Amines and Amides

Amines

Amines are organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups. They are functional groups characterized by a nitrogen atom with a lone pair of electrons. Amines have a general molecular formula of RNH₂ and can form up to three bonds like ammonia, giving them a pyramidal shape due to sp³ hybridization.

They naturally occur in proteins, vitamins, and hormones, and are also synthetically prepared for use in dyes, drugs, and polymers.

Examples include methylamine (CH₃NH₂), ethylamine (C₂H₅NH₂), and aniline (C₆H₅NH₂).

Alkyl & Aromatic Amines

Alkylamines

These contain tetrahedral nitrogen centers. The C–N bond length is slightly shorter than a C–C bond. Alkylamines can be chiral due to the lone pair on nitrogen and the different groups attached.

Aromatic Amines

In aromatic amines like aniline, nitrogen adopts a planar structure due to delocalization of the lone pair with the aryl ring. The C–N bond is shorter due to partial double-bond character.

Naming of Amines

Amines are named by replacing the suffix “e” in the parent hydrocarbon name with “amine.”

• Primary amine: Methylamine (CH₃NH₂)
• Secondary/Tertiary: Named with “di-” or “tri-” prefixes (e.g., dimethylamine, trimethylamine)
• Aromatic amines: Named as derivatives of aniline, e.g., N,N-dimethylaniline

Classification of Amines

• Primary (1°): One hydrogen replaced. e.g., CH₃NH₂
• Secondary (2°): Two hydrogens replaced. e.g., (CH₃)₂NH
• Tertiary (3°): All three hydrogens replaced. e.g., N(CH₃)₃

Credit: scienceready

Cyclic amines such as aziridine and piperidine fall under secondary or tertiary amines depending on substitution.

Preparation of Amines

1. From Haloalkanes

Haloalkanes react with ammonia in ethanol under sealed conditions:

CH₃CH₂Br + NH₃ → CH₃CH₂NH₃⁺Br⁻

Then, the salt reacts with excess ammonia:

CH₃CH₂NH₃⁺Br⁻ + NH₃ → CH₃CH₂NH₂ + NH₄Br

2. Alkylation of Alcohols

Alcohols react with ammonia to give amines:

ROH + NH₃ → RNH₂ + H₂O

3. Gabriel Phthalimide Synthesis

Used to prepare primary amines by heating potassium phthalimide with alkyl halides and hydrolyzing the product. Not suitable for aromatic amines.

4. Benzoylation

Methylamine reacts with benzoyl chloride:

CH₃NH₂ + C₆H₅COCl → CH₃NHCOC₆H₅ + HCl

5. Reduction of Nitriles

Nitriles are reduced with LiAlH₄ to give primary amines.

6. Reductive Amination

Aldehydes or ketones react with ammonia or amines and are reduced to form amines.

7. Reduction of Amides

Amides are reduced using LiAlH₄ to form amines:

• Primary amides → Primary amines
• N-substituted amides → Secondary or tertiary amines

Physical Properties of Amines

• Lower amines have fishy odour.
• Lower ones are gases; higher ones are liquids or solids.
• Aliphatic amines are more basic than aromatic ones.
• Boiling points increase with molar mass.
• They turn red litmus paper blue.
• Aryl amines like aniline oxidize and darken on exposure to air.

Chemical Properties of Amines

1. Dissociation in Water

They ionize to form hydroxide ions:

CH₃NH₂ + H₂O → CH₃NH₃⁺ + OH⁻

2. Basicity

Amines react with acids to form salts. These can be regenerated with NaOH:

CH₃NH₃⁺Cl⁻ + NaOH → CH₃NH₂ + NaCl + H₂O

3. Alkylation

Amines react with alkyl halides via nucleophilic substitution to give higher amines.

4. Sulphonation

Aniline reacts with concentrated H₂SO₄ to form sulphanilic acid upon heating.

5. Acylation

Primary and secondary amines react with acyl chlorides or anhydrides in the presence of pyridine:

RNH₂ + R'COCl → RNHCO–R' + HCl

Uses of Amines

• Production of polyamides (e.g., nylon)
• Manufacture of dyes and pigments
• Used as corrosion inhibitors and water treatment agents
• Key ingredients in analgesics (e.g., morphine)
• Used in insecticides and pesticides
• Important in producing amino acids and synthetic fibers
• Used as developers in photography

Amides

An amide is a functional group containing a carbonyl group (C=O) attached to a nitrogen atom. Amides are derived from carboxylic acids and have the general formula RCONH₂ or RCONR₂', where R and R' can be hydrogen or organic groups such as alkyl or phenyl.

