Amides.pptx

AMIDES

1. Amides

2. N a t u r a l Ami d e s 2 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.

3. Preparation of amides FROM ACID ANHYDRIDES An acid anhydride is obtained when a molecule of water is removed from two c a rb o x y li c a c i d ( - C O O H g r o up ) The reaction of acid anhydrides are like those of acylchlorides except that a molecule of carboxylic acid is produced

4. 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.

5. 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:

6. (1) Hydrolysis. The amides can be hydrolysed in acid or in alkaline solution on prolonged heating.

7. 2) Reaction with Nitrous Acid (NaNO2 + HCI) Amides on treatment with nitrous acid form carboxylic acids.

8. (3) Reduction Lithium aluminium hydride (LiAlH4) reduces amides to primary amines

9. (4) Dehydration with P2O5. When heated with a strong dehydrating agent such as phosphorus oxide, they form nitriles .

10. (5)Hofmann's Degradation of Amides Treatment of an amide with bromine and sodium hydroxide solution produces a primary amine.

11. (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.