AMIDES

What are amides?

Amides are made by a chain or carbon atoms with one as a carbonyl, double bonded to oxygen, bonded to an amine group, a Nitrogen, which is then bonded to the next carbon to continue the chain and one hydrogen.

The carbonyl group must be adjacent to the amine group to make it an amide.

Since they form chains, they can continue for a long number of atoms, this allows for one chain to have many amide groups in one chain.

So . . . amides contain the -CONH2 group.

GENERAL STRUCTURE OF AMIDES

The most commonly discussed amide is ethanamide, CH3CONH2

(old name: acetamide).

NOMENCLATURE

Notice that in each case, the name is derived from the acid by replacing the

"oic acid" ending by "amide".

If the chain was branched, the carbon in the -CONH2 group counts as the number 1 carbon atom. For example:

PREPARATIONS OF AMIDES

PREPARATION OF AMIDES

Preparation of Amides

From Carboxylic acids

FromAcyl Chlorides

FromAcid anhydrides

i. Making amides from carboxylic acids

The carboxylic acid is first converted into an ammonium salt which then produces an amide on heating.

The ammonium salt is formed by adding solid ammonium carbonate to an excess of the acid.

For example: Ammonium ethanoate is made by adding ammonium carbonate to an excess of ethanoic acid.

2CH3COOH + (NH4) 2CO3 → 2CH3COONH4+ H2O + CO2

Why excess?Prevent dissociation of the ammonium salt before it dehydrates.

Ammonium ethanoate

When the reaction is complete, the mixture is heated and the ammonium salt dehydrates to produced ethanamide.

CH3COONH4 → CH3CONH2 + H2O

ii. Making Amides from acyl chlorides Acyl chlorides (acid chlorides) have the general formula RCOCl. The chlorine atom is very easily replaced by other things such as -

NH2 group to make an amide.

First stage: Ammonia reacts with acyl chloride (ethanoyl chloride) to produced ethanamide and hydrogen chloride gas.

CH3COCl + NH3 → CH3CONH2 + HCl

Second stage: Hydrogen chloride will reacts with excess ammonia to give ammonium chloride.

NH3 + HCl → NH4Cl Overall equation:

CH3COCl + 2NH3 → CH3CONH2 + NH4Cl

iii. Making amides from acid anhydrides An acid anhydride produced from removing a molecule of water

from two carboxylic acid -COOH groups.

Example: Two ethanoic acid molecules and removed a molecule of water

between them to obtain the acid anhydride, (ethanoic anhydride).

Then ethanoic anhydride is added to conc. ammonia solution, Ethanamide + ammonium ethanoate will produced. The reaction happens in two stages.

PROPERTIES OF AMIDES

i. MELTING POINTS

Methanamide is a liquid at room temperature (melting point: 3°C).

Ethanamide forms colourless crystals with a melting point of 82°C.

The melting points of the amides are high because they can form hydrogen bonds which is come from the amine group( -NH2 )

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 large heat energy to break up the bonding and so the melting points of the amides are quite high.

ii. SOLUBILITY IN WATER

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

The solubility of amides is greater than that of corresponding hydrocarbons. Because ??

The presence of N-H dipoles allows amides to function as H-

bond donors as well.

Thus amides can participate in hydrogen bonding with water and other monoprotic solvents; the oxygen and nitrogen atoms can accept hydrogen from water and the N-H hydrogen atoms can donate H-bonds. (Meant: can accept and donate hydrogen)

As a result of interactions such as these, the water solubility of amides is greater than corresponding hydrocarbons.

But…amides have water solubility roughly comparable to esters.

Typically amides are less soluble than comparable amines and carboxylic acids, because amides can donate and accept hydrogen bonds, and can ionize at appropriate pHs to further enhance solubility

iii. BASICITY1. Compared to amines, amides are very weak bases.

2. Because : The electron-withdrawing nature of the carbonyl group .

3. The lone pair of electrons on the nitrogen is delocalized, to forming a partial double bond with the carbonyl carbon and oxygen atom become a negative charge.

4. On the other hand, amides are much stronger bases than carboxylic acids, esters, aldehydes, and ketones.

USES OF AMIDES Amides are found in the plastic and rubber industry, paper industry, water

and sewage treatment and colour, in crayons, pencils and inks.

Acrylamide and polyacrylamide are the products most widely used in these industries. However, acrlamide is a carcinogen, so can only be used if the chemicals are not intended for consumption.

Polyacrylamide is used in its place, mainly therefore, in the treatment of drinking water and sewage, as these are intended for consumption.

The paper industry takes a lot of polyacrylamide produced, being used as a binder and for retention aids for fibers. It also retains the colour pigments on the paper.

Acrylamide is used to stabilize soil and is used in sand to help keep free sand able to flow, so can fit moulds.

It is used as a coating on many household appliances and car parts with thermosetting acrylics.

Other minor uses of acrylamide are in the cosmetic industry to prepare soaps, hair products and pre-shave lotions.

It is also used for explosives, adhesives, printing inks, latex thickeners and emulsion stabilizers, but these are on a much lower scale to the ones stated before.