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Author: Subject: ethylbenzene by Wolff-Kishner (Huang-Minlon)
Magpie
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[*] posted on 24-8-2016 at 13:25
ethylbenzene by Wolff-Kishner (Huang-Minlon)


I intend to do this preparation shortly but am perplexed by the procedure in Vogel's 3rd ed, p. 516. Here is the procedure:

"Place 36.0g of redistilled acetophenone, bp 201°, 300mL of diethylene glycol, 30mL of 90% hydrazine hydrate and 40g of potassium hydroxide pellets in a 500mL Claisen flask provided with a reflux condenser and a thermometer dipping into the liquid. Warm the mixture on a boiling water bath until most of the potassium hydroxide has dissolved then reflux (free flame) for one hour. Arrange the apparatus for distillation and distill until the temperature of the mixture rises to 175° (note 1); keep the distillate (ca 50 mL). Replace the condenser in the flask and continue the refluxing for 3 hours.

Separate the upper hydrocarbon layer from the distillate and extract the aqueous layer twice with 20mL portions of ether; dry the combined upper layer and ethereal extracts with anhydrous magnesium sulfate, remove the ether on a water bath and distill the residue from a 50mL Claisen flask. Collect the ethylbenzene at 135-136°; the yield is 20g. By extracting the syrupy liquid in the reaction flask with three 30mL portions of ether a further 2g of ethylbenzene, bp 136°, may be obtained.

Note
1. The reduction takes place at low temperature and is fairly rapid for acetophenone. With higher ketones the upper layer of the initial distillate should be returned to the contents of the reaction flask and refluxing continued for 3-5hrs. The reactant mixture and aqueous distillate are then combined, extracted with ether, etc. "

It is fine up to the point where the pot residue is refluxed for another 3 hours. But then nothing is done with this product. Only as almost an afterthought is it mentioned that another 2g of ethylbenzene can be extracted from the "syrupy residue."

Am I missing something?




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AvBaeyer
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[*] posted on 24-8-2016 at 17:35


Magpie,

I do not think that the additional reflux is only done to give an extra 10% in yield. It appears that the additional reflux time is necessary for higher homologs of acetophenone. This comes from parsing Note 1.

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[*] posted on 24-8-2016 at 17:42


Quote: Originally posted by AvBaeyer  
Magpie,
I do not think that the additional reflux is only done to give an extra 10% in yield.


I agree.

Quote: Originally posted by AvBaeyer  

It appears that the additional reflux time is necessary for higher homologs of acetophenone. This comes from parsing Note 1.


I agree with your reasoning here. But doesn't it look like Vogel was a little sloppy in his presentation of this procedure for ethylbenzene?

I have looked up the original work by Huang-Minlon in JACS. There are two articles, the first in 1946, volume 68, shows preparations for n-probylbenzene and cyclohexane. These also use the 3hr addtional reflux. In these cases the distillate and the residue are combined and extracted with ether.

The 2nd article shows how this procedure can be used to reduce nitro groups, 1948, volume 70.

Attachment: Wolf-Kishner (Huang-Minlon) 1946.pdf (281kB)
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Attachment: Wolff-Kishner (Huang-Minlon) nitro reduction 1948.pdf (498kB)
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[Edited on 25-8-2016 by Magpie]

[Edited on 25-8-2016 by Magpie]




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[*] posted on 24-8-2016 at 18:10


Magpie,

I find a good deal of Vogel to be sloppy and sometimes less than fully correct. Best to find reaction conditions in Org Syn, which Vogel largely plagiarizes anyway. That's not to say that Vogel does not have utility, which in general it does. Never in my professional career did I see anyone using Vogel as a primary reference. Just my rant.

Now to be more helpful. There is a synthesis procedure for 4-methylethylbenzene in Organic Reactions, vol 4, p 389 which is in the forum library. Not exactly the procedure you found in Vogel but looks good. See also JACS, 68, 2487 (1946) which is attached.

AvB



Attachment: Wolff-Kishner Redn huang-minlon1946.pdf (281kB)
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[*] posted on 24-8-2016 at 18:13


Magpie,

I did not see that you had already posted the article that I attached. Sorry.

