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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 ...
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 ...
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 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.
Hammond's postulate is useful for understanding the relationship between the rate of a reaction and the stability of the products. While the rate of a reaction depends just on the activation energy (often represented in organic chemistry as ΔG ‡ “delta G double dagger”), the final ratios of products in chemical equilibrium depends only ...
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.
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Nucleophile strength is also affected by charge and electronegativity: nucleophilicity increases with increasing negative charge and decreasing electronegativity. For example, OH − is a better nucleophile than water, and I − is a better nucleophile than Br − (in polar protic solvents).