:Trimer (chemistry)

{{Further|Alkyne trimerisation}}

File:Acetylene trimer.svg

In 1866, Marcellin Berthelot reported the first example of cyclotrimerization, the conversion of acetylene to benzene.{{cite encyclopedia|author=Hillis O. Folkins|chapter=Benzene|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley-VCH|place=Weinheim|year=2005|doi=10.1002/14356007.a03_475|isbn=3527306730}} This process was commercialized:

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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. The chloride gives cyanuric chloride:

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The bromide has an extended shelflife when refrigerated. Like the chloride, it undergoes ab exothermic trimerization to form cyanuric bromide. This reaction is catalyzed by traces of bromine, metal salts, acids and bases.{{cite book|last1=Morris|first1=Joel|title=Encyclopedia of Reagents for Organic Synthesis|last2=Kovács|first2=Lajos|last3=Ohe|first3=Kouichi|chapter=Cyanogen Bromide|year=2015|pages=1–8|doi=10.1002/047084289X.rc269.pub3|isbn=9780470842898}} For this reason, experimentalists avoid brownish samples.{{cite journal|title=Cyanogen Bromide|author1=Joel Morris|author2= Lajos Kovács|journal=Encyclopedia of Reagents for Organic Synthesis|year=2008|doi=10.1002/047084289X.rc269.pub2|isbn=978-0471936237}}

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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: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

:{{chem2 | 3 H2N\sCO\sNH2 -> [C(O)NH]3 + 3 NH3 }}

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:

:{{chem2 | (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.

:{{chem2 | 3 HOCN -> [C(O)NH]3 }}

:{{chem2 | [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.

:{{chem2 | 3 HOCN + 3 H2O -> 3 CO2 + 3 NH3 }}

In total, the second step is exothermic:

:{{chem2 | 6 HCNO + 3 NH3 -> C3H6N6 + 3 CO2 + 3 NH3 }}

but the overall process is endothermic.

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:Industrial Organic Chemistry, Klaus Weissermel, Hans-Jurgen Arpe John Wiley & Sons; 3rd 1997 {{ISBN|3-527-28838-4}}

:Image:Synthesis of 1,5,9-cyclododecatriene.svg

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.Bost, R. W.; Constable, E. W. "sym-Trithiane" Organic Syntheses, Collected Volume 2, p.610 (1943). {{cite web |url=http://www.orgsyn.org/orgsyn/pdfs/CV2P0610.pdf |title=Archived copy |accessdate=2014-05-05 |url-status=dead |archiveurl=https://web.archive.org/web/20120329051447/http://www.orgsyn.org/orgsyn/pdfs/CV2P0610.pdf |archivedate=2012-03-29 }}

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{{cite book

|last= Cumming

|first= W. M.

|date= 1937

|title= Systematic organic chemistry (3E)

|url= http://www.prepchem.com/synthesis-of-mesitylene/

|location= New York, USA

|publisher= D. Van Nostrand Company

|page=57

}}

Dimethylsilanediol dehydrates to a trimer of {{chem2|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 {{chem2|Me2Si\dO}}, although this species is not an intermediate.

The dithiobenzoate complexes {{chem2|[M(S2CPh)2]}} crystallize as trimers (M = Ni, Pd).{{cite journal|author1=Bonamico, M. |author2=Dessy, G. |author3=Fares, V. |author4=Scaramuzza, L. |title=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=Journal of the Chemical Society, Dalton Transactions|issue=21|year=1975|pages=2250–2255|doi=10.1039/DT9750002250}}

File:DTBNIT11.png

• Protein trimer

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{{commons category|Trimers}}

{{DEFAULTSORT:Trimer (Chemistry)}}

Category:Trimers (chemistry)