Propenylbenzene synth. Comments and suggestions please
Example 1.
500g P1P (3.73 mole)
43g NaBH4 (1.14 mole)
1250ml MeOH
746ml dH2O
4.5 7g NaOH
565ml 2M HCl
1500ml DCM
1340ml dH2O
Dehydration:
19.4g KHSO4
1500ml DCM
450ml 10% NaHCO3
10g 10% NaHCO3
450ml Brine
500g P1P and 1250ml MeOH was poured in a 3l erlnmyer flask with stir bar and thermometer, sitting in an ice bath without water. was placed. 4.5g NaOH
and 746ml dH2O was cooled then added 43g NaBH4. Temp was 6 deg. Poured in a 2l sep funnel and started addition.
Solution temp was 22 deg when water was added to ice bath and temp held between 26 - 28 deg. Addition took 39 minutes. After addition, kept temp at
that range by removing some ice water and stired for 3 hrs. Temp fell after about 1hr from end of addition.
565ml cold 2M HCl was added till pH 7, temp kept below 28 deg. 2 layers slowly appeared. Top was slightly yellow. Poured into a 4l sep, separated then
extracted aqueous 2 x 500ml DCM. Added to the organic layer along with another 500ml DCM and washed with 2 x 670 ml dH2O. Organic layer became very
clear. Backwashed aqueous layer and put in a 3l RBF.
Set up for simple distillation and removed DCM with bath @ 50 - 60 deg. Stoped when it came over at 41 deg. Transfered to a 1l RBF, added 12.5 ground
KHSO4 and distilled with fractionator. Then removed column, washed dowm with some DCM first and replaced distillation head with recovery tube and
dehydrated the alcohol.
Bath Temp...... .Event
Up to 128 deg..Rest of DCM came over
133......... ..... ..Some water started coming over
134......... ........Used head gun on recovery tube
190.......... .......H2O evolution was very slowed right down
194.......... .......Oily appearence with water. No more need for heat gun
198............. ....Propenyl benzene and H2O started coming over
196........... ......2 drops/sec
198........... ......3 drops/sec
202........ .........distillation stopped
208........ .........nothing
214.......... .......nothing
Distillation took about 5 hours. 3 layers in distillate. Propenyl, water, DCM. Propenyl layer was was grey/green. 17.8 g coffee coloured residue.
This was poured in a 2l sep and washed with 450 dH2O and 50g NaHCO3 then washed with 450ml brine that resulted in a horrible emulsion that did not
clear after 2 hours so 55ml 32% HCl with 500ml dH2O was added and after a while the emulsion shrunk very well.
Washed again with plain dH20 but got bad emulsion again. Added MgSO4 solution which cleared it abit then CaCl2 solution which did the trick.
Backwashed all aqueous layers. Dried with CaCl2
Filtered into a 3l RBF with fractionator and removed most DCM with bath up to 70 deg. Transfered to a 1l RBF with fractionator and removed DCM, bath
up to 100 deg then set up for vacuum fractional distillation and removed the rest of the DCM with vacuum at room temp.
Collected 395.9g Propenylbenzene @45 deg, 6.5 mbar, with 12.1g light yellow/green residue.
Yield 89.9%
Example 2.
774.3g P1P (5.77 mole)
65.5g NaBH4 (1.73 mole)
1935ml MeOH
1155ml dH2O
7g NaOH
866ml 2M HCl
2320ml Benzene
2065ml dH2O
Dehydration:
19.4g KHSO4
2320ml Benzene
1030ml 10% NaHCO3
1030ml Brine
774.3g P1P and 1935ml MeOH was placed in a 5l bottle with thermometer and overhead stirrer sitting in an ice bath.
7g HaOH and 1155ml dH2O was cooled in the freezer then 65.5g NaBH4 was added and poured in a 2l sep funnel. temp was -2 degC. This was dripped in to
the bottle and solution was kept between 25 and 27 deg which took 63 min. Ice water was removed while not letting temp climb and stirred for 2 3/4
hrs.
866ml 2M HCl @ 10 deg was added keeping temp below 27 deg till pH 7. Two layers formed. Bottom aqueous layer was siphoned off and extracted 2 x 775ml
benzene, 1 x 600ml and 175ml to wash out all bottles used. This was added to the organic layer, which was washed with 2 X 1030ml dH20. Should have
been 3 to extract all traces MeOH, but 2 worked ok. Washed were backwashed with 100ml benzene which in turn was washed.
