medchem 9 Flashcards
a ring system containing one or more hetroatoms is
hetrocyles
- hetroatoms are atoms other than carbon as nitrogen oxygen or sulphur
aromatic hetrocyles are
hetraomatic compounds
once the ch group is replaced w/ nitrogen its called
-pyridine and its isoelectronic w/ benzene
the geometry or pyridine is
triagonal planer
pyridine is basic bc
the lone pairs in the nitrogen are not in the aromatic sextet, can be protonated , good ligand
protonation of pyridine uses —- and it does not affect the — and it produces
- nitrogens lone pairs
- aromatic sextet
- Pyridium ion, its aromatic and the pyridine stability is not affected by the protonation
structure and properties of pyrrole
- there is seeningly only 4 pi electrons but is 6 electrons in total w/ the nitrogen’s lone pair
- traigonal planer
- bond angle 120
- aromatic stability can be qualified by the heats of hydrogenation
pyrrole is a very weak — bc —
Basic bc the lone pairs are not available for bonding
the aromatic stability of the pyrrole is — due to —
- lost
- protonation
- so its not basic
— contains 2N-containing hetrocyles
- imidazole
- contains pyridine like N ( basic ) and pyrrole like N (very weak basic )
- the lone pairs in N is not available for covelant bonding
- The ‘pyridine-like’ nitrogen is the only N protonated in this equilibrium.
— is part of side chain of amino acid histamine
imidazole
In many cases the ‘pyridine-like’ nitrogen is complexed to
metal ions e.g. in haemoglobin, myoglobin.
At physiological pH the imidazole group and its conjugate acid are in —– and this accounts for the —
- equilibrium
- buffer properties to keep the ph constant
Does Pyridine Undergo Electrophilic Aromatic Substitution ?
Rarely.
Pyridine has low reactivity to electrophilic substitution as electron withdrawal
by N makes p system less electron rich and less attractive to electrophiles. (Rem. Electronegativity value for N = 3.04 and for C = 2.55).
Does Pyrrole Undergo Electrophilic Aromatic Substitution ?
Yes.
Pyrrole undergoes electrophilic substitution and does so much faster than benzene
(e.g. nitration occurs at 0 °C without catalyst).
In contrast to benzene no catalyst or heat is required.
The high reactivity of pyrrole is possibly because the aromatic sextet is spread over five centres (not six) and each centre is very ‘p electron rich’ and reactive with electrophiles.
