June 30, 2022, 12:59:20 PM
Forum Rules: Read This Before Posting


Topic: acidity of nitro pyrrole and nitro imidazole  (Read 4584 times)

0 Members and 1 Guest are viewing this topic.

Offline PRIYA1022

  • Regular Member
  • ***
  • Posts: 61
  • Mole Snacks: +5/-2
acidity of nitro pyrrole and nitro imidazole
« on: May 18, 2007, 01:00:08 AM »
I am working with nitro imidazoles and nitro pyrroles, and in one of the steps, I had to replace the N-H with a Methyl group. I followed literature procedures ( DMF, anhy K2CO3, CH3I for the nitroimidazole and aq KOH, CH3I for the nitro pyrrole)  and I found nitropyrrole gane the  N-methyl product in 1 Hour and the yield was almost 100%, but the nitroimidazole took 24 hrs , and my yield was just 55%( in this case I get 2 isomers).
Does this mean that the nitro pyrrole is more acidic than the nitro pyrrole?

Offline movies

  • Organic Minion
  • Retired Staff
  • Sr. Member
  • *
  • Posts: 1973
  • Mole Snacks: +222/-20
  • Gender: Male
  • Better living through chemistry!
Re: acidity of nitro pyrrole and nitro imidazole
« Reply #1 on: May 18, 2007, 02:14:02 AM »
Not necessarily.  You aren't comparing apples to apples.  Aciditiy (in terms of pKa) varies depending on the solvent you are using, and it is likely different in DMF vs water because of the difference in dielectric constant (39 vs 80, respectively).  Also, the equilibrium you are looking at is significantly different because your conjugate acids and bases are significantly different as well.

Solvent can also have a big effect on the kinetics of a process by favoring different transition states.  This again often depends on the dielectric constant of the medium, but can also be affected by other things like the density of the medium.  Acidity measured by pKa is also a thermodynamic property because it refers to a system at equillibrium, while rate of reaction is a kinetic property.

Imidazoles are usually more acidic than pyrroles, but I can't say for sure which one is more acidic in this case.  What I can say is that you can't tell from the data you presented.

Offline PRIYA1022

  • Regular Member
  • ***
  • Posts: 61
  • Mole Snacks: +5/-2
Re: acidity of nitro pyrrole and nitro imidazole
« Reply #2 on: May 18, 2007, 03:04:17 AM »
Thanks Movies,
 You gave me a clue about the solvent effects. and also I had a typo in the post( What I meant was Is the nitropyrrole more acidic than the nitroimidazole)

Sponsored Links