Transcript Reactions of Aliphatic and Aromatic Amines File

What do each of these have in common ?
H
H2N
C
H
COOH
Amines
L.O: To be able to describe and explain reactions of
aliphatic and aromatic amines
Homework: Activity – Paracetamol; an alternative to aspirin
Due: Mon 22nd Feb
Structure
Contain the NH2 group
Classification
H
R
N:
H
R
H
primary (1°) amines
R
secondary (2°) amines
R
R
N:
R
tertiary (3°) amines
N:
R
R
+
N
R
R
quarternary (4°) ammonium salts
Aliphatic
methylamine, ethylamine, dimethylamine
Aromatic
NH2 group attached directly to benzene ring (phenylamine)
Nomenclature
Named after the groups surrounding the nitrogen + amine
C2H5NH2
ethylamine
(CH3)2NH
dimethylamine
(CH3)3N
trimethylamine
C6H5NH2
phenylamine (aniline)
Physical properties
The LONE PAIR on the nitrogen atom in 1°, 2° and 3° amines makes them ...
LEWIS BASES - they can be lone pair donors
BRØNSTED-LOWRY BASES - they can be proton acceptors
RNH2
+
H+
RNH3+
NUCLEOPHILES - provide a lone pair to attack an electron deficient centre
Physical properties
Boiling point
Boiling points increase with molecular mass
Amines - higher boiling
points than corresponding
alkanes because of their
intermolecular hydrogen bonding
Solubility
Lower mass compounds are
soluble in water due to hydrogen
bonding with the solvent.
Solubility decreases as the
molecules get heavier.
Aromatic amines are only very slightly soluble in water.
Soluble in organic solvents.
Basic properties
Bases
The lone pair on the nitrogen atom makes amines basic;
RNH2
+
H+
RNH3+
a proton acceptor
Strength depends on the availability of lone pair and ability to pick up protons
• The greater the electron density on the N, the better it can pick
up protons
• Affected by the groups attached to the nitrogen
Electron withdrawing substituents (benzene rings)
decrease basicity as the electron density on N is
lowered and the lone pair is less effective
Electron releasing substituents (CH3 groups)
increase basicity as the electron density is
increased and the lone pair is more effective
H
C 6H 5
N:
H
H
CH3
N:
H
Preparation
Aliphatic Amines can be prepared from halogenoalkanes
Reagent
Aqueous, alcoholic ammonia
Conditions
Reflux in aqueous, alcoholic solution under pressure
Product
Amine (or its salt from reaction with the acid produced, which
Nucleophile
Ammonia (NH3)
Equation
the amine is then liberated from with addition of NaOH (aq))
C2H5Br + NH3 (aq/alc)
C2H5NH2 + HBr
( or C2H5NH3+Br¯ )
Preparation
Aromatic Amines can be prepared via reduction of nitrobenzene
Reagent
Nitrobenzene, Sn metal, conc HCl
Conditions
Boiling under reflux
Product
Aromatic amine salt (dissolve in the excess HCl).
Free amine is liberated by addition of NaOH(aq) and then
collected using steam distillation.
Equation
C6H5NO2 + Sn + HCl
C6H5NH2
Reactions as weak bases
Amines which dissolve in water produce weak alkaline solutions
CH3NH2(g)
+
H2O(l)
CH3NH3+(aq)
+
OH¯(aq)
Amines react with acids to produce salts.
C6H5NH2(l)
+
HCl(aq)
C6H5NH3+Cl¯(aq) phenylammonium chloride
This reaction allows an amine to dissolve in water as its salt.
Addition of aqueous sodium hydroxide liberates the free base from
its salt
C6H5NH3+Cl¯(aq)
+
NaOH(aq)
C6H5NH2(l)
+
NaCl(aq)
+ H2O(l)
Nucleophilic reactions
Amines react as nucleophiles – why?
Reagent
Product
haloalkanes
substituted amines
acyl chlorides
N-substituted amides
Mechanism
nucleophilic substitution
addition-elimination
Nucleophilic substitution
HALOALKANES
Acting as a nucleophile, amines can attack halogenoalkanes to produce a 2°
amine.
This is also a nucleophile and can react further producing a 3° amine
C2H5NH2
+
C2H5Br
HBr
+ (C2H5)2NH
diethylamine, 2°amine
Amine reactions
With acid
C2H5NH2
+
H+(aq)
C2H5NH3+ (aq)
The product is an ionic salt that will dissolve in water.
The amine can be regenerated from the salt with the addition of
STRONG ALKALI.
With acid chlorides
C2H5NH2
+ CH3COCl
CH3CONHC2H5 + HCl
The product is a substituted amide.
Amino acids
Amino acids contain 2 functional groups
amine
R1
NH2
carboxyl
H2N
COOH
C
COOH
R2
They all have a similar structure - the identity of R1 and R2 vary
H
H2N
C
H
H
COOH
H2N
C
CH3
COOH
Amino acids – optical isomers
Amino acids can exist as optical isomers
If they have different R1 and R2 groups
Glycine doesn’t exhibit optical isomerism as
there are two H attached to the C atom
H
H2N
C
CH3
H
H2N
C
COOH
H
GLYCINE
2-aminoethanoic acid
COOH
Amino acids - Zwitterions
• a dipolar ion
• has a plus and a minus charge in its structure
• amino acids exist as zwitterions
R1
• give increased inter-molecular forces
• melting and boiling points are higher
H3N+
C
R2
COO¯
Amino acids
• amino acids possess acidic and basic properties
• this is due to the two functional groups
• COOH gives acidic properties
• NH2 gives basic properties
• they form salts when treated with acids or alkalis.
R1
H2N
C
R2
COOH
Amino acids
Basic properties: with H+
HOOCCH2NH2
HOOCCH2NH2
+ H+
HOOCCH2NH3+
+ HCl
HOOCCH2NH3+ Cl¯
Acidic properties: with OH¯
HOOCCH2NH2 + OH¯
HOOCCH2NH2
+ NaOH
¯OOCCH2NH2
+
Na + ¯OOCCH2NH2
H2O
+
H2O