Trimer (chemistry)

In chemistry, a trimer (/ˈtraɪmər/) (tri-, "three" + -mer, "parts") is an oligomer derived from three identical precursors. An example is the procedure of production of polymers. At first, a monomer is made. By combining two monomers, a dimer is produced. With further additions, a trimer and eventually a polymer is made. Often, trimerization competes with polymerization; for example, dimethylsilanediol polymerizes to polydimethylsiloxane, even though a trimer is made:

Me₂Si(OH)₂ + (HO)₂SiMe₂ → Me₂(OH)Si-O-Si(OH)Me₂ – dimer

Me₂(OH)Si-O-Si(OH)Me₂ + (HO)₂SiMe₂ → Me₂(OH)Si-O-SiMe₂-O-Si(OH)Me₂ – trimer

Me₂(OH)Si-O-SiMe₂-O-Si(OH)Me₂ + (HO)₂SiMe₂ → Me₂(OH)Si-O-SiMe₂-O-SiMe₂-O-Si(OH)Me₂ – tetramer

et cetera, until Me₂(OH)Si-O-[SiMe₂-O-]_nSi(OH)Me₂ (n>100) – polymer

Trimers are typically cyclic. Chemical compounds that often trimerise are aliphatic isocyanates and cyanic acids.

Trimerisation via breaking triple bond

Alkyne trimerisation

The trimerization cyclization reaction can be understood with this scheme.

In 1866, Marcellin Berthelot reported the first example of cyclotrimerization leading to aromatic products, the cyclization of acetylene to benzene.^[1] In the Reppe synthesis, the trimerization of acetylene gives benzene:

Breaking carbon-hetero triple bonds forms symmetrical unsaturated 1,3,5-heterocycles

Symmetrical 1,3,5-triazines are prepared by trimerization of certain nitriles such as cyanogen chloride or cyanimide.

Cyanuric chloride is prepared in two steps from hydrogen cyanide via the intermediacy of cyanogen chloride, which is trimerized at elevated temperatures over a carbon catalyst:

HCN + Cl₂ → ClCN + HCl

In 2005, approximately 200,000 tonnes were produced.^[2]

Cyanuric bromide is analogously prepared from cyclotrimerization of cyanogen bromide:

3 BrCN → (BrCN)₃

An industrial route to cyanuric acid entails the thermal decomposition of urea, with release of ammonia. The conversion commences at approximately 175 °C:^[2]

3 H₂N-CO-NH₂ → [C(O)NH]₃ + 3 NH₃

Synthesis of melamine

The endothermic synthesis of melamine can be understood in two steps. First, urea decomposes into cyanic acid and ammonia in an endothermic reaction:

(NH₂)₂CO → HOCN + NH₃

Then in the second step, cyanic acid polymerizes to form cyanuric acid which condenses with the liberated ammonia from the first step to release melamine and water.

3 HOCN → [C(O)NH]₃

[C(O)NH]₃ + 3 NH₃ → C₃H₆N₆ + 3 H₂O

This water then reacts with cyanic acid present, which helps drive the trimerization reaction, generating carbon dioxide and ammonia.

3 HOCN + 3 H₂O → 3 CO₂ + 3NH₃

In total, the second step is exothermic:

6 HCNO + 3 NH₃ → C₃H₆N₆ + 3 CO₂ + 3NH₃

but the overall process is endothermic.

-ene trimerisation

Diene trimerisation

The 1,5,9-trans-trans-cis isomer of cyclododecatriene C₁₂H₁₈ has some industrial importance ^[3] and is obtained by cyclotrimerization of butadiene with titanium tetrachloride and an organoaluminium co-catalyst:^[4]

Catalyzing and dehydrating by sulfuric acid, trimerization of acetone via aldol condensation affords mesitylene.

Breaking carbon-hetero double bonds forms symmetrical saturated 1,3,5-heterocycles

Cyclotrimerization of formaldehyde affords 1,3,5-Trioxane:

1,3,5-Trithiane is the cyclic trimer of the otherwise unstable species thioformaldehyde. This heterocycle consists of a six-membered ring with alternating methylene bridges and thioether groups. It is prepared by treatment of formaldehyde with hydrogen sulfide.^[5]

Three molecules of acetaldehyde condense to form “paraldehyde,” a cyclic trimer containing C-O single bonds.

See also

• Oligomer
• Protein trimer

References

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