Aromaticity 5&6 Flashcards
What reagents are required to form pyrrole?
Hexane-2,5-dione and ammonia
What reagents are required to form furan?
Hexane-2,5-dione + acid + Heat
What reagents are required to form Thiophene?
Hexane-2,5-dione + P2S5 (Lawesson’s reagent)
Thiocarbonyls tend to be unstable due to weak C=S bond allowing fro the rapid formation of the cyclised product thiophene. TRUE OR FALSE?
TRUE
Lawesson’s reagent does not allow for the formation of Thiocarbonyls. TRUE OR FALSE?
FALSE
What reagents are required to form Pyridine?
Heptane-2,6-dione + ammonia
What are the suitable oxidising agents for formation of pyridine?
HNO3, or Ce(IV), or quinone
Why is not oxidation required in order to form the heteroaromatic ring in formation of pyridine?
because of water is lost from the dihydropyridine when using hydroxylamine under acidic conditions
which other reagents can be used to form pyridine?
heptane-2,6-dione + hydroxylamine, HCl & EtOH
What are some of properties of pyridine?
- a very unreactive aromatic imine (a stable imine)
- weakly basic (pKa = 5.5)
- toxic & foul smelling
- a cheap, popular solvent
- a reasonable nucleophile for C=O groups
- Used in reactions between acid chlorides and alcohol to make esters
Pyridine is less reactive than benzene in Electrophilic Aromatic Substitution (EAS) reactions. TRUE OR FALSE?
TRUE
Pyridine is less reactive than benzene in Nucleophilic Aromatic Substitution (NAS) reactions. TRUE OR FALSE?
FALSE
What reagents are required to form Tetrazole?
nitrile (RCN) and sodium azide
The reaction is slower when R is an EWG for formation of Tetrazole. TRUE OR FALSE?
FALSE
What other reagents can be used to form Tetrazole by ring cycloaddition?
Via synthesis of broperamole
Why can the reaction occur on any of the nitrogens when making tetrazole from synthesis of broperamole ?
This is because the anion intermediate is
delocalised within the ring
How can Triazole be formed from?
Azides and alkynes and heat
Isoxazole can be formed from nitrile oxide and alkynes. TRUE OR FALSE?
TRUE
Electrophilic aromatic substitution reactions:
• work very well on pyrroles, furans & thiophenes
• occur best at the 2- & 5- positions
• occur nearly as well at the 3- & 4- positions
Are all these statements about 5 membered heterocyclic TRUE OR FALSE?
TRUE
Pyrrole is the best at performing EAS reactions. TRUE OR FALSE?
TRUE
Which is the most and least powerful electron donor?
Between nitrogen and sulfur
Nitrogen most powerful
Sulfur least powerful
Does thiophene have similar EAS reactivity to benzene?
YES
Why is Thiophene less reactive than furan or pyrrole?
due to 3p-2p orbital overlap, which is not as good as 2p-2p overlap
Why can Furan react with molecular bromine?
Because Furan is less aromatic than pyrrole, this allows for reactions other than substitutions
What is the pKa of pyrrole as a base?
-4
What is the pKa of pyrrole NH ?
16.5
will Increasing ring delocalisation increase or reduced basicity?
Reduce
what does Pyrroles + strong acid form?
Polymers
What is the reason for the formation of a polymer when using pyrroles and strong acid?
pyrrole will act as a base
What does Pyrroles + Br2 form?
polybromination
For Pyrrole: Reaction with molecular
bromine requires a Lewis acid & leads
to substitution at all 4 positions. TRUE OR FALSE?
FALSE
For Pyridine: Molecular bromine reacts
with the ring nitrogen. TRUE OR FALSE?
TRUE
What is the main difference between pyrroles and indoles?
Preferred EAS reactions at the 2-position rather than the 3-position
Which of the positions (2 or 3) is more stable and produces a linear conjugate system in the pyrroles EAS?
2 position
Which conjugation system does the 3 position in EAS reaction of pyrroles form?
Cross conjugated system
When does EAS reaction work well for indoles?
when performing halogenation,
nitration, sulfonation, Friedel-Crafts acylation &
alkylation
What position does Indoles EAS reaction occur?
position 3
Indoles in EAS reactions can give 2-position product if electrophile migrates from 3-position. TRUE OR FALSE?
TRUE
Nucleophilic aromatic substitution reactions work well for both pyridine & quinoline. TRUE OR FALSE?
TRUE
Give the reason why Nucleophilic aromatic substitution reactions work well for both pyridine & quinoline?
Because they give substitution products when the negative charge can rest on the nitrogen in the intermediate species
Pyridine does undergo electrophilic aromatic substitution (EAS) reactions. TRUE OR FALSE?
FALSE
What are the reasons to which pyridine does not undergo EAS reactions?
Because:
(1) N de-stabilises the cationic would-be intermediate
(2) Ring is unreactive to electrophiles
(3) Electrophilic reagents instead attach to the N (e.g. AlCl3 from Friedel-Crafts reactions
complexes with the N (so pyridine is a good ligand for metals such as AlCl3
Pyridine does undergo nucleophilic aromatic substitution (NAS) reactions at the 2- and 4-position. TRUE OR FALSE?
TRUE
Why can pyridine undergo NAS reactions?
Because:
(1) the negative charge in the intermediate can rest on N
(2) of similarities between this chemistry & that already described involving nucleophilic attack on carbonyl compounds (especially acid chlorides)
which one between piperidine, imine and pyridine is the weakest base?
Pyridine
Saturated amines are better bases than imines. TRUE OR FALSE?
TRUE
The weaker the base the stronger the acidity of the conjugate acid, which explains why the conjugate acid of pyridine (the pyridinium ion) shows similar acidity to RCOOH. TRUE OR FALSE?
TRUE
Pyridones – reaction with POCl3 followed by a nucleophilic substitution. TRUE OR FALSE?
TRUE
2-Pyridone & 4-pyridone exist in the ‘amide’ form. TRUE OR FALSE?
TRUE
