Dr. Wolf's CHM 201 & Chapter 22 Amines

Dr. Wolf's CHM 201 & Amine Nomenclature

Dr. Wolf's CHM 201 & Alkylamine N attached to alkyl group Arylamine N attached to aryl group Primary, secondary, or tertiary determined by number of carbon atoms directly attached to nitrogen Classification of Amines

Dr. Wolf's CHM 201 & Two IUPAC styles 1)analogous to alcohols: replace -e ending by -amine 2)name alkyl group and attach -amine as a suffix Nomenclature of Primary Alkylamines (RNH 2 )

Dr. Wolf's CHM 201 & Examples: some primary alkylamines CH 3 CHCH 2 CH 2 CH 3 NH2NH2NH2NH2 (RNH 2 : one carbon directly attached to N) CH 3 CH 2 NH 2 NH2NH2NH2NH2 ethylamine or ethanamine cyclohexylamine or cyclohexanamine 1-methylbutylamine or 2-pentanamine

Dr. Wolf's CHM 201 & Name as derivatives of aniline. Nomenclature of Primary Arylamines (ArNH 2 ) p-fluoroaniline 5-bromo-2-ethylaniline NH2NH2NH2NH2F NH2NH2NH2NH2Br CH 2 CH 3

Dr. Wolf's CHM 201 & Amino groups as substituents p-aminobenzaldehyde amino groups rank below OH groups and higher oxidation states of carbon in such cases name the amino group as a substituent NH2NH2NH2NH2 HCHCHCHCO HOCH 2 CH 2 NH 2 2-aminoethanol

Dr. Wolf's CHM 201 & Name as N-substituted derivatives of parent primary amine. (N is a locant-it is not alphabetized, but is treated the same way as a numerical locant) Parent amine is one with longest carbon chain. Secondary and Tertiary Amines

Dr. Wolf's CHM 201 & Examples CH 3 NHCH 2 CH 3 N-methylethylamine NHCH 2 CH 3 NO 2 Cl 4-chloro-N-ethyl-3-nitroaniline CH 3 N N,N-dimethylcycloheptylamine

Dr. Wolf's CHM 201 & A nitrogen with four substituents is positively charged and is named as a derivative of ammonium ion (NH 4 + ). Ammonium Salts CH 3 NH 3 + Cl– methylammonium chloride N CH 3 H CH 2 CH 3 + CF 3 CO 2 – N-ethyl-N-methylcyclopentylammonium trifluoroacetate

Dr. Wolf's CHM 201 & When all four atoms attached to N are carbon, the ion is called a quaternary ammonium ion and salts that contain it are called quaternary ammonium salts. Ammonium Salts + CH 2 N CH 3 I – benzyltrimethylammonium iodide

Dr. Wolf's CHM 201 & Structure and Bonding

Dr. Wolf's CHM 201 & pm 106° 112° Alkylamines

Dr. Wolf's CHM 201 & Most prominent feature is high electrostatic potential at nitrogen. Reactivity of nitrogen lone pair dominates properties of amines. Alkylamines

Dr. Wolf's CHM 201 & Compare geometry at N of methylamine, aniline, and formamide. sp 3 sp 2 Geometry at N Pyramidal geometry at sp 3 -hybridized N in methylamine. Planar geometry at sp 2 -hybridized N in formamide. C O NH2NH2NH2NH2 H C NH2NH2NH2NH2 H H H

Dr. Wolf's CHM 201 & Compare geometry at N of methylamine, aniline, and formamide. sp 3 sp 2 Geometry at N Pyramidal geometry at sp 3 -hybridized N in methylamine. Planar geometry at sp 2 -hybridized N in formamide.

Dr. Wolf's CHM 201 & Angle that the C—N bond makes with bisector of H—N—H angle is a measure of geometry at N. sp 3 sp 2 Geometry at N ~125° 180° Note: this is not the same as the H—N—H bond angle

Dr. Wolf's CHM 201 & Angle that the C—N bond makes with bisector of H—N—H angle is a measure of geometry at N. sp 3 sp 2 Geometry at N ~125° 180° 142.5°

Dr. Wolf's CHM 201 & Geometry at N in aniline is pyramidal; closer to methylamine than to formamide. Geometry at N 142.5°

Dr. Wolf's CHM 201 & Geometry at N 142.5° Hybridization of N in aniline lies between sp 3 and sp 2. Lone pair of N can be delocalized into ring best if N is sp 2 and lone pair is in a p orbital. Lone pair bound most strongly by N if pair is in an sp 3 orbital of N, rather than p. Actual hybridization is a compromise that maximizes binding of lone pair.

