Editing SN2 reaction (section)

===Nucleophile===
Like the substrate, steric hindrance affects the nucleophile's strength. The [[methoxide]] anion, for example, is both a strong base and nucleophile because it is a methyl nucleophile, and is thus very much unhindered. [[Potassium tert-butoxide|''tert''-Butoxide]], on the other hand, is a strong base, but a poor nucleophile, because of its three methyl groups hindering its approach to the carbon. Nucleophile strength is also affected by charge and [[electronegativity]]: nucleophilicity increases with increasing negative charge and decreasing electronegativity. For example, OH<sup>−</sup> is a better nucleophile than water, and I<sup>−</sup> is a better nucleophile than Br<sup>−</sup> (in polar protic solvents). In a polar aprotic solvent, nucleophilicity increases up a column of the periodic table as there is no hydrogen bonding between the solvent and nucleophile; in this case nucleophilicity mirrors basicity. I<sup>−</sup> would therefore be a weaker nucleophile than Br<sup>−</sup> because it is a weaker base. Verdict - A strong/anionic nucleophile always favours S<sub>N</sub>2 manner of nucleophillic substitution.