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Transcript
Spectroscopy of Amines - IR Characteristic N–H stretching absorptions 3300 to 3500 cm-1. NH2 group shows an irregular doublet, NH - weak multiple bands. Ammonium ions show N-H at 2600 cm-1. Amine absorption bands are sharper and less intense than hydroxyl bands. 1o amines show NH2 deformation band at 1650-1590 cm-1. C-N stretching vibrations are found at 1090-1068 cm-1 in 1o amines with a 1o a carbon, 1140-1080 cm-1 with a 2o a carbon, and at 1240-1170 cm-1 with a 3o a carbon. 3o amines show no C-N vibrations. In aromatic amines, this band is at 1330-1260 cm-1. N-H wagging bands are found at 850-750 cm-1 as strong, broad, multiple bands. These are weak in aromatic amines. 1 2 3 Mass Spectrometry Since N is a compound with an odd number of nitrogen atoms has an odd-numbered molecular weight and a corresponding parent ion Alkylamines cleave at the C–C bond nearest the nitrogen to yield an alkyl radical and a nitrogencontaining cation 4 Mass Spectrum of N-Ethylpropylamine The two modes of a cleavage give fragment ions at m/z = 58 and m/z = 72. 5 1H - NMR Spectroscopy N–H hydrogens appear as broad signals, either fully coupled to neighboring C–H hydrogens, or more frequently with no coupling (just like the H-bonded OH signals) Hydrogens on C next to N and absorb at lower field than alkane hydrogens N-CH3 gives a sharp three-H singlet at δ 2.2 to 2.6 6 C3H9NO 7 C4H11NO2 8 C8H11NO 9 C9H13N 10 C15H17N 11 C6H15N - IR 12 C6H15N - 1H-NMR 13 Structure, Properties and Reactivity of Amines Organic derivatives of ammonia, NH3 Nitrogen atom with a lone pair of electrons, making amines both basic and nucleophilic Occur in plants and animals 14 IUPAC nomenclature of simple amines For simple amines, the suffix -amine is added to the name of the alkyl substituent The suffix -amine can also be used in place of the final -e in the name of the parent compound 15 IUPAC nomenclature of simple amines Alkyl-substituted (alkylamines) or aryl-substituted (arylamines) Classified: 1° (RNH2), methyl (CH3NH2), 2° (R2NH), 3° (R3N) 16 IUPAC nomenclature of complex amines. Amines with more than one functional group. Consider the NH2 as an amino substituent on the parent molecule 17 IUPAC nomenclature of complex amines. Amines with multiple alkyl groups. Symmetrical secondary and tertiary amines are named by adding the prefix di- or tri- to the alkyl group 18 IUPAC nomenclature of complex amines. Amines with multiple different alkyl groups. Named as N-substituted primary amines Largest alkyl group is the parent name, and other alkyl groups are considered N-substituents 19 Common Names Alkylamines do not have common names unless they are biological molecules, such as putrycine (1,6-hexanediamine) or cadaverine (1,7-heptanediamine) Simple arylamines have common names 20 Common Names of Heterocyclic Amines If the nitrogen atom occurs as part of a ring, the compound is designated as being heterocyclic Each ring system has its own parent name 21 Quaternary Ammonium Ions A nitrogen atom with four attached groups is positively charged Compounds are quaternary ammonium salts 22 Structure and Bonding in Amines Bonding to N is similar to that in ammonia N is sp3-hybridized C–N–C bond angles are close to 109° tetrahedral value 23 Basicity of Amines The lone pair of electrons on nitrogen makes amines basic and nucleophilic They react with acids to form acid–base salts and they react with electrophiles Amines are stronger bases than alcohols, ethers, or water Amines establish an equilibrium with water in which the amine becomes protonated and hydroxide is produced 24 Amines as Acids Loss of the N–H proton requires a very strong base 25 Synthesis of Amines SN2 Reactions of Alkyl Halides Ammonia and other amines are good nucleophiles 26 Uncontrolled Multiple Alkylation are Unavoidable when Sterically Un-hindered Amines React 27 Reduction of nitriles and amides 28 Reduction Aryl Nitro Compounds Arylamines are prepared from nitration of an aromatic compound and reduction of the nitro group Reduction by catalytic hydrogenation over platinum is suitable if no other groups can be reduced Iron, zinc, tin, and tin(II) chloride are effective in acidic solution 29 Selective Preparation of Primary Amines: the Azide Synthesis Azide ion, N3- displaces a halide ion from a primary or secondary alkyl halide to give an alkyl azide, RN3 Alkyl azides are not nucleophilic (but they are explosive) Reduction gives the primary amine 30 Gabriel Synthesis of Primary Amines A phthalimide alkylation for preparing a primary amine from an alkyl halide The N-H in imides (-CONHCO-) can be removed by KOH followed by alkylation and hydrolysis 31 Hofmann and Curtius Rearrangements Carboxylic acid derivatives can be converted into primary amines with loss of one carbon atom by both the Hofmann rearrangement and the Curtius rearrangement 32 Hofmann Rearrangement (Mechanism) Part I RCONH2 reacts with Br2 and base to give electron deficient nitrogen O O R N O R H OHR H N N H + H2O H + Br - Br + H2O O H Br2 R N Br O O R N H OHR N 33 Br Hofmann Rearrangement (Mechanism) Part II Alkyl group (-R) migrates to the neighboring electron-deficient nitrogen. Hydration of the resultant isocyanate gives carboxamic acid. O O R N R Br O R R N N H2O R N + Br - N C O C (isocyanate) O R N H O (carboxamic acid) C OH 34 Hofmann Rearrangement (Mechanism) Part III Deprotonation-reprotonation produces a protonated ammonium zwitterion (a good leaving group). It’s elimination produces the amine and carbon dioxide by-product. O R N N OH R O O O R H H N H2O C O O R N C H O O R H O H2O + C H H C H O R C H N OH- H N + C 35 O Curtius Rearrangement Heating an acyl azide prepared from substitution an acid chloride This rearrangement also involves migration of R from C=O to the neighboring electron-deficient nitrogen with simultaneous loss of a leaving group 36 Reactions of Amines Acylation leads to amides (1o, 2o, or 3o.) 37 Hofmann Elimination Converts amines into alkenes NH2- is very a poor leaving group so it converted to an alkylammonium ion, which is a good leaving group 38 Silver Oxide Is Used for the Elimination Step Exchanges hydroxide ion for iodide ion in the quaternary ammonium salt, thus providing the base necessary to cause elimination 39 Orientation in Hofmann Elimination We would expect that the more highly substituted alkene product predominates in the E2 reaction of an alkyl halide (Zaitsev's rule) However, the less highly substituted alkene predominates in the Hofmann elimination due to the large size of the trialkylamine leaving group The base must abstract a hydrogen from the most sterically accessible, least hindered position 40 Steric Effects Control the Orientation 41