Resonance is a useful concept in chemistry. Earlier, we looked at the following carbocation, and explained it in terms of a spreading of charge. An alternative (and sometimes more useful) way of descibing this phenomenon of the lowering of the energy of this species is by use of resonance. Let us draw the molecule again, without the p orbitals … Read more
A carbocation is a positively charged species. The positive charge is localised on an sp2 hybridised carbon atom. Hence the carbocation itself is planar around this carbon. The p orbital of the sp2 carbon is empty. Typically they are significantly stabilised if the positive charge can be delocalised across the molecule, and thus they form a part of our discussion … Read more
The oxidation and reduction of organic molecules forms a very important part of many organic reactions and syntheses. There is a massive variety of reagents available and a vast array of systems that can be oxidised and/or reduced. However, the simple ‘oxidation is losing’ ‘reduction is gaining’ (‘oilrig‘) way of identifying electron transfer redox processes in inorganic chemistry is difficult to use in terms of organic chemistry. … Read more
The Birch Reduction: This extremely useful reaction uses the alkali metal/ammonia solution again to partially reduce aromatic rings. The basic reaction scheme is as follows; However, the reaction becomes more interesting when substituents on the ring are involved – it seems that EWGs and EDGs cause a different arrangement of the final two double bonds: The key to … Read more
Hydrogenation (see electrophilic addition) Homogeneous Catalysis The term ‘homogeneous catalysis’ refers to a catalyst that is present in the same medium as the reaction mixture – i.e. in solution. Most often used as catalysts for this are complexes of 2nd and 3rd row transition metals e.g. Rh, Ir, Pt etc. – a good example of such a complex is the much used Wilkinson’s … Read more
Full reduction with LiAlH4: All the way through to amine Partial reduction with LiAlH(OEt)3: Through to aldehyde
Esters with LiAlH4 LiAlH4 is reactive enough to reduce first to aldehyde and then through to alcohol – not able to stop at the aldehyde unless strictly one equivalent is added at low temperature (-780C). with DIBAL-H (Di-Iso-Butyl-Aluminium-Hydride) This reagent is useful because it will reduce an ester to the aldehyde and go no further; Carboxylic Acids These are difficult … Read more
Probably the most common type of reagent used for reductions in organic chemistry is the hydride reagent – i.e. one which reacts as ‘H–‘ – a hydride nucleophile. It is worth noting, however, that simple hydrides e.g. NaH are not very nucleophilic and are far more likely to act as bases. Therefore the reagentsmost likely to be encountered here are of … Read more
Oxidising Phenols with Transition Metals Note as in the Manganese Dioxide mechanism (page 1), half-hook arrows are used to represent the movements of single electrons. Oxidising Pyridine with Transition Metals
Allylic Oxidation The first step is a pericyclic ‘ene’ reaction. Diketone from ketone ‘Ene’ reaction involved again – this time in the second step.