Trimer (chemistry)

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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

Examples[edit]

Trisiloxanes[edit]

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 trimerisation[edit]

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:

Reppe-chemistry-benzene.png

Nitrile trimerization[edit]

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]

Cyanurchloride Synthesis V.1.svg

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
Melamine2.svg

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 trimerisation[edit]

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]

Cyclododeca-1,5,9-triene

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

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

Trioxane Synthesis V.1.svg

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 chemistry[edit]

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

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

See also[edit]

References[edit]

  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.