SN1 Reaction

In SN1 reaction ‘S’ stands for substitution, ‘N’ stands for nucleophilic, and ‘1’ for unimolecular. It is proceeded by the ionization mechanism. This reaction first involves bond breaking between carbon and leaving group. Later, the bond is formed between carbon and nucleophile.

What is the SN1 reaction?

Hughes and coworkers observed the hydrolysis of tert-butyl chloride. They found that the formation of tert-butyl alcohol is different from other substitutions reactions.  They observed that the reaction rate only depends on the concentration of tert-butyl chloride. Hence, neither water nor hydroxide is involved in the rate-determining step. Thus the reaction is 1st order. It is known as the SN1 reaction (substitution nucleophilic unimolecular).

Mechanism of an SN1 reaction

There are the following steps are involved in the SN1 mechanism:

Step 1

In the first step, heterolytic cleavage of the carbon-halogen bond occurs. It results in the formation of a carbocation. It is a highly endothermic step. This is because no other bond is formed.

Step 2

In the second step, the carbocation combines with the nucleophile. It results in the formation of a tert-butyloxonium ion.  However, mostly the solvent itself acts as a nucleophile.

Step 3

The proton transfer from the oxonium ion to the water is the last step. Hence, the formation of tert-butyl alcohol takes place.

Rate determining step

This mechanism involves more than one step. However, the slow step determines the rate of reaction. Therefore, it is known as the rate-determining step. In SN1 reaction first step is the slow step. Hence, its rate only depends upon the concentration of alkyl halide.

Stability of carbocation in sn1 reaction

The tertiary carbocations are most stable. This is because three alkyl groups stabilize positive charge by electron-donating effect.  On the other hand, primary carbocations are the least stable.

The overall order of stability is as follows:

The SN1 mechanism involves carbocation formation. Therefore tertiary alkyl halide is preferable for the SN1 reaction.

Stereochemistry of sn1 reaction

The SN1 mechanism involves the formation of carbocation as an intermediate. However, the central carbon of the intermediate is sp2 hybridized. Therefore, it has trigonal planar geometry and achiral. The unhybridized p-orbital on central carbon is perpendicular to the plane of the molecule. A carbocation is a symmetrical molecule. Therefore, the nucleophile can attack from either side of the plane. Hence, the product is formed as a racemic mixture.

 

Solvolysis

” A nucleophilic substitution in which nucleophile is the molecule of the solvent is known as solvolysis reaction.”

The SN1 reaction of an alkyl halide with water is an example of solvolysis.

  • When the solvent is water it is known as hydrolysis.
  • When alcohol act as a solvent it is called methanolysis.

Examples of solvolysis 

(CH3)3C-Br + H2O → (CH3)3C-OH + HBr

(CH3)3C-Br + CH3OH → (CH3)3-OCH3 + HBr

Factors affecting SN1 reaction

There is a number of factors that affect this reaction. The most important are the following:

The effect of the substrate

In this reaction, the reactivity of the substrate depends on the stability of carbocation. The order of stability of carbocation is the following:

tertiary > secondary > primary > methyl

Therefore, the SN1 mechanism is more favorable for the tertiary subtrate.

The effect of the nucleophile

The nucleophile is not involved in the rate-determining step of the SN1 reactions. Therefore, its concentration and nature do not influence the rate of this reaction. For example, the rate of hydrolysis of tert-butyl chloride is not affected by changing the nucleophile from H2O to OH.

Concepts Berg

Why is it called SN1?

SN1 is a unimolecular nucleophilic substitution reaction. Therefore, it is represented by the S(substitution), N(nucleophilic), and 1(unimolecular).

What are the characteristics of an SN1 reaction?

There are the following characteristics of SN1 reaction:

  • It is a two-step reaction.
  • Carbocation formation occurs
  • The first step is the rate-determining step.
  • The tertiary alkyl halides are preferable.
  • It has an ionization mechanism.

What favors an SN1 reaction?

This reaction is favored by the polar protic solvent.

Is SN1 one or two steps?

SN1 mechanism is a two-step process.

Does an excellent leaving group favor SN1 or SN2?

SN1 and SN2 both require good leaving groups. Therefore, this factor does not affect the decision of the mechanism.

What are the similarities of SN1 and SN2 reaction mechanisms?

There are the following similarities between the SN1 and SN2 mechanisms:

  • Both are nucleophilic substitution reactions.
  • Alkyl halides show both mechanisms.
  • Both are not required high temperatures.

Reference Books 

  • Organic Chemistry by T.W. Graham Solomons (University of South Florida), Craig B. Fryhle (Pacific Luthern University), and Scott A. Snyder (University of Chicago)

Reference Links 

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