Unimolecular nucleophilic substitution reactions SN1

 

What are Nucleophilic substitution reactions S_N?

Explain unimolecular nucleophilic substitution reactions S_N1.

Nucleophilic substitution reactions:

The reaction of alkyl halide with aqueous alkali involves the replacement of the halide ion by the hydroxide ion to yield alcohol.

            CH₃__Cl + OH^-  →  CH₃__OH + Cl^-1

       Substrate  nucleophile    product  leaving group

 When the bond between methyl group and chlorine is broken, the latter takes electron pairs with it rendering relatively positive site, i.e. carbon in methyl group easily attacked by hydroxide ion. An organic compound , like methyl chloride attacked by reagent is called substrate.

Nucleophile : A reagent with electron pair, like hydroxide ion that tends to attack an electron deficient center is called a nucleophile(nucleus loving). Nucleophile can be neutral(H₂O) or negatively charged specie(OH^-).

Electrophile : An electron deficient specie that loves electrons is called electrophile. Examples include positive charged ions and atom with incomplete octet, for example: H⁺, CH₃⁺,BH₃, BeF₂, AlCl₃.

Leaving group : The part of substrate that departs along with the electron pair like chlorine is called leaving group(nucleofuge).

Nucleophilic substitution : The type of reaction in which part of molecule is substituted by a nucleophile is called nucleophilic substitution and is denoted by S_N.

Alkyl halides are good substrates for nucleophilic substitution reactions because the halide ion is a good leaving group. Sometimes solvent itself functions as a nucleophile the reaction is then called solvolysis.

S_N1 Reactions Mechanism:

The S_N1 mechanism consists of two steps. The first step involves slow ionization of the substrate resulting in the formation of a carbocation that rapidly combines with the nucleophile to form the product in the second step.

                         

In a reaction involving more than one steps the slowest step determines the rate of the reaction and is therefore called the rate determining step. So, in this reaction the ionization of substrate is the rate determining step.

Kinetics of S_N1 Reactions:

In reaction following the S_N1 mechanism, only the substrate is involved in the rate determining step. The rate therefore should be depending only on the concentration of the substrate, i.e. it should be a first order reaction with the following rate law.

               Rate = k₁[(CH₃)₃CCl]

Since only one molecule , i.e. the substrate is involved in the formation of the activated complex in the rate determining step , the reaction is called a unimolecular reaction.

Stereochemical evidence:

This mechanism involves the formation of a carbocation as an intermediate. Since the central carbon atom of the carbocation is sp²hybridized, it is a planar molecule. The unhybridized p orbital on carbon is perpendicular to the plane of the molecule with one lobe on each side of the plane.

The nucleophile can therefore attack from either side of the plane to form a bond with the carbocation. The chances of attack on either sides are equal and if we start with optically active substance, we are expected to get a racemic mixture of the product.

 

 Rearrangement of carbocation:

The formation of carbocation as an intermediate is also supported by the fact that it can undergo rearrangement to a more stable carbocation, before it combines with the nucleophile.

                      

Factors affecting S_N1 Reactions:

Effect of substrate: S_N1 mechanism involves the formation of carbocation, the easier is the formation of carbocation more rapidly it is formed. Because it depends upon stability of carbocation. The order of stability is tertiary >> secondary > primary> methyl. Thus S_N1 reaction is more suitable for tertiary substrates than secondary and primary.

Effect of nucleophile: Nucleophile is not involved in the rate determining step of an S_N1 reaction. The rate of S_N1 reaction is therefore not influenced by the nucleophile. For example, rate of hydrolysis of tert-butyl bromide, which follows S_N1 mechanism, is not affected by change of nucleophile.

Effect of leaving group: The ability of a group to act as a leaving group is inverse of its basicity; the weakest base is the best leaving group. Thus, among halides iodide is the best leaving group. Since S_N1 reactions do not require powerful nucleophile but require good leaving groups.

Effect of solvent: The greater the polarity of a solvent the greater its ability to stabilize a charged specie. The S_N1 reactions in which carbocation is formed in the rate determining step, are more favorable in polar solvents.

Energy Diagram for S_N1 mechanism: The following energy diagram shows energy changes for reaction between tert-butyl chloride and OH^-.

  

It is a two step process so diagram has two curves.

Some important MCQs:

1.   1. The S_N1 mechanism for the hydrolysis of an alkyl halide to an alcohol involves the formation of

(a)carbocation               (b)carbanion

(c)free radical                (d)pentavalent carbon in the  transition state

     Ans. (a)

 2.  Isopropyl chloride undergoes hydrolysis by the mechanism

 (a)S_N1                         (b)S_N1 and S_N2

 (c)S_N2                          (d)neither S_N1 norS_N2

    Ans. (b) because it is a secondary alkyl halide

2.  

 3. In the reaction CH₃CH(Br)CH(CH₃)CH₃ +C₂H₅O^-  → X ^(S_N¹ mechanism) the major product is

                                                 

    (a)CH₃CH₂C(OC₂H₅)(CH₃)CH₃               

    (b)CH₃CH(CH₃)CH₂CH₂OC₂H₅

     (c)CH₃CH(OC₂H₅)CH(CH₃)CH₃                       

     (d)none of these

               Ans. (a) first a secondary carbocation is formed after rearrangement most stable carbocation, tertiary carbocation is formed then second step of attack of nucleophile takes place.

For details on S_N² reactions visit Bimolecular Nucleophilic substitution(SN2) Reactions