Question

Case study : Nucleophilic Substitution

Nucleophilic Substitution reaction of haloalkane can be conducted according to both S_N1 and S_N2 mechanisms. S_N1 is a two step reaction while S_N2 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 S_N1 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 S_N2 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 (S_N1) of tertiary chlorobutane at 25°C in water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_N2) 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 S_N1 and S_N2 reaction, but with different results. Generally speaking weak polar solvent is favourable for S_N2 reaction, while strong polar solvent is favourable for S_N1. Generally speaking the substitution reaction of tertiary haloalkane is based on S_N1 mechanism in solvents with a strong polarity (for example ethanol containing water).

Answer the following questions :

(a) Why racemisation occurs in S_N1 ?

(b) Why is ethanol less polar than water ?

(c) Which one of the following in each pair is more reactive towards S_N2 reaction ?

(i) \(CH_3 - CH_2 -I \ \text{or}\ CH_3CH_2 - Cl\) 

(ll)

  

OR

(c) Arrange the following in the increasing order of their reactivity towards S_N1 reactions :

(i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane

(ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3-methylbutane

Answer 1

(a) In S_N1 reaction, carbocation intermediate formed is a planar specie (sp² hybridised). Therefore, attack of nucleophile can occur from both front and back side with same ease, giving racemic mixture

Example: 

 

 

(b) Ethanol is less polar than water as O is attached to two hydrogen atoms in case of water and O is attached to one hydrogen atom and one carbon atom. Electronegativity of carbon atom is more than that of hydrogen atom making it less polar.

(c) (i) CH₃ — CH₂ — I is more reactive than CH₃ — CH₂ — Cl towards S_N2 reaction because \(I^{\ominus}\) is a good leaving group as compared to Cl^— because of its large size.

(ii)

 

is more reactive than

towards SN2 reaction because it is primary halide

OR

(c) (i) Order of reactivity towards S_N1 reaction is 1-Bromopentane < 2-Bromopentane < 2-Bromo-2-methylbutane

Carbocation intermediate formed during S_N1 in case of 2-Bromo-2-methyl butane is more stable (\(3 ^\circ\) carbocation formed) than 2-Bromopentane in which \(2^\circ\) carbonation is formed and 1-bromopentane is least reactive due to formation of less stable \(1^\circ\) carbocation.

(ii) Order of reactivity is governed by stability of carbocation formed is step 1 in S_N1 reaction. Stability of alkyl carbocations is

\(3 ^\circ\)> \(2^\circ\) > \(1^\circ\)

\(\therefore\) Order of reactivity is

1-Bromo-3-methylbutane < 2-Bromo-3-methylbutane < 2-Bromo-2-methylbutane