amide

 

An aliphatic compound containing the group –CO.NH₂ (the amide group), derived from carboxylic acids and ammonia by replacing the acid hydroxyl group by the amino group (NH₂). N-substituted amides are derived from primary or secondary amines instead of ammonia. Other amides are derived analogously from inorganic oxy-acids or from sulfonic acids. Amides are prepared by reaction of acid chlorides, acid anhydrides, or esters with ammonia or amines, or by partial hydrolysis of nitriles.  

Most simple amides are low-melting solids with strong hydrogen bonding, soluble in water. Methanamide (formamide) and N-substituted amides are liquids widely used as solvents. Amides are both weak acids and weak bases. They may be hydrolyzed to carboxylic acids, and dehydrated to nitriles. Metallic hydrides convert them to amines, and treatment with bromine and sodium hydroxide (the Hofmann degradation) yields amines with one fewer carbon atom. Polymeric amides (such as nylon) are used as synthetic fibers, and similar amid linkages join amino acids in proteins and peptides.

is it true Most simple amides are low-melting solids ? why ?

why amides are soluble in water ?

Amides are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an amide the -OH part of that group is replaced by an -NH₂ group.

The three simplest amides are :

HCONH2	methanamide
CH3CONH2	ethanamide
CH3CH2CONH2	propanamide

Methanamide is a liquid at room temperature (melting point: 3°C), but the other amides are solid.

For example, ethanamide forms colourless deliquescent crystals with a melting point of 82°C. A deliquescent substance is one which picks up water from the atmosphere and dissolves in it. Ethanamide crystals nearly always look wet.

The melting points of the amides are high for the size of the molecules because they can form hydrogen bonds. The hydrogen atoms in the -NH₂ group are sufficiently positive to form a hydrogen bond with a lone pair on the oxygen atom of another molecule.

As you can see, there is the potential for lots of hydrogen bonds to be formed. Each molecule has two slightly positive hydrogen atoms and two lone pairs on the oxygen atom.

These hydrogen bonds need a reasonable amount of energy to break, and so the melting points of the amides are quite high.

Solubility in water

The small amides are soluble in water because they have the ability to hydrogen bond with the water molecules.

It needs energy to break the hydrogen bonds between amide molecules and between water molecules before they can mix - but enough energy is released again when the new hydrogen bonds are set up to allow this to happen.

~~forgiveness if there are mistakes~~