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[*] posted on 24-8-2016 at 18:21


So I think what you are saying is that for ethylbenzene I don't need to do the 3hr reflux unless I want the 2g in the syrup? Or maybe its in the syrup without the 3hr reflux?

What worried me about extracting the syrup was its large volume and the fact that DEG and DCM (my substitute for ether) are mutually soluble. So I did a test tube experiment: I added 1mL of DCM to 1mL of DEG. They are indeed mutually soluble. Then I added 1mL of water. Again everything was mutually soluble. Then I added ~30mg of KOH, which went into solution. Then 2 phases appeared: a larger upper phase and about 1mL of the lower phase. So I conclude that extracting the syrup would be practical. Do you agree?




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[*] posted on 24-8-2016 at 20:53


I'd extract it, perhaps keeping it separate from the distillate, and see what you get - no harm in trying to gain that extra bit of material. At the least, hold on to the syrup until you're sure you've isolated most of the product in the distillate, so that you can process the syrup further if you find that you're missing the material you expected.
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[*] posted on 25-8-2016 at 19:21


Magpie,

*So I think what you are saying is that for ethylbenzene I don't need to do the 3hr reflux unless I want the 2g in the syrup? Or maybe its in the syrup without the 3hr reflux?*

The extra 3 hrs of reflux are probably not productive for ethylbenzene. I would not fight for the extra 2 grams of product, with or without extra reflux, if the initial distillate gives you an acceptable yield. It's going to be a messy extraction. In my opinion, it comes down to time value.

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[*] posted on 25-8-2016 at 20:22


AvB you are right, of course. But I decided to just follow the procedure as I had the time, and the ice. The 3hr reflux was finished this afternoon.

The distillate from the initial distillation looks as expected both in quantity and in showing 2 phases nicely. Tomorrow I'll do the extractions. If the extraction of the syrup (over 300mL) goes OK I'll combine it with the other extraction and the hydrocarbon phase of the distillate, dry, and then distill again to get the ethylbenzene. If the extraction of the syrup looks like junk I will simply discard it.





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[*] posted on 26-8-2016 at 17:27


Magpie,

Curious to learn how this all worked out for you.

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[*] posted on 26-8-2016 at 17:53


AvB,

There will be a full report in a day or two. :)




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[*] posted on 27-8-2016 at 12:52


Ethylbenzene by Wolff-Kishner reduction of acetophenone

Ethylbenzene was produced via a Wolff-Kishner (Huang-Minlon) reduction of acetophenone following the procedure in Vogel (ref 1).

Wolff-Kishner eqns.gif - 4kB

The Wolff-Kishner reduction exists in an alkaline medium. This provides a valuable alternative to the Clemmensen reduction which exists in an acid medium.

1. Reagents
36g of freshly distilled acetophenone
34mL of 79.6% hydrazine hydrate
300mL of diethylene glycol (DEG)
40g of KOH
dichloromethane (DCM) in place of ether
anhydrous magnesium sulfate

The DEG can be obtained OTC as a cooking fuel for camping. A picture of the two cans I used is shown below:

OTC diethylene glycol (DEG).jpg - 135kB
Couglan’s camp heat

2. Procedure
A 500mL RBF was setup with an electric heating mantle and a magnetic stirrer. A Claisen adapter was attached. In the center neck a thermocouple was installed to allow temperature measurement of the liquid. To the long neck was attached a condenser.

The flask was loaded with the DEG, acetophenone, hydrazine hydrate, and KOH in that order. Heating was begun but stirring was not possible until the KOH had sufficiently dissolved a short time later.

a. initial reflux Continued heating brought the yellow product mix to a steady reflux for 1 hour. Photos of the lagged and un-lagged apparatus are shown below:

Wolff-Kishner 1hr reflux.jpg - 133kB
lagged reflux

Wolff-Kishner 1hr reflux - unlagged.jpg - 131kB
un-lagged reflux

The un-lagged picture shows the yellow color of the intermediate hydrazone.

b. initial distillation After 1hr the condenser was reconfigured to allow downward distillation. This was continued until the pot liquid reached 175°C. Something over 50mL of 2-phase distillate was collected as shown below:

Attachment: phpqz9s4H (139kB)
This file has been downloaded 665 times
initial distillation picture

c. final reflux The condenser was again configured for reflux. This was continued for 3hrs. Following this it can be seen that most all of the hydrazone has been converted to ethylbenzene as the yellow color is virtually gone. After cooling to room temperature this had the consistency of corn syrup.

