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HSAB is an acronym for "hard and soft (Lewis) acids and bases".HSAB is widely used in chemistry for explaining the stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species.
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 ...
Phenylmagnesium bromide is a strong nucleophile as well as a strong base. It can abstract even mildly acidic protons, thus the substrate must be protected where necessary. It often adds to carbonyls, such as ketones, aldehydes. [1] [3] With carbon dioxide, it reacts to give benzoic acid after an acidic workup.
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.
Butyllithium is a strong base (pK b ≈ -36), but it is also a powerful nucleophile and reductant, depending on the other reactants. Furthermore, in addition to being a strong nucleophile, n -BuLi binds to aprotic Lewis bases, such as ethers and tertiary amines , which partially disaggregate the clusters by binding to the lithium centers.
Protic solvents react with strong nucleophiles with good basic character in an acid/base fashion, thus decreasing or removing the nucleophilic nature of the nucleophile. The following table shows the effect of solvent polarity on the relative reaction rates of the S N 2 reaction of 1-bromobutane with azide (N 3 – ).
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.
Organometallic compounds of electropositive metals are superbases, but they are generally strong nucleophiles. Examples include organolithium and organomagnesium ( Grignard reagent ) compounds. Another type of organometallic superbase has a reactive metal exchanged for a hydrogen on a heteroatom , such as oxygen (unstabilized alkoxides ) or ...