Dr. Wolf's CHM 201 & Electrostatic Potential Maps of Aniline Nonplanar geometry at N. Region of highest negative potential is at N. Planar geometry at N. High negative potential shared by N and ring.

Dr. Wolf's CHM 201 & Physical Properties

Dr. Wolf's CHM 201 & Amines are more polar and have higher boiling points than alkanes; but are less polar and have lower boiling points than alcohols. Physical Properties CH 3 CH 2 CH 3 CH 3 CH 2 NH 2 CH 3 CH 2 OH dipole moment (  ): boiling point: 0 D 1.2 D 1.7 D -42°C17°C78°C

Dr. Wolf's CHM 201 & Boiling points of isomeric amines decrease in going from primary to secondary to tertiary amines. Primary amines have two hydrogens on N capable of being involved in intermolecular hydrogen bonding. Secondary amines have one. Tertiary amines cannot be involved in intermolecular hydrogen bonds. Physical Properties CH 3 CH 2 NHCH 3 CH 3 CH 2 CH 2 NH 2 (CH 3 ) 3 N boiling point: 50°C34°C3°C

Dr. Wolf's CHM 201 & Basicity of Amines

Dr. Wolf's CHM 201 & Effect of Structure on Basicity 1. Alkylamines are slightly stronger bases than ammonia.

Dr. Wolf's CHM 201 & AmineConj. AcidpK a NH 3 NH CH 3 CH 2 NH 2 CH 3 CH 2 NH Table 22.1 (page 920) Basicity of Amines in Aqueous Solution CH 3 CH 2 NH 3 + is a weaker acid than NH 4 + ; therefore, CH 3 CH 2 NH 2 is a stronger base than NH 3.

Dr. Wolf's CHM 201 & Effect of Structure on Basicity 1. Alkylamines are slightly stronger bases than ammonia. 2. Alkylamines differ very little in basicity.

Dr. Wolf's CHM 201 & AmineConj. AcidpK a NH 3 NH CH 3 CH 2 NH 2 CH 3 CH 2 NH (CH 3 CH 2 ) 2 NH(CH 3 CH 2 ) 2 NH (CH 3 CH 2 ) 3 N(CH 3 CH 2 ) 3 NH Table 22.1 (page 920) Basicity of Amines in Aqueous Solution Notice that the difference separating a primary, secondary, and tertiary amine is only 0.3 pK units.

Dr. Wolf's CHM 201 & Effect of Structure on Basicity 1. Alkylamines are slightly stronger bases than ammonia. 2. Alkylamines differ very little in basicity. 3. Arylamines are much weaker bases than ammonia.

Dr. Wolf's CHM 201 & AmineConj. AcidpK a NH 3 NH CH 3 CH 2 NH 2 CH 3 CH 2 NH (CH 3 CH 2 ) 2 NH(CH 3 CH 2 ) 2 NH (CH 3 CH 2 ) 3 N(CH 3 CH 2 ) 3 NH C 6 H 5 NH 2 C 6 H 5 NH Table 22.1 (page 920) Basicity of Amines in Aqueous Solution

Dr. Wolf's CHM 201 & H2NH2NH2NH2N Decreased basicity of arylamines + HNHH + NH2NH2NH2NH2 + + H3NH3NH3NH3N pK a = 4.6 pK a =10.6 Stronger acid Weaker acid Stronger base Weaker base

Dr. Wolf's CHM 201 & H2NH2NH2NH2N Decreased basicity of arylamines + HNHH + NH2NH2NH2NH2 + + H3NH3NH3NH3N Stronger acid Weaker acid When anilinium ion loses a proton, the resulting lone pair is delocalized into the ring.

