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Lindlar catalyst is commercially available but can also be created by reducing palladium chloride in a slurry of calcium carbonate (CaCO 3) and adding lead acetate. [1] [2] A variety of other "catalyst poisons" have been used, including lead oxide and quinoline. [3] The palladium content of the supported catalyst is usually 5% by weight.
After ring closure the new triple bond is stereoselectively reduced with hydrogen and the Lindlar catalyst in order to obtain the Z-alkene (cyclic E-alkenes are available through the Birch reduction). An important driving force for this type of reaction is the expulsion of small gaseous molecules such as acetylene or but-2-yne.
2-Butyne (dimethylacetylene, crotonylene or but-2-yne) is an alkyne with chemical formula CH 3 C≡CCH 3. Produced artificially, it is a colorless, volatile, pungent liquid at standard temperature and pressure .
The C≡C bond distance of 118 picometers (for C 2 H 2) is much shorter than the C=C distance in alkenes (132 pm, for C 2 H 4) or the C–C bond in alkanes (153 pm). [ 3 ] Illustrative alkynes: a , acetylene, b , two depictions of propyne, c , 1-butyne, d , 2-butyne , e , the naturally occurring 1-phenylhepta-1,3,5-triyne, and f , the strained ...
If the catalyst and reaction conditions are indicative of low effectiveness, selective poisoning may be observed, where poisoning of only a small fraction of the catalyst's surface gives a disproportionately large drop in activity. [4] If η is the effectiveness factor of the poisoned surface and h p is the Thiele modulus for the poisoned case:
Steps in the hydrogenation of a C=C double bond at a catalyst surface, for example Ni or Pt : (1) The reactants are adsorbed on the catalyst surface and H 2 dissociates. (2) An H atom bonds to one C atom. The other C atom is still attached to the surface. (3) A second C atom bonds to an H atom. The molecule leaves the surface.
An alkyne trimerisation is a [2+2+2] cycloaddition reaction in which three alkyne units (C≡C) react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applicable to organic synthesis. [1] Being a cycloaddition reaction, it has high atom economy.
Phenylacetylene is a prototypical terminal acetylene, undergoing many reactions expected of that functional group. It undergoes semihydrogenation over Lindlar catalyst to give styrene. In the presence of base and copper(II) salts, it undergoes oxidative coupling to give diphenylbutadiyne. [6]