Electrophilicity

In chemistry, electrophilicity (from the Greek philos "friend") is an abstract measure of the ability of an electrophilic (electron-loving) molecule to react with another molecule that has an increased electron density . The electron-poor particle is called the electrophile , the reaction partner is called the nucleophile . Electrophilic particles are positively charged or have a strong positive partial charge . As a result of the reaction, the formation of a covalent bond can be observed due to the charge difference between the two particles .

Electrophilic particles

Electrophilic particles or atoms are positively charged ( cations ), positively polarized ( dipoles ) or generally tend to take up electrons . They are therefore attracted to negative charges . Typical electrophiles are carbocations , protons (H ⁺ ) and halogens, as well as carbonyl compounds and Lewis acids .

Electrophilic reactions

Electrophiles preferably enter into addition reactions ( electrophilic addition , A _E ) on a double bond with two carbon atoms or substitution reactions ( electrophilic aromatic substitution , S _E ) on an aromatic ring .

Electrophobia

The opposite of electrophilic is electrophobic (Greek phobos = fear). Such a molecule or ion repels electrons because it has a lone pair of electrons . Electrophobia is mostly called nucleophilicity .

Electrophility scale

Electrophilicity index some selected values (dimensionless)
fluorine	3.86
chlorine	3.67
bromine	3.40
Iodine	3.09
Hypochlorites	2.52
Sulfur dioxide	2.01
Carbon disulfide	1.64
benzene	1.45
sodium	0.88

There are several methods of ordering the degree of electrophilicity in an order of reactivity , one of which is indicated by the electrophilicity index ω compiled by Robert Ghormley Parr :

{\ displaystyle \ omega = {\ frac {\ chi ^ {2}} {2 \ eta}} \,}

with the electronegativity and HSAB , H ard and S often A cids and B ases . This equation is analogous to the classic equation about the relationships for electrical power : ${\ displaystyle \ chi \,}$ ${\ displaystyle \ eta \,}$

{\ displaystyle P = {\ frac {U ^ {2}} {R}} \,}

where is the electrical resistance and the electrical voltage . ${\ displaystyle R \,}$ ${\ displaystyle U \,}$

The electrophilicity index also applies to free radicals . Strongly electrophilic residues such as those of the halogens react with electron-rich reaction sites and strongly nucleophilic residues such as the 2-hydroxypropyl-2-yl and tert- butyl residues, which themselves react with a preference for electron-deficient reaction sites.

Individual evidence

^ Electrophilicity Index Parr, RG; Szentpaly, L. v .; Liu, SJ Am. Chem. Soc .; 1999; 121 (9); 1922–1924, doi : 10.1021 / ja983494x .
↑ Electrophilicity Index Chattaraj, PK; Sarkar, U .; Roy, DR Chem. Rev .; 2006 ; 106 (6); 2065-2091, doi : 10.1021 / cr040109f .
^ Electrophilicity Index Parr, RG; Szentpaly, L. v .; Liu, S. J. Am. Chem. Soc. ; 1999 ; 121 (9); 1922–1924, doi : 10.1021 / ja983494x .
↑ Electrophilicity and Nucleophilicity Index for Radicals Freija De Vleeschouwer, Veronique Van Speybroeck, Michel Waroquier, Paul Geerlings, and Frank De Proft Org. Lett .; 2007; 9 (14) pp. 2721-2724, doi : 10.1021 / ol071038k .

[1] Electrophilicity Index Parr, RG; Szentpaly, L. v .; Liu, SJ Am. Chem. Soc .; 1999; 121 (9); 1922–1924, doi : 10.1021 / ja983494x .

[2] Electrophilicity Index Chattaraj, PK; Sarkar, U .; Roy, DR Chem. Rev .; 2006 ; 106 (6); 2065-2091, doi : 10.1021 / cr040109f .

[Parr-3] Electrophilicity Index Parr, RG; Szentpaly, L. v .; Liu, S. J. Am. Chem. Soc. ; 1999 ; 121 (9); 1922–1924, doi : 10.1021 / ja983494x .

[4] Electrophilicity and Nucleophilicity Index for Radicals Freija De Vleeschouwer, Veronique Van Speybroeck, Michel Waroquier, Paul Geerlings, and Frank De Proft Org. Lett .; 2007; 9 (14) pp. 2721-2724, doi : 10.1021 / ol071038k .