Isocyanate

Key takeaways

Isocyanate belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Isocyanate to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Isocyanate from memory before moving on to harder problems.

Reference excerpt

In organic chemistry, isocyanate is the functional group with the formula R−N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for the production of polyurethanes, a class of polymers.

Structure and bonding In terms of bonding, isocyanates are closely related to carbon dioxide (CO2) and carbodiimides (C(NR)2). The C−N=C=O unit that defines isocyanates is planar, and the N=C=O linkage is nearly linear. In phenyl isocyanate, the C=N and C=O distances are respectively 1.195 and 1.173 Å. The C−N=C angle is 134.9° and the N=C=O angle is 173.1°. Isocyanates are isomers of cyanate functional group-containing molecules, with the difference being the connectivity with the rest of the molecule through the oxygen atom rather than the nitrogen atom in isocyanates. Cyanates usually convert to isocyanates due to better thermodynamic stability.

Production Isocyanates are usually produced from amines by phosgenation, i.e. treating with phosgene:

RNH2 + COCl2 → RNCO + 2 HCl These reactions proceed via the intermediacy of a carbamoyl chloride (RNHC(O)Cl). Owing to the hazardous nature of phosgene, the production of isocyanates requires special precautions. A laboratory-safe variation masks the phosgene as oxalyl chloride. Also, oxalyl chloride can form acyl isocyanates from primary amides, which phosgene typically dehydrates to nitriles instead. Another route to isocyanates entails addition of isocyanic acid to alkenes. Complementarily, alkyl isocyanates form by displacement reactions involving alkyl halides and alkali metal cyanates. Aryl isocyanates can be synthesized from reductive carbonylation of nitro- and nitrosoarenes; a chalcogen or palladium catalyst is necessary to avoid side-reactions of the nitrene intermediate. Three rearrangement reactions involving nitrenes give isocyanates:

Schmidt reaction, a reaction where a carboxylic acid is treated with ammonia and hydrazoic acid yielding an isocyanate. Curtius rearrangement degradation of an acyl azide to an isocyanate and nitrogen gas. Lossen rearrangement, the conversion of a hydroxamic acid to an isocyanate via the formation of an O-acyl, sulfonyl, or phosphoryl intermediate. An isocyanate is also the immediate product of the Hofmann rearrangement, but typically hydrolyzes under reaction conditions.

Reactivity

With nucleophiles Isocyanates are electrophiles, and as such they are reactive toward a variety of nucleophiles including alcohols, amines, and even water. Upon treatment with an alcohol, an isocyanate forms a urethane linkage:

ROH + R'NCO → ROC(O)N(H)R' R and R' are alkyl or aryl groups. If a diisocyanate is treated with a compound containing two or more hydroxyl groups, such as a diol or a polyol, polymer chains are formed, which are known as polyurethanes.

Isocyanates react with water to form carbon dioxide:

RNCO + H2O → RNH2 + CO2 This reaction is exploited in tandem with the production of polyurethane to give polyurethane foams. The carbon dioxide functions as a blowing agent. Isocyanates also react with amines to give ureas:

R2NH + R'NCO → R2NC(O)N(H)R' The addition of an isocyanate to a urea gives a biuret:

R2NC(O)N(H)R' + R''NCO → R2NC(O)NR'C(O)NHR'' Reaction between a di-isocyanate and a compound containing two or more amine groups produces long polymer chains known as polyureas. Carbodiimides are produced by the decarboxylation of alkyl and aryl isocyanate using phosphine oxides as a catalyst:

C6H11NCO → (C6H11N)2C + CO2 Ketenimines can be prepared by reactions of isocyanates with Wittig reagents: When the substituents on the terminal carbon differ, the ketenimines are intrinsically chiral owing to the non-linearity of the C=N-R group.

Ph3P=CR2 + R'N=C=O → R2C=C=NR' + Ph3PO (Ph = phenyl)

Cyclization Isocyanates also can react with themselves. Aliphatic diisocyanates can trimerise to from substituted isocyanuric acid groups. This can be seen in the formation of polyisocyanurate resins (PIR) which are commonly used as rigid thermal insulation. Isocyanates participate in Diels–Alder reactions, functioning as dienophiles.

Rearrangement reactions Isocyanates are common intermediates in the synthesis of primary amines via hydrolysis:

Hofmann rearrangement, a reaction in which a primary amide is treated with a strong oxidizer such as sodium hypobromite or lead tetraacetate to form an isocyanate intermediate.

Commercial isocyanates

The simplest isocyanate, methyl isocyanate (MIC), is used in the manufacture of pesticides. Diisocyanates are produced on a particularly large scale because they are precursors to polyurethanes. The global market for diisocyanates in the year 2000 was 4.4 million tonnes, of which 61.3% was methylene diphenyl diisocyanate (MDI), 34.1% was toluene diisocyanate (TDI), 3.4% was the total for hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI), and 1.2% was the total for various others. MDI is used in the manufacture of rigid foams and surface coating. Polyurethane foam boards are used in construction for insulation. TDI is used in applications where flexible foams are used, such as furniture and bedding. Both MDI and TDI are used in the making of adhesives and sealants due to weather-resistant properties. Isocyanates, both MDI and TDI are widely used in as spraying applications of insulation due to the speed and flexibility of applications. Foams can be sprayed into structures and harden in place or retain some flexibility as required by the application. HDI is commonly utilized in high-performance surface-coating applications, including automotive paints.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Isocyanate

Start with the simplest possible case. Write down what Isocyanate claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Isocyanate before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Isocyanate ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Isocyanate

In research: Isocyanate appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Isocyanate in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Isocyanate is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical hazards, Commodity chemicals, Functional groups, so understanding it makes those chapters shorter.
In everyday life: Look for Isocyanate outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Isocyanate in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Isocyanate means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Isocyanate out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Isocyanate in simple terms?

In organic chemistry, isocyanate is the functional group with the formula R−N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates.

Why does Isocyanate matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Isocyanate?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Isocyanate.