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Organic Chemistry II - Amines

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    Study Guide Organic Chemistry II – Amines 1. Introduction to Amines Amines are organic compounds derived from ammonia (NH ₃ ) . In amines, one or more hydrogen atoms of ammonia are replaced by alkyl or aryl groups . Amines can be aliphatic (attached to alkyl groups) or aromatic (attached to aromatic rings). Like ammonia, amines behave as weak bases with basicity values (K ᵦ ) typically ranging from 10 ⁻ ⁴ to 10 ⁻ ⁶ . This basic nature is due to the lone (unshared) pair of electrons on the nitrogen atom , which can accept a proton. 1. Classification of Amines Amines are classified based on the number of carbon - containing groups attached to the nitrogen atom . Types of Amines • Primary (1°) amines : One alkyl or aryl group attached to nitrogen Example: CH ₃ NH ₂ (methylamine) • Secondary (2°) amines : Two alkyl or aryl groups attached Example: (CH ₃ ) ₂ NH (dimethylamine) • Tertiary (3°) amines : Three alkyl or aryl groups attached

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    Study Guide Example: (CH ₃ ) ₃ N (trimethylamine) As the number of alkyl groups increases, the structure around nitrogen becomes more crowded. 2. Nomenclature of Amines Common System of Naming In the common naming system , amines are named by: • Naming the group(s) attached to nitrogen • Adding the word “amine” Examples: • CH ₃ NH ₂ → methylamine • C ₆ H ₅ NHCH ₃ → methyl phenyl amine • (CH ₃ ) ₃ N → trimethylamine IUPAC System of Naming The IUPAC system follows clear rules: 1. Choose the longest carbon chain attached to the nitrogen. 2. Take the alkane name of that chain and replace the final – e with – amine . 3. Number the chain starting from the end closest to the – NH ₂ group .

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    Study Guide 4. Any substituent attached directly to nitrogen (not the carbon chain) is indicated by a capital N . Examples: • CH ₃ NH ₂ → methanamine • (CH ₃ ) ₂ CHNH ₂ → 1,1 - dimethylethanamine • CH ₃ CH ₂ N(CH ₃ ) ₂ → N,N - dimethylethanamine 3. Aromatic Amines When an – NH ₂ group is directly attached to a benzene ring , the compound is called aniline , which acts as the parent aromatic amine. Examples: • Aniline (C ₆ H ₅ NH ₂ ) – parent compound • N - methylaniline – N - substituted • N,N - dimethylaniline – N,N - disubstituted

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    Study Guide Aromatic amines show behavior different from aliphatic amines due to resonance effects . 4. Basicity of Amines Amines are basic because the lone pair on nitrogen can accept a proton. The greater the electron density around nitrogen, the stronger the base. Effect of Substituents • Electron - donating groups (like alkyl groups) increase basicity • Electron - withdrawing groups decrease basicity Basicity Order in the Gas Phase In the gas phase, only inductive effects operate. Alkyl groups donate electrons and increase basicity. Most basic → Least basic 5. Basicity Order in Aqueous Solution In water, solvation effects become important. Here, secondary amines are the most basic. Why Does This Change? In aqueous solution: • Amines exist as ammonium ions • Primary and secondary ammonium ions form strong hydrogen bonds with water • Tertiary ammonium ions are less solvated due to steric hindrance

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    Study Guide This stronger solvation stabilizes primary and secondary ammonium ions, increasing their effective basicity. 6. Formation of Ammonium Salts When amines react with water, they form ammonium salts : • Primary amine → Primary ammonium salt • Secondary amine → Secondary ammonium salt • Tertiary amine → Tertiary ammonium salt Solvation by water is strongest for primary and secondary ammonium ions . 7. Basicity of Aromatic Amines Aromatic amines are weaker bases than aliphatic amines . Reason: Resonance Effect In aromatic amines such as aniline , the lone pair on nitrogen becomes delocalized into the benzene ring through resonance. This delocalization makes the lone pair less available to accept a proton .

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    Study Guide Figure 1 As a result: • Electron density on nitrogen decreases • Basicity decreases This is why aniline is less basic than cyclohexylamine . Key Takeaway • Amines are derivatives of ammonia and act as weak bases. • Classification depends on the number of carbon groups attached to nitrogen. • IUPAC naming uses “amine” as the suffix and N - notation for nitrogen substituents. • Basicity depends on electron density, solvation, and resonance . • Gas - phase and aqueous basicity orders are different. • Aromatic amines are weaker bases due to resonance delocalization. 2. Preparation of Amines Amines can be prepared using several well - known laboratory methods. The most commonly used methods include:

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