Basicity of Amines, Effect of Substituents on Basicity, and Synthetic uses of Aryl Diazonium Salts

Basicity of Amines

The relative basicity of amines is discussed in this section. An organic functional group that contains nitrogen should be assessed for its basicity by determining the lone pair on the nitrogen and its reactivity (and thus its nucleophilicity). In other words, do you think that lone pair wants to form a new bond with hydrogen or stay connected to the nitrogen nucleus. The lone pair electrons in nitrogen in amines cause the chemical to be electron dense. This is depicted in the electrostatic potential map below left as a red color. Acids, which are electron poor, can easily react with amino acids, since they are basic.



Nitrogen contains lone pair electrons that cause amines to be considered basis groups, which is a consequence of its neutral nature. A N-H bond is formed when the single electrons attack an acidic hydrogen. Ammonium salts containing four single bonds and positive charges result from this reaction.



The reaction of amines with water leads to the formation of the hydroxide ion and an ammonium salt, as outlined in this general equation:



This reaction requires an equilibrium constant called the base ionization constant (Kb), also known as the base dissociation constant:



Based on the acidity constant of a carboxylic acid, one can calculate the amine's base strength. When Kb is large and pKb is small, the proton-transfer equilibrium is favorable and the base is stronger. When discussing relative basicity of amines, Kb values are often omitted. By comparing the Ka of their conjugate acids, the Ka of amines can be used to compare their basicity. For amines, the Ka and Kb values have a direct relationship.

Effects of Substituents on Basicity

• Because the nitrogen atom has a single electron, aromatic amines are basic in nature. In comparison to aliphatic amines, they tend to be more acidic.
• Aromatic amines gain their basic character from electron-releasing groups.
• (For electron-releasing groups) Parasubstituted anilines are more basic than orthosubstituted anilines. The para effect is responsible for this.
• Electron withdrawing groups transform aromatic amines, causing them to lose their basic characteristics.
• N,N- dimethyl aniline > N- methyl aniline >Aniline
• In general, the more electrons present in a molecule, the more basic it is.
• Alkyl groups (-CH3, etc.) that donate electrons (inducing a positive inductive effect) increase the basicity of amine molecules while nitrogen-containing groups such as -NO2, (by inducing a negative inductive effect) decrease its basicity.
• Because of the electron-donating properties of alkyl groups attached to nitrogen atoms, such as CH3- and CH3CH2-, the electron density around nitrogen atoms increases along with its ability to release electrons.
• Alkyl amines are more basic than ammonia because of the greater and easier release of electrons.

Synthetic uses of Aryl Diazonium Salts

• Among their applications are dyes and pigments and dyed fabrics.
• Documents are reproduced using them as they tend to degrade when exposed to ultraviolet light.
• A wide variety of organic compounds are synthesized with them, including aryl derivatives.
• Preparation of aryl iodides and fluorides cannot be done directly through halogenation. It is not possible to substitute cyano groups for chlorine in chlorobenzene. Cyanobenzene can be easily formed from diazonium salts.
• Substituting benzene directly with substituted aromatic compounds is not possible. Using diazonium salts with diazo groups as substitutes, we can synthesize these compounds.

Get subject wise printable pdf documentsView Here

Share

Tweet

Share

Pin it

Share