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In chemistry, a trimer is a molecule or an 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.

Contents

ExamplesEdit

Alkyne trimerisationEdit

 
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.[1] This process was commercialized:

 

Nitrile trimerizationEdit

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

Cyanuric chloride and the bromide trimerizes at elevated temperatures over a carbon catalyst:[1]

 

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
 

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.

Diene trimerisationEdit

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:[3]

 

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

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.[4]

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.

TrisiloxanesEdit

Dimethylsilanediol dehydrates to a trimer of Me2SiO 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 Me2Si=O, although this species is not an intermediate.

Coordination chemistryEdit

The dithiobenzoate complexes [M(S2CPh)2] crystallizes as a trimers (M = Ni, Pd).[5]

 
Structure of the trimer [Ni(S2CPh)2]3.

EtymologyEdit

/ˈtrmər/ tri-, "three" + -mer, "parts".

See alsoEdit

ReferencesEdit

  1. ^ a b Hillis O. Folkins (2005). "Benzene". Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a03_475. 
  2. ^ Klaus Huthmacher, Dieter Most "Cyanuric Acid and Cyanuric Chloride" Ullmann's Encyclopedia of Industrial Chemistry" 2005, Wiley-VCH, Weinheim. doi 10.1002/14356007.a08 191
  3. ^ Industrial Organic Chemistry, Klaus Weissermel, Hans-Jurgen Arpe John Wiley & Sons; 3rd 1997 ISBN 3-527-28838-4
  4. ^ Bost, R. W.; Constable, E. W. "sym-Trithiane" Organic Syntheses, Collected Volume 2, p.610 (1943). "Archived copy" (PDF). Archived from the original (PDF) on 2012-03-29. Retrieved 2014-05-05. 
  5. ^ Bonamico, M.; Dessy, G.; Fares, V.; Scaramuzza, L. (1975). "Structural Studies of Metal Complexes with Sulphur-Containing Bidentate Ligands. Part I. Crystal and Molecular Structures of Trimeric Bis-(dithiobenzoato)-nickel(II) and -palladium(II)". Journal of the Chemical Society, Dalton Transactions: 2250–2255. doi:10.1039/DT9750002250.