[Nitrous_Acid]

Hi!

Henry reaction? Under which circumstances the nitroalcohol dehydrates to form nitroalkene?

From OrganicSyntheses.org NITROSTYRENE =>Nitrostyrene can be prepared by the condensation of benzaldehyde and nitromethane. This may be accomplished by the use of small amounts of a primary aliphatic amine,1 a method which requires a number of days for the reaction to go to completion, or preferably by the use of alkali as first discovered by Thiele.2 Alternatively, nitrostyrene can be prepared from benzil, nitromethane, and sodium ethoxide,3 or from styrene and nitrosyl chloride.4

Isn't there a contradiction between the above and the below writing?

From => Organic-chemistry.org
If acidic protons are available (i.e. when R = H), the products tend to eliminate water to give nitroalkenes. Therefore, only small amounts of base should be used if the isolation of the ß-hydroxy nitro-compounds is desired.

Because one (the one theat's above) governs the use of small amounts of base if the isolation of NiroStyrenes is desired but the below writing governs that small amounts of base should be used if the isolation of the ß-hydroxy nitro-compounds is desired.

As far as I can understand to get the NitroAlcohols if the reaction is done with strong bases like NaOH he is done under low temperatures. But if he is done with weaker bases such as Amines he is done under high temperatures to get the NitroAlkenes. This might be due to the formation of Na salts of the NitroAlcohols (in the middle stage) (which is possibly the white preticipate that forms duiring the addition) which are converted into Nitrostyrenes upon the addition of the acid. -OH from the Na goes with the acidic proton of the Nitroalkane thereby creating a double bond betwen the alpha-carbon of the Nitro Compound and the alcoholic -OH group is coverted into H2O while Na is converted into NaCl creating Nitrostyrene!

But the point that I can't understand is that how is the NitroAlcohol synthesis made by the usage of NaOH? (I have got many examples for these type of reactions) Patent: EP0960876

Basically I want you to compare the above patent (Nitroalcohol Synthesis) and the preperations given below (For NitroAlkene Synthesis) and answer my question:

Organic Syntheses, Coll. Vol. 1, p.413 (1941); Vol. 9, p.66 (1929). (http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV1P0413)
JOC 15, 8 (1950)

SO THE MAIN QUESTION IS THIS: Under which circumstances the nitroalcohol dehydrates to form nitroalkene? (Without the usage of dehydrating catalysts)

Thanks!

[macman104]

I guess I would say that each site has it's own definition of "small".  Also, one of those cites the reaction needs to take place over several days, implying the slow reaction and not rushing to the alkene dehydration product.

Unfortunately, I can only speculate, I don't have any hard answers for you (and there may not in fact be any, as it may depend partially on the substrate, type of base, etc.)

[Nitrous_Acid]

So can we say that harsh reaction conditions cause dehydration?

[Nitrous_Acid]

Like high temperatures fast reactions?

[macman104]

I would say typically yes, more harsh conditions, and higher temperatures will lead to dehydration.  You need to treat your compound nicely if you don't want to dehydrate it

If you keep the energy in the reaction (eithe mild reagents or low temperatures), you lower the likelihood of providing enough "power" for the compound to dehydrate.

[Nitrous_Acid]

Thanx! This queston has been puzzling my mind. And if someone can inform me about Knoevenagel condensation I'd be glad!

[macman104]

Thanx! This queston has been puzzling my mind. And if someone can inform me about Knoevenagel condensation I'd be glad!

Start a topic.  What about it do you want to know?

[Nitrous_Acid]

I would say typically yes, more harsh conditions, and higher temperatures will lead to dehydration.  You need to treat your compound nicely if you don't want to dehydrate it

If you keep the energy in the reaction (eithe mild reagents or low temperatures), you lower the likelihood of providing enough "power" for the compound to dehydrate.

Are these valid before neutralizing the catalyst (base)? After we neutralize him we can put the NitroAlcohol in any condition and he won't dehydrate. Are these right?

[macman104]

In the general sense, I would believe yes.  There may be some exception, I'm overlooking.

[Nitrous_Acid]

What kind of exceptions? Correct me if I'm wrong but I think that the reaction would finish after the base has been neutralized. If the reaction doesn't finish all the Nitro-Alcohols would be suspectible to dehydration under harsh conditions.

[macman104]

I don't know of any.  I was just trying to say that I was not 100% sure.  There may be some exception that I am not aware of, but I cannot think of any.

[Nitrous_Acid]

So as a general rule can we say that "Make the reaction as non-harsh as possible before neutralizing the base to not to dehydrate your compound (to get Nitro-Alcohol) before neutralizing the base?"

[macman104]

I would generally agree with that statement, yes.

[Nitrous_Acid]

Sorry I've written it twice!
I want to make this issue clear (and I want to be absolutely sure) so please answer this as a last question for this thread!

So can we say that "Make the reaction as non-harsh as possible before neutralizing the base to not to dehydrate your compound (to get the Nitro-Alcohol)?"

[macman104]

I said that already, yes!