Nucleophiles and Electrophiles
Somewhat less known than acids and bases
Nucleophile = nucleus-loving. It refers to an area of a molecule with high electron density. It resembles a base.
Electrophile = electron-loving. It refers to an area of a molecule with low electron density. It resembles an acid
The concepts of nucleophile and base on the one hand, and electrophile and acid on the other, are closely related.
Reactions between molecules can be understood based on the electronic density they possess.
The electronic density, like the pressure in the atmosphere (depressions -low pressure- and anticyclones -high pressure-), is not distributed equally in the molecules.
Electronegative atoms "concentrate" electronic density around them and deprive nearby atoms of electronic density.
In methyl chloride (H3CCl) chlorine is the most electronegative atom and is the one that concentrates the negative charge.
The carbon and hydrogens of the methyl group (CH3) are much less electronegative than chlorine.
The negative part of one molecule will tend to react with the positive part of another and vice versa.
The electron density can be calculated with a computer program. The red color represents high electron density.
Blue color indicates electron density defect. With which atoms of methyl chloride do the colors match?
Electrophile
Species "electron-loving", eager for molecular areas with excess electrons, that is, with relative negative charge density. Logically, an electrophile will have an electron density defect.
Nucleophile
Species "nucleus-loving", eager for molecular areas defective of electrons, that is, with relative positive charge density. Logically, a nucleophile will have an electron density excess.
Does this reaction seem logical to you?
The hydroxide anion (HO−) comes from water due to the loss of a proton. It has a great electronic density. It is the nucleophile (red).
It reacts with methyl chloride, attacking the molecule on the methyl side which, due to the chlorine, has an electron density defect. It is the electrophile (blue).
The C-Cl bond is broken and the C-O bond is formed. The C-O bond is stronger than the C-Cl and the reaction is energetically favorable.