Wolff-Kishner syrup.jpg - 132kB
Wolff-Kishner syrup.

The EPDM rubber stopper used as a thermocouple adapter was heavily degraded as can be seen in the picture below.

Wolff-Kishner TC adapter.jpg - 98kB
TC adapter

d. solvent extractions

1. The aqueous layer from the initial distillate was extracted with 2x20mL of DCM. The extract was combined with the hydrocarbon phase from the initial distillate in an Erlenmeyer flask. A scoop of anhydrous MgSO4 was added for drying.

2. An extraction of the syrup was also attempted using 50mL of DCM. However, no phase separation occurred. Four test tube tests using 2mL ea of the syrup were made under the following conditions:

1. 1mL of water was added
2. K2CO3 was added
3. 1mL of water and K2CO3 were added
4. 1mL of ether was added

Only test # 4 using ether produced a phase separation. Therefore I added 30mL of ether (all I had on hand) to the syrup and attempted an extraction. The ether phase separated but by the time I returned to the hot lab most of the ether had evaporated. Not wishing to pursue this any further, extraction of the syrup was abandoned. However, I feel that the 2g of ethylbenzene reported by Vogel is indeed likely to be in this syrup. My conclusion is based on the color change of the syrup from yellow to colorless following the 3hr reflux.

e. 2nd distillation

The dried extract/distillate was again distilled. The fraction boiling in the range 135°-136°C was taken as ethylbenzene.

distillation of ethylbenzene.jpg - 149kB
Distillation of ethylbenzene

ethylbenzene at bp.jpg - 90kB
Ethylbenzene boiling point

ethylbenzene receiver.jpg - 129kB
Ethylbenzene in receiver

2. Results
25.0g of ethyl benzene was obtained for a % yield of 78.5%. The Vogel expected yield is 20g.

3. Discussion
There was some confusion on my part concerning the procedure as presented by Vogel. This is discussed earlier in this thread. In the end I decided to follow the procedure “as is” except I used DCM in place of ether. Although time consuming, this is a facile method for the reduction of acetophenone. And according to Huang-Minlon it works well for the preparation of n-propyl benzene and cyclohexane (ref 2) as well.

4. References
1. Practical Organic Chemistry by Arthur I. Vogel, 3rd ed, (1956), p. 516
2. “A Simple Modification of the Wolff-Kishner Reduction,” by Huang-Minlon, JACS, 68, (1946), pp. 2487-2488.







[Edited on 27-8-2016 by Magpie]

[Edited on 27-8-2016 by Magpie]

[Edited on 28-8-2016 by Magpie]





[Edited on 28-8-2016 by Magpie]




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[*] posted on 28-8-2016 at 19:45


Here's a good reason not to reflux the reaction mix for 3hrs. It seems the hot (183°C) alkaline mix permanently etched my 500mL RBF. I have tried alcoholic NaOH and chromic acid cleaners to no avail.

500mL Kontes flask.jpg - 141kB




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[*] posted on 17-9-2016 at 10:47


Your degraded stopper makes me think of the original Huang-Minlon variation. If I'm not mistaken he discovered this method when he did a Wolff-Kishner reduction with a cork where you had the rubber stopper, and the cork degraded leaving a gap where the vapors were able to slowly escape during reflux. He came back to find what he thought was a ruined reaction, but worked it up anyway and got a great yield. If memory serves me right I found that anecdote in Feiser's TOPICS IN ORGANIC CHEMISTRY, an entertaining read, but maddening in a way because it's a supplement to another book which I don't have, so some of the chapters start in the middle of the story.

I'm still hoping that somebody else with a hyphenated name discovers a useful variation on this process we can have a reaction with an even more imposing name.
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