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Chapter 24

Amines - Lecture and Textbook Notes Chapter 24

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Cecilia Kutas

CHM247H1 Jasmyn Lee Chapter 24: Amines RNH 2 R2NH, 3 N Organic derivatives o3 NH Lone pair of electrons on N: defines much of amine chemistry Amines you’ve heard of Common (Trivial) Names 24.1 Naming Amines Nomenclature of Amines  Note: when speaking of “tertiary” alcohol or alkyl halide – referring to the degree of substitution at the alkyl carbon atom; but when speaking of “tertiary” amine – referring to the degree of substitution at nitrogen  For 1° amines, use the suffix “amine”  2° and 3° amines o If symmetrical, add prefix “di” or “tri”  Eg/ Triethylamine, diphenylamine N N H o If asymmetrical, name as N-substituted primary amine  Longest alkyl group is parent; list others alphabetically  Eg/ N-methyl-N-propylbutylamine N  When N is not the highest priority group (i.e. amine as a substituent) o Use prefix “amino” 24.2 Structure and Properties of Amines Structure and Classification  Defined by degree of substitution on N o Primary, 1° (R-NH 2 o Secondary, 2° (R2NH) o Tertiary, 3° (3 N) + - o Quaternary ammonium salt, 4° (R 4 X ) o Note: lone pair is used up to accommodate 4 substituent on N 1 CHM247H1 Jasmyn Lee  Nitrogen atom is sp hybridized o Tetrahedral o 3 substituents in 3 corners of tetrahedron o One lone pair of electrons in the fourth corner  Chiral amine o Unlike chiral carbon compounds – can’t be resolved because two enantiomer forms rapidly interconvert by a pyramidal inversion  Amines with fewer than five carbon atoms are generally water soluble  1° and 2° amines form H-bonds 24.3 Basicity of Amines  Lone pair on N is donated in an acid-base reaction  Acid-base reaction with water gives the base ionization constant, b  Amines are much stronger bases than alcohols and ethers  Quantifying Basicity K H b + R N H O H RNH 3 + HO - H  Usually use pKb= -log10 b o Eg/ if Kb= 10 ,3- pKb= 3  Stronger bases have smaller values of pK b  Kband K aor any conjugate acid-base pair are related  i.e. b x a = [3 O ] [HO ] = w = 1.0 x 1014 o pK b pK =a14 o pK = 14 - pK a  Kaof corresponding ammonium ion is often quoted, to relate concept to acid strengths  Acid-base equilibrium between ammonium ion and water  Weaker base = smaller pK far ammonium ion (ammonia stronger acid)  Stronger base = larger pKafor ammonium ion  Any structural feature that stabilizes the ammonium ion relative to the free amine makes the amine a stronger base  +NH 4pK 9a3) gives up H more easily than CH NH3(pK 30.64a because the CH - subst3tuent donated electron + density inductively to stabilize the 3H NH3ion  Additional R groups do not increase basicity significantly – the charged ammonium ions are strongly solvated by water, and the energy of solvation contributes to their stability additional R groups hinder solvation  Inductive stabilization vs. steric hindrance of solvation 2 CHM247H1 Jasmyn Lee  Contrast Lone Pair Availability in Amines vs. Amides o Amides are non-basic, are not protonated by aqueous acid and are poor nucleophiles o Reason for difference of basicity between amide and amine – amide is stabilized by delocalization of the nitrogen lone-pair electrons through orbital overlap with the carbonyl group  Amides are more stable and less reactive and amines because they are hybrids of two resonance forms  Acidity of Amines o 1° and 2° amines can act as very weak acids – N-H proton can be removed by a sufficiently strong base o Used to convert esters and ketones to enolates 24.4 Basicity of Arylamines Resonance Effects on Basicity  Aryl amines are less basic than alkyl amines – nitrogen lone-pair electrons are delocalized by interaction with the aromatic ring π electron system and are less available for bonding to H o Aryl amines are stabilized because of their 5 resonance structures  pKaof aromatic ammonium ion is 4-5  Much less basic than N3 , because nitrogen lone pair is delocalized into aromatic ring 3 CHM247H1 Jasmyn Lee  Much of the resonance stabilization is lost on protonation – energy difference between protonated and nonprotonated forms is higher for aryl amines than for alkyl amines  Substituted arylamines can be either more or less basic than aniline, depending on the substituent o Electron-donating substituents (eg/ -3H , -2H , -3CH ) – increase reactivity of an aromatic ring toward electrophilic substitution; increase basicity of the corresponding arylamine o Electron-withdrawing substituents (eg/ -Cl, -2O , -CN) – decrease ring reactivity toward electrophilic substitution; decrease arylamine basicity  Hybridization Effects on Basicity o Electrons are held more tightly by orbitals with more S character (recall terminal alkynes) 24.5 Biological Amines and the Henderson-Hasselbalch Equation 4 CHM247H1 Jasmyn Lee 24.6 Synthesis of Amines Preparation of Amines (already seen) – Reduction of Nitriles, Amides and Nitro Compounds 1. Nitrile Reduction 2. Amide Reduction 3. Reduction of nitro aromatics to arylamine Preparation of Amines – S 2NReactions of Alkyl Halides 1. Direct Alkylation  Simplest method of alkylamine synthesis is by S 2 alkylation of ammonia or an alkylamine with an alkyl halide N  Generally gives complex mixtures; can’t stop at desired stage  Gives good yield of desired product
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