The synthesis of furan thiophene and pyrrole are not very different, since we talk about etrocicli to 5 terms with one heteroatom. The easiest way to synthesize these molecules is from 1-4 carbonyl compounds . The most common method of synthesis for these three compounds was proposed by chemical Carl Paal and Ludwig Knorr.
Paal - Knorr Furan synthesis
The reaction is acid-catalyzed, to enhance the electrophilic character of one of the two carbonyl groups. What happens is that the non-protonated carbonyl acts as a nucleophile towards the carbonyl protonated. We arrive at an intermediate hemiacetal, which rapidly dehydrates to give the furan .
To understand and remember this reaction and all reactions of this type have to focus not so much on the reaction mechanism in and of itself (though useful). What drives this type of reactions is the tendency to flavor (or should I say thermodynamics). When they the atoms that serve us in the right relationship, the cyclization and aromatization will certainly result in the desired compound. Here it becomes quite easy to understand what are the reagents necessary to obtain an aromatic heterocycle such as furan, thiophene orpyrrole .
Paal - Knorr synthesis of thiophene
The reaction perfectly reproduces that of furan synthesis, if not for the fact that to insert a sulfur atom in the cycle is necessary to use a solforante agent, such as phosphorus pentasulfide, P2S5 or Lawesson reagent , which in addition to generate tiochetone do at the same time from dehydrating.
Why is sulfur to act as a nucleophile? First, because it is more nucleophilic, and secondly because the carbon bonded to oxygen is more electrophilic.