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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:
Contents
- Alkyne trimerisation
- Breaking carbon hetero triple bonds forms symmetrical unsaturated 135 heterocycles
- Diene trimerisation
- Breaking carbon hetero double bonds forms symmetrical saturated 135 heterocycles
- References
Trimers are typically cyclic. Chemical compounds that often trimerise are aliphatic isocyanates and cyanic acids.
Alkyne trimerisation
In 1866, Marcellin Berthelot reported the first example of cyclotrimerization leading to aromatic products, the cyclization of acetylene to benzene. 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 + HClIn 2005, approximately 200,000 tonnes were produced.
Cyanuric bromide is analogously prepared from cyclotrimerization of cyanogen bromide:
3 BrCN → (BrCN)3An industrial route to cyanuric acid entails the thermal decomposition of urea, with release of ammonia. The conversion commences at approximately 175 °C:
3 H2N-CO-NH2 → [C(O)NH]3 + 3 NH3The 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 + NH3Then 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 H2OThis water then reacts with cyanic acid present, which helps drive the trimerization reaction, generating carbon dioxide and ammonia.
3 HOCN + 3 H2O → 3 CO2 + 3NH3In total, the second step is exothermic:
6 HCNO + 3 NH3 → C3H6N6 + 3 CO2 + 3NH3but the overall process is endothermic.
Diene trimerisation
The 1,5,9-trans-trans-cis isomer of cyclododecatriene C12H18 has some industrial importance and is obtained by cyclotrimerization of butadiene with titanium tetrachloride and an organoaluminium co-catalyst:
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.
Three molecules of acetaldehyde condense to form “paraldehyde,” a cyclic trimer containing C-O single bonds.