Organic Reaction Mechanisms.
2. Haloalkane reaction with electron rich species.
b. The SN1 mechanism.

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Experimental observations.

The overall reaction involves the substitution of one electron-rich atom or group (the leaving group) by another (the nucleophile) with the following observations:

1. Kinetics:
Rate = [RX] (first order in haloalkane, first order overall.)
2. The incoming electron-rich group - the "nucleophile":
is a good nucleophile and not a base - with even a weakly basic attacking group the competing elimination reaction dominates.
3. The leaving electron-rich atom or group - the "leaving group":
is a good leaving group - that is it is a reasonably stable entity.
4. The halogen bearing carbon - the substitution site:
is tertiary rather than secondary.
is not primary.
can undergo skeletal rearrangements.
is stereochemically racemized (often) where this can be observed.

Accounting for the experimental observations.

1. The kinetics.

The first order kinetics indicates that only the haloalkane is involved in the rate determining step. This suggests that the ionization of the haloalkane to form a carbocation in a slow step occurs first. As a consequence, the intermediate (and the transition state leading to it) is less crowded than either the reactant or the product having three atoms round the carbon undergoing reaction.

2. The basicity of the nucleophile.

The elimination reaction, on the other hand requires the site of attack to be the more accessible beta-hydrogens on the "outside" of the molecule. Since the intermediate undergoing step two of this reaction (the carbocation) is of high energy, any base will encounter the beta-hydrogens first and quickly react to produce the more stable alkene product.

3. The leaving group.

The less stable the leaving group, the more energy is required for it to leave and the reaction slows, or stops.

4. The substitution site.

The more crowded the substitution site, the slower is the SN2 mechanism and the faster the SN1 mechanism, with its reduced crowding in the intermediate. Additionally, carbocations are stabilized by substitution, tertiary carbocations being more stable than secondary, which in turn are more stable than primary.

4a. The stereochemistry.

The carbocation produced in the first step is planar, and has lost the original chirality. Addition of the nucleophile can occur from either side of the cation producing the racemic mixture.

The animated mechanism. | Reaction summary.

Date created: 2005 06 26.