Author Topic: Dehydration of P-1-POl with p-TsOH, how much?  (Read 1128 times)

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  • Guest
Dehydration of P-1-POl with p-TsOH, how much?
« on: June 15, 2003, 07:45:00 PM »
Hi there,

Does it matter how much p-toluene sulfonic acid and toluene I use per mole of P-1-POl to dehydrate? So far I've only used the KHSO4 method and I dislike it. Does p-TsOH take longer?

Pschokitty mentions the French from the 50's and 60's useing Tosic to make isosafrole. Does anyone have any references?



  • Guest
dehydration/elimination by tosic acid
« Reply #1 on: June 16, 2003, 03:38:00 AM »
I believe that TFSE contains tosic acid catalyzed eliminations, but I have no excellent keywords to provide you with... Try an assortment of keywords, starting with for example: "elimin* AND tos*" or "dehydrat* AND tos*"

Suggested procedure: 1-2% tosic acid in a 20-25% solution of P1Pol in refluxing toluene (following the reaction on TLC), alternatively by using 1% tosic acid in neat P1Pol (with good mag-stirring), and continous distillation of the formed propenylbenzene using an oil bath set at 10-20°C above the bp of propenylbenzene (see TFSE).

Workup: Wash the solution with 1/3 its volume of 10% bicarbonate followed by brine, then dry the solution over anhydrous CaCl2 or similar agent and fractionately redistill. When you are done, please write a detailed report with your isolated yield, and precise amounts & reaction times used and post it here. Thanks!


  • Guest
Propenylbenzene by TsOH dehydration of P-1-Pol
« Reply #2 on: June 20, 2003, 09:56:00 PM »
1) 50.3g (0.369 mol) redistilled P-1-POl, 1.002g TsOH and approx 150ml Toluene was placed in a 2 neck 500ml RBF with stir bar and thermometer in one neck just to see the vapour tempreture, then up for reflux with a Dean-Stark trap.

2) Oil bath was set to 130°C and reflux rate was less than 2 drops/sec. Reflux slowed when 6.2ml of H2O was collected. Vapour tempreture climbed from 113°C to 116°C and oil bath climbed to 140°C with reflux starting  again, washing the rest of the water droplets from the condensor  and surounding area into the trap. Heat was stopped and a total of 6.5ml H2O was collected. This took about 4 hours.

3) The solution was transfered to a 500ml separation funnel and washed with 65ml 10% NaHCO3 then 65ml Brine. The solution was light yellow and very clear.

4) This was poured in a 400ml beaker with stir bar and dried with some CaCl then filtered into a 500ml RBF and set up for fractional distillation with a claison adaptor packed with Rashig rings.

5) Oil bath was heated to 70°C and toluene vacuum distilled

6) Cooled and transfered to a 100ml RBF and washed 500ml flask with a little toluene. Heated at atm pressure to collect last of the Toluene then at 174°C-175°C came the propenylbenzene. Oil bath was at 210°C

7) When distillation stopped, vacuum was applied to get the last of the propenyl over. Recovered 34.2g B-Methylstyrene. 6.7g of brown viscous mushy liquid remained which was acid to litmus.

Yield was 78.4%. Is that and reasonable yield for TsOH? Can I improve on it, because I got 80.3% using KHSO4. Anyway, thanks for the proceedure. It was very easy.



  • Guest
Yes, there is still room for improvement
« Reply #3 on: June 22, 2003, 01:18:00 PM »
Congratulations to a successful run!

Yes, there is still room for improvement, as my suggested procedure was just off the top of my head. Try playing with the TsOH amount and see what happens if you use 1.5 grams instead, or use xylene as the solvent (bp 140°C instead of toluenes 110°C). Using Toluene, I don't think there is any need for a fractional distillation in the workup, a slow ordinary distillation should suffice and there you could probably save another milliliter.

The difference between 78.4 and 80.3% yield isn't more than a single gram, so that should easily be found somewhere.

Edit: You say that you have been using KHSO4 for the dehydration in 80.3% yield before, do you have notes of that too which you can post here too? I'm sure that people would appreciate it. I will upload them to my page too when I have the time.


  • Guest
Better yield with KHSO[sub]4[/sub]
« Reply #4 on: September 10, 2003, 03:31:00 AM »
55g P1POL, with all DCM removed, was placed in a 250ml RBF with stir bar, and 1.2g KHSO4 was added and set up for simple distillation using a recovery tube instead of a distillation head.

With oil bath at 147°C, small bubbles were seen. At 158°C, water began forming in the recovery tube and falling back into the flask causing fizzing and white smoke. This problem was effectively curtailed by the use of a heat gun aimed at the recovery tube every time water appeared.

At about 183°C, the water droplets started to appear oily. At 198°C no more water was seen to evolve and propenylbenzene started coming over (along with water), but no more need for the heat gun. Over 205-210°C, the benzene was coming over fast, then slowed.

Oil bath was taken up to 220°C when the rest of the benzene appeared to come over. After that, temp was taken to 228°C when a light yellow liquid appeared. Distillation was stopped. 56.7g total was collected and 2.8g remained in the reaction flask.

Transfered to a 250ml sep funnel and removed approx 6.1g water. Tapped the propenylbenzene into a 250ml erlnmyer flask, washed RB receiver flask and sep funnel with 10ml DCM and added that to the propenyl.

Added some CaCl2, left for an hour and filtered into a 100ml RBF with stir bar. Washed Erlnmyer flask and filter with 3x20ml DCM and added that to the propenyl as well.

