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Sources, Preparation and Properties of Amides

Acid amides are the derivatives of carboxylic acids in which-OR part of the carboxylic group is replaced by the Hz group. These amides are known as primary amides because they have only one carbon atom attached to the nitrogen atom of the amide.

 

 SOURCES, PREPARATION AND PROPERTIES OF AMIDES

SOURCES OF AMIDES

Amides occur naturally in plant~ and animals. For example, urea is found in the urine of mammals A normal adult produces about 30 g of ute3 per day. Nicotinamide is found in many plants and animals as a source of vitamin B. Yeast, liver extract kontomire and milk are important sources.

PREPARATION OF AMIDES

1. From Acid Chlorides or Acid Anhydrides. Amide:. are generally prepared by reaction of ammo11ia or amines with acid ,chlorides or acid anhydrides.

. From Ammonium Salts of Carboxylic Acids Amides may also be prepared by the decomposition of ammonium salts of carboxylic acids

PHYSICAL PROPERTIES OF AMIDES

All amides except formamide, are crystalline solids at room temperature. They have relatively high melting and boiling points. It is due to association of amide molecules by intermolecular hydrogen bonds.

(a) Hydrogen bonding in amides gives them relatively high melting and boiling points

(b) Amides dissolve in water by hydrogen bond association. Hydrogen bonding in amides and with water

 

Comparison of physical properties of amides and alkanoic acids

CHEMICAL PROPERTIES OF AMIDES

1. Amphoteric Character. Am ides are very feeble bases. This is due to the fact that the lone pair of electrons on nitrogen atom is involved in resonance with carbonyl group. It is due to the contribution of resonating structure II.

Thus, electron pair of nitrogen is not easily available for protonation. Consequently, the basic character is considerably decreased. However, under suitable conditions amides can also exhibit a feeble acidic character. The amphoteric character of amides is illustrated by the following reactions:

(i) Basic character. In accordance with resonating structure I already shown, it is evident that nitrogen atom of amide molecule has a lone pair of electrons. Therefore, it can act as a base. For example, acetamide (as base) reacts with hydrochloric acid (an acid) to form a salt.

CH3 CONH2 + HCl à CH3 CONH2 HCl

(ii) Acidic character. In accordance with resonating structure II shown earlier, it is clear that the development of positive character on nitrogen atom facilitates the release of proton. Thus, amide can act as acid. For example, acetamide (as acid) reacts with mercuric oxide (a base) to form mercury salt and water.

2CH3 COHN2 + HgO → (CH3 CONH)2 Hg + H2O

Similarly, acetamide reacts with metallic sodium to form sodium salt and hydrogen.

Amides arc associated by strong hydrogen bonds at both the polar oxygen of the C = 0 bond and the nitrogen of the – NH2 group points. The extent of hydrogen bonding appears to be greater in amides than in alkanoic acids.

Amides, especially the lower members, are very soluble in water by hydrogen bond association. They are also soluble in common organic solvents.

2. Reduction. Amides on reduction with sodium and ethyl alcohol or with lithium aluminium hydride, form primary amines. For example:

3. Dehydration. Amides on heating with phosphorus pentoxide get dehydrated to form nitriles. For example:

4. Hydrolysis. On boiling with dilute acid or alkali, amides rapidly undergo hydrolysis. For example:

5. Reaction with Nitrous Acid. Amides react with nitrous acid to give carboxylic acids. and nitrogen gas. Nitrous acid required is prepared in situ by reaction of NaNO2 and HCI.

6. Hoffmann’s Bromamide Reaction. Amides (carrying primary nitrogen atom) react with bromine in the presence of alkali to form a primary amine carrying one carbon atom Jess than the parent amide.

This reaction is useful in the preparation of a lower homologue from the higher one (descending of series).

 

EXPERIMENT TO TEST THE PRESENCE OF AMI DES

1. Take a pinch of ethanamide in a dry test tube

2. Add 5- 10 ml of 40% caustic soda (NaOH) solution  to it

3. To the mouths of test tube, attach the gas detector containing Nesseler’s reagent (Fig. 47.1)

4. Warm the test tube on bunsen flame

Observations and Inference

A colourless gas with ammoniacal smell is evolved, On passing the gas through Nesseler’s reagent, it turns it brown,