blogfast25
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t-butyl benzene by Friedel-Crafts: some questions...
I’d like to test some home prepared anh. AlCl<sub>3</sub> and anh. SnCl<sub>4</sub> by alkylating benzene with t-butyl
chloride and possibly later 2-chloropropane (yielding cumene). I now have more or less all the starting materials and hope to have a first run in a
couple of weeks.
I found a method for this synthesis in Vogel (library of this site, Ch. Aromatic Compounds, p. 513) but find it a bit puzzling.
It calls for 0.375 mol of anh. AlCl<sub>3</sub> (50 g) and 2.24 mol of benzene (200 ml) to be combined and chilled on an ice bath. 3 – 4
ml of t-butyl chloride is then added to prevent freezing of the benzene (5.5 C MP).
In total 0.54 mol of t-butyl chloride (59 ml) is then to be added over a period of, wait for it… 4 to 5 hours!
The requirement for chilling indicates a strongly exothermic reaction but such a long addition time? Is it really that exothermic? I calculated the
Standard Enthalpy of Reaction for:
C<sub>6</sub>H<sub>6</sub> + CH<sub>3</sub>Cl === >
C<sub>6</sub>H<sub>5</sub>-CH<sub>3</sub>+ HCl, (as a simple but impractical model) and found it to be modestly
exothermic , about - 47 kJ/mol, driven mainly by the Heat of Formation of HCl(g).
The entry also offers an alternative preparation using 0.154 mol of FeCl<sub>3</sub> (25 g) (instead of 50 g
AlCl<sub>3</sub> . There the mixture is cooled to about 10 C and (I
quote) “the 50 g of t-butyl chloride is added”. No mention of addition time here. It goes on: “The mixture is slowly warmed to 25 C and
maintained at this temperature until no more hydrogen chloride is evolved.”
Because FeCl<sub>3</sub> is a catalyst it should not affect the overall exotherm. Perhaps FeCl<sub>3</sub> catalysed reaction
proceeds slower?
I’m also slightly surprised by the high molar ratio of catalyst to alkylating agent: 0.69 for the AlCl<sub>3</sub> catalysis, 50 g of
anh. AlCl<sub>3</sub> that needs to be destroyed during work up.
Any insights welcome.
[Edited on 5-7-2014 by blogfast25]
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mnick12
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My guess is the slow addition is to keep the concentration of t-butyl chloride low during the reaction. Remember alkyl groups are electron donating,
so alkylation of the ring activates it to polyalkylation. That is just a guess though.
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Metacelsus
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In my organic chemistry class, we alkylated toluene (much safer than benzene!) with t-butyl chloride using iron chloride. We added everything all at
once in an ice bath, let it react for 30 minutes, and then warmed it up to room temperature.
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Magpie
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Quote: Originally posted by blogfast25  | I’d like to test some home prepared anh. AlCl<sub>3</sub> and anh. SnCl<sub>4</sub> by alkylating benzene with t-butyl
chloride and possibly later 2-chloropropane (yielding cumene).
|
Does your AlCl3 fume in moist air and make a hissing noise when added to water?
You could test your AlCl3 by making acetophenone by an FC reaction if you happen to have some acetic anhydride. I have done this and it worked well
IIRC.
The single most important condition for a successful synthesis is good mixing - Nicodem
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blogfast25
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| Quote: Originally posted by mnick12 | | My guess is the slow addition is to keep the concentration of t-butyl chloride low during the reaction. Remember alkyl groups are electron donating,
so alkylation of the ring activates it to polyalkylation. That is just a guess though. |
Avoiding polyalkylation