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A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH + 3, methanium CH + 5, acylium ions RCO +, and vinyl C ...
Stabilization is ranked the order, -F < -Cl < -CH 3 < -CH=CH 2. All substituents impart stability except for fluorine which destabilized the vinyl cation by 7 kcal/mole. This phenomenon can be explained by comparing a-fluorine substituent effects on vinyl and ethyl cations. In ethyl cations, fluorine stabilizes the carbocation.
The Hammond–Leffler postulate suggests that this, too, will increase the rate of carbocation formation. The S N 1 mechanism therefore dominates in reactions at tertiary alkyl centers. An example of a reaction proceeding in a S N 1 fashion is the synthesis of 2,5-dichloro-2,5-dimethylhexane from the corresponding diol with concentrated ...
The driving force for this rearrangement step is believed to be the relative stability of the resultant oxonium ion. Although the initial carbocation is already tertiary, the oxygen can stabilize the positive charge much more favorably due to the complete octet configuration at all centers.
Highly substituted carbocations are more stable than methyl or primary substituted cations. Such stability gives time for the two-step E1 mechanism to occur. If S N 1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat. Specific features : Rearrangement possible; Independent of concentration and basicity of base
The ability of polar solvents to increase the rate of S N 1 reactions is a result of the polar solvent's solvating the reactant intermediate species, i.e., the carbocation, thereby decreasing the intermediate energy relative to the starting material.
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This regioselectivity is rationalized by the resonance stabilization of a neighboring carbocation by a lone pair on the initially installed halogen. Depending on relative rates of the two steps, it may be difficult to stop at the first stage, and often, mixtures of the mono and bis hydrohalogenation products are obtained.