In organic chemistry, an imide is a functional group consisting of two acyl groups bound to nitrogen. These compounds are structurally related to acid anhydrides, although imides are less reactive. In terms of commercial applications, imides are best known as components of high-strength polymers.
Most imides are cyclic compounds derived from dicarboxylic acids and their names reflect the parent acid. Examples are succinimide derived from succinic acid and phthalimide derived from phthalic acid. For imides derived from amines (vs. ammonia), the N-substituent is indicated by a prefix, e.g., N-ethylsuccinimide is derived from succinic acid and ethylamine. Isoimides are isomaric with normal imides, they have the formula RC(O)OC(NR')R. They are often intermediates that convert to the more symmetrical imides. Organic compounds called carbodiimides have the formula RN=C=NR, they are unrelated to imides.
Imides in coordination chemistry
Being highly polar, imides exhibit good solubility in polar media. The N-H center for imides derived from ammonia is acidic and can participate in hydrogen bonding. Unlike the structurally related acid anhydrides, they resist hydrolysis and some can even be recrystallized from boiling water.
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 (TM), 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 has been phased out because of its carcinogenic properties, and Procymidone.
- (RCO)2O + R'NH2 → (RCO)2NR' + H2O
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.
Certain imides can also be prepared in the isoimide-to-imide Mumm rearrangement.
For imides derived from ammonia, the N-H center is acidic. Thus, alkali metal salts of imides are well known, a well-known example being potassium phthalimide. These salts can be alkylated to give N-alkylimides, which in turn can be degraded to release the primary amine. Strong nucleophiles, such as potassium hydroxide or hydrazine are used in the release step.
The nitrogen in imides is not very basic, which allows it to form stable compounds with halogens. Treatment of imides with halogens and base gives the N-halo derivatives. Examples that are useful in organic synthesis are N-chlorosuccinimide and N-bromosuccinimide, which respectively serve as sources of "Cl+" and "Br+" in organic synthesis.
- Walter W. Wright and Michael Hallden-Abberton "Polyimides" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a21_253
- Peter Ackermann, Paul Margot, Franz Müller “Fungicides, Agricultural“ in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a12_085
- Vincent Rodeschini, Nigel S. Simpkins, and Fengzhi Zhangi (2009), Illustrative imide formation from amine and anhydride, Org. Synth.; Coll. Vol. 11: 1028