Sciencemadness Discussion Board

2,5-dimethoxynitrostyrene with wrong mp

Bronstein - 13-5-2012 at 03:06

Hi,

I made some presumed 2,5-dimethoxynitrostyrene by condensing 2,5-dimethoxybenzaldehyde with nitromethane in isopropanol, with dilute aqueous methylamine as catalyst. According to the method posted by Barium on the hive. I heated for 1 hour, and it was then removed from the heat. It solidified to a yellow-orange mass almost immediately. It was recrystallized once from a methanol/isopropanol mixture, producing orange needles. (I got 1.8g from 3g aldehyde, nowhere near the yields claimed by the method i followed. But I might have heated for to long time.)

I wanted to be sure of the identity of the product so I took a melting point. It was taken in a capillary tube sealed in one end, "glued" to the thermometer with capillary force, in a heating bath. I got a mp of 109C, while it should be around 119C (source).

So it must be impure, and I recrystallized again, this time from 50% acetic acid. Again nice orange needles, with the mother liquid becoming yellowish. So, another melting point test. Still the same, 109C!

Why wasn't the melting point improved? Is this something else than the nitrostyrene? The crystals was after both recrystallizations allowed to dry in the open air over night, and then put in the refridgerator. I heated more slowly towards the end nearing the hoped for mp.

To be sure of the thermometer and the starting material, I tried a mp of the aldehyde. It melted at 46-48C, which is correct according to this source. But now I find some other sources claiming the mp of 2,5-dimethoxybenzaldehyde should be upwards of 50C or even 53C (Combined chemical dictionary)?

turd - 13-5-2012 at 03:16

Maybe you have a mixture of cis- and trans-beta-nitrostyrene?
Or you're heating too quickly when taking the mp.
Or your product is not dry after all.

The reported melting points of substituted benzaldehydes feature a notorious scattering.

If you get nice orange needles, I wouldn't bother too much.

PS: Recrystallization from 50% AcOH? Really? Why not use EtOAc?

solo - 13-5-2012 at 18:42

Quote: Originally posted by Bronstein  
Hi,

I made some presumed 2,5-dimethoxynitrostyrene by condensing 2,5-dimethoxybenzaldehyde with nitromethane in isopropanol, with dilute aqueous methylamine as catalyst. According to the method posted by Barium on the hive. I heated for 1 hour, and it was then removed from the heat. It solidified to a yellow-orange mass almost immediately. It was recrystallized once from a methanol/isopropanol mixture, producing orange needles. (I got 1.8g from 3g aldehyde, nowhere near the yields claimed by the method i followed. But I might have heated for to long time.)

I wanted to be sure of the identity of the product so I took a melting point. It was taken in a capillary tube sealed in one end, "glued" to the thermometer with capillary force, in a heating bath. I got a mp of 109C, while it should be around 119C (source).

So it must be impure, and I recrystallized again, this time from 50% acetic acid. Again nice orange needles, with the mother liquid becoming yellowish. So, another melting point test. Still the same, 109C!

Why wasn't the melting point improved? Is this something else than the nitrostyrene? The crystals was after both recrystallizations allowed to dry in the open air over night, and then put in the refridgerator. I heated more slowly towards the end nearing the hoped for mp.

To be sure of the thermometer and the starting material, I tried a mp of the aldehyde. It melted at 46-48C, which is correct according to this source. But now I find some other sources claiming the mp of 2,5-dimethoxybenzaldehyde should be upwards of 50C or even 53C (Combined chemical dictionary)?



.....what was your reference for your procedure....your own or followed the work of someone else ...?

Bronstein - 15-5-2012 at 08:04

Quote:

If you get nice orange needles, I wouldn't bother too much.

PS: Recrystallization from 50% AcOH? Really? Why not use EtOAc?


Ok, I will not worry then. I just hope it isn't contaminated with some of the starting aldehyde.
I tried recrystallization with dilute acetic acid as I read it in an article, and decided it would perhaps be more optimal.

Quote: Originally posted by solo  



.....what was your reference for your procedure....your own or followed the work of someone else ...?


As I said in the first post I followed a method posted by Barium on the hive. I haven't found an archive of it online but I'll quote part of it here:
Quote:
A general method for nitrostyrenes is as follows:

100 mmol of a aromatic aldehyde
105 mmol nitromethane or nitroethane
15-20 mmol methylamine as a 10-20% aqueous solution
Enough MeOH, EtOH or IPA to make the mixture stirable. Usually about 25 ml

To a 250 ml rb-flask equipped with a magnetic stirbar is added the aromatic aldehyde, nitromethane and the alcohol. Sirring is started and the aqueous methylamine solution is added in one portion. The reaction flask is placed in a water bath and heated to 40-50°C. The reaction progress can be monitored by TLC to see when the aldehyde is consumed. In most cases 45 minutes in the water bath is enough.
When the reaction is over 25 mmol GAA is added to the reaction mixture and the flask is placed in the freezer until the crystallisation is complete [1]. The solids are then broken up with a spatula, transferred to a filter funnel and washed with water to remove most of the methylamine acetate. The nitrostyrene is then recrystallised from MeOH, EtOH or IPA.

arsphenamine - 19-5-2012 at 10:51

Quote: Originally posted by turd  
Maybe you have a mixture of cis- and trans-beta-nitrostyrene?
I kept wondering about that one.
When an aromatic Henry reaction product dehydrates, you usually get the trans geometry under ordinary conditions,
particularly when there are adjacent substituents on the ring.

For the cis form, MMFF structure optimization rotates the nitro-ethylene moiety out of the ring plane ~45o because of
steric crowding and then the ethylenic π bond can't conjugate with the ring.

My quick and dirty ab initio computations say the trans geometry is 4-6kcal more stable than the cis,
so my suspicion is that you'll almost never see the cis form without some clever photochemical reaction conditions.


[Edited on 19-5-2012 by arsphenamine]

Nicodem - 19-5-2012 at 11:19

Quote: Originally posted by arsphenamine  
My quick and dirty ab initio computations say the trans geometry is 4-6kcal more stable than the cis,
so my suspicion is that you'll almost never see the cis form without some clever photochemical reaction conditions.

The beta-nitrostyrenes formed via any of the Knoevenagel condensation methods, such as the one described here, are always present only in the trans configuration. There is absolutely no cis isomer observable in the 1H NMR of such compounds and I have seen a lot of their spectra. This is normal for such push-pull double bond systems which rapidly equilibrate to the thermodynamically more stable form. So even if you would have a reaction mechanism that would favour the formation of the cis isomer, it is quite likely that this could not be isolated at normal temperatures before thermodynamic equilibration (though perhaps it could be detected by some fast spectroscopy). If the difference in energies is high, even as high as you calculated, then it is comprehensible that the insignificant equilibrium amounts of the cis form can not be detected by NMR.

arsphenamine - 19-5-2012 at 12:47

Quote:
The beta-nitrostyrenes formed via any of the Knoevenagel condensation methods, such as the one described here, are always present only in the trans configuration.
That was my thought, too, but I didn't want to shout "Inconceivable!"

Since I'm doing lots of computations these days, looking at the cis vs. trans energies seemed an obvious thing to do.
It may not be the best place to look but the light's good.

I have to hedge on formation of cis/Z nitrostyrenes, and simply wonder
if they may be formed at all by reactions other than condensations.

chemrox - 20-5-2012 at 01:00

It may just be wet product. You have to get the cryx really dry to get a good mp. Clibrate your thermometer too. Usually good for a degree or two.
As above, I also would prefer a volatile solvent. Benzene should work. DCM too.