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Amide

Definition of amide

Amides (RCONH2) are functional group where carbonyl group attached to a amine group. In simple amides nitrogen attached with two hydrogen atoms. And in complex amides nitrogen attached with one or two aliphatic or aromatic group replacing the hydrogen atom.

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The carbon-nitrogen bond in amide is known as amide linkage. This bond is also present in protein molecules where it is known as peptide linkage.

Structure of amide

The nitrogen, carbon and oxygen atoms in amide group, are sphybridized. Thus the electrons of three p orbitals on three atoms oxygen, carbon and nitrogen are are on the same plane and delocalized. The resonance structures of amide is as follows:

By Ckalnmals (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Nomenclature

As amides are the derivatives of acid, by changing the end -oic acid to -amide we can name the corresponding amides. Normally the name starts with the name of long chain aliphatic. As for example, the name of the first compound is ethanamide. When aliphatic or aromatic group is attached to the nitrogen atom, the name of these groups are placed at first, like in the second one, the name is N,N-dimethylmethanamide.

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Properties of amides

Melting point

Although the melting point of methanamide is liquid but the other higher carbon chain amides are solid at room temperature. This is because of the presence of intermolecular hydrogen bond between the partially positive hydrogen atoms attached with nitrogen of one molecule and partially negative charged oxygen atom in another molecule. Each molecule has two partially positive hydrogens, makes it more possible sites to have hydrogen bonding. To break these hydrogen bonds more energy is needed and thus the melting point increases.

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Solubility

Low amides are soluble in water because they can have hydrogen bond with water molecules. Amides both act as a hydrogen bond donor and acceptor. The nitrogen and oxygen atom act as acceptor and the hydrogens attached with nitrogen atom act as donor. As the aliphatic carbon chain increases the solubility decreases because of the long chain get in the way of hydrogen bond.

Basicity

Unlike amines, amides show no measurable basicity because of the presence of more electronegative atom near to it. The lone pair of electron on the nitrogen atom is pulled and delocalized through the oxygen, carbon and nitrogen atoms. As a result there are no available electrons on nitrogen to show the basicity of the compound.

Synthesis of amides

Amides are usually formed from the reaction between an amine and a carboxylic acid. This is an condensation reaction and water molecule is removed during the reaction.

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This reaction is occurred in few steps:

Step 1: A nucleophilic nitrogen attacks the carbonyl carbon center of carboxylic acid and oxygen atom pulls the electron pair from the pi-bond simultaneously. Thus a zwiterrion is formed (a positive ion on nitrogen atom and a negative ion on oxygen atom).

Step 2: The electron pair on oxygen reform the pi-bond and kicks the hydroxyl group (OH) out from the zwiterrion.

Step 3: The negatively charged hydrogen takes one of the hydrogen (H+) attached to nitrogen atom to get rid of the charge from this stage. And thus the amide is formed.

Reaction of amides

Amide breaks down to its corresponding amine and carboxylic acid. Amides are comparatively unreactive because of delocalization of electrons. However an electronegative atom can attack on the carbonyl carbon and break the pi-bond. The tetrahedral intermediate eventually breaks to form amine and carboxylic acid under acidic condition.

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Summary

  • Amides are functional group where carbonyl group attached to a amine group.
  • The electrons of three p orbitals on three atoms oxygen, carbon and nitrogen are are on the same plane and delocalized.
  • Normally the name starts with the name of long chain aliphatic and ends with -amide.
  • Although the melting point of methanamide is liquid but the other higher carbon chain amides are solid at room temperature.
  • Short chain amide dissolves in water while the long chain amides are not.
  • Amides are not basic in nature.
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