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These strategies can be grouped into approaches for managing reaction sequences. Reaction Sequences: Multistep synthesis involves sequential chemical reactions, each requiring its own work-up to isolate intermediates before proceeding to the next stage. [4] For example, the synthesis of paracetamol typically requires three separate reactions.
In the gas phase, the comproportionation reaction is much faster because of the much higher mobility of the reacting species as illustrated, e.g., in the Claus reaction where H 2 S and SO 2 react together to form elemental sulfur. Various classical comproportionation reactions are detailed in the series of examples here below.
Using the twenty natural amino acids, for example, in a tripeptide creates 8,000 (20 3) possibilities. Solid-phase methods for small molecules were later introduced and Furka devised a "split and mix" approach [2] [4] In its modern form, combinatorial chemistry has probably had its biggest impact in the pharmaceutical industry. [5]
Often cross-coupling reactions require metal catalysts. One important reaction type is this: R−M + R'−X → R−R' + MX (R, R' = organic fragments, usually aryl; M = main group center such as Li or MgX; X = halide) These reactions are used to form carbon–carbon bonds but also carbon-heteroatom bonds.
The Ugi reaction has been applied in combination with an intramolecular Diels-Alder reaction [16] in an extended multistep reaction. A reaction in its own right is the Ugi–Smiles reaction with the carboxylic acid component replaced by a phenol. In this reaction the Mumm rearrangement in the final step is replaced by the Smiles rearrangement. [17]
Also, some relatively minor additions to the reaction can be indicated above the reaction arrow; examples of such additions are water, heat, illumination, a catalyst, etc. Similarly, some minor products can be placed below the arrow, often with a minus sign. An example of organic reaction: oxidation of ketones to esters with a peroxycarboxylic acid
The reaction can also be quenched with pyridine, which will scavenge ZnI 2 and excess reagents. [24] Methylation of heteroatoms is also observed in the Simmons–Smith reaction due to the electrophilicity of the zinc carbenoids. For example, the use of excess reagent for long reaction times almost always leads to the methylation of alcohols. [25]
The reverse of an acid–base reaction is also an acid–base reaction, between the conjugate acid of the base in the first reaction and the conjugate base of the acid. In the above example, ethanoate is the base of the reverse reaction and hydronium ion is the acid.