Question ID - 175083 | SaraNextGen Top Answer

Nucleophilic substitution reaction of haloalkane can be conducted according to both $\mathrm{S}_{\mathrm{N}}{ }^{1}$ and $\mathrm{S}_{\mathrm{N}}^{2}$ mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent.
Influences of halogen: No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is, the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom; that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is $\mathrm{I}^{-}<\mathrm{Br}^{-}<\mathrm{Cl}^{-}<\mathrm{F}^{-}$and the order of their leaving tendency should be $\mathrm{I}^{-}>\mathrm{Br}^{-}>\mathrm{Cl}^{-}>\mathrm{F}^{-}$. Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI $>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}$. In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on $\mathrm{S}_{\mathrm{N}}^{1}{ }^{1}$ mechanism. If the leaving group is not easy to leave, the reaction is based on $\mathrm{S}_{\mathrm{N}}^{2}$ mechanism.

Influences of solvent polarity: In $\mathrm{S}_{\mathrm{N}}{ }^{1}$ reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In $\mathrm{S}_{\mathrm{N}}^{2}$ 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 $\left(\mathrm{S}_{\mathrm{N}}^{1}\right)$ of tertiary chlorobutane in $25^{\circ} \mathrm{C}$ water (dielectric constant 79$)$ is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate $\left(\mathrm{S}_{\mathrm{N}}^{2}\right)$ of 2-bromopropane and $\mathrm{NaOH}$ in ethanol containing $40 \%$ water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both $\mathrm{S}_{\mathrm{N}}{ }^{1}$ and $\mathrm{S}_{\mathrm{N}}{ }^{2}$ reactions, but with different results. Generally speaking, weak polar solvent is favorable for $\mathrm{S}_{\mathrm{N}}{ }^{2}$ reaction, while strong polar solvent is favorable for $\mathrm{S}_{\mathrm{N}}{ }^{1}$ reaction, because only under the action. of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on $\mathrm{S}_{\mathrm{N}}^{1}$ mechanism in solvents with a strong polarity (for example, ethanol containing water).
(Ding, Y. (2013). ABrief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.)

1.$\mathrm{S}_{\mathrm{N}}{ }^{1}$ mechanism is favoured in which of the following solvents:
(a) benzene
(b) carbon tetrachloride
(c) acetic acid
(d) carbon disulphide
2.Nucleophilic substitution will be fastest in case of:
(a) 1-Chloro-2,2-dimethyl propane
(b) 1-Iodo-2,2-dimethyl propane
(c) 1-Bromo-2,2-dimethyl propane
(d) 1-Fluoro-2,2-dimethyl propane
3.$\mathrm{S}_{\mathrm{N}}^{1}$ reaction will be fastest in which of the following solvents?
(a) Acetone (dielectric constant 21)
(b) Ethanol (dielectric constant 24 )
(c) Methanol (dielectric constant 32 )
(d) Chloroform (dielectric constant 5)
4.Polar solvents make the reaction faster as they:
(a) destabilize transition state and decrease the activation energy
(b) destabilize transition state and increase the activation energy

(c) stabilize transition state and increase the activation energy
(d) stabilize transition state and decrease the activation energy

5.$\mathrm{S}_{\mathrm{N}}^{1}$ reaction will be fastest in case of:
(a) 1-Chloro-2-methyl propane
(b) 1-Iodo-2-methyl propane
(c) 1-Chlorobutane
(d) 1-Iodobutane

a)c,b,c,d,b   b)c,b,c,a,b  c)c,a,c,d,b  d)c,b,c,d,d