Information AboutNucleophile |
| CATEGORIES ABOUT NUCLEOPHILE | |
| physical organic chemistry | |
|
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. Nucleophilic is an Adjective that describes the affinity of a nucleophile to the nuclei, while '''nucleophilicity''' or ''nucleophile strength'' refers to the nucleophilic character. Nucleophilicity is often used to compare an atom's relative affinity to another's. In general, the more basic the ion (the higher the pKa of the conjugate acid), the more reactive it is as a nucleophile. 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 or the . 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 ). The terms ''nucleophile'' and ''electrophile'' were introduced by Christopher Kelk Ingold in 1929,Ingold, C. K. Recl. TraV. Chim. Pays-Bas 1929 replacing the terms ''cationoid'' and ''anionoid'' proposed earlier by A. J. Lapworth in 1925.Lapworth, A. Nature '''1925''', 115, 625 COMMON EXAMPLES In the example below, the Oxygen of the hydroxide ion donates an electron to bond with the Carbon at the end of the Bromopropane molecule. The bond between the carbon and the Bromine then undergoes Heterolytic Fission , with the bromine atom taking the donated electron and becoming the Bromide ion (Br−): Carbon nucleophiles Carbon nucleophiles are alkyl metal halides found in the Grignard Reaction , Blaise Reaction , Reformatsky Reaction , and Barbier Reaction , Organolithium Reagent s and Anion s of a Terminal Alkyne . Enols are also carbon nucleophiles. The formation of an enol is catalyzed by acid or base. Enols are ambident nucleophiles, but generally nucleophilic at the alpha carbon atom. Enols are commonly used in condensation reactions, including the Claisen condensation and the aldol condensation reactions. Oxygen nucleophiles Examples of oxygen nucleophiles are Water (H2O) and Alcohol s. Sulphur nucleophiles Sulphur nucleophiles are Thiol s (HS−). Sulphur is generally very nucleophilic because of its large size, which makes it easily polarizable, and its lone pairs of electrons (in some cases). Nitrogen nucleophiles Nitrogen nucleophiles are Ammonia and Amine s. NUCLEOPHILICITY SCALES Many schemes have been devised attempting to quantify relative nucleophilic strength. The following Empirical data have been obtained by measuring Reaction Rate s for a large number of reactions involving a large number of nucleophiles and electrophiles and Linear Regression . Nucleophiles displaying the so-called Alpha Effect are usually omitted in this type of treatment. Swain-Scott equation The first such attempt is found in the so-called Swain-Scott equation''Quantitative Correlation of Relative Rates. Comparison of Hydroxide Ion with Other Nucleophilic Reagents toward Alkyl Halides, Esters, Epoxides and Acyl Halides'' C. Gardner Swain, Carleton B. Scott definition Link derived in 1953:
This Free-energy Relationship relates the Pseudo First Order Reaction Rate Constant (in water at 25°C), k, of a reaction, normalized to the reaction rate, '''k0''', of a standard reaction with water as the nucleophile, to a '''nucleophilic constant''' '''n''' for a given nucleophile and a '''substrate constant''' '''s''' that depends on the sensitivity of a substrate to nucleophilic attack (defined as 1 for Methyl Bromide ). This treatment results in the following values for typical nucleophilic anions: Acetate 2.7, Chloride 3.0, Azide 4.0, Hydroxide 4.2, Aniline 4.5, Iodide 5.0 and Thiosulfate 6.4. Typical substrate constants are 0.66 for Ethyl Tosylate , 0.77 for β-propiolactone , 1.00 for 2,3-epoxypropanol , 0.87 for Benzyl Chloride and 1.43 for Benzoyl Chloride . The equation predicts that in a Nucleophilic Displacement on Benzyl Chloride , the Azide anion reacts 3000 times faster than water. Richie equation The Richie equation named after its creator and derived in 1972 is another definition Link ''Nucleophilic reactivities toward cations'' Calvin D. Ritchie Acc. Chem. Res.; '''1972'''; 5(10); 348-354. Abstract ''Cation-anion combination reactions. XIII. Correlation of the reactions of nucleophiles with esters'' Calvin D. Ritchie J. Am. Chem. Soc. ; 1975; 97(5); 1170-1179. Abstract or where N+ is the nucleophile dependent parameter and k0 the Reaction Rate Constant for water. In this equation a substrate dependent parameter like s in the Swain-Scott equation is absent. The equation states that two nucleophiles react with the same relative reactivity regardless of the nature of the electrophile which is in violation of the Reactivity–selectivity Principle . For this reason this equation is also called the '''constant selectivity relationship'''. In the original publication the data were obtained by reactions of selected nucleophiles with selected electrophilic Carbocation s such as Tropylium cations: : or Diazonium cations: : or (not displayed) ions based on Malachite Green . Subsequently many other reaction types were described. Typical Richie N+ values (in , 5.9 for the Cyanide anion, 7.5 for the Methoxide anion , 8.5 for the Azide anion and 10.7 for the Thiophenol anion. The values for the relative cation reactivities are -0.4 for the malachite green cation, +2.6 for the benzenediazonium cation and +4.5 for the tropylium cation. Mayr-Patz equation In the Mayr-Patz equation (1994):''Scales of Nucleophilicity and Electrophilicity: A System for Ordering Polar Organic and Organometallic Reactions'' Angewandte Chemie International Edition in English Volume 33, Issue 9, Date: May 18, 1994, Pages: 938-957 The Second Order Reaction Rate Constant k at 20°C for a reaction is related to a '''nucleophilicity parameter N''', an '''electrophilicity parameter E''' and a '''nucleophile-dependent slope parameter s'''. The constant s is defined as 1 with ''2-methyl-1-pentene'' as the nucleophile. Many of the constants have been derived from reaction of so-called benzhydrylium ions as the ; 2001; 123(39) pp 9500 - 9512; (Article) : and a diverse collection of π-nucleophiles: :. Typical E values are +6.2 for R = Chlorine , +5.90 for R = Hydrogen , 0 for R = Methoxy and -7.02 for R = Dimethylamine . Typical N values with s in parenthesis are -4.47 (1.32) for Electrophilic Aromatic Substitution to Toluene (1), -1.41 (1.12) for Electrophilic Addition to 1-phenyl-2-propene (2) and 0.96 (1) for addition to 2-methyl-1-pentene (3), -0.13 (1.21) for reaction with triphenylallylsilane (4), 3.61 (1.11) for reaction with 2-methylfuran (5), +7.48 (0.89) for reaction with isobutenyltributylstannane (6) and +13.36 (0.81) for reaction with the Enamine 7.An internet database for reactivity parameters maintained by the Mayr group is available at http://www.cup.uni-muenchen.de/oc/mayr/ The range of organic reactions also include Volume 45, Issue 23 , Pages 3869 - 3874 2006 : With E = -9.15 for the ''S-methyldibenzothiophenium ion'', typical nucleophile values N (s) are 15.63 (0.64) for Piperidine , 10.49 (0.68) for Methoxide and 5.20 (0.89) for water. In short: nucleophilicities towards sp2 or sp3 centers follow the same pattern. Unified equation In an effort to unify the above described equations the Mayr equation is rewritten as: with sE the electrophile-dependent slope parameter and sN the nucleophile-dependent slop parameter. This equation can be rewritten in several ways:
: :or the original Scott-Swain equation written as: :
: :or the original Ritchie equation written as: : SEE ALSO REFERENCES |
|
|