Question

H.W 10 Explain the reasons of each of the following: 1-pyridine is more basic than pyrrole. 2-pyridine is less active than aliphatic Amines. pyridine give E substitution reaction at 3rd position-3

Answer 1

Page No Date TONormally a spz nitrogen centre is less bosic than sp-nitrogen centre due to I the fact that spa centres alle more Telectronegative than spa-centres But the case of pyrole and pyridine is quite different In pyridine we can simply say that nitrogen is sp2.hybull- Idised but in case of pyrole it seem to be a sp3- centres but it's not, in Itact it also a spa centue the season I for this is that the lone pain of IN in pyrole is in conjugation with 2- double bond in the ring and making the sing aromatic Now at both N aure spa, Both pyridine and pyrole are aromatic molecules. Now, at time pasicity is compared on the basis of how easily and effectively a compound can share its one pain.. In pyridine as the lone paire of nitrogen is not involved in the aromat- ization of the ring it is available far donation. Teacher's Signature

The sone pair of w in py eqide is localised and hence ready for donation In pyrole. the tone pain of Mirrogen is already involved in the cyclic conjugation and its completely declocalized cuRU 6 hatom making its unavailable for donation Teacher's Signature

the tone pair of N in pyMmole is in gesonance so this done paire can't be donated so, pyridine is more basic than pyrole. In s-ryridine Jone pall are placed in nitrogen distributed over the arom- atic ming due to resonance, after taking sesonance some lone pally demain on placed of Nitrogen, But in case of aliphatic amines the tone pair of nitrogen don't participate in meson- ance due to this magnitude of lone pain is mare in aliphatic amine as comported to pyride. Due to this the protonation of e- is more easily in aliphatic amines & pyxidine is more stable than aliphatic amine becallse pysyidine don't leaves its lone paill easily. so has pyridine is Jess active aliphatic amines. than I - pyridine R - NH₂ al om Teacher's Signature

Teacher's Signature

I've shown all thuee possible electrophilic substitution on pyridine if you look! at second position substitution and I fouth position substitution then you'll tind one structure in both which I've marked with waves. In these two structure the position charge is on nitrogen. However, there is no positivel charged on nitrogen in third position substitution if you remember the ander of stability of resonating structure then you must know that if there is positive changed on an electronegative atom, it becomes unstable Hence, substitution at 2 and 2 unstable a prieduces intermediate while substitution on 3 produces stable intermediate .. Thus 3-positich substitution is favowed