl 1-chloro cyclohexane is less stable one extra methyl group is attached which make it more reactive towards SN1 mechanism.
OR
(c) (i) 2-Bromo-2-methylbutane >2-Bromopentane>1-Bromopentane
(ii) 1-Bromo-3-methyl butane > 2-Bromo-3methyl butane > 2-Bromo-2-methyl butane.

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