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In chemistry, a trimer (/ˈtrmər/) (tri-, "three" + -mer, "parts") is an oligomer derived from three identical precursors. Often, trimerization competes with polymerization

Contents

ExamplesEdit

TrisiloxanesEdit

Dimethylsilanediol polymerizes to polydimethylsiloxane, even though a trimer is made:

Me2Si(OH)2 + (HO)2SiMe2 → Me2(OH)Si-O-Si(OH)Me2dimer
Me2(OH)Si-O-Si(OH)Me2 + (HO)2SiMe2 → Me2(OH)Si-O-SiMe2-O-Si(OH)Me2trimer
Me2(OH)Si-O-SiMe2-O-Si(OH)Me2 + (HO)2SiMe2 → Me2(OH)Si-O-SiMe2-O-SiMe2-O-Si(OH)Me2tetramer
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.

Alkyne trimerisationEdit

 
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:

 

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:[2]

 

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[how?] 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.

Coordination chemistryEdit

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

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

See alsoEdit

ReferencesEdit

  1. ^ Schetter, M. C. R. (1866). Hebd. Seances Acad. Sci. 62: 905.  Missing or empty |title= (help)
  2. ^ a b 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). http://www.orgsyn.org/orgsyn/pdfs/CV2P0610.pdf
  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.