Organic Nitrogen Compounds - Study Notes
Chapter Summary
Organic nitrogen compounds form a critical class of molecules essential for life and chemical industry, encompassing nitro compounds, aliphatic and aromatic amines, cyanides, and isocyanides. Nitroalkanes are aliphatic structures where one or more hydrogens are replaced by a nitro group, exhibiting chain, position, and functional isomerism along with a distinctive nitro-aci tautomerism for primary and secondary species. Amines, organic derivatives of ammonia, carry a trivalent sp3-hybridized nitrogen with a lone pair that renders them basic and nucleophilic. Their basicity in aqueous media is dictated by an intricate balance of inductive +I effects, steric factors, and solvent hydration. Primary aromatic amines can undergo diazotization under low temperatures to yield highly versatile diazonium salts, which are indispensable in synthetic organic chemistry as precursors to phenols, haloarenes, and azo dyes. Cyanides and isocyanides represent isomeric derivatives of hydrocyanic acid, serving as valuable synthetic intermediates for carboxylic acids, amides, and amines.
Learning Objectives
- Distinguish between primary, secondary, and tertiary nitro compounds and amines structurally.
- Analyze the chemical nature and equilibrium of nitro-aci tautomerism in nitroalkanes.
- Explain key organic preparation reactions including Gabriel phthalimide and Hoffmann bromamide synthesis.
- Evaluate the structural, inductive, steric, and solvation factors that determine amine basicity.
- Use diagnostic tests like Hinsberg, carbylamine, mustard oil, and nitrous acid to identify amines.
- Recognize the industrial and synthetic importance of diazonium coupling and nitrile hydrolysis.
Key Concepts and Definitions
Nitro-Aci Tautomerism
Primary and secondary nitroalkanes having at least one alpha-hydrogen exist in equilibrium with an aci-form (nitronic acid). The nitro form is less acidic, electrically low-conducting, and slowly soluble in base, whereas the aci-form is more acidic, highly conducting, and dissolves instantly in sodium hydroxide.
Amine Basicity
Basicity represents the availability of the nitrogen lone pair for donation to an electron-deficient proton. Basicity is quantified by the pKb value; a smaller pKb value represents a stronger base.
Diazotization
The reaction of primary aromatic amines with nitrous acid (generated in situ from sodium nitrite and hydrochloric acid) at 273 to 278 K to produce stable benzenediazonium chloride.
Hinsberg Test
A chemical method to distinguish primary, secondary, and tertiary amines using benzenesulfonyl chloride. Primary amines yield a sulfonamide soluble in alkali, secondary amines yield an insoluble sulfonamide, and tertiary amines do not react.
Carbylamine Reaction
Primary amines (both aliphatic and aromatic) react with chloroform and alcoholic potassium hydroxide to form highly offensive, foul-smelling alkyl or aryl isocyanides.
Worked Methods
Method for Gabriel Phthalimide Synthesis
This reaction prepares pure primary aliphatic amines. First, phthalimide is reacted with potassium hydroxide to form potassium phthalimide. Next, potassium phthalimide undergoes nucleophilic substitution with an alkyl halide to produce N-alkylphthalimide. Finally, complete hydrolysis or hydrazinolysis of N-alkylphthalimide yields the pure primary amine and phthalhydrazide, ensuring no secondary or tertiary amine contamination.
Method for Identifying Amine Mixtures
When presented with an unknown amine sample, treat it with benzenesulfonyl chloride. If a precipitate forms that dissolves in aqueous sodium hydroxide, the sample is a primary amine. If the precipitate remains insoluble in base, it is a secondary amine. If no reaction takes place initially but the amine dissolves in hydrochloric acid, it is a tertiary amine.
Common Exam Traps
The Basicity Trend Trap
Trap: Ranking aqueous basicity of methyl-substituted amines solely by their +I inductive effect (predicting tertiary > secondary > primary).
Correction: In aqueous solution, hydration and steric effects counteract the inductive effect. The correct basicity sequence for methyl-substituted amines is secondary > primary > tertiary > ammonia ((CH3)2NH > CH3NH2 > (CH3)3N > NH3).
The Aromatic Gabriel Synthesis Trap
Trap: Attempting to prepare aniline using the Gabriel phthalimide method.
Correction: Aryl halides (like chlorobenzene) do not undergo nucleophilic substitution with the phthalimide anion under normal conditions due to partial double bond character of the C-X bond and resonance stabilization.
Exam Tips
- Remember that the Hoffmann mustard oil reaction produces alkyl isothiocyanates which carry a distinctive mustard oil smell, and is unique to primary amines.
- Always use cold conditions (0 to 5 degrees Celsius) for diazotization; exceeding this temperature causes the diazonium salt to decompose, releasing nitrogen gas and forming phenol.
- The reduction of nitrobenzene under neutral medium (Zn/NH4Cl) yields N-phenylhydroxylamine, while acidic reduction (Sn/HCl) yields aniline; pay close attention to the reaction conditions specified in pathways.