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Chapter 15 Amines Structure & Classification Amines are classified as 1°, 2°, or 3° depending on the number of carbon groups bonded to nitrogen CH3 -NH2 Methylamin e (a 1° amine) NH2 Anilin e (a 1° aromatic amine) H CH3 -N-CH3 D imeth ylamine (a 2° amine) H N-CH3 N-Methylan iline (a 2° aromatic amine) CH3 CH3 -N-CH3 Trimeth ylamine (a 3° amine) CH3 CH2 -N-CH3 Ben zyldimethylamine (a 3° aliph atic amin e) Structure & Classification heterocyclic amine: an amine in which the nitrogen atom is part of a ring N N N H N H Pyrrolidin e Pip eridin e (h eterocyclic aliph atic amin es) N N N N H H Pyridin e Pyrimidin e Imid azole Purine (heterocyclic aromatic amines ) N N N Nomenclature IUPAC names Similar to alcohols drop the final -e of the parent alkane and replace it by -amine use a number to locate the amino group on the parent chain NH2 CH3 CHCH3 2-Propanamine NH2 H2 N Cyclohexanamine NH2 1,6-Hexan ediamine Nomenclature IUPAC names (cont’d) IUPAC nomenclature retains the common name aniline for C6H5NH2, the simplest aromatic amine use numbers to locate substituents or, alternatively, use the prefixes ortho (o), meta (m), and para (p) common names are still widely used NH2 NH2 NH2 CH3 A niline NO2 4-N itroaniline (p-N itroan iline) 3-Methylaniline (m-Tolu idine) Nomenclature IUPAC names (cont’d) name unsymmetrical secondary and tertiary amines as N-substituted primary amines take the largest group bonded to nitrogen as the parent amine smaller group(s) are substituents indicated by using the prefix N CH3 NHCH3 N-Methylanilin e N CH3 N,N-Dimethylcyclopentan amine Nomenclature Common names for most aliphatic amines, list the groups bonded to nitrogen in alphabetical order in one word ending in the suffix -amine NH2 Propylamine NH2 NH2 N sec-Butylamin e D ieth ylmethylamin e Cyclohexylamine Nomenclature Amine salts when four things are bonded to a nitrogen atom, the nitrogen bears a positive charge name the compound as a salt replace the ending -amine (or aniline or pyridine or the like) by -ammonium (or anilinium or pyridinium or the like) and add the name of the anion ( CH3 CH2 ) 3 NH+ ClTriethylammonium chloride Physical Properties Like ammonia, low-molecular-weight amines have very sharp, penetrating odors trimethylamine, for example, is the pungent principle in the smell of rotting fish two other particularly pungent amines are 1,4-butanediamine (putrescine) and 1,5pentanediamine (cadaverine) Physical Properties Amines are polar compounds both 1° and 2° amines have N-H bonds, and can form hydrogen bonds with one another Physical Properties an N-H---N hydrogen bond is weaker than an O-H---O hydrogen bond compare the boiling points of ethane, methanamine, and methanol MW (amu) bp (°C) CH3 CH3 CH3 NH2 CH3 OH 30.1 -88.6 31.1 -6.3 32.0 65.0 Physical Properties all classes of amines form hydrogen bonds with water and are more soluble in water than are hydrocarbons of comparable molecular weight most low-molecular-weight amines are completely soluble in water higher-molecular-weight amines are only moderately soluble in water or are insoluble Basicity of Amines Like ammonia, amines are weak bases, and aqueous solutions of amines are basic the acid-base reaction between an amine and water involves transfer of a proton from water to the amine H + CH3 -N-H :O-H H Meth ylammoniu m hydroxide : : : : H CH3 -N: + H-O-H H Methylamin e (a bas e) Basicity of Amines The base dissociation constant, Kb, for the reaction of an amine with water has the following form, illustrated for the reaction of methylamine with water to give methylammonium hydroxide pKb is defined as the negative logarithm of + Kb [CH3 NH3 ][ OH ] -4 Kb = = 4.37 x 10 [ CH3 NH2 ] pKb = - log 4.37 x 10-4 = 3.360 Basicity of Amines aliphatic amines SS bases than NH3 aromatic amines are weaker bases Clas s pKb Example N ame Aliph atic Ammonia 3.0 - 4.0 CH3 CH2 NH2 4.74 Ethan amin e Stron ger bas e Aromatic 8.5 - 9.5 C6 H5 NH2 Anilin e Weaker base Basicity of Amines We can determine which form of an amine exists in body fluids, pH 7.40 if an aliphatic amine is dissolved in blood, it is present predominantly as its protonated (conjugated acid) form HO NH2 HO D opamine HO NH3 + HO Con jugate acid of d op amine (the major form p res ent in b lood p lasma) Reactions of Amines The most important chemical property of amines is their basicity amines, whether soluble or insoluble in water, react quantitatively with strong acids to form water-soluble salts HO H HO HO NH2 + HCl HO (R)-N orepinep hrin e (on ly s ligh tly solub le in w ater) H2 O HO H + - NH3 Cl HO (R)-N orep ineph rine h yd roch loride (a w ater-soluble s alt) Reactions of Amines example: complete each acid-base reaction and name the salt formed (a) ( CH3 CH2 ) 2 NH + HCl + CH3 COOH (b ) N Reactions of Amines example: complete each acid-base reaction and name the salt formed solution: (a) (CH3 CH2 ) 2 NH + HCl (CH3 CH2 ) 2 NH2 + ClD ieth ylammoniu m chloride CH3 COO- + CH3 COOH (b) N N+ H Pyridinium acetate Nucleophylic Substitution SN2 Displacement of a Leaving Group LG = Weak base LG attached to “small” tetrahedral C Nucleophile Lone pair and neutral Negatively charged Other (Later) Synthesis of Amines 1) Alkyl halides react with ammonia to make primary amines – SN2 This can lead to multiple amination products Example Reduction Non-polar Reduction H2 plus metal catalysts (Pt, Pd, Ni, Zn, etc) Polar Reduction H- source LiAlH4 or NaBH4) Biochem NaDH or NaDPH2 Cyanide SN2 and Reduction 2) Alkyl halides may be reacted with cyanide and then hydrogenated to form the primary amine exclusively. Reactions of Amines 3) Aldehydes and ketones can condense with primary amines and then be hydrogenated to for secondary amines Ketone to Amine - Biochem Biological Application Transamination - Biochem Amine Transfer Amines End Chapter 15