Dr. Wolf's CHM 201 & H2NH2NH2NH2N Decreased basicity of arylamines + HNHH + NH2NH2NH2NH2 + + H3NH3NH3NH3N Aniline is a weaker base because its lone pair is more strongly held. Stronger base Weaker base

Dr. Wolf's CHM 201 & Decreased basicity of arylamines C6H5NH2C6H5NH2C6H5NH2C6H5NH2 (C 6 H 5 ) 2 NH (C 6 H 5 ) 3 N pK a of conjugate acid: ~-5 Increasing delocalization makes diphenylamine a weaker base than aniline, and triphenylamine a weaker base than diphenylamine.

Dr. Wolf's CHM 201 & Effect of Substituents on Basicity of Arylamines 1. Alkyl groups on the ring increase basicity, but only slightly (less than 1 pK unit). X NH 2 XpK a of conjugate acid H4.6 CH 3 5.3

Dr. Wolf's CHM 201 & Effect of Substituents on Basicity of Arylamines 2. Electron withdrawing groups, especially ortho and/or para to amine group, decrease basicity and can have a large effect. X NH 2 XpK a of conjugate acid H4.6 CF O 2 N1.0

Dr. Wolf's CHM 201 & p-Nitroaniline NH2NH2NH2NH2 O N O – + O N O – – NH2NH2NH2NH2 + + Lone pair on amine nitrogen is conjugated with p-nitro group—more delocalized than in aniline itself. Delocalization lost on protonation.

Dr. Wolf's CHM 201 & Effect is Cumulative Aniline is 3800 times more basic than p-nitroaniline. Aniline is ~1,000,000,000 times more basic than 2,4-dinitroaniline.

Dr. Wolf's CHM 201 & Heterocyclic Amines N H N is more basic than piperidinepyridine pK a of conjugate acid: (an alkylamine) (resembles an arylamine in basicity)

Dr. Wolf's CHM 201 & Heterocyclic Amines N is more basic than imidazolepyridine pK a of conjugate acid: NH N

Dr. Wolf's CHM 201 & Imidazole NH N Which nitrogen is protonated in imidazole? H+H+H+H+ H+H+H+H+ NH N H + N H N H +

Dr. Wolf's CHM 201 & Imidazole NH N Protonation in the direction shown gives a stabilized ion. H+H+H+H+ NH NH+ NH N H +

Dr. Wolf's CHM 201 & Tetraalkylammonium Salts as Phase-Transfer Catalysts

Dr. Wolf's CHM 201 & Phase-Transfer Catalysis Phase-transfer agents promote the solubility of ionic substances in nonpolar solvents. They transfer the ionic substance from an aqueous phase to a non-aqueous one. Phase-transfer agents increase the rates of reactions involving anions. The anion is relatively unsolvated and very reactive in nonpolar media compared to water or alcohols.

Dr. Wolf's CHM 201 & Phase-Transfer Catalysis Quaternary ammonium salts are phase-transfer catalysts. They are soluble in nonpolar solvents. N H3CH3CH3CH3C CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 + Cl – Methyltrioctylammonium chloride

Dr. Wolf's CHM 201 & Phase-Transfer Catalysis Quaternary ammonium salts are phase-transfer catalysts. They are soluble in nonpolar solvents. Cl – Benzyltriethylammonium chloride N CH 2 CH 3 +

Dr. Wolf's CHM 201 & Example The S N 2 reaction of sodium cyanide with butyl bromide occurs much faster when benzyl- triethylammonium chloride is present than when it is not. CH 3 CH 2 CH 2 CH 2 Br + NaCN CH 3 CH 2 CH 2 CH 2 CN + NaBr benzyltriethylammonium chloride

Dr. Wolf's CHM 201 & MechanismMechanism Cl – (aqueous) N CH 2 CH 3 + (aqueous) CN – + Cl – N CH 2 CH CN – (aqueous) (aqueous)

Dr. Wolf's CHM 201 & N CH 2 CH 3 + CN – (aqueous) (in butyl bromide) N CH 2 CH 3 + CN – MechanismMechanism

Dr. Wolf's CHM 201 & (in butyl bromide) N CH 2 CH 3 + CN – MechanismMechanism CH 3 CH 2 CH 2 CH 2 Br + N CH 2 CH 3 + Br – (in butyl bromide) CH 3 CH 2 CH 2 CH 2 CN +