3. NaturalAmides
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Amides are prevalent in nature and serve diverse but key structural and physiological
roles.
These are present in complex structural proteins such as keratin or in key hormones
such as insulin. Peptide bonds of peptides and proteins are amide bonds.
Amides also occur naturally in simple systems such as caffeine and melatonin, a
hormone biosynthesized from tryptophan, that controls the sleep-wake cycle.
People with high levels of melatonin sleep longer and more soundly than those
with low levels.
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13. Preparation of amides
• FROM ACID ANHYDRIDES
•An acid anhydride is obtained when a molecule of water is removed
from two carboxylic acid (-COOH group)
•The reaction of acid anhydrides are like those of acylchlorides except
that a molecule of carboxylic acid is produced
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15. PHYSICAL PROPERTIES
(1) Almost all amides are white, odorless (when pure), crystalline solids.
(2) They have sharp melting points that are higher than those of
corresponding acids, owing to intermolecular hydrogen bonding.
(3) Simple amides upto six-carbons are very soluble in water but the
water-solubility declines as we ascend the series.
(4) The characteristic C=O stretch band in the IR spectrum occurs at
1680 cm¹.
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17. CHEMICAL PROPERTIES
• Amides are least reactive of the acid derivatives. They owe their
chemical stability to resonance
• Since the electron-pair on N atom is not readily available to an acid,
amides are much less basic than amines. In fact, they are neutral
compounds and do not form salts with acids. Some of their general
reactions are listed below:
18. (1) Hydrolysis.
• The amides can be hydrolysed in acid or in alkaline solution on
prolonged heating.
19. 2) Reaction with Nitrous Acid (NaNO2 + HCI)
• Amides on treatment with nitrous acid form carboxylic acids.
21. (4) Dehydration with P2O5.
• When heated with a strong dehydrating agent such as phosphorus
oxide, they form nitriles.
22. (5)Hofmann's Degradation of Amides
• Treatment of an amide with bromine and sodium hydroxide solution
produces a primary amine.
23. (5)Hofmann's Degradation of Amides
• This reaction is also called Hofmann's Rearrangement. Notice that the
overall result is the removal of the carbonyl group from the amide.
The product contains one carbon less than the original amide.
