Organic Chemistry-3: Basicity, Nucleophilicity, and Reactions of Amines

Definition of a Base: According to the Bronsted-Lowry and Lewis definitions, a base is a species that accepts a proton ( $H^+$ ).
Nature of Amines: An amine acts as a Lewis base because of the lone pair of electrons on the nitrogen atom.
General Reaction (Basicity):
- $\text{Amine (Lewis base)} :N- + \text{Acid } H:A \rightarrow \text{Salt } [-N-H]^+ :A^-$
Comparison of Basicity (Amines vs. Alcohols and Ethers):
- While alcohols and ethers containing oxygen also possess lone pairs, amines are significantly stronger bases.
- Reasoning: Nitrogen is less electronegative than oxygen. Consequently, nitrogen holds its lone pair less tightly than oxygen does.
- Result: The lone pair on the nitrogen atom is more available to form a bond with a proton ( $H^+$ ).
- Relative Basicity Order: \text{Amines} > \text{Alcohols} \approx \text{Ethers}

Inductive Effect:
- Alkyl groups are electron-donating groups ( $EDG$ ).
- The presence of alkyl groups increases the electron density on the nitrogen atom, making the lone pair more available for donation.
- Comparison: Methylamine ( $CH_3NH_2$ ) is more basic than ammonia ( $NH_3$ ).
Resonance Effect:
- In aromatic amines like Aniline, the nitrogen lone pair is delocalized into the aromatic ring through resonance.
- Because the lone pair is involved in resonance, it is less available for protonation compared to aliphatic amines.
- Comparison: Aniline is significantly less basic than methylamine.
Hybridization: The hybridization of the nitrogen atom affects how tightly the lone pair is held (e.g., $sp^3$ vs. $sp^2$ vs. $sp$ ).
Solvation: The ability of the solvent to stabilize the resulting conjugate acid (ammonium ion) via hydrogen bonding or other interactions.
Typical Basicity Order:
- CH_3NH_2 > NH_3 > \text{Aniline}

Definition: A nucleophile is a species that donates an electron pair ( $:$ ) to attack an electrophilic (electron-deficient) center.
Attack Mechanism: Amines typically attack electrophilic carbon atoms ( $C^+$ ).
Dual Nature of Amines: The chemical behavior of amines is primarily determined by the lone pair of electrons on the nitrogen atom.
- As Bases: They accept a proton ( $H^+$ ) from an acid to form ammonium salts.
  - Example: $CH_3NH_2 + HCl \rightarrow CH_3NH_3^+Cl^-$
- As Nucleophiles: The nitrogen lone pair attacks electrophilic atoms, especially carbon. This leads to participation in various nucleophilic substitution and nucleophilic addition reactions.

Basicity Profile: Amines are categorized as weak bases.
Reaction: They react with mineral acids to produce ammonium salts.
General Reaction Formula: $R-NH_2 + HX \rightarrow R-NH_3^+ X^-$
Applications:
- Used to increase the water solubility of organic molecules.
- Crucial in the pharmaceutical industry for the formation of medicinal salts.

Reaction Pathway: Amines react with alkyl halides via a nucleophilic substitution reaction ( $S_N2$ ).
Mechanism: The lone pair on the nitrogen atom attacks the electrophilic carbon of the alkyl halide.
* General Reaction: $R-NH_2 + R'-X \rightarrow R-NH-R'$
Possible Products (Sequential Alkylation):
- Secondary amine
- Tertiary amine
- Quaternary ammonium salt (formed when an excess of alkyl halide is present).

Reagents: Amines react with acid chlorides or acid anhydrides.
Product: Amides are formed.
General Reaction: $R-NH_2 + R'COCl \rightarrow R-NH-COR' + HCl$
Importance:
- Standard method for the preparation of amides.
- Used for the "protection" of amino groups during complex organic syntheses.

Reagents: $R-NH_2 + NaNO_2 + 2 HX \xrightarrow{H_2O} [R-N \equiv N^+ X^-] + NaX + 2 H_2O$
Product: Aliphatic diazonium salts.
Stability: These are highly unstable.
Outcome: They spontaneously lose Nitrogen gas ( $N_2$ ) to form a carbocation ( $R^+$ ), which subsequently reacts to form various products including Alkenes, Alcohols, and Alkyl halides.

Reagents: $Ar-NH_2 + NaNO_2 + 2 HX \rightarrow Ar-N \equiv N^+ X^- + NaX + 2 H_2O$
Product: Arenediazonium salts.
Stability: These salts are stable if kept in cold conditions (below $5\,^\circ\text{C}$ ).

Product: N-Nitrosamines.
Visual Characteristic: Typically appear as a yellow oil.
Specific Examples:
- Dimethylamine: $(CH_3)_2NH + HCl + NaNO_2 \xrightarrow{H_2O} (CH_3)_2N-N=O \text{ (N-Nitrosodimethylamine)}$
- N-Methylaniline: C_6H_5NHCH_3 + HCl + NaNO_2 \xrightarrow{H_2O} C_6H_5N(CH_3)-N=O \text{ (N-Nitroso-N-methylaniline, 87-93% yield)}

Specific Example (Aromatic): Reaction of N,N-dimethylaniline with nitrous acid at $8\,^\circ\text{C}$ .
Product: $p\text{-Nitroso-N,N-dimethylaniline}$ (yield of 80-90%).

Purpose: To chemically distinguish between Primary ( $1^\circ$ ), Secondary ( $2^\circ$ ), and Tertiary ( $3^\circ$ ) amines.
Reagent: Benzenesulfonyl chloride ( $C_6H_5SO_2Cl$ ).
Reaction Principle: Based on the formation of sulfonamides.

Primary Amines ( $1^\circ$ ):
- React with the sulfonyl chloride to form an N-substituted sulfonamide.
- The resulting sulfonamide contains an acidic hydrogen attached to the nitrogen.
- Solubility: Because of this acidic hydrogen, the product reacts with sodium hydroxide ( $NaOH$ ) to form a soluble salt and dissolves in the alkali solution.
Secondary Amines ( $2^\circ$ ):
- React to form an N,N-disubstituted sulfonamide.
- This product does not contain an acidic hydrogen.
- Solubility: It remains insoluble in alkali solutions.
Tertiary Amines ( $3^\circ$ ):
- Reaction: No reaction occurs.
- Reasoning: Tertiary amines lack the necessary $N-H$ bond required for the formation of the sulfonamide.

Organic Chemistry: A Tenth Edition by John McMurry, Cornell University (Emeritus).
Solomon-Fryhle's Organic Chemistry.