Ch 8 : SN1 mechanism

Chapter 8: Nucleophilic Substitution

S_N1 mechanism

S_N1 indicates a substitution, nucleophilic, unimolecular reaction, described by the expression rate = k [R-LG].
This implies that the rate determining step of the mechanism depends on the decomposition of a single molecular species.

This pathway is a multi-step process with the following characteristics:

		step 1: slow loss of the leaving group, LG, to generate a carbocation intermediate, then

		step 2 : rapid attack of a nucleophile on the electrophilic carbocation to form a new s bond

reaction coordinate diagram for a two step process

Multi-step reactions have intermediates and a several transition states (TS).

In an S_N1 there is loss of the leaving group generates an intermediate carbocation which is then undergoes a rapid reaction with the nucleophile..

reaction coordinate diagram for an S<sub>N</sub>1

General case

S_N1 reaction

Let's look at how the various components of the reaction influence the reaction pathway:

R-
Reactivity order : (CH₃)₃C- > (CH₃)₂CH- > CH₃CH₂- > CH₃-

In an S_N1 reaction, the rate determining step is the loss of the leaving group to form the intermediate carbocation. The more stable the carbocation is, the easier it is to form, and the faster the S_N1 reaction will be. Some students fall into the trap of thinking that the system with the less stable carbocation will react fastest, but they are forgetting that it is the generation of the carbocation that is rate determining.
Since a carbocation intermediate is formed, there is the possibility of rearrangements (e.g. 1,2-hydride or 1,2-alkyl shifts) to generate a more stable carbocation. This is usually indicated by a change in the position of the substituent or a change in the carbon skeleton of the product when compared to the starting material.

The following JSMOL images show a series of alkyl bromides and their relative rates of reaction in an S_N1 hydrolysis.
Try to correlate the structure of the alkyl bromide with the type of carbocation that will be formed.
If you need help, click the button to show you where the carbocation would be formed - it will highlight the electrophilic center.

	Highlight C+ center	Highlight C+ center	Highlight C+ center	Highlight C+ center
Relative rate of hydrolysis	1	2	43	100,000,000

You should have found that the carbocations get more stable as you go left to right in the table. As the carbocation gets easier to form, so the rate of reaction increases.

-LG
The only event in the rate determining step of the S_N1 is breaking the C-LG bond. Therefore, there is a very strong dependence on the nature of the leaving group, the better the leaving, the faster the S_N1 reaction will be.

Nu
Since the nucleophile is not involved in the rate determining step, the nature of the nucleophile is unimportant in an S_N1 reaction. However, the more reactive the nucleophile, the more likely an S_N2 reaction becomes.

Stereochemistry

In an S_N1, the nucleophile attacks the planar carbocation. Since there is an equally probability of attack on each face there will be a loss of stereochemistry at the reactive center as both products will be observed.

Nu can attack either face of a C+ giving products that are mirror images

Solvent
Polar solvents which can stabilise carbocations which can favour the S_N1 reaction (e.g. H₂O, ROH)

Summary
This pathway is most common for systems with good leaving groups, stable carbocations and weaker nucleophiles. A typical example is the reaction of HBr with a tertiary alcohol.

S_N1 MECHANISM FOR REACTION OF ALCOHOLS WITH HBr

Step 1:
An acid/base reaction. Protonation of the alcoholic oxygen to make a better leaving group. This step is very fast and reversible. The lone pairs on the oxygen make it a Lewis base.

Step 2:
Cleavage of the C-O bond allows the loss of the good leaving group, a neutral water molecule, to give a carbocation intermediate. This is the rate determining step (bond breaking is endothermic)

Step 3:
Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkyl bromide.

S_N1 MECHANISM FOR REACTION OF ALKYL HALIDES WITH H₂O

Step 1:
Cleavage of the already polar C-Br bond allows the loss of the good leaving group, a halide ion, to give a carbocation intermediate. This is the rate determining step (bond breaking is endothermic)

Step 2:
Attack of the nucleophile, the lone pairs on the O atom of the water molecule, on the electrophilic carbocation creates an oxonium species.

Step 3:
Deprotonation by a base yields the alcohol as the product.

Note that this is the reverse of the reaction of an alcohol with HBr.

In principle, the nucleophile here, H₂O, could be replaced with any nucleophile, in which case the final deprotonation may not always be necessary.