Nucleophile

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 Gold Book definition Link. 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 such as with 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.

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.

The more basic the ions (high pK_a) the more reactive they are as a nucleophile. Polarizability is also important in the determination of the nucleophilicity. Larger atoms contain more electrons; therefore distortion is easier so they are more nucleophilic, 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 or the . For this reason, the S_N2 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 pair of electrons 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 reagents and anions of a terminal alkyne

Oxygen nucleophiles

Examples of oxygen nucleophiles are Water (H₂O) and Alcohols

Sulphur nucleophiles

Sulphur nucleophiles are Thiols (HS⁻)

Nitrogen nucleophiles

Nitrogen nucleophiles are Ammonia and Amines

Nucleophilicity scales

Many schemes have been devised attempting to quantify relative nucleophilic strength. The following empirical data have been obtained by measuring reaction rates for a large number of reactions involving a large number of nucleophiles and electrophiles and linear regression.

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 J. Am. Chem. Soc.; 1953; 75(1); 141-147. Abstract Gold Book definition Link derived in 1953:

$\ log(k/k_0) = s*n$

This free-energy relationship relates the pseudo first order reaction rate constant (in water at 25°C) of a reaction k to the reaction rate of a standard reaction (k₀) 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 free-energy relationship Gold Book 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:

$\ log(k/k_0) = N^+$

$\ log(k) = N^+ + log(k_0)$

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 carbocations 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 methanol) are: 0.5 for methanol, 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 :

$\ log(k) = s(N + E)$

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 electrophiles Reference Scales for the Characterization of Cationic Electrophiles and Neutral NucleophilesHerbert Mayr, Thorsten Bug, Matthias F. Gotta, Nicole Hering, Bernhard Irrgang, Brigitte Janker, Bernhard Kempf, Robert Loos, Armin R. Ofial, Grigoriy Remennikov, and Holger Schimmel J. Am. Chem. Soc.; 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 SN2 reactions Towards a General Scale of Nucleophilicity? Thanh Binh Phan, Martin Breugst, Herbert Mayr, Angewandte Chemie International Edition 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 :

$\ log(k) = s_Es_N(N + E)$

with s_E the electrophile-dependent slope parameter and s_N the nucleophile-dependent slop parameter. This equation can be rewritten in several ways:

with s_E = 1 for carbocations this equation is equal to the original Mayr-Patz equation of 1994,
with s_N = 0.6 for most n nucleophiles the equation becomes

\ log(k) = 0.6s_EN + 0.6s_EE

or the original Scott-Swain equation written as:

\ log(k) = log(k_0) + s_EN

with s_E = 1 for carbocations and s_N = 0.6 the equation becomes:

\ log(k) = 0.6N + 0.6E

or the original Ritchie equation written as:

\ log(k) - log(k_0) = N^+

References

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