Set up for fractional vacuum distillation. With bath at 50°C removed the DCM then with bath at 54°C @5.5 mbar came the product at 42-43°C. Crystal clear B-Methylstyrene Yield 83.8%. Yeah!

Better than I've ever done and I feel sure I can get over 90%. I have a feeling that water falling back into the reaction flask lowers the yield and some white stuff can be seen in the condenser and recovery tube when this happens. Using a distillation head and thermometer will guarantee the water falling back in, even with a geat gun. A heat tape is a probably a better idear.



  • Guest
P1Pol dehydration doc
« Reply #5 on: September 10, 2003, 01:03:00 PM »
Good Post, FriendlyFinger, I have now added both your variations to a doc:

Dehydration of Phenyl-1-propanol to Propenylbenzene


BTW, which method(s) have you been using to synthesize the P1Pol?


  • Guest
Anhydr. CuSO4: Alcohol Dehydration Catalyst
« Reply #6 on: September 18, 2003, 04:00:00 PM »
Anhydrous copper(II)sulfate: an efficient catalyst for the liquid-phase dehydration of alcohols

J. Org. Chem. 45(5), 917-919 (1980)


A fundamental organic transformation for which a great number of methods have been reported is the conversion of alcohols to olefins. New procedures and reagents appear regularly, each of which offers certain advantages over other methods.[2] We wish to report that anhydrous copper(II)sulfate, prepared by heating copper(II)sulfate pentahydrate at 200-300°C for 2 days, serves as an effective catalyst for the dehydration of secondary, tertiary, benzylic, and allylic alcohols to the corresponding olefins.  We find that it is a valuable alternative to other dehydration reagents and works for a variety of alcohol types.

Of greatest note is the extreme simplicity of the method. The neat alcohol and the solid catalyst are heated at an appropriate temperature (vide infra), and the olefin distills from the reaction mixture. Dehydration occurs in the liquid phase so that gas-phase flow systems are unnecessary, the conditions are fairly mild (100-160°C), and the time required is relatively short (0.5-15 h). Furthermore, the olefin product is uncontaminated with byproducts other than the evolved water; filtration through a cotton pad gives pure product.

Experimental Guidelines. Anhydrous copper(II)sulfate can be prepared in large batches and stored indefinitely in a desiccator. It was found that an alcohol:catalyst ratio of 1:0.75 gave the best results. The temperature required for smooth dehydration varies with the alcohol structure - the reaction temperatures found to be best were about 120-130°C for the tertiary, benzylic, and allylic types and 160-180°C for secondary alcohols. Higher temperatures lead to somewhat decreased yields in cases of easily polymerized olefins.

The olefin product was collected at atmospheric pressure or under reduced pressure for olefins with boiling points greater than ~120°C. The pressure was chosen so that the reaction temperature was at or near the boiling point of the starting alcohol, allowing smooth distillation of the olefin from the reaction mixture.

In addition, the dehydrations of 1-phenylethanol and 1-phenylpropanol lead to side products uniquely indicative of carbenium ion intermediates. Heating either of these alcohols with anhydrous copper(II)sulfate yields first the bis ether 1, which then undergoes smooth cracking to the olefin. Furthermore, examination of the reaction residue reveals that the major byproduct is a styrene dimer, 2.

The exact catalytic species is not known at this time, nor is the role of copper understood. When very small amounts of pyridine are added to the reaction mixture, nearly complete destruction of the catalytic activity follows. It is therefore possible that in the preparation of anhydrous copper(II) sulfate, small amounts of strong, protic acids are formed which are responsible for the catalytic activity.

What is notable is that anhydrous copper(II)sulfate is a general dehydration catalyst that equals or surpasses other common catalysts in yields and is often more efficient (lower temperatures and/or shorter times).[6] For example, beta-methylstyrene is produced in 69% yield in the present work while potassium bisulfate gives 55-60% yields.[7]

Experimental Section

Anhydrous copper(II) sulfate was prepared by heating CuSO4.5 H2O (ACS grade) in a porcelain dish at 275°C for 2 days. The off-white solid wm stirred several times during heating to help break up the lumps and was stored in a desiccator.

General Dehydration Procedure

The alcohol to be dehydrated was placed in a round-bottomed flask containing a magnetic stirring bar and anhydrous copper(II)sulfate (0.75 molar equiv). A 7-cm Vigreux column and condenser was attached, and the mixture was immersed with vigorous stirring in an oil bath preheated to the desired reaction temperature. Heating was continued until the distillation of volatile products into an ice-cooled receiver stopped. The majority of the water layer of the product was removed by pipet, and the olefii product was filtered through a plug of cotton. In all cases the olefin product was pure as determined by NMR spectroscopy, which was also used to calculate ratios of isomeric products. Where the olefin product was higher boiling (>120°C), aspirator vacuum (25 mm) was applied to the same experimental setup.

Thus 1-Phenyl-1-propanol, at 125°C/25mmHg for 2h, gave beta-methylstyrene (trans/cis, 91:9) in 69% yield.


[2] See for example: (a) Ashby, E. C.; Williard, G. F.; Goel, A. B. J. Org. Chem. 44, 1221 (1979) (b) Brieger, G.; Watson, S. W.; Barar, D. G.; Shene, A. L. J. Org. Chem. 44, 1340 (1979)
[6] For a good recent review of alcohol dehydrations, see: Askani, R. Methoden Org. Chem. (Houben-Weyl), 4th Ed. 1972, 5(1b), 44.
[7] Wittig, G.; Harborth, G. Chem. Ber.  77, 315 (1944)