Sn1 Reaction and Characteristics

Unimolecular Nucleophilic Substitution Reaction (S_N¹)

Unimolecular substitution reactions occur on substrates where the back side of the leaving group is not accessible. In general, tertiary alkyl halides react the best in S_N¹ reactions.

These reactions take place in two steps and proceed via a carbocation intermediate. The first step of the reaction is the formation of a carbocation after the removal of the leaving group (usually a halide ion). The carbocation is then attacked by the nucleophile forming the final substituted product.

The rate-determining (slow) step of the reaction is the first step where the halide ion is removed and the carbocation is formed.
Since the transition state of the slow step only involves the substrate (alkyl halide), the rate of the reaction depends only on the concentration of the alkyl halide. The reaction is a first-order reaction.

Rate = k[RX]

 

Stereochemistry of the Reaction

In addition to the mechanism, the S_N¹ reaction is also different from the S_N² reaction in terms of the product stereochemistry. This reaction results in the racemization of the stereocenter at which the substitution takes place.

The racemization is due to the formation of the planar carbocation. Unlike the S_N² reaction where the nucleophile only attacks from the back side, the nucleophile in S_N¹ reaction can attack the carbocation from either side to create two different enantiomers in equal amounts (racemization).

 

Factors Affecting the Rate of S_N¹ Reaction

The S_N¹ reaction takes place in two steps: Generation of the carbocation and the reation of carbocation with the nucleuophile to yield the final product. The first step of the reaction is the rate-determining step and that does not include the nucleophile. So the rate of an S_N¹ reaction is unaffected by the concentration or the nucleophilicity of the nucleophile.

 

Leaving Group Effect

The transition state for the slow step of an SN₁ reaction includes the substrate with the leaving group partially attached to the prospective carbocation. The rate of this step depends upon how fast or slow the bond between the carbon atom and the leaving group breaks. Substates with good leaving groups react fast while substrates with poor leaving groups react slowly. The leaving group ability is dependent upon the electronegativity and the size of the leaving group. Highly electronegative or small atoms are poor leaving groups while atoms with low electronegativty and large size are good leaving groups.

Nature of the Substrate

The nature of the substate itself is a major deciding factor in the S_N¹ reactivity of a molecule. Substrates that can generate stable carbocations react easily under S_N¹ reactions. A general trend of carbocation stability is shown below:

The reason behind the stability of secondary and tertiary carbocations is hyperconjugation. Hyperconjugation is the name given to a stabilizing interaction between a hybridized filled orbital and an unhybridized empty orbital.

As can be seen from the figure above, the electrons in the C–H σ-bonds stabilize the empty p-orbital of the carbocation. The greater the number of these C–H σ-bonds, the greater the stability. That is why tertiary carbocation are the most stable because they provide the greatest degree of hyperconjugation.

 

Solvent Effect

The S_N¹ reactions are similar to the S_N² reactions in a way that their rates are dependent upon the nature of the solvent. However, the reactivity trend is entirely opposite: S_N¹ reactions proceed the fastest in polar protic solvents. This trend in reactivity is due to the mechanism of the S_N¹ reaction. Since the mechanism of an S_N¹ reaction involves the formation of a carbocation, a solvent that can facilitate the bond cleavage between the carbon atom and the solvate works best in these reactions.

Polar protic solvent increase the rate of S_N¹ reactions because they can solvate both the carbocation and the leaving group. This solvation process results in the weakening of the bond between the carbon atoms and the leaving group and the rate of the reaction is increased. The figure below shows how water, a polar protic solvent, can solvent both a carbocation and a leaving group.

Some Notable Features of the Solvents

Not only a solvent can alter the rate of a substitution reaction, it can even alter the mechanism of the reaction for some substrates. It has been discussed that primary substrates cannot react under S_N¹ conditions due to the unstable nature of primary carbocations and that tertiary substrates react preferably by the S_N¹ mechanism. But what about secondary substrates? Secondary carbocations are neither very unstable nor very stable. In most cases, secondary substrates react via both mechanisms. However, the predominant mechanism is decided by the nature of the solvent.

Polar protic solvents promote the S_N¹ mechanism while polar aprotic solvents promote the S_N² mechanism.

Take an example of this benzylic substate.

When this reaction is carried out in acetone (an aprotic solvent or medium polarity), the major product is the one where inversion has taken place. Although there is some S_N¹ mechanism operating because as evident by the product with retained configuration, the major mechanism operating here is S_N². This shows how a polar aprotic solvent has shifted the choice of mechanism for a secondary substate.

Here's an example of how a polar protic solvent (H₂O) shifts the mechanism towards S_N¹ and results in racemization of the product.