In chemistry, a trimer (; ) is a molecule or polyatomic anion formed by combination or association of three molecules or ions of the same substance. In technical jargon, a trimer is a kind of oligomer derived from three identical precursors often in competition with polymerization.
Examples
Alkyne trimerization
thumb|The trimerization cyclization reaction can be understood with this scheme.
In 1866, Marcellin Berthelot reported the first example of cyclotrimerization, the conversion of acetylene to benzene. This process was commercialized:
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Nitrile trimerization
Symmetrical 1,3,5-triazines are prepared by trimerization of certain nitriles such as cyanogen chloride.
Cyanogen chloride and cyanogen bromide each trimerize at elevated temperatures over a carbon catalyst. For this reason, experimentalists avoid brownish samples.
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An industrial route to cyanuric acid entails the thermal decomposition of urea, with release of ammonia. The conversion commences at approximately 175 °C:
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The endothermic synthesis of melamine can be understood in two steps.
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First, urea decomposes into cyanic acid and ammonia in an endothermic reaction:
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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.
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This water then reacts with cyanic acid present, which helps drive the trimerization reaction, generating carbon dioxide and ammonia.
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In total, the second step is exothermic:
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but the overall process is endothermic.
Diene trimerization
The 1,5,9-trans-trans-cis isomer of cyclododecatriene, which has some industrial importance is obtained by cyclotrimerization of butadiene with titanium tetrachloride and an organoaluminium co-catalyst:
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Breaking carbon-hetero double bonds forms symmetrical saturated 1,3,5-heterocycles
Cyclotrimerization of formaldehyde affords 1,3,5-Trioxane:
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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.
Catalyzing and dehydrating by sulfuric acid, trimerization of acetone via aldol condensation affords mesitylene
Trisiloxanes
Dimethylsilanediol dehydrates to a trimer of as well as polydimethylsiloxane. The reaction illustrates the competition between trimerization and polymerization. The polymer and trimer are formally derived from the hypothetical sila-ketone , although this species is not an intermediate.
Coordination chemistry
The dithiobenzoate complexes crystallize as trimers (M = Ni, Pd).
thumb|Structure of the trimer .
See also
- Protein trimer