September 17, 2019, 09:15:26 PM
Forum Rules: Read This Before Posting


Topic: Lewis Acid  (Read 8268 times)

0 Members and 1 Guest are viewing this topic.

Offline newbiesig

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
Lewis Acid
« on: July 14, 2013, 02:12:49 AM »
Is CH2O a Lewis acid or base? pretty sure it's a base but can't seem to explain. then again I'm quite bad with lewis acids and bases. Any help would be much appreciated in explaining how to tell if its a Lewis acid or base.

Offline magician4

  • Chemist
  • Full Member
  • *
  • Posts: 567
  • Mole Snacks: +70/-11
Re: Lewis Acid
« Reply #1 on: July 15, 2013, 10:29:40 AM »
It depends on how strictly you stick with the definition of "Lewis acid / base"

basically, a Lewis acid would be a substance with empty orbitals to store an (external) electron pair in it, like with AlCl3 or B(OH)3 , and a Lewis base would be something to donate one of its free electron pairs to an external medium, like :NH3 for example

in this sense , formaldehyde would be a Lewis base, as is might donate the free electrons from its oxygen, but doesn't have an electron gap somewhere else

from a more relaxed point of view, we would write the structure of H2CO like H2C[itex] ^{ \delta + }[/itex]O[itex] ^{ \delta - }[/itex] , and say that it had Lewis-acid properties at the carbon, but Lewis base properties at the oxygen


regards

Ingo
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Offline opsomath

  • Chemist
  • Full Member
  • *
  • Posts: 469
  • Mole Snacks: +50/-8
Re: Lewis Acid
« Reply #2 on: July 15, 2013, 11:57:59 AM »
Most of formaldehyde's reactions are as a Lewis acid. For instance, it accepts electrons from a Grignard reagent to make a new hydroxymethylene group where the MgBr group was. Similarly, it condenses with ammonia to make hexamethylene tetraamine; here, ammonia is the Lewis base, and again formaldehyde is the acid.

Bearing this out, a solution of 37% formaldehyde will have a strongly acidic (pH 2-4) pH: http://www.sigmaaldrich.com/etc/medialib/docs/Sigma/Product_Information_Sheet/f1635pis.Par.0001.File.tmp/f1635pis.pdf

The issue is confused by the oxidation of formaldehyhde to formic acid, which would obviously drop the pH. But the pH is low even with methanol present, which inhibits that oxidation.

The reaction with water to form acid is as follows:

CH2O + H2O -> H+ + HO-CH2-O-

Offline magician4

  • Chemist
  • Full Member
  • *
  • Posts: 567
  • Mole Snacks: +70/-11
Re: Lewis Acid
« Reply #3 on: July 15, 2013, 01:20:07 PM »
(...)Bearing this out, a solution of 37% formaldehyde will have a strongly acidic (pH 2-4) pH: http://www.sigmaaldrich.com/etc/medialib/docs/Sigma/Product_Information_Sheet/f1635pis.Par.0001.File.tmp/f1635pis.pdf
(...)

what's described here are not the properties of formaldehyde, but of it's hydrate, the 1,1 dihydroxymethane, i.e. the reaction product of formaldehyde and water (the reaction in solution is next to quantitative, i.e. 100%)

the C-H bonds in formaldehyde are not acidic at all  (i.e. are in the range of other non-activated carbon-hydrogen bonds)
the O-H bonds in 1,1 dihydroxymethane on the other hand are: they show a pKa ~ 13.7

... but that's a different story alltogether

hence, if we discussed the properties of formaldehyde, we shouldn't confuse this with the properties of 1,1 dihydroymethane

regards

Ingo
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Offline opsomath

  • Chemist
  • Full Member
  • *
  • Posts: 469
  • Mole Snacks: +50/-8
Re: Lewis Acid
« Reply #4 on: July 15, 2013, 02:54:57 PM »
Hi Ingo, interesting points. This is almost a philosophical question in that formaldehyde almost does not exist in the "pure" form, but is mostly sold in the form of 37% aqueous solution.

I'd say it is appropriate to consider the acidity of formaldehyde hydrate, since that is the way we evaluate other sorts of Lewis acids. For instance, when my students first learn about Lewis acidity, I show them the molecular formula of ferric nitrate, and explain that it makes an acidic solution in water. They know about Bronsted-Lowry acids, so this is wonderfully confusing at first. The end result is explaining how there's a Lewis acid-base reaction between water and Fe(III), and the resulting complex is a Bronsted-Lowry acid.

This is exactly the situation in formaldehyde as well. The water reacts with CH2O, making a Bronsted-Lowry acid which is CH2(OH)2. An excellent confirmation of the fact that CH2O is, by itself, a Lewis acid.

Where'd you get the pKa value for those OH groups in the hydrate? I didn't bother to look them up because I thought it would be hard. :)

Offline magician4

  • Chemist
  • Full Member
  • *
  • Posts: 567
  • Mole Snacks: +70/-11
Re: Lewis Acid
« Reply #5 on: July 15, 2013, 04:34:11 PM »
I agree with you that if you're looking for commercially available formaldehyde, you'd mostly be sold the aqueous solution, i.e. the 1,1 dihydroxymethane , instead

this system has properties in its own right, yes .
...but those will differ from formaldehyde immensely

though of cause it is quite interesting to discuss those properties  (esp. the gem. dihydroxy situation violating Erlenmeyer's rule) , my impression was that "newbiesig" wanted to know about formaldehyde, not about 1,1 dihydroxymethane instead

Another point i tend to disagree with you, in all respect , is that formaldehyde might be more or less nothing but a purely philosophical concept, whereas 1,1 dihydroxymethane would be the one and only real thing.
To the best of my knowledge, formaldehyde is a well known , stable substance , and even abundant in earth's atmosphere ( approx. 1 ppb , mostly from rotting organic materials ).
In technical processes it is widely used as pure gas - where available.
... because that's the problem with this material: you can't compress it very well without facing immediate polymerisation, and hence you'll have to look for a convenient transports form, which the waterbased system is.

Quote
Where'd you get the pKa value for those OH groups in the hydrate? I didn't bother to look them up because I thought it would be hard.
For example, Wikipedia (Ger ) has it (that's where I did look it up) , and Wikipedia (Eng), too, as I just found out , if you search for "Formaldehyde"


regards

Ingo
« Last Edit: July 15, 2013, 05:06:20 PM by magician4 »
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Offline opsomath

  • Chemist
  • Full Member
  • *
  • Posts: 469
  • Mole Snacks: +50/-8
Re: Lewis Acid
« Reply #6 on: July 15, 2013, 05:19:04 PM »
Quote
my impression was that "newbiesig" wanted to know about formaldehyde, not about 1,1 dihydroxymethanal instead

I apologize if I was not clear, I was making the case that the Bronsted-Lowry acidity of CH2(OH)2 is evidence for the Lewis acidity of CH2O.

You are right, I was certainly exaggerating when I said that about formaldehyde being a philosophical concept.

If the original poster could clarify their question, it would be good so that we can provide more specific information. Nonetheless, I would say that in most contexts that make sense formaldehyde is a Lewis acid.

Sponsored Links