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In chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a reagent that forms a chemical bond to its reaction partner (the electrophile) by donating both bonding electrons. [1] Because nucleophiles donate electrons, they are by definition Lewis bases (see acid-base reaction theories). All molecules or ions with a free pair of electrons can act as nucleophiles, although negative ions (anions) are more potent than neutral reagents. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis. Nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge on an element and displaces the group it is bonded to. In general, in a row across the periodic table, the more basic the ion (the higher the pKa of the conjugate acid), the more reactive it is as a nucleophile. In a given group, polarizability is more important in the determination of the nucleophilicity the easier it is to distort the electron cloud around an atom or molecule, the more readily it will react. e.g., the iodide ion (I-) is more nucleophilic than the fluoride ion (F-). An ambident nucleophile is one that can attack from two or more places, resulting in two or more products. For example, the thiocyanate ion (SCN-) may attack from either the S or the N. For this reason, the SN2 reaction of an alkyl halide with SCN- often leads to a mixture of RSCN (an alkyl thiocyanate) and RNCS (an alkyl isothiocyanate). Similar considerations apply in the Kolbe nitrile synthesis.
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