Organic Reaction Mechanisms.
2. Haloalkane reaction with electron rich species.
e. The E1 mechanism.

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Experimental observations.

The overall reaction involves the loss of an electron-rich atom or group (the leaving group) and a proton in the presence of a base with the following observations:

1. Kinetics:
Rate = [RX] (first order in haloalkane, first order overall.)
2. The incoming electron-rich group - the base:
With even a weakly basic attacking group the elimination reaction occurs. With strong bases, the reaction tends to be E2 rather than E1.
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 leaving group site:
is tertiary rather than secondary.
is not primary.
can undergo skeletal rearrangements during the reaction.
no stereoselectivity.

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 base.

The elimination reaction 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 elimination site.

Elimination takes place rapidly with any base, producing mixtures, the most stable alkene in greatest amount.

4a. The stereochemistry.

The carbocation produced in the first step is planar, and has lost the original chirality if present. Conformational requirements found for the E2 mechanism are not found here.

The animated mechanism. | Reaction summary.

Date created: 2005 06 26.