Chapter 19 AMINES PROBLEMS 2a,b,e,3a,b,c,6a,c16b,c,17a,c,20b,d21b, 25a,c,e,g,26a,c27b,36a,b,37ab,41c,d,I, 42a,b,c44a,c,g INTRODUCTION Organic derivatives of ammonia 70% of pharmaceuticals contain nitrogen Alkaloids - obtained from plants and have strong biological properties
Examples Primary RNH 2 DOPAMINE Parkinson's Disease - inadequate concentration of dopamine Dopamine is a worthless drug since it can't cross blood-brain barrier i.e. can't get in cerebrospinal fluid An amino acid that can cross barrier
Basicity of amines The greater the availability of the lone pair electrons on nitrogen, the greater the base. Kb = [RNH 3 + ] [OH - ] / RNH 2 In the old days, pK b was a measure of base strength. pK b = - log K b The stronger the base the lower the pK b EFFECTS ON AMINE BASICITY 1. INDUCTIVE EFFECT - ALKYL SUBSTITUTION METHYL GROUP INCREASES ELECTRON DENSITY ON N METHYLS ARE BETTER THAN ONE WATCH OUT THREE METHYL GROUPS DECREASES BASICITY pK b = Steric inhibition of solvation of HOH with the NH + of the R3NH + cation.
The greater the availability of the lone pair electrons on nitrogen, the greater the base. Kb = [RNH 3 + ] [OH - ] / RNH 2 In the old days, pK b was a measure of base strength. pK b = - log K b The stronger the base the lower the pK b EFFECTS ON AMINE BASICITY 1. INDUCTIVE EFFECT - ALKYL SUBSTITUTION METHYL GROUP INCREASES ELECTRON DENSITY ON N METHYLS ARE BETTER THAN ONE WATCH OUT THREE METHYL GROUPS DECREASES BASICITY pK b = Steric inhibition of solvation of HOH with the NH + of the R3NH + cation.
2. RESONANCE EFFECT Base weakening Why?Delocalizes electron pair on N!! pK b = 15 not aromatic Hybridization The greater the % of s character The closer the lone pair is to N The weaker the base sp 2 sp 3 sp
Sections to be omitted 19-7 (salts of amines) 19-8 (amines as PT catalysts) 19-9 (Spectroscopy)
Reaction of Amines to be reviewed Reaction with Aldehydes and Ketones Aromatic Substitution of Arylamines 19-11
Alkylation of Amines Alkylation of amines by alkyl halides - Only two situations of importance Excess Methyl iodide - all the way to quaternary salt Excess ammonia - stops are monoalkylation stage PhCH 2 Br + excess NH 3 PhCH 2 NH 2 Propyl-NH 2 + ex MeI Propyl-N(Me) 3 +
Acylation of Amines AMIDE N-methylbenzamide
Amines as Leaving Groups Hofmann Elimination HEAT ANTI ELIMINATION - Less stable (less substituted) alkene
Stereochemistry H CH 3 H H looking down C2-C3 rotation about C-3 counter clockwise More stable; not Suitable for E-2 Suitable for E-2;less stable
Looking down C1-C2 Rotating clockwise about C-2 all suitable for E-2
Examples Note: NMe3 has no beta hydrogens thus complicating the E-2 process
COPE REACTION N-OXIDE LESS HINDERED BETA HYDROGEN SYN ELIMINATION
COPE EXAMPLE Mild conditions
Reaction of Primary Amines with HNO 2 Preparation of HNO 2 NaNO 2 + HCl HNO 2 NaCl +
Reaction Aliphatic amines RN 2 +R+ Hot carbocation mixture of products
Aromatic Primary Amines ArN o C stable undergoes SN1 with nucleophiles
Examples ArBr ArCl ArF ArCN ArOH ArH
Conversion of NO 2 to NH 2 Good way to introduce NH 2 RED = SnCl 2 / HCl
Another one 1. NaNO3/HCl 2. KI Br 2 / Fe HNO 3 H 2 SO 4 CH 3 I / AlCl 3
Synthesis of Amines Reductive Amination NOTE: carbonyl has been reduced and aminated - most general method
Examples Primary Amines Could use NH 3 but it is a gas and inconvenient LAH is usual reducing agent
Secondary Amines Note: Primary amine is converted to secondary amine.
Tertiary Amines iminium salt Very unstabile So reaction is run with reducing presence at all times. This means that there will both product and carbonyl compounds presence. Must use reducing agent that only reduces iminium salt It is: sodium triacetoxyborohydride!!
Preparation of Tertiary, Con’t Na(MeCOO) 3 ) 3 BH tertiary amine NOTE: Secondary amine converts to tertiary amine
Mechanism of iminium salts -H 2 O
EXAMPLE Na(MeCOO) 3 BH
Other Ways to Prepare Primary amines REDUCTION OF NITROANILINES - as before REDUCTION OF NITRILES SN2 conditions RCH 2 CH 2 NH essentially replaces halogen with a CH 2 NH 2 group. Increases carbon chain length by one carbon
Don’t do problem 27b ch.19 Please do problem 30b, REDUCTION OF AZIDES RCH 2 N 3 RCH 2 NH 2 essentially replaces halogen with an NH 2 group. No increase in carbon chain length NOTE 1-bromopentane to 1-pentanamine - azide but 1-bromopentane to1-hexanamide - CN-