Author Topic: 2,5-dimethoxy-4-ethylbenzaldehyde Distill: Purity?  (Read 4504 times)

0 Members and 1 Guest are viewing this topic.

scarmani

  • Guest
2,5-dimethoxy-4-ethylbenzaldehyde Distill: Purity?
« on: October 01, 2003, 01:17:00 AM »
As Swim stared into infinite parallel ports, he recieved the following information:

"The following procedure was done under a fume hood.

The crude, dark, and very viscous product from the treatment of 2,5-dimethoxy-1-ethylbenzene with SnCl4 and Dichloromethyl Methyl Ether (a formylation reaction?) were dissolved in a small volume of Dichloromethane for easier handling, and the solution was poured into a 100 mL round bottomed flask which it filled approximately 1/2 way.

The small flask was then clamped in place so that its bottom was in gentle, but direct contact with the heating surface of a sad excuse for a hotplate-stirrer.  A stirbar was added to the flask.

A distillation adapter and thermometer were placed in the neck of the distilling flask, and a short Liebig condenser was attached to the distilling adapter.  A recieving flask with capilary bleed was attached to the setup by way of two-way adapter.  Everything was clamped and Keck clipped.

Flow of room temperature water was started through the condenser, rapid stirring was started, the hotplate heat was turned on, and vacuum was applied.  The Dichloromethane began boiling off vigorously and soon the sides of the roundbottomed flask were covered in a thin layer of frost.

Attempts to recondense the DCM vapor in a cold trap with an ice/salt bath had not been successful in past attempts.  Since dry ice was not at hand, the DCM was reluctantly allowed to be pulled into the vacuum pump.  As the stripping of DCM continued, the initial vacuum of 0.5 - 0.6 mm Hg weakened to 1.6 mm Hg.  It was suspected that this may have been due to DCM contamination of pump oil.

Once DCM had been stripped, temperature began climbing in flask and stabilized at 110 °C.  A clear oily liquid could be seen refluxing rapidly in the small flask, but heat loss along the short path to the Liebig condenser was too great to permit the vapor to condense into the recieving flask. 

Due to the limited heating capacity of the stirrer/hotplate, it was necessary to wrap the flask and the distilling adapter first with cotton towel, and then with a large "outer shell" of aluminum foil, which trapped hot air from the hotplate and "chimneyed" it up along the flask and distilling adapter.

After these insulation steps were taken, the rapidly stirring liquid began to boil and splatter slightly, the thermometer temperature began rising, and the refluxing clear oil (presumably the desired aldehyde) began approaching the distillation adapter and eventually condensing in the Liebig and down into the recieving flask. 

The initial temperature of the condensing vapors was 125°C and the pressure was 1.6 mm Hg.  The droplets coming over were very pale yellow at first; as the vacuum distillation progressed, the temperature rose through 130°C and the drops coming over became increasingly yellow, towards the end slightly tinged with green as well.  At about 135°C, the condensing droplets slowed to a crawl and essentially stopped.  The temperature was allowed to rise to 142°C without further distillate, and then distillation was ceased.  The clear, yellowish distillate constituted approximately 1/2 of the initial crude product.

It was noted with pleasure that towards the end of the distillation, the distillate began solidifying both in the recieving flask, and then in the Liebig condenser itself.  This indicated that the desired product was probably being obtained, since in "the literature", 2,5-dimethoxy-4-ethylbenzaldehyde had a reported mp of 47°C.  

It was also encouraging that the growing lumps/crystals seemed to be mostly white/pale in color, and that the yellow color was thus concentrated more in the liquid that had not yet crystalized.  This seemed to indicate that the desired product was indeed colorless, and it Was in the distillate, but that the observed distillate was yellow in color due to an impurity.

However, as the distillate continued to crystallize, the yellow, liquid portion eventually solidified as well.

Once the distillation was complete, the vacuum was gradually released, the heat and stirring was turned off and the setup dissasembled.  The solidified distillate was washed into the recieving flask with room temperature Ethanol, which it dissolved in gradually after repeated shaking.

In addition to having a melting point consistent with the "literature", the distillate was also obtained over a temperature range (125-135°C at 1.6 mm Hg) in reasonable accordance with the 90-110°C at 0.2 mm Hg reported "in the literature".

Overall, the vacuum distillation was uneventful and encouraging.  The problem of bumping, which had been difficult to control in past distillations, was eliminated by very rapid magnetic stirring.  Some difficulties in attaining sufficient heat input to push vapor over into the recieving flask were solved by insulating the distilling flask and distillation adapter with cotton towel and aluminum foil.  It seemed likely that the desired product had been obtained.

