Transcript m4 amines and peptides

AMINES
CONTENTS
• Prior knowledge
• Structure and classification
• Nomenclature
• Physical properties
• Basic properties
• Nucleophilic properties
• Amino acids
• Peptides and proteins
AMINES
Before you start it would be helpful to…
• know the functional groups found in organic chemistry
• know the arrangement of bonds around atoms
• recall and explain nucleophilic substitution reactions
STRUCTURE & CLASSIFICATION
Structure
Contain the NH2 group
Classification
H
R
N:
H
R
H
primary (1°) amines
R
secondary (2°) amines
R
R
N:
R
R
R
tertiary (3°) amines
N:
+
N
R
R
quarternary (4°) ammonium salts
Aliphatic
methylamine, ethylamine, dimethylamine
Aromatic
NH2 group is attached directly to the benzene ring (phenylamine)
NOMENCLATURE
Nomenclature
Named after the groups surrounding the nitrogen + amine
C2H5NH2
ethylamine
(CH3)2NH
dimethylamine
(CH3)3N
trimethylamine
C6H5NH2
phenylamine (aniline)
PREPARATION
Amines can be prepared from halogenoalkanes
Reagent
Aqueous, alcoholic ammonia
Conditions
Reflux in aqueous, alcoholic solution under pressure
Product
Amine (or its salt due to a reaction with the acid produced)
Nucleophile
Ammonia (NH3)
Equation
C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr ( or C2H5NH3+Br¯ )
PHYSICAL PROPERTIES
The LONE PAIR on the nitrogen atom in 1°, 2° and 3° amines makes them ...
NUCLEOPHILES - provide a lone pair to attack an electron deficient centre
CHEMICAL REACTIONS - WEAK BASES
Water
Amines which dissolve in water produce weak alkaline solutions
CH3NH2(g)
Acids
+
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 one to dissolve an amine 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)
AMINO ACIDS
Structure
Amino acids contain 2 functional groups
amine
NH2
carboxyl
R1
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 ISOMERISM
Amino acids can exist as optical isomers
If they have different R1 and R2 groups
Optical isomers exist when a molecule
Contains an asymmetric carbon atom
H
H2N
Asymmetric carbon atoms have four
different atoms or groups attached
C
COOH
CH3
Two isomers are formed - one rotates plane
polarised light to the left, one rotates it to the right
H
Glycine doesn’t exhibit optical isomerism as
there are two H attached to the C atom
H2N
C
COOH
H
GLYCINE
2-aminopropanoic acid
AMINO ACIDS - ZWITTERIONS
Zwitterion
• a dipolar ion
• has a plus and a minus charge in its structure (see below)
• amino acids exist as zwitterions
• give increased inter-molecular forces
• melting and boiling points are higher
R1
H3N+
C
R2
COO¯
AMINO ACIDS - ACID-BASE PROPERTIES
• amino acids possess acidic and basic properties
• this is due to the two functional groups (see above)
• COOH gives acidic properties
• NH2 gives basic properties
• they form salts when treated with acids or alkalis.
R1
H2N
C
R2
COOH
AMINO ACIDS - ACID-BASE PROPERTIES
Acidic properties:
with H+
HOOCCH2NH2
+ H+
——>
HOOCCH2NH3+
with HCl
HOOCCH2NH2
+ HCl
——>
HOOCCH2NH3+ Cl¯
Basic properties:
+ OH¯
——> ¯OOCCH2NH2 + H2O
with OH¯
HOOCCH2NH2
with NaOH
HOOCCH2NH2 + NaOH ——> Na+ ¯OOCCH2NH2 + H2O
PEPTIDES - FORMATION & STRUCTURE
Amino acids can join together to form peptides via an amide or peptide link
2 amino acids joined
dipeptide
3 amino acids joined
tripeptide
many amino acids joined
polypeptide
a dipeptide
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
• attack takes place at the slightly positive C of the C=O
• the C-N bond is broken
• hydrolysis with water is very slow
• hydrolysis in alkaline/acid conditions is quicker
• hydrolysis in acid/alkaline conditions (e.g. NaOH) will produce salts
with
HCl
H+
NaOH
OH¯
NH2
NH2
COOH
COOH
becomes
becomes
becomes
becomes
NH3+Cl¯
NH3+
COO¯ Na+
COO¯
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
H
H2N
H
C
CO NH
CH3
C
H
CH3
CO NH
C
CH3
Which amino acids are formed?
COOH
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
H
H2N
C
H
CO NH
CH3
C
CH3
CO NH
C
COOH
CH3
H
H
H
H2N
C
CH3
COOH
+
H2N
C
H
CH3
COOH
+
H2N
C
CH3
COOH
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
H
H2N
H
C
CO NH
CH3
C
H
H
CO NH
C
CH3
Which amino acids are formed?
COOH
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
H
H2N
C
H
CO NH
CH3
H
CO NH
C
H2N
C
CH3
COOH
CH3
H
H
H
2x
C
COOH
+
H2N
C
H
COOH
PROTEINS
• are polypeptides with high molecular masses
• chains can be lined up with each other
• the C=O and N-H bonds are polar due to a difference in electronegativity
• hydrogen bonding exists between chains
dotted lines ---------- represent hydrogen bonding