In amides, the nitrogen atom forms single bonds with either hydrogen or carbon atoms. Naming amides involves replacing the "-ic" suffix of the acid with "amide".

• Acetic acid → Acetamide
• Ethanoic acid → Ethanamide

Nomenclature of Amides

• The root name is based on the longest carbon chain including the carbonyl group.
• Amides are derived by replacing the -OH of carboxylic acids with -NH₂.
• The suffix "-e" of alkanes is replaced by "amide".
• Substituents on nitrogen are indicated with "N-" prefix.

Examples:

• Methanamide (HCONH₂ or CH₃NO)
• Ethanamide / Acetamide (CH₃CONH₂ or C₂H₅NO)
• Propanamide (CH₃CH₂CONH₂ or C₃H₇NO)
• Butanamide (CH₃CH₂CH₂CONH₂ or C₄H₉NO)

Carbamide / Urea

Urea (Carbamide) is an organic compound with the formula CO(NH₂)₂. It contains two –NH₂ groups joined by a carbonyl (C=O) group. Urea is the main nitrogenous waste in mammalian urine and plays a major role in nitrogen metabolism.

It is used in fertilizers and as a raw material in the chemical industry.

Classification of Amides

Primary Amides

Contain the group RCONH₂. The nitrogen is bonded to two hydrogen atoms.

Examples: Methanamide (HCONH₂), Ethanamide (CH₃CONH₂), Benzamide (C₆H₅CONH₂)

Secondary Amides

Contain the group RCONHR'. The nitrogen is bonded to one hydrogen and one carbon group. Named using the prefix "N-" for the nitrogen substituent.

Examples: N-methylpropanamide (CH₃CH₂CONHCH₃), N-phenylpropanamide (CH₃CH₂CONHC₆H₅)

Tertiary Amides

Contain the group RCONR'R". Nitrogen is bonded to two carbon atoms with no hydrogen. Named using "N,N-" prefix.

Examples: N,N-dimethylformamide (HCON(CH₃)₂), N-ethyl-N-methylethanamide (CH₃CON(CH₃)C₂H₅)

Preparation of Amides

1. From Acyl Chlorides

Acid chlorides (RCOCl) react with ammonia to form amides:

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl

2. From Ammonium Salts

Carboxylic acids react with ammonium carbonate to form ammonium salts, which dehydrate to amides upon heating:

CH₃COOH + (NH₄)₂CO₃ → CH₃COONH₄ + H₂O + CO₂

CH₃COONH₄ → CH₃CONH₂ + H₂O

Physical Properties of Amides

• Formamide is a liquid; others are crystalline solids.
• Form hydrogen bonds.
• Lower aliphatic amides are water-soluble.
• High boiling and melting points.
• Polar in nature.

Chemical Properties of Amides I

1. Hydrolysis

In acidic or basic conditions, amides hydrolyze to carboxylic acids and ammonia:

RCONH₂ + H₂O → RCOOH + NH₃

CH₃CH₂CONH₂ + H₂O → CH₃CH₂COOH + NH₃

2. Hofmann Degradation

Amides react with Br₂ and NaOH to give primary amines with one carbon less:

RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O

3. Amphoteric Character

Amides act as both acids and bases:

As Acid: 2CH₃CONH₂ + HgO → (CH₃CONH₂)₂Hg + H₂O

As Base: CH₃CONH₂ + HCl → CH₃COCl + NH₃

Chemical Properties of Amides II

4. Dehydration

Amides are dehydrated to nitriles using phosphorus(V) oxide:

RCONH₂ → RCN + H₂O

5. Reaction with Nitrous Acid

Amides react with nitrous acid to form carboxylic acids and nitrogen gas:

NaNO₂ + HCl → NaCl + HNO₂

CH₃CONH₂ + HNO₂ → CH₃COOH + N₂ + H₂O

6. Reduction

Amides are reduced to primary amines using lithium aluminium hydride:

RCONH₂ + 4[H] → RCH₂NH₂ + H₂O

Uses of Amides

• Used in rubber, paper, plastics, crayons, pencils, and ink industries.
• Polyacrylamide is used in water treatment.
• Acrylamide is used in coatings for appliances and vehicles.
• Used to produce primary amines.
• Used in cosmetics: soaps, lotions, hair products.
• Used in explosives, adhesives, emulsions, and inks.

Test for Amides

1. Test with Alkali

When heated with sodium hydroxide, amides release ammonia (ammoniacal smell).

2. Nitrous Acid Test

Amides heated with HCl and NaNO₂ cause effervescence due to nitrogen gas release.

3. Burning Test

Amides burn with a non-sooty flame and leave a white residue.