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~~
