Download AMIDES AND AMINO ACIDS

AMIDES AND AMINO
ACIDS
AMIDES
Structure
derivatives of carboxylic acids
amide group is
Nomenclature
-CONH2
White crystalline solids named from the corresponding acid
(remove oic acid, add amide)
CH3CONH2
ethanamide (acetamide)
CH3CONHC2H5
N - ethyl ethanamide -
the N tells you the
substituent is on
the nitrogen
Nylons are examples of polyamides
Preparation
Acyl chloride + ammonia
CH3COCl
+
ethanoyl chloride
NH3
——>
CH3CONH2 + HCl
ethanamide
Physical properties
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The N-H bond is polar and results in
extensive intermolecular hydrogen
bonding.
The melting and boiling points of amides
are relatively high.
Apart from methanamide (a liquid) all
amides are white crystalline solids.
They are all very soluble in water.
AMIDES - CHEMICAL PROPERTIES
Hydrolysis
general reaction
acidic soln.
alkaline soln.
Identification
CH3CONH2
CH3CONH2
CH3CONH2
+
+
+
H2O ——> CH3COOH + NH3
H2O + HCl ——> CH3COOH + NH4Cl
NaOH ——> CH3COONa + NH3
Warming an amide with dilute sodium hydroxide solution and
testing for the evolution of ammonia using moist red litmus paper
is used as a simple test for amides.
Reduction
Reduced to primary amines: CH3CONH2 + 4[H] ——>
(LiAlH4 in ethoxyethane is used as the reducing agent)
CH3CH2NH2 + H2O
Polyamides – condensation
polymers
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The first polyamide (Nylon) was developed in the 1940s.
Hexanedioic acid was condensed with 1,6diaminohexane to produce nylon-66.
In the 1960s another polyamide known as “Kevlar” was
developed by the same laboratories.
Kevlar has high temperature resistance and low thermal
conductivity, high tensile strength, doesn’t shrink in the
wash, is flame, chemical and cutting resistant.
Applications so far include – bullet proof vests vehicle
hoses, structural parts in aircraft, space shuttles and
boat, protective clothing, ropes etc
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
Two isomers are formed - one rotates plane
polarised light to the left, one rotates it to the right
C
COOH
CH3
ALANINE
2-aminopropanoic acid
H
Glycine doesn’t exhibit optical isomerism as
there are two H attached to the C atom
H2N
C
COOH
H
GLYCINE
2-aminoethanoic 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
Basic properties:
with H+
HOOCCH2NH2
+ H+
——>
HOOCCH2NH3+
with HCl
HOOCCH2NH2
+ HCl
——>
HOOCCH2NH3+ Cl¯
Acidic 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
THE END