It is envisaged that the product will next be reacted with nitromethane and ammonium acetate to form the nitrostyrene.  Further progress will be reported"

Swim was left with a few questions after considering the beamed account.  First, swim was curious about the best means of separating the yellow impurity from the desired aldehyde product.

It seemed that melting and slow crystallization might be an option to exclude some of the yellow impurity.  Other potential options might be a recrystalization from hot solvent, or a redistillation. 

Swim is not sure what a suitable recrystallization solvent might be; finding one would probably require trial and error.  Redistillation may be useful, but swim thinks that this may not totally eliminate the yellow impurity, since it is likely to have a similar boiling point as the desired aldehyde.

Swim wonders whether it would be advisable to perform any of these purification steps (or other ones not contemplated) before the instigator of the transmission proceeds to the nitrostyrene.  Which one would likely be most efficaceous?

Swim is also curious if there is any way short of acetone/dry ice to condense volatile solvents such as DCM under vaccum and prevent them from contaminating vacuum pump oil or being exhausted.  Swim wonders if DCM is the likely cause of reduced vacuum pump efficiency.  Could DCM cause corrosion or harm to a rotary vane vacuum pump?  Is there an effective way to remove it from vacuum pump oil, or is it best simply to replace vacuum pump oil frequently?


Osmium

  • Guest
> The small flask was then clamped in place
« Reply #1 on: October 01, 2003, 06:10:00 AM »
> The small flask was then clamped in place so that its bottom was in gentle, but direct contact
> with the heating surface of a sad excuse for a hotplate-stirrer.

Dunno how to say this, but have you ever heard about oil baths?

> vacuum was applied.  The Dichloromethane began boiling off vigorously

ARRGH! DCM has a boiling point of 40°C. There is no need to vacuum distill it!

> Attempts to recondense the DCM vapor in a cold trap with
> an ice/salt bath had not been successful in past attempts.

I wonder why?!  

> the initial vacuum of 0.5 - 0.6 mm Hg weakened to 1.6 mm
> Hg.  It was suspected that this may have been due to DCM
> contamination of pump oil.

I guess you got that right!

> swim was curious about the best means of separating the
> yellow impurity from the desired aldehyde product. It
> seemed that melting and slow crystallization might be an
> option to exclude some of the yellow impurity.

No, that is the least efficient way to do it.

> Other potential options might be a recrystalization from hot solvent,

Yup.

> Swim is not sure what a suitable recrystallization solvent
> might be; finding one would probably require trial and error.

Or having a look into PIHKAL, and see which solvents Mr. Shulgin would use for a benzaldehyde closely related to yours.

> Swim is also curious if there is any way short of
> acetone/dry ice to condense volatile solvents such as DCM
> under vaccum and prevent them from contaminating vacuum
> pump oil or being exhausted.

Distill it at ambient pressure!

> Swim wonders if DCM is the likely cause of reduced vacuum
> pump efficiency.

Yes.

> Could DCM cause corrosion or harm to a rotary vane vacuum pump?

Yes.

> Is there an effective way to remove it from vacuum pump oil,

Let the pump run for several hours, with the gas ballast open and the vacuum inlet closed. That will get rid of most of the DCM, but the pump oil will not revert back into pristine, brand new condition.

> or is it best simply to replace vacuum pump oil frequently?

Depends how often you abuse your pump like this.


hypo

  • Guest
DCM
« Reply #2 on: October 01, 2003, 06:22:00 AM »
i never fully get rid of DCM at 1 bar. i always use
an aspirator to remove the last of it before using
the oil pump.


abolt

  • Guest
Or
« Reply #3 on: October 02, 2003, 09:08:00 PM »
Or you could place your reaction material into a beaker and heat and stir to say 60-80 Celcius for say, 30 minutes, and allow the residual DCM to fume off into the atmosphere via the use of an exhaust fan.

Swim has thought about using aspirator for this, but Swim doesn't like to waste water and uses a recirculating pump for Swim's aspirator station. The downside to this is that the DCM will sit on top of the water reservoir and fume off into the work area. The idea of passing DCM through an electric motor doesn't appeal to Swim much either.


foxy2

  • Guest
The idea of passing DCM through an electric...
« Reply #4 on: October 04, 2003, 09:21:00 AM »
The idea of passing DCM through an electric motor doesn't appeal to Swim much either.

Why?  DCM is not flammable.

Rhodium

  • Guest
no fire, but toxic gasses
« Reply #5 on: October 04, 2003, 10:07:00 AM »
No, but phosgene may form by the action of electric sparks on halogenated solvents.