Chapter 4: Alcohols and Alkyl Halides

S_N1 mechanism

S_N1 indicates a substitution, nucleophilic, unimolecular reaction, described by the kinetic expression : rate = k [R-LG]

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

• step 1: rate determining (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 σ bond

	Multi-step reactions have intermediates and several transition states (TS).  In an SN1 reaction loss of the leaving group generates an intermediate carbocation which then undergoes a rapid reaction with the nucleophile.  The reaction profiles shown here are simplified and do not include the equilibria for protonation of the -OH.
General case		SN1 reaction

The following issues are relevant to the S_N1 reactions of alcohols, ROH :

The effect of the alkyl group: R-
Reactivity order :   (CH₃)₃C-  >  (CH₃)₂CH-   >  CH₃CH₂-  >  CH₃-

In an S_N1 reaction, the key 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.

More about carbocations

The effect of the leaving group: -LG
The only event in the rate determining step of the S_N1 is breaking the C-LG bond. For alcohols it is important to remember that -OH is a very poor leaving. In the reactions with HX, the -OH is protonated first to give an oxonium, providing the much better leaving group, a water molecule (see scheme below).

The effect of the nucleophile: Nu
Since the nucleophile is not involved in the rate determining step of an S_N1 reaction (since rate = k [R-LG]), the nature of the nucleophile is unimportant. 
In the reactions of alcohols, ROH, with HX, the reactivity trend of HI > HBr > HCl > HF is not due to the nucleophilicity of the halide ion but the acidity of HX which is involved in generating the leaving group prior to the rate determining step.

Stereochemistry

In an SN1 reaction, the nucleophile attacks the planar carbocation. Since there is an equally probability of attack on either face there will be a loss of stereochemistry at the reactive center and both possible products will be observed.

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 (see later).  This is usually indicated by a change in the position of the halide compared to that of the original -OH group, or a change in the carbon skeleton of the product when compared to the starting material.
 

SN1 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.

© Dr. Ian Hunt, Department of Chemistry