thumb|A general linear imide functional group

In organic chemistry, an imide is a functional group consisting of two acyl groups bound to nitrogen. The compounds are structurally related to acid anhydrides, although imides are more resistant to hydrolysis. In terms of commercial applications, imides are best known as components of high-strength polymers, called polyimides. Inorganic imides are also known as solid state or gaseous compounds, and the imido group (=NH) can also act as a ligand.

Examples

The simplest example is diacetamide with the formula , formally the diacetylated derivative of ammonia. Commonly encountered imides, however, are cyclic, being derived from dicarboxylic acids. A common example is succinimide derived from succinic acid and ammonia. The names of these cyclic imides reflect the parent acid.

Many imides are derived from primary amines as opposed to ammonia. These are indicated by N-substituent in the prefix. For example, N-ethylsuccinimide is derived from succinic acid and ethylamine.

:{| class="wikitable skin-invert-image"

|+Cyclic imides.

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|+

!n!!Common name!!Systematic name!!Structure!!PubChem!!Parent acid!!Structure

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|2||Succinimide||Pyrrolidine-2,5-dione||60px||11439||Succinic acid||120px

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|2, unsaturated, cis carbon-carbon double bonds||Maleimide||Pyrrole-2,5-dione|| 120px||10935||Maleic acid||100px

|-

|3||Glutarimide||Piperidine-2,6-dione||90px||70726||Glutaric acid||150px

|-

|6||Phthalimide||Isoindole-1,3-dione||100px||6809||Phthalic acid||120px

|}

Properties

Being highly polar, imides exhibit good solubility in polar organic solvents. Unlike the structurally related acid anhydrides, they resist hydrolysis and some can even be recrystallized from boiling water.

The N–H center for imides derived from ammonia is acidic and can participate in hydrogen bonding. The N-H group is weakly acidic as indicated in the case of maleimide, with a pKa estimated at 10.

Occurrence and applications

Many high strength or electrically conductive polymers contain imide subunits, i.e., the polyimides. One example is Kapton where the repeat unit consists of two imide groups derived from aromatic tetracarboxylic acids. Another example of polyimides is the polyglutarimide typically made from polymethylmethacrylate (PMMA) and ammonia or a primary amine by aminolysis and cyclization of the PMMA at high temperature and pressure, typically in an extruder. This technique is called reactive extrusion. A commercial polyglutarimide product based on the methylamine derivative of PMMA, called Kamax, was produced by the Rohm and Haas company. The toughness of these materials reflects the rigidity of the imide functional group.

Interest in the bioactivity of imide-containing compounds was sparked by the early discovery of the high bioactivity of the Cycloheximide as an inhibitor of protein biosynthesis in certain organisms. Thalidomide, famous for its adverse effects, is one result of this research. A number of fungicides and herbicides contain the imide functionality. Examples include Captan, which is considered carcinogenic under some conditions, and Procymidone.

thumb|center|upright=2.5|class=skin-invert-image|Illustrative imides, from left: [[N-ethylmaleimide|N-ethylmaleimide, a biochemical reagent; phthalimide, an industrial chemical intermediate; Captan, a controversial pesticide; thalidomide, a drug that once caused many birth defects; a subunit of Kapton, a high strength polymer used to make space suits.]]

In the 21st century new interest arose in thalidomide's immunomodulatory effects, leading to the class of immunomodulators known as immunomodulatory imide drugs (IMiDs).

Preparation

Most common imides are prepared by heating dicarboxylic acids or their anhydrides and ammonia or primary amines. The result is a condensation reaction:

: (RCO)<sub>2</sub>O + R′NH<sub>2</sub> → (RCO)<sub>2</sub>NR′ + H<sub>2</sub>O

These reactions proceed via the intermediacy of amides. The intramolecular reaction of a carboxylic acid with an amide is far faster than the intermolecular reaction, which is rarely observed.

They may also be produced via the oxidation of amides, particularly when starting from lactams.

:

Organic compounds called carbodiimides have the formula RN=C=NR. They are unrelated to imides.

References

  • IUPAC: imides

See also

  • Inorganic imide