The oxygen of hydroxide ion bears a formal charge of -1. The hydrogen of hydrogen chloride bears a d+ charge because chlorine is more electronegative than hydrogen and thus the H-Cl bonding electron pair is unequally shared. We can envision the start of the acid-base reaction between hydroxide ion and hydrogen chloride as an electrostatic attraction between the opposite charges. As the reaction proceeds, the oxygen atom of hydroxide ion shares a lone electron pair with the hydrogen atom of hydrogen chloride, as shown with the curved arrow in the reaction above. This simple reaction shares one feature in common with the majority of reactions that you will encounter in your study of elementary organic chemistry. One species in the reaction shares an electron pair (a Lewis base) with another species (a Lewis acid) to make a new covalent bond. Application of this electron sharing idea to any reaction gives you an excellent chance at starting to figure out the mechanism for most common organic reactions.
Because of the ubiquity of electron pair sharers and acceptors in organic reactions, we assign special and distinct terms to these species. A molecule or ion that accepts a pair of electrons to make a new covalent bond is called an electrophile (from the Greek for "electron loving"). An electrophile is the same thing as a Lewis acid. Any molecule, ion or atom that is electron deficient in some way can behave as an electrophile. Electron deficiency would include a formal positive charge (methyl carbocation), a partial positive charge (d+), usually in conjunction with a polar bond (such as H-Cl) or an open octet (borane). "E" or "E+" are common abbreviations for generic electrophiles.
A molecule or ion that donates a pair of electrons to form a new covalent bond is called a nucleophile (from the Greek for "nucleus loving"). A nucleophile is the same thing as a Lewis base. Any molecule, ion or atom that has electrons that can be shared can be a nucleophile. The most common indications that electrons are available to be shared are formal negative charge (iodide ion), a partial negative charge (d-), usually in conjunction with a polar bond (methyl magnesium bromide), a p bond (isobutylene) or lone pairs (ammonia). "Nuc" or "Nu" are common abbreviations for generic nucleophiles.
The study of reaction mechanisms is central to the study of organic chemistry at any level. Therefore identification of electrophiles and nucleophiles is a critical organic chemistry survival skill. Examination of a structure for the features discussed above is one way to identify how a molecule or ion might behave in a reaction. Another way is by considering the curved arrows. Because electrons flow from an electron source to a place of electron deficiency, a curved arrow points away from a nucleophile and to an electrophile. This does not work in every reaction, however. In some reactions, the electron flow could go in either direction, and there are no distinct nucleophiles and electrophiles. Such reactions are uncommon in a course of this level.
Electrons always flow from nucleophile to electrophile:
Example 1: Using the curved arrows shown below, label each reactant as a nucleophile or electrophile.
Solution 1: A nucleophile is a molecule or ion that donates an electron pair to form a new covalent bond. In this example, chloride ion is donating a lone pair to form a new bond with carbon. The chloride ion is at the origin on the curved arrow that indicates this bond change. That the chloride ion bears a formal negative charge further suggests it should function as a nucleophile. Because nucleophile must react with an electrophile that leaves the other molecule (a chlorosulfite ester) to be the electrophile. The ester carbon is accepting an electron pair from the nucleophile to form the new C-Cl bond. The other bond changes within the chlorosulfite ester molecule are inconsequential when defining its role as a nucleophile or electrophile. The chlorine atom that is expelled as chloride ion accepts and electron pair from the S-Cl bond, but it does not make a new covalent bond, so it is neither electrophile or nucleophile. In this reaction, chloride ion is the leaving group.
Example 2: Decide if each molecule or ion shown below will react as a nucleophile or electrophile, or both.
Solution 2: Examine each structure for the charge distribution and electronic features discussed above.
a. Bromide ion: This atom has four lone pairs and a formal negative charge, suggesting it is electron-rich and can therefore function as a nucleophile. It has none of the features that would suggest it might behave as an electrophile.
b. Ammonium ion: This ion has a formal positive charge, suggesting it is electron-poor and can therefore function as an electrophile. It has no lone pairs or areas of negative charge, suggesting it will not function as a nucleophile.
c. Water: The oxygen atom of water has two lone pairs and a d-
charge (oxygen is more electronegative than hydrogen). This
that water can behave an a nucleophile. Each hydrogen atom bears
a d+ charge, so the molecule can behave as
electrophile as well. Many molecules can be both nucleophiles and
electrophiles. How they behave depends upon what they react with.
For example, if water is reacted with an electrophile, the water will
as a nucleophile.
Identify the nucleophiles and electrophiles in each mechanism step shown below.
Decide if each molecule or ion shown below will react as a nucleophile or electrophile, or both.