8 - HETEROCYCLES Flashcards
what is the order of reactivity towards electrophilic aromatic substitution of pyrrole, furan, thiopene and benzene?
how can this order be explained?
can be explained by the stability of the intermediate, the atom size and the electronegativity:
oxygen is more electronegative than nitrogen, holds onto electrons more tightly, so furan is a poorer electron donor than pyrrole
the low reactivity of thiophene can be explained by orbitals:
the 3p orbital does not overlap as well with the 2p orbital when making a 2p-3p pi bond because of their different sizes, so the intermediate is not as stable and therefore less reactive
benzene is the least reactive because it has no electrons to push towards an electrophile without breaking aromaticity awfully
at which carbon does substitution most readily happen in 5 membered heterocycles?
at the C2 position
how is tetrahydrofuran made from corn cubs and sugar canes and what is it?
THF is a polar aprotic solvent
acetal bonds are broken from the sugars to make a mixture of sugars, but mainly xylose
by distillation you get furfural, responsible for the smell of almonds
by heating you get furan
and hydrogenation gives you THF
where can furan be found?
DNA contains furanose rings
explain the indophenin test and how it lead to the discovery of thiophene
scientists would mix isatin with crude benzene from coal tar, acid catalysis, and that would make a blue dye
this was used as a positive “benzene” test
but when you used pure benzene in this reaction, this blue dye would not appear
the blue dye would only be formed if you used thiophene
the conclusion is that thiophene is what reacts with isatin to give the blue polymer
this lead to the discovery of thiophene
how would thiophene react with isatin?
since the Paal Knorr synthesis of furan is reversible, how do you make sure it goes forward?
increase the yield by shifting to the right: trap the water or distill the furan as its formed to induce reaction to make some more
mechanism of paal knorr synthesis of furan
what is the starting product of paal knorr synthesis?
1,4-diketones
why are esters less reactive to nucleophiles than ketones?
they are conjugated
draw a mechanism for the synthesis of pyrrole (the weird knorr synthesis slide idk)
how does the commercial method of thiophene synthesis work?
what is hydrodesulfurization?
used to remove thiophene from oil, natural gas, coal etc to reduce SO2 emissions from those fuels in cars trains etc
the H2S produced in hydrodesulfurization is used to produce S8, sulfuric acid etc
what is the paal knorr synthesis of thiophene?
how does furan undergo electrophilic aromatic substitution?
C2 is preferred
adding an acid to furan will lead to its polymerization
an acetyl group (EWG) needs to be added to the furan, this will decrease its reactivity with an acid and prevents polymerization
how is furan brominated?
how does furan undergo electrophilic reaction followed by an addition reaction?
what is a meso compound?
how does a furan reaction go after metallation at C2?
how does thiophene undergo electrophilic aromatic substitution and then Raney Ni?
what is the hydrogenation of benzene with Raney Ni reaction?
what is the oxidation step of the hydrogenation of benzene?
how is a ketone reduced with a wolff kishner reduction?
