Organic Chemistry II - Amines | Chemistry

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Study GuideOrganic Chemistry II–Amines1.Introduction to AminesAminesare organic compounds derived fromammonia (NH₃). In amines, one or more hydrogenatoms of ammonia are replaced byalkyl or aryl groups. Amines can bealiphatic(attached to alkylgroups) oraromatic(attached to aromatic rings).Like ammonia, amines behave asweak baseswith basicity values (Kᵦ)typically ranging from10⁻⁴ to10⁻⁶.This basic nature is due to thelone (unshared) pair of electrons on the nitrogen atom, which canaccept a proton.1.Classification of AminesAmines are classified based on thenumber of carbon-containing groups attached to the nitrogenatom.Types of Amines•Primary (1°) amines: One alkyl or aryl group attached to nitrogenExample: CH₃NH₂(methylamine)•Secondary (2°) amines: Two alkyl or aryl groups attachedExample: (CH₃)₂NH (dimethylamine)•Tertiary (3°) amines: Three alkyl or aryl groups attached

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Study GuideExample: (CH₃)₃N (trimethylamine)As the number of alkyl groups increases, the structure around nitrogen becomes more crowded.2.Nomenclature of AminesCommon System of NamingIn thecommon naming system, amines are named by:•Naming the group(s) attached to nitrogen•Adding the word“amine”Examples:•CH₃NH₂→ methylamine•C₆H₅NHCH₃→ methyl phenyl amine•(CH₃)₃N → trimethylamineIUPAC System of NamingTheIUPAC systemfollows clear rules:1.Choose thelongest carbon chainattached to the nitrogen.2.Take the alkane name of that chain and replace the final–ewith–amine.3.Number the chain starting from the end closest to the–NH₂group.

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Study Guide4.Any substituent attached directly to nitrogen (not the carbon chain) is indicated by acapitalN.Examples:•CH₃NH₂→ methanamine•(CH₃)₂CHNH₂→ 1,1-dimethylethanamine•CH₃CH₂N(CH₃)₂→ N,N-dimethylethanamine3.Aromatic AminesWhen an–NH₂group is directly attached to a benzene ring, the compound is calledaniline, whichacts as the parent aromatic amine.Examples:•Aniline (C₆H₅NH₂)–parent compound•N-methylaniline–N-substituted•N,N-dimethylaniline–N,N-disubstituted

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Study GuideAromatic amines show behaviordifferent from aliphatic amines due toresonance effects.4.Basicity of AminesAmines are basic because thelone pair on nitrogencan accept a proton.Thegreater the electron densityaround nitrogen, the stronger the base.Effect of Substituents•Electron-donating groups(like alkyl groups) increase basicity•Electron-withdrawing groupsdecrease basicityBasicity Order in the Gas PhaseIn the gas phase, onlyinductive effectsoperate. Alkyl groups donate electrons and increase basicity.Most basic → Least basic5.Basicity Order in Aqueous SolutionIn water,solvation effectsbecome important.Here,secondary aminesare the most basic.Why Does This Change?In aqueous solution:•Amines exist asammonium ions•Primary and secondary ammonium ions form stronghydrogen bonds with water•Tertiary ammonium ions are less solvated due to steric hindrance

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Study GuideThis stronger solvation stabilizes primary and secondary ammonium ions, increasing their effectivebasicity.6.Formation of Ammonium SaltsWhen amines react with water, they formammonium salts:•Primary amine → Primary ammonium salt•Secondary amine → Secondary ammonium salt•Tertiary amine → Tertiary ammonium saltSolvation by water is strongest forprimary and secondary ammonium ions.7.Basicity of Aromatic AminesAromatic amines are weaker bases than aliphatic amines.Reason: Resonance EffectIn aromatic amines such asaniline, the lone pair on nitrogen becomesdelocalized into the benzeneringthrough resonance. This delocalization makes the lone pairless available to accept a proton.

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Study GuideFigure 1As a result:•Electron density on nitrogen decreases•Basicity decreasesThis is whyaniline 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 andN-notationfor nitrogen substituents.•Basicity depends onelectron density, solvation, and resonance.•Gas-phase and aqueous basicity orders are different.•Aromatic amines are weaker bases due to resonance delocalization.2.Preparation of AminesAmines can be prepared using several well-known laboratory methods. The most commonly usedmethods include:

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