December 05, 2021, 09:30:02 AM
Forum Rules: Read This Before Posting


Topic: SN1 reactions  (Read 21261 times)

0 Members and 1 Guest are viewing this topic.

Offline Ilovescience

  • New Member
  • **
  • Posts: 3
  • Mole Snacks: +0/-0
SN1 reactions
« on: February 23, 2009, 02:41:30 PM »
Hi! I've performed a lab exercise where we mixed alkyl halides with silver nitrate dissolved in ethanol. I know the mechanism of this reaction. ethanol serves as the nucleophile and the silver reacts with the halogen to form an insoluble compound. My question is: why cant the nitrate ion act as the nucleophile? why doesnt the nitrate ion undergo a substitution reaction of some sort?

Thanks.

Offline macman104

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 1644
  • Mole Snacks: +168/-26
  • Gender: Male
Re: SN1 reactions
« Reply #1 on: February 23, 2009, 04:13:37 PM »
The nitrate is a poor nucleophile.  It has 3 oxygen atoms pulling electron density away from the nitrogen, and there is also a fair amount of resonance that lowers the nucleophilicity considerably as well.

Check out the resonance on the wikipedia page.

http://en.wikipedia.org/wiki/Nitrate

Offline Doom91

  • Regular Member
  • ***
  • Posts: 54
  • Mole Snacks: +0/-3
  • Gender: Male
Re: SN1 reactions
« Reply #2 on: February 24, 2009, 02:07:31 AM »
nitric acid is a very strong acid hence eqm is greatly to the right , hence the nitrate ion is very stable, no reason for it to get bonded to carbon through the Sn reaction. Adding on to what macman104 said , Ois much more electro-ve than N , therefore dipole point from N to O , from electro+ve to electro-ve Hence it removes the electron from nitrogen , causing the nitrogen to be less -ve , as the nitrogen is less-ve , it is stabilised , Also due to resonance forms aka(mesomeric forms), the 3 oxygen atoms share the -ve charge of nitrogen hence stabilising it

Offline IFP

  • New Member
  • **
  • Posts: 4
  • Mole Snacks: +1/-0
Re: SN1 reactions
« Reply #3 on: February 26, 2009, 10:46:16 PM »
"The nitrate is a poor nucleophile.  It has 3 oxygen atoms pulling electron density away from the nitrogen, and there is also a fair amount of resonance that lowers the nucleophilicity considerably as well."

It's not true. Negative charge in nitrate anion is localized (more correctly delocalized between 3) on oxygen, not on nitrogen, so  no sense to consider nucleophilicity of nitrogen (which is BTW positively charged, 4 valent, so it can't be a nucleophile). Nucleophilisity of oxygen in nitrate anion is low due to electron withdrawing effect of NO2 in O2N-O(-).
To understand difference in nucleophilicities between alkoxide (RO-)/ hydroxide(HO-)  /nitrate - just compare Hammett constants for substitution in benzene ring for R(alkyl) / H / NO2...

Offline IFP

  • New Member
  • **
  • Posts: 4
  • Mole Snacks: +1/-0
Re: SN1 reactions
« Reply #4 on: February 26, 2009, 10:55:33 PM »
"nitric acid is a very strong acid hence eqm is greatly to the right , hence the nitrate ion is very stable, no reason for it to get bonded to carbon through the Sn reaction"

This is not correct again... You mix definitions of acidity/basicity and nucleophilisity/electriophilicity. Although sometimes you can see some correlations beween them, in general case these are different properties.
As example (related to your logics that if the acid stron then its anion is poor nucleophile):
In the series of halogen-containing acids in water the acidity is increased in the range HF<HCl<HBr<HI, i.e HI is the strongest acid !!! Following your explanations onecould extect that F- is the strongest nucleophile and I- is weakest... However, I- is the strongest nucleophile, as you probably know...

Offline macman104

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 1644
  • Mole Snacks: +168/-26
  • Gender: Male
Re: SN1 reactions
« Reply #5 on: February 27, 2009, 12:12:40 AM »
"The nitrate is a poor nucleophile.  It has 3 oxygen atoms pulling electron density away from the nitrogen, and there is also a fair amount of resonance that lowers the nucleophilicity considerably as well."

It's not true. Negative charge in nitrate anion is localized (more correctly delocalized between 3) on oxygen, not on nitrogen, so  no sense to consider nucleophilicity of nitrogen (which is BTW positively charged, 4 valent, so it can't be a nucleophile). Nucleophilisity of oxygen in nitrate anion is low due to electron withdrawing effect of NO2 in O2N-O(-).
To understand difference in nucleophilicities between alkoxide (RO-)/ hydroxide(HO-)  /nitrate - just compare Hammett constants for substitution in benzene ring for R(alkyl) / H / NO2...
I'm confused why you think that's not what I said...

I agree with your second post, that a strong acid does not necessarily indicate a bad nucleophile as evidenced by your example.

Also, for future reference, if you are going to respond to 2 different posts at the same time, please combine them into one post.

Sponsored Links