Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Nucleophilic Substitution Reactions at sp3 Carbons
15.2 The Electrophile

The substitution of an electronegative atom, such as chlorine, for hydrogen of an alkane introduces a polar covalent bond into the molecule. The carbon—halogen bond is a polar covalent bond since halogens are more electronegative than carbon, the bonding electrons are attracted closer to the halogen (X) than to the carbon. This concept is illustrated in the figure below.

The result is that the carbon has a partial positive charge (δ⁺) and of course the halogen, which is more electronegative, has a partial negative charge (δ⁻). Since an electrophile is described as an electron-loving species, in the above example, the carbon bonded to the electronegative halogen is an electrophilic atom and the molecule is classified as an electrophile, as shown below.

Some electrophiles are fully positive, as is the case of a proton, which is a cation whereas others are neutral molecules, but carry a partially positive atom, as in the example above. It is important to be able to easily recognize electrophiles in organic chemistry. Examples of molecules that have electrophilic atoms are shown below, the partial positive charge (δ⁺) indicates the electrophilic atoms.

Problem 15.1

Using the symbol (δ⁺), identify the electrophilic atom in each of the following molecules.