Trimer (chemistry)

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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.

Examples[edit]

Alkyne trimerisation[edit]

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:

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:[1]

Cyanurchloride Synthesis V.1.svg

Cyanogen bromide has an extended shelflife when refrigerated. Similar to some other cyanogen compounds, cyanogen bromide undergo an exothermic trimerisation to cyanuric bromide ((BrCN)3). This reaction is catalyzed by traces of bromine, metal salts, acids and bases.[2] For this reason, experimentalists avoid brownish samples.[3]

Cyanuric bromide synthesis.PNG

An industrial route to cyanuric acid entails the thermal decomposition of urea, with release of ammonia. The conversion commences at approximately 175 °C:[4]

3 H2N-CO-NH2 → [C(O)NH]3 + 3 NH3

The endothermic synthesis of melamine can be understood in two steps.

Melamine2.svg

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:[5]

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.[6]

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[7]

Trisiloxanes[edit]

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 chemistry[edit]

The dithiobenzoate complexes [M(S2CPh)2] crystallizes as a trimers (M = Ni, Pd).[8]

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

Etymology[edit]

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

See also[edit]

References[edit]

  1. ^ a b Hillis O. Folkins (2005). "Benzene". Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a03_475.
  2. ^ Morris, Joel; Kovács, Lajos; Ohe, Kouichi (2015). "Cyanogen Bromide": 1–8. doi:10.1002/047084289X.rc269.pub3.
  3. ^ Joel Morris; Lajos Kovács (2008). "Cyanogen Bromide". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rc269.pub2.
  4. ^ 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
  5. ^ Industrial Organic Chemistry, Klaus Weissermel, Hans-Jurgen Arpe John Wiley & Sons; 3rd 1997 ISBN 3-527-28838-4
  6. ^ 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.
  7. ^ Cumming, W. M. (1937). Systematic organic chemistry (3E). New York, USA: D. Van Nostrand Company. p. 57.
  8. ^ 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.