Editing HSAB theory (section)

== Kornblum's rule ==
An application of HSAB theory is the so-called '''Kornblum's rule''' (after [[Nathan Kornblum]]) which states that in reactions with [[ambident nucleophile]]s (nucleophiles that can attack from two or more places), the more [[electronegative]] atom reacts when the [[reaction mechanism]] is [[SN1 reaction|S<sub>N</sub>1]] and the less electronegative one in a [[SN2 reaction|S<sub>N</sub>2]] reaction. This rule (established in 1954)<ref>''The Mechanism of the Reaction of Silver Nitrite with Alkyl Halides. The Contrasting Reactions of Silver and Alkali Metal Salts with Alkyl Halides. The Alkylation of Ambident Anions'' Nathan Kornblum, Robert A. Smiley, Robert K. Blackwood, Don C. Iffland [[J. Am. Chem. Soc.]]; '''1955'''; 77(23); 6269-6280. {{doi|10.1021/ja01628a064}}</ref> predates HSAB theory but in HSAB terms its explanation is that in a S<sub>N</sub>1 reaction the [[carbocation]] (a hard acid) reacts with a hard base (high electronegativity) and that in a S<sub>N</sub>2 reaction tetravalent carbon (a soft acid) reacts with soft bases.

According to findings, [[Kolbe nitrile synthesis|electrophilic alkylations at free CN<sup>−</sup>]] occur preferentially at carbon, regardless of whether the S<sub>N</sub>1 or S<sub>N</sub>2 mechanism is involved and whether hard or soft electrophiles are employed. Preferred N attack, as postulated for hard electrophiles by the HSAB principle, could not be observed with any alkylating agent. Isocyano compounds are only formed with highly reactive electrophiles that react without an activation barrier because the diffusion limit is approached. It is claimed that the knowledge of absolute rate constants and not of the hardness of the reaction partners is needed to predict the outcome of alkylations of the cyanide ion.<ref>{{cite journal |journal= Angewandte Chemie International Edition |year= 2004 |volume= 44 |issue= 1 |pages= 142–145 |title= Ambident Reactivity of the Cyanide Ion: A Failure of the HSAB Principle |doi= 10.1002/anie.200461640 |first1= Alexander A. |last1= Tishkov |first2= Herbert |last2= Mayr |pmid=15599920}}</ref>

=== Criticism ===
Reanalysis of a large number of various most typical ambident organic system reveals that thermodynamic/kinetic control describes reactivity of organic compounds perfectly, whereas the HSAB principle fails and should be abandoned in the rationalization of ambident reactivity of organic compounds.<ref>{{cite journal|doi=10.1002/anie.201007100 | pmid=21726020 | volume=50 | issue=29 | title=Farewell to the HSAB Treatment of Ambident Reactivity | journal=Angewandte Chemie International Edition | pages=6470–6505 | last1 = Mayr | first1 = Herbert| year=2011 }}</ref>