Organic Compounds Containing Nitrogen

Definition

Amines are organic compounds derived from ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups. Essentially, if ammonia is the "parent," amines are its "children."

Classification

• Primary (1°) Amines: Nitrogen is attached to one carbon group. (Formula: R-NH₂)
• Secondary (2°) Amines: Nitrogen is attached to two carbon groups. (Formula: R₂-NH)
• Tertiary (3°) Amines: Nitrogen is attached to three carbon groups. (Formula: R₃-N)

Nomenclature (Naming)

• Common Name: Name the alkyl groups followed by the word "amine."
  Example: Ethylamine.
• IUPAC Name: Use the suffix "-amine." The carbon attached to nitrogen is always C1.
  Example: Ethanamine.

Structure

The nitrogen atom in amines is sp³ hybridized. It possesses a pyramidal shape due to the lone pair of electrons on the nitrogen, which repels the other three bonds downward.

Preparation of Amines

• Reduction of Nitriles: Treating a nitrile (R-CN) with LiAlH₄ or H₂/Pd.
  CH₃CN → CH₃CH₂NH₂
• Gabriel Phthalimide Synthesis: Used specifically to prepare pure primary aliphatic amines, preventing the mixture of 1°, 2°, and 3° amines.

Physical and Chemical Properties

• Physical: Lower molecular weight amines are liquids and water-soluble due to their ability to form hydrogen bonds.
• Chemical (Basicity): Amines are basic because the lone pair on nitrogen can accept a proton (H⁺).
• Basicity Order: In aqueous solution, the order is usually: Secondary > Primary > Tertiary (for methyl groups).

Identification: The Hinsberg Test

Using Benzenesulphonyl chloride to distinguish amine types:

• Primary Amine: Forms a precipitate that is soluble in alkali.
• Secondary Amine: Forms a precipitate that is insoluble in alkali.
• Tertiary Amine: Does not react.

Real-Life Use: Pharmaceutical Drugs

Amines are the backbone of the medicine industry. Examples include Ephedrine (asthma) and Morphine (painkiller). Most antidepressants and antipsychotics are designed as amines to cross the blood-brain barrier and interact with neurotransmitters like Serotonin.

Definition

Diazonium salts are organic compounds containing the functional group -N₂⁺ X^-. They are typically prepared from aromatic primary amines like Aniline.

Preparation (Diazotization)

Aniline reacts with nitrous acid (NaNO₂ + HCl) at very low temperatures (0-5°C).

Chemical Reactions

• Sandmeyer Reaction: Replacing the -N₂ group with Cl, Br, or CN using copper salts.
• Gattermann Reaction: Similar to Sandmeyer but utilizes copper powder.
• Coupling Reactions: Reacting a diazonium salt with phenol or aniline to create brightly colored dyes.

Importance in Synthetic Organic Chemistry

Diazonium salts act as a "bridge." Since it is difficult to place halogen atoms precisely on a benzene ring, chemists first create an amine, convert it to a diazonium salt, and then substitute the nitrogen for the desired group.

Real-Life Use: The Textile Industry (Azo Dyes)

Intense reds, oranges, and yellows in fabrics are often Azo Dyes. These are created by "coupling" a diazonium salt with another aromatic compound, creating a long chain of alternating double bonds (conjugation) that reflects brilliant colors.

Meaning and Classification

These compounds feature a triple bond between carbon and nitrogen.

• Cyanides (Nitriles): Carbon is attached to the R-group. (Formula: R-C ≡ N)
• Isocyanides (Isonitriles): Nitrogen is attached to the R-group. (Formula: R-N⁺ ≡ C^-)

Comparison Table

Real-Life Use: Polymer Chemistry

Acrylonitrile (a cyanide) is the monomer for Acrylic fibers (e.g., Orlon). These fibers are warm, lightweight, and moth-resistant, serving as a synthetic alternative to wool.

• Basicity Equation: R-NH₂ + H₂O ⇌ R-NH₃⁺ + OH^-
• Diazotization Temp: Must be 0-5°C (otherwise salt decomposes into phenol).
• Sandmeyer Reaction: Ar-N₂⁺Cl^- ⟶_CuCl/HCl Ar-Cl + N₂↑

In modern chemistry, HPLC (High-Performance Liquid Chromatography) is used to separate and identify amines in biological samples, such as testing for banned stimulants in athletes.

On an industrial scale, catalytic hydrogenation using Raney Nickel is the gold standard for converting nitriles into primary amines.

Imagine creating Lidocaine. The process begins with 2,6-dimethylaniline (a primary aromatic amine). To build the drug, the amine reacts with chloroacetyl chloride to create an amide linkage.

To explore derivatives, chemists convert the amine into a Diazonium Salt using NaNO₂ and HCl at 0-5°C. Using the Sandmeyer reaction, they can introduce a halogen or cyano group (CN) at the para-position.

The final step involves reacting the intermediate with a secondary amine (like diethylamine). The resulting molecule contains both an amide and a tertiary amine group. The tertiary amine ensures the drug is a soluble salt (Lidocaine HCl), while the aromatic ring allows it to penetrate fatty nerves to block pain signals.

