Amines and Amides: Structure, Classification, and Properties

Chapter 14 – Carboxylic Acids, Esters, Amines, and Amides

Overview

This chapter focuses on the chemistry of amines and amides, two important classes of organic compounds derived from ammonia and carboxylic acids, respectively. The following notes cover their structure, classification, nomenclature, and key properties relevant to GOB Chemistry.

Amines

Definition and Structure

Amines are organic derivatives of ammonia (NH3) in which one or more hydrogen atoms are replaced by alkyl or aromatic groups. The nitrogen atom in amines is bonded to one or more carbon-containing groups.

• General formula: R-NH2 (primary), R2NH (secondary), R3N (tertiary)

• Example: Ethylamine (CH3CH2NH2), Dimethylamine (CH3NHCH3), Aniline (C6H5NH2)

Classification of Amines

Amines are classified based on the number of carbon groups attached to the nitrogen atom:

• Primary (1°) amine: One carbon group bonded to nitrogen. Example: CH3NH2 (Methylamine)

• Secondary (2°) amine: Two carbon groups bonded to nitrogen. Example: CH3NHCH3 (Dimethylamine)

• Tertiary (3°) amine: Three carbon groups bonded to nitrogen. Example: (CH3)3N (Trimethylamine)

Naming Amines

Simple amines are named as alkylamines:

• List the names of the alkyl groups bonded to the N atom in alphabetical order, followed by "amine".

• Use prefixes (di-, tri-) for duplicate alkyl groups.

• Examples:

  • CH3CH2NH2: Ethylamine

  • CH3NHCH3: Dimethylamine

  • CH3CH2N(CH3)CH3: Ethyldimethylamine

Aromatic Amines

The amine of benzene is called aniline. Alkyl groups attached to the nitrogen use the prefix N- with the alkyl name.

• Examples:

  • N-Methylaniline: C6H5NHCH3

  • 3-Chloroaniline: C6H4ClNH2

• Application: Aniline is used in the manufacture of dyes for textiles such as wool, cotton, and silk.

Physical Properties of Amines

Amines exhibit hydrogen bonding due to the presence of a polar N–H bond:

• Primary and secondary amines (with fewer than six carbons) are soluble in water due to hydrogen bonding.

• Tertiary amines can form hydrogen bonds with water, but not between amine molecules.

• As the hydrocarbon chain length increases, solubility in water decreases due to the nonpolar nature of the chain.

Chemical Properties of Amines

Amines act as bases in water (Bronsted-Lowry bases):

• The lone pair on nitrogen accepts a proton (H+) from water, forming an ammonium ion and a hydroxide ion.

Equation:

• $\text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4^+ + \text{OH}^-$

• $\text{CH}_3\text{NH}_2 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{NH}_3^+ + \text{OH}^-$

Neutralization and Amine Salts

Amines react with acids to form ammonium salts:

• Amine + Acid → Ammonium Salt

• Example: $\text{CH}_3\text{NH}_2 + \text{HCl} \rightarrow \text{CH}_3\text{NH}_3^+ \text{Cl}^-$ (Methylammonium chloride)

Properties of Ammonium Salts:

• Solid at room temperature

• Odorless and soluble in water

• Used as active ingredients in medications (e.g., Sudafed, Benadryl)

Amides

Definition and Structure

Amides are derivatives of carboxylic acids in which the hydroxyl group (–OH) is replaced by an amino group (–NH2, –NHR, or –NR2).

• General structure: R–CO–NH2 (primary), R–CO–NHR (secondary), R–CO–NR2 (tertiary)

• Example: Acetaminophen (an amide used as a pain reliever)

Preparation of Amides (Amidation)

Amides are formed by reacting a carboxylic acid with ammonia or a primary/secondary amine and heat.

• Equation: $\text{RCOOH} + \text{NH}_3 \xrightarrow{\text{heat}} \text{RCONH}_2 + \text{H}_2\text{O}$

Naming Amides

Amides are named by replacing the "oic acid" (IUPAC) or "ic acid" (common) ending of the parent carboxylic acid with "amide". Alkyl groups attached to the nitrogen are named with the prefix N- followed by the alkyl name.

• Example: CH3CONH2: Acetamide (IUPAC: ethanamide)

• Example: CH3CH2CONHCH3: N-methylpropanamide

Hydrolysis of Amides

Amides undergo hydrolysis under acidic or basic conditions:

• Acid hydrolysis: Produces a carboxylic acid and an ammonium salt.

• Base hydrolysis: Produces a carboxylate salt and an amine or ammonia.

Equations:

• Acid hydrolysis: $\text{RCONH}_2 + \text{H}_2\text{O} + \text{HCl} \rightarrow \text{RCOOH} + \text{NH}_4^+ \text{Cl}^-$

• Base hydrolysis: $\text{RCONH}_2 + \text{NaOH} \rightarrow \text{RCOO}^- \text{Na}^+ + \text{NH}_3$

Biological and Medicinal Importance

• Urea: An amide that is the end product of protein metabolism, excreted in urine.

• Barbiturates: Cyclic amides used as sedatives and sleep inducers.

• Acetaminophen: An amide used as a pain reliever and fever reducer.

Summary Table: Classification of Amines

Type	General Structure	Example	Solubility in Water
Primary (1°)	R–NH2	CH3NH2 (Methylamine)	High (if R is small)
Secondary (2°)	R2NH	CH3NHCH3 (Dimethylamine)	High (if R groups are small)
Tertiary (3°)	R3N	(CH3)3N (Trimethylamine)	Moderate (can H-bond with water only)

Key Concepts

• Amines are classified by the number of carbon groups attached to nitrogen.

• Amides are formed from carboxylic acids and amines/ammonia.

• Both amines and amides have important biological and pharmaceutical roles.