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A hydroxide ion acting as a nucleophile in an S N 2 reaction, converting a haloalkane into an alcohol. In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they ...
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). In a polar aprotic solvent, nucleophilicity increases ...
In this type of substitution reaction, one group of the substrate participates initially in the reaction and thereby affects the reaction. A classic example of NGP is the reaction of a sulfur or nitrogen mustard with a nucleophile, the rate of reaction is much higher for the sulfur mustard and a nucleophile than it would be for a primary or secondary alkyl chloride without a heteroatom.
Also, because the intermediate is partially bonded to the nucleophile and leaving group, there is no time for the substrate to rearrange itself: the nucleophile will bond to the same carbon that the leaving group was attached to. A final factor that affects reaction rate is nucleophilicity; the nucleophile must attack an atom other than a hydrogen.
In the case below, tosylate is the best leaving group when ethoxide is the nucleophile, but iodide and even bromide become better leaving groups in the case of the thiolate nucleophile. [ 7 ] Relative rates for leaving groups ( k X / k Br ) in each reaction
The second step is the loss of a sulfur dioxide molecule and its replacement by the chloride, which was attached to the sulphite group. The difference between S N 1 and S N i is actually that the ion pair is not completely dissociated , and therefore no real carbocation is formed, which else would lead to a racemisation.
In the presence of certain reducing agents, one of the sulfur-carbon bonds of the sulfonyl group is cleaved, leading to sulfur-free organic products. Depending on the nature of the substrate and reaction conditions, alkyl sulfones afford either the corresponding alkanes or olefins (the Julia olefination ).
As a general rule, while saturated carbon is best attacked by nucleophiles that are neutral compounds, those based on nonmetals far down on the periodic table (e.g. sulfur, selenium, or iodine), or even both, silicon is best attacked by charged nucleophiles, particularly those involving such highly electronegative nonmetals as oxygen, fluorine ...