1. Which of the following is a tertiary amine?
   a) CH₃NH₂ | b) (CH₃)₂NH | c) (CH₃)₃N | d) C₆H₅NH₂
   Answer: c
2. The catalyst used in the Sandmeyer reaction is:
   a) Pd/C | b) Cu₂Cl₂ | c) LiAlH₄ | d) Ni
   Answer: b
3. Hinsberg's reagent is:
   a) Benzenesulphonyl chloride | b) Benzoyl chloride | c) Phenyl isocyanide | d) Nitrous acid
   Answer: a
4. Which gas is evolved during the reaction of a diazonium salt with H₃PO₂?
   a) O₂ | b) CO₂ | c) N₂ | d) H₂
   Answer: c
5. The basicity of amines is due to:
   a) Presence of Nitrogen | b) Lone pair on Nitrogen | c) sp³ hybridization | d) Hydrogen bonding
   Answer: b
6. Gabriel Phthalimide synthesis is used to prepare:
   a) Secondary amines | b) Tertiary amines | c) Pure primary aliphatic amines | d) Aromatic amines
   Answer: c
7. Azo dyes are formed by the reaction of diazonium salts with:
   a) Alkanes | b) Phenols/Amines | c) Alcohols | d) Ethers
   Answer: b
8. What is the correct order of basicity for methyl amines in aqueous solution?
   a) 1° > 2° > 3° | b) 2° > 1° > 3° | c) 3° > 2° > 1° | d) 3° > 1° > 2°
   Answer: b
9. Isocyanides are also known as:
   a) Nitriles | b) Isonitriles | c) Amides | d) Azides
   Answer: b
10. Diazotization occurs at what temperature?
    a) 100°C | b) 25°C | c) 0-5°C | d) -78°C
    Answer: c

1. Q: Why are aromatic amines less basic than aliphatic amines?
   Ans: In aromatic amines (like aniline), the lone pair of electrons on the nitrogen atom is delocalized into the benzene ring through resonance. This makes the lone pair less available for donation to a proton, thus decreasing basicity.
2. Q: Describe the Hinsberg test to distinguish between 1°, 2°, and 3° amines.
   Ans: The amine is reacted with benzenesulphonyl chloride. 1° amines form a sulfonamide that dissolves in KOH (clear solution). 2° amines form a sulfonamide that is insoluble in KOH (precipitate remains). 3° amines do not react.
3. Q: How is Benzene Diazonium Chloride prepared? Give the equation.
   Ans: It is prepared by reacting aniline with a mixture of NaNO₂ and HCl at 0-5°C.
   Equation: C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl^- + NaCl + 2H₂O.
4. Q: What happens when a diazonium salt is warmed with water?
   Ans: The diazonium salt undergoes hydrolysis to form phenol and evolves nitrogen gas.
   Equation: C₆H₅N₂⁺Cl^- + H₂O ⟶_Δ C₆H₅OH + N₂ + HCl.
5. Q: Compare Cyanides and Isocyanides.
   Ans: Cyanides (R-CN) have the carbon atom attached to the organic group and are used in making acrylic fibers. Isocyanides (R-NC) have the nitrogen atom attached to the organic group and are known for their extremely foul smell.
6. Q: What is the role of LiAlH₄ in the preparation of amines?
   Ans: LiAlH₄ acts as a strong reducing agent. It reduces nitriles (R-CN) and amides (R-CONH₂) to primary amines by adding hydrogen across the C-N bond.
7. Q: Why is the Gabriel Phthalimide synthesis preferred over the reduction of nitriles for 1° amines?
   Ans: Reduction of nitriles or alkylation of ammonia often leads to a mixture of 1°, 2°, and 3° amines. Gabriel synthesis specifically ensures only a pure primary amine is produced.
8. Q: What is a Coupling Reaction? Give an example.
   Ans: A coupling reaction is the reaction of a diazonium salt with a phenol or aromatic amine to form an azo compound (dye). Example: Benzene diazonium chloride + Phenol → p-hydroxyazobenzene (Orange dye).
9. Q: Explain the structure of amines.
   Ans: Amines have a pyramidal structure. The Nitrogen is sp³ hybridized with one lone pair and three sigma bonds. The bond angle is slightly less than 109.5° because the lone pair repels the bonding pairs.
10. Q: What is the effect of electron-withdrawing groups on the basicity of aniline?
    Ans: Electron-withdrawing groups (like -NO₂ or -Cl) decrease the electron density on the nitrogen atom further, making the lone pair even less available. This significantly decreases the basicity of the amine.

• Basicity Order (Aqueous Methyl): Remember "2-1-3". (Secondary → Primary → Tertiary).
• Diazonium Temperature: Think "Ice Cold". If it's not 0-5°C, the nitrogen "flies away" (decomposes).
• Hinsberg Test:
  • 1° → Soluble (Single carbon group).
  • 2° → Insoluble (Inside two groups).
  • 3° → No reaction (Not reacting).