Nucleophiles and electrophiles: Organic structures

ORGANIC STRUCTURES

Key Notes

Alkanes

Alkanes have no nucleophilic or electrophilic centers and are unreactive.

Polar functional groups

The nucleophilic and electrophilic centers of functional groups can be iden-tified by identifying polar bonds. Bonds such as C–H and C–C are consid-ered to be nonpolar. Carbon bonded to oxygen or halogen is an electrophilic center. Some electrophilic and nucleophilic centers are weak and are not usually important.

Unsaturated Unsaturated

Alkenes, alkynes, and aromatic rings are nucleophiles. The multiple bonds in these functional groups represent an area of high electron density and are therefore nucleophilic centers.

Alkanes

Alkanes are made up of carbon–carbon and carbon–hydrogen single bonds and are unreactive compounds. This is because C–C and C–H bonds are covalent in nature and so there are no electrophilic or nucleophilic centers present. Since most reagents react with nucleophilic or electrophilic centers, alkanes are unreactive molecules.

Polar functional groups

It is possible to identify the nucleophilic and electrophilic centers in common functional groups, based on the relative electronegativities of the atoms present.

The following guidelines are worth remembering:

·           C–H and C–C bonds are covalent. Therefore, neither carbon nor hydrogen is a nucleophilic or electrophilic center;

·           nitrogen is immediately to the right of carbon in the periodic table. The nitro-gen is more electronegative but the difference in electronegativity between these two atoms is small and so the N–C bond is not particularly polar. There-fore, the carbon atom can usually be ignored as an electrophilic center; 

·           N–H and O–H bonds are polar covalent. Nitrogen and oxygen are strong nucleophilic centers. Hydrogen is a weak electrophilic center;

·           C=O, C=N and C==N bonds are polar covalent. The O and N are nucleophilic centers and the carbon is an electrophilic center;

·           C–O and C–X bonds (X halogen) are polar covalent. The oxygen atom is moderately nucleophilic whereas the halogen atom is weakly nucleophilic. The carbon atom is an electrophilic center.

Using the above guidelines, the nucleophilic and electrophilic centers of the com-mon functional groups can be identified, where atoms having a slightly negative charge are nucleophilic centers and atoms having a slightly positive charge are electrophilic centers (Fig. 1).

Not all the nucleophilic and electrophilic centers are of equal importance. For example, a nitrogen atom is more nucleophilic than an oxygen atom. Also halogen atoms are very weakly nucleophilic and will not usually react with electrophiles if there is a stronger nucleophilic center present. Hydrogen atoms attached to halogens are more electrophilic than hydrogen atoms attached to oxygen. Hydro-gen atoms attached to nitrogen are very weakly electrophilic.

Taking this into account, some functional groups are more likely to react as nucleophiles while some functional groups are more likely to react as electro-philes. For example, amines, alcohols and ethers are more likely to react as nucle-ophiles, since they have strong nucleophilic centers and weak electrophilic centers. Alkyl halides are more likely to react as electrophiles since they have strong electrophilic centers and weak nucleophilic centers. Aldehydes and ketones can react as nucleophiles or electrophiles since both electrophilic and nucleophilic centers are strong.

Some functional groups contain several nucleophilic and electrophilic centers. For example, carboxylic acids and their derivatives fall into this class and so there are several possible centers where a nucleophile or an electrophile could react.

Unsaturated hydrocarbons

Not  all  functional  groups  have  polar  bonds.  Alkenes,  alkynes,  and  aromatic compounds are examples of functional groups which have covalent multiple bonds. The space between the multiple bonded carbons is rich in electrons and is therefore nucleophilic. Thus, the nucleophilic center in these molecules is not a specific atom, but the multiple bond (Fig. 2)!