Organic Reaction Mechanisms.
2. Haloalkane reaction with electron rich species.
b. The SN2 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][N:] (first order in each reactant, second order overall)
2. The incoming electron-rich group - the "nucleophile":
is a good nucleophile and a weak base - the more basic the attacking group the more the competing elimination reaction occurs.
is not too bulky.
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 primary rather than secondary.
is not tertiary.
is not "crowded" by other atoms close by.
is stereochemically inverted where this can be observed.

Accounting for the experimental observations.

1. The kinetics.

The second order kinetics can be explained if the nucleophile reacts at the carbon bearing the halogen, displacing the halogen in a single step process (no intermediate is formed). As a consequence, the transition state is more crowded than either the reactant or the product having five atoms round the carbon undergoing reaction.

2. The basicity and size of the nucleophile.

Since in the proposed mechanism the transition state is more crowded than reactants any thing which causes even more crowding will slow the substitution. 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. A bulky electron-rich species will have difficulty reaching the carbon in a substitution process, but not be so hindered in removing a beta-hydrogen.

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 transition state again explains why the carbon at the site of substitution must be as easy to access as possible, which is the case with a primary site, and not the case with a tertiary site.

4a. The stereochemistry.

For the required stereochemical outcome (inversion at the carbon undergoing substitution) to be explained, the nucleophile must react on the opposite side of the carbon to the side with the leaving group (the "back side"). This can be explained as follows:

1. The bulkiness of the incoming and leaving groups interfere with each other if the are both on the same side of the molecule.

2. The charge, or partial charge on the incoming and leaving groups is the same, and so they would repel each other.

3. From a molecular orbital point of view, the lowest unoccupied molecular orbital (lumo) of the haloalkane into which electrons must first go, is the antibonding mo of the C-X bond which has its largest lobe opposite the X group, making the back side approach of the nucleophile the best for bond formation.

The animated mechanism. | Reaction summary.

Date created: 2005 06 26.