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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 ...
2,6-Di-tert-butylpyridine, a weak non-nucleophilic base [2] pK a = 3.58; Phosphazene bases, such as t-Bu-P 4 [3] Non-nucleophilic bases of high strength are usually anions. For these species, the pK a s of the conjugate acids are around 35–40. Lithium diisopropylamide (LDA), pK a = 36
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.
The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged. An example of nucleophilic substitution is the hydrolysis of an alkyl bromide , R-Br under basic conditions, where the attacking nucleophile is hydroxyl ( OH − ) and the leaving group is bromide ( Br − ).
An application of HSAB theory is the so-called Kornblum's rule (after Nathan Kornblum) which states that in reactions with ambident nucleophiles (nucleophiles that can attack from two or more places), the more electronegative atom reacts when the reaction mechanism is S N 1 and the less electronegative one in a S N 2 reaction.
with regards to acid-base reactions, electron-rich species have high pKa's and react with weak Lewis acids. [ 4 ] with regards to nucleophilic substitution reactions , electron-rich species are relatively strong nucleophiles, as judged by rates of attack by electrophiles.
Here, a strong Lewis acid is required to generate either a carbocation from an alkyl halide in the Friedel-Crafts alkylation reaction or an acylium ion from an acyl halide. In the vast majority of cases, reactions that involve leaving group activation generate a cation in a separate step, before either nucleophilic attack or elimination.
the base is a poor nucleophile. Bases with steric bulk, (such as in potassium tert-butoxide), are often poor nucleophiles. For example, when a 3° haloalkane is reacts with an alkoxide, due to strong basic character of the alkoxide and unreactivity of 3° group towards S N 2, only alkene formation