The isocyanate functional group

Isocyanate is the functional group of atoms –N=C=O (1 nitrogen, 1 carbon, 1 oxygen), not to be confused with the cyanate functional group which is arranged as –O–C≡N. In Organic chemistry, functional groups are specific groups of Atoms within Molecules that are responsible for the characteristic Chemical reactions Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the The cyanate Ion is an Anion consisting of one Oxygen Atom, one Carbon atom and one Nitrogen atom − in that Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An organic compound is any member of a large class of Chemical compounds whose Molecules contain Carbon. An isocyanate may have more than one isocyanate group. An isocyanate that has two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for reaction with polyols in the production of polyurethanes. The name polyols refers to Alcohols containing multiple Hydroxyl groups A polyurethane, commonly abbreviated PU, is any Polymer consisting of a chain of organic units joined by urethane links

Synthesis

The normal industrial route to the manufacture of diisocyanates, is to synthesise the desired molecular structure with amine functional groups at the positions to be occupied by the isocyanate groups. Amines are Organic compounds and Functional groups that contain a basic Nitrogen Atom with a Lone pair. Reaction with phosgene replaces the amine groups with isocyanate groups. Phosgene is the Chemical compound with the formula COCl2 This colorless gas gained infamy as a Chemical weapon during World War I ^[1]

Common methods for the synthesis of isocyanates in the laboratory are the Curtius rearrangement of acyl azides and the Lossen rearrangement of hydroxamic acids. The Curtius rearrangement (or Curtius reaction or Curtius degradation) as first defined by Theodor Curtius, is a Chemical reaction that involves The Lossen rearrangement is the Chemical reaction of a Hydroxamic acid 1 with a dehydration agent (such as Tosyl chloride) to form the O-derivative

Chemistry

The isocyanate group reacts with the hydroxyl functional group to form a urethane linkage. Hydroxyl in Chemistry stands for a molecule consisting of an Oxygen atom and a Hydrogen atom connected by a Covalent bond. Carbamates, or Urethanes are a group of Organic compounds sharing a common Functional group with the general structure -NH(COO- If a diisocyanate is reacted with a compound containing two or more hydroxyl groups (a polyol), long polymer chains are formed, known as polyurethanes. A polyurethane, commonly abbreviated PU, is any Polymer consisting of a chain of organic units joined by urethane links

The isocyanate group also reacts with the amine functional group. Reaction between a diisocyanate and a compound containing two or more amine groups, produces long polymer chains known as polyureas. Polyurea is a type Elastomer that is derived from the reaction product of an Isocyanate component and a Synthetic resin blend component

The isocyanate group can react with itself. Aliphatic diisocyanates can form trimers, known as biurets. In Organic chemistry, compounds composed of Carbon and Hydrogen are divided into two classes Aromatic compounds which contain Benzene rings In Chemistry a trimer is a reaction product of three identical Molecules Trimers are typically encountered as cyclic trimers Biuret is a condensation compound of Urea, equivalent to two molecules of urea less one of Ammonia.

Common compounds

Methylene diphenyl 4,4'-diisocyanate (MDI)
Numbering of the ring atoms shown with blue numbers

A monofunctional isocyanate of industrial significance is methyl isocyanate (MIC), which is used in the manufacture of pesticides. Methyl isocyanate (MIC is an organic compound with the molecular formula C2H3NO arranged as H3C-N=C=O

The global market for diisocyanates in the year 2000 was 4. 4 million tonnes, of which 61. 3% was methylene diphenyl diisocyanate (MDI), 34. Methylene diphenyl diisocyanate, most often abbreviated as MDI, is an aromatic diisocyanate. 1% was toluene diisocyanate (TDI), 3. Toluene diisocyanate (TDI is an aromatic diisocyanate. It is produced for reaction with Polyols to form Polyurethanes It exists in two 4% was the total for hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI), and 1. Hexamethylene diisocyanate (HDI is an organic compound in the class known as Isocyanates More specifically it is an aliphatic diisocyanate Isophorone diisocyanate (IPDI is an organic compound in the class known as Isocyanates More specifically it is an aliphatic diisocyanate 2% was the total for various others. ^[1]

Hazards

The reactivity of isocyanates makes them harmful to living tissue. They are toxic and are known to cause asthma in humans, both through inhalation exposure and dermal contact. Toxicity is the degree to which a substance is able to damage an exposed organism Asthma is a chronic Condition involving the Respiratory system in which the airways occasionally constrict become inflamed, and are Exposure to isocyanates and their vapors should be avoided. For the safe handling of isocyanates,information is available from ISOPA^[2], the European Diisocyanate and Polyol Producers Association.

See also

• Isothiocyanate

References

1. ^ ^{a b} Randall, D (2002). Isothiocyanate is the Chemical group - N = C = S, formed by substituting Sulfur for Oxygen in the Isocyanate The Polyurethanes Book. Wiley. ISBN 0-470-85041-8.  
2. ^ http://www.isopa.org ISOPA

External links

• NIOSH Safety and Health Topic: Isocyanates, from the website of the National Institute for Occupational Safety and Health (NIOSH)

• Health and Safety Executive, website of the UK Health and Safety Executive, useful search terms on this site - isocyanates, MVR, asthma