1,2,3,4-tetrahydrocarbazole

In this experiment the 1,2,3,4-tetrahydrocarbazole was prepared at 50 mmol scale which caused some challenges in comparison with 1 mol scale in the original method.

calculations:
cyclohexanone 4,9 g (50 mmol), M = 98,15 g/mol
phenylhydrazine 5,4 g (50 mmol), M = 108,14 g/mol
acetic acid 18 g

Into 50 ml round bottomed flask was added cyclohexanone 4,9 g (50 mmol), acetic acid 18,0 g and boiling stones. Liebig condenser was attached and the mixture was heated. When it started to reflux, during 1 hour was dropwise added phenylhydrazine 5,4 g (50 mmol) through the top of condenser (using 2 neck flask and dropping funnel would be more advanced but the simple design used here worked well too). Some solid precipitate formed in the bottom of condenser which melted after stopping cooling water circulation for a while. Then the reaction was refluxed for another 1 hour. Then it was allowed to cool down to room temperature (it started to crystallize very soon while still very hot) and overnight in a fridge at +4 C (acetic acid does not yet crystallize at +4 C as it contains some water from the reaction 50 mmol of H2O = 0,9 g H2O with 18,0 g acetic acid = 95% acetic acid).
The solid product was vacuum filtered using Buchner funnel and filter paper, the mass was quite thick and hard to get from the reaction flask, using wide ground glass joint flask is recommended (29/32 joint size was used), scraping the mass using glass rod and rinsing the flask with a little of water which was added to the funnel was necessary. The product was rinsed with a little of mixture of water + methanol 1:1 on the filter paper which improved its color.
The product was air dried for 2 weeks, smell of acetic acid in the beginning was observed which soon faded away, but the weight continued to decrease slowly for a long time. It really lasted so long, only after that long time the weight ceased to decrease and stayed stable for few days.
8,4 g of product obtained (yield 98%). 100% yield at 50 mmol scale = 8,56 g (M = 171.24 g/mol).
The crude product was recrystallized from 45 ml of methanol (it dissolved well in only 35 ml on boiling but after cooling down the crystalline mass was too thick so finally 45 ml of methanol was used by adding extra 10 ml to original 35 ml). Solution was cooled down slowly to room temperature and then to +4 C in fridge, crystals were vacuum filtered on sinter, washed with 5 ml cold methanol (+4 C) on sinter, air dried until stable weight during 3 days.
Yield 6,0 g = 70%.
m.p. 118-120 C (according literature the m.p. depends on the rapidness/slowness of heating)

Notes:
The stirring was not necessary as stated in the original method. Refluxing was enough. Adding boiling stones was necessary then.
Addition of phenylhydrazine through top of condenser avoided clogging of additional funnel stem, but some phenylhydrazine acetate crystallized in the condenser which had to be melted by temporarily stopping cooling water circulation through condenser. The weaker the reflux, the better (less phenylhydrazine acetate in the condenser and all the acid condenses in the bottom of condenser and not higher). It is recommended to use short and wide condenser, mounted as straightly as possible, dropping the phenylhydrazine through the middle of the condenser tube, not on its walls. A medicinal syringe was successfully used for that and it stayed in the top of the condenser for the whole addition and just pressing its piston to add phenylhydrazine drop by drop.
Mother liquor remaining after the recrystallization from methanol was boiled with activated charcoal but its color did not change significantly. It was only a test whether it was feasible to use activated charcoal during recrystallization. When using charcoal the filtration would require heated funnel at 1 mol scale so at 20 times smaller scale the product would certainly crystallize heavily during filtration so the charcoal was not used during recrystallization. Testing charcoal later even did not improve the color of mother liquor remaining after recrystallization so its usage seems to be not much advantageous. If you want to use charcoal anyway, use much more methanol and reduce its volume after filtering off the activated charcoal.



the beginning, cyclohexanone + acetic acid in RBF with wide ground glass joint, boiling stones (5 small brown glass balls) in the flask





after addition of first few drops of phenylhydrazine





after addition of all phenylhydrazine during 1 hour





after 1 h of reflux and removing from heating mantle the product started to crystallize immediately, the amount of acetic acid is at the edge, even in the first picture just after removing from heating mantle there was already a little of crystallized product, in the last picture the content in the flask was completely solidified





vacuum filtration, rinsing the product with a little of mixture of water+methanol 1:1 on filter paper improved its color (second picture)





air drying the product for 2 weeks





recrystallization from methanol (45 ml used although it dissolved in minimal amount of 35 ml)





vacuum filtration on sinter (methanol has low b.p. and partly evaporates at reduced pressure so filtering very cold mass is recommended)





air dried final product



[Edited on 13-11-2022 by Fery]

Quote: Originally posted by Fery

Hi Boffis, the experiment was for the beauty of chemistry, to synthesize some heterocycle. Of course I will try the dehydrogenation, just I would like to avoid Pb3O4 and try something else.