All was placed in a 5l RBF with fractionator and benzene removed. Oil bath up to 143 deg. Transfered to a 2l RBF with recovery tube and heated slowly.
Bath Temp..Event
127 deg C...Benzene azeotrope came over
162......... ...More H2O than benzene started coming over
164........ ....H2O came over
169......... ...H2O started slowing. Heat gun used.
180........ ....H2O evolution was very slow
186........ ....Propenyl benzene and H2O came over. Azeotrope I'm guessing.
187........ ....Held at this temp, 2 drops/sec till azeotrope slowed.
189....... .....Azeotrope done and then came over clearer.
189 - 192....Most propenyl came over in that range
Cooled and transfered to a 250ml RBF and removed KHSO4
194...........Propenyl came over slow. Cooled, added 2.1g KHSO4, heated again.
194 -198....A few drops came over, but no advantage with added KHSO4.
51.7g was left as a dark drown residue. In the receiver was a layer of water and the propenyl with benzene was milky and seemed less green than
before. Put in a 5l Bottle with 2040 benzene and washed with 900ml 10%NaHCO3. Added 9g NaCO3 because of the 90ml water already present. Then washed
with 900ml brine. This time no emulsion.
Did not bother drying, so poured in a 5l RBF and distilled off most benzene. Transfered to a 1l RBF with fractionator keeping at exactly 80 deg C.
When temp went up, changed receiver and collected propenyl benzene at 175 - 176 deg C collecting 582.5g. When no more came over, set up on a water
bath and vacuum distilled the rest collecting 15.6g
The dark brown residue was vacuum distilled collecting 13.1g.
Total propenyl benzene collected was about 611.2g. Total residue was 52.3.
Yield was about 89.6%
I say "about" because even though the propenyl distilled at 175 - 176, it still had some water in it. It's light green when it has water. A tiny spoon
of CaCl turned it crystal clear. The benzene azeoptrope came over first as expected, then at 80. That's why I didn't bother drying it, but I'm
guessing that the propenyl hung on to some water anyway. Mother fucker!
Vacuum is the way to go, breaks the azeotrope, less problems. Getting a good vacuum started in the first place is a major problem for me. That's why I
chose benzene. I thought it would remove all the water, but as I discovered, when I applied vacuum in this synth, the water was still there in the
residue.
Advantage of benzene;
Aqueous layer on bottem is easier to siphon off.
No emulsions
Dehydration easier, less water to deal with because of azeotrope. Less heat gun use.
loose less to aquaeous layer.
Disadvantages;
Cancer probably hurts,
Harder to get.
takes longer to distill
Questions;
1) Is the amount of MeOH ok or should more be used.?
2) Is the amount of NaBH4 ok?
3) Is there an ideal reaction temp for reduction with NaBH4?
4) Should the borohydride solution be more dilute or concentrated?
5) Should more time be given after addition, does stiring matter?
6) can you recycle KHSO4. Does it break down in any way below 196 deg?
Example 3.
Almost same as example 2 that NaBH4 solution was split into 2 lots. One was kept in an ice bath till needed. Addition took 55 minutes. Left without
stirring for 3 hours then extracted with 3 x 775ml DCM and washed 3 x dH2O. Didn't dry it. This was not as brown as example 2
Dehydrated as before but didn't distill al of it thinking that there was no point because it probably al done. Combined residue and distillate, mixed
with 2325 ml DCM before realising that there's no need to neutralise and wash, so removed DCM in a 5L RBF with a boiling water bath, transfered to a
1l RBF which was filled almost to the brim. Never done that before, so I was very nervous.
Set up for vacuum fractional distillation and started to remove rest of the DCM with bath at RT and set stirrer on very high. This took hours, so
pumped up the heat to 60 deg with high vacuum and waited another hr or so till vacuum was ok, 8 mbar. Overall took 7-8 hrs. Too long man. Maybe vacuum
isn't the way to go.
Collected 606.6g Propenyl benzene with 64.6g orange residue. (12.3g more than ex 2)
Yield, 88.9%
I don't think benzene offers any advantage. DCM is safer and residue isn't dark. I also think that the residue contains P1P and P1POl and could be
reprocessed. I doubt that washing after dehydrating is necessary so this saves a lot of time. So what if it's a bit acidic, it's going into the
performic anyway. I may be wrong.
Questions;
1) What's the right way to remove DCM
_________________
Be joyful, though you've concided the facts!
[Edited on 24-4-2006 by FriendlyFinger]
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