Amido and Amide

1.0Introduction 

The terms "amido" and "amide" are often used in organic chemistry, but they refer to slightly different concepts. Amides are a class of organic compounds, whereas the amido group is a substituent or a ligand in coordination chemistry. Understanding the distinction is crucial for accurate chemical naming and understanding.

2.0What is the Amide Functional Group?

An amide is an organic compound containing a carbonyl group (>C=O) bonded to a nitrogen atom. The general formula for an amide is R−CO−NR′R′′, where R, R', and R'' can be hydrogen atoms, alkyl groups, or aryl groups. This functional group is derived from a carboxylic acid where the hydroxyl (-OH) group is replaced by an amino group $(-N{H}_2)$

3.0Structure and Bonding of Amides

The amide group has a unique structure due to the interaction between the carbonyl group and the nitrogen atom's lone pair of electrons. This leads to resonance, where the lone pair on the nitrogen delocalizes into the carbonyl π-system.

• Resonance in Amides: The resonance structure contributes a partial double bond character to the C-N bond and a partial positive charge on the nitrogen and a negative charge on the oxygen. This makes the C-N bond shorter and stronger than a typical single bond and restricts rotation around it.
• Geometry: The atoms in the amide group (O, C, N, and the atoms attached to them) are typically coplanar due to the $s{p}^2$ hybridization of the carbonyl carbon and the resonance effect.

4.0Classification of Amides

Amides are classified based on the number of hydrogen atoms on the nitrogen atom that have been replaced by alkyl or aryl groups.

• Primary Amides ($1^o$): The nitrogen atom is bonded to two hydrogen atoms. General formula: R−CONH2​.
• Secondary Amides ($2^o$): The nitrogen atom is bonded to one hydrogen atom and one alkyl/aryl group. General formula: R−CONHR′.
• Tertiary Amides ($3^o$): The nitrogen atom is bonded to two alkyl/aryl groups. General formula: R−CONR′R′′.

5.0Nomenclature of Amides

• IUPAC Naming: Amides are named by replacing the '-oic acid' suffix of the parent carboxylic acid with '-amide'.
  Example: $C{H}_{3}CON{H}_2$(derived from ethanoic acid) is named Ethanamide.
• N-Substitution: For secondary and tertiary amides, the groups attached to the nitrogen atom are named as prefixes using 'N-' to indicate their position.
  Example: ​ $C{H}_{3}CON(C{H}_3{)}_2$ is named N,N-dimethylethanamide.

6.0Preparation of Amides

• Reaction of Acyl Chlorides with Amines: This is a common and efficient method. An acyl chloride reacts with a primary or secondary amine (or ammonia) to form an amide and HCl.$R-COCl+R^{\prime }-N{H}_2\to R-CONH{R}^{\prime }+HCl$
• Heating Ammonium Carboxylates: Carboxylic acids react with ammonia to form ammonium salts, which when heated, lose a water molecule to form amides.$R-COOH+N{H}_3\to R-COON{H}_4\stackrel{\Delta }{\to }R-CON{H}_4+H_{2}O$

7.0Properties of Amides

• Physical Properties: Primary and secondary amides can form strong intermolecular hydrogen bonds, leading to high boiling points and melting points. Lower members are soluble in water. Tertiary amides cannot form hydrogen bonds and thus have lower boiling points.
• Chemical Properties: Amides are less reactive than other carboxylic acid derivatives due to the resonance stabilization. They can be hydrolyzed in acidic or basic conditions to yield carboxylic acids and amines.

8.0What is the Amido Group?

The term amido is used in two main contexts:

1. Prefix in IUPAC Nomenclature: It's used as a prefix when the amide group is not the highest priority functional group. The prefix is amido- and the structure is $-N{H}_2$ when it's attached to a carbonyl, for instance, in a complex molecule.
2. Coordination Chemistry: In coordination chemistry, the amido group is a ligand with the formula$N{H}_2^{-}$. It is the conjugate base of ammonia and acts as a strong Lewis base.

9.0Distinction between Amido and Amide

• Amide: Refers to a class of compounds containing the −CONH− functional group. The term itself is a suffix in IUPAC nomenclature.
• Amido: Refers to the $-N{H}_2$ group as a substituent or as a specific ligand $N{H}_2^{-}$ in coordination compounds. It is a prefix in IUPAC nomenclature. In simpler terms, you have a compound class (amide) and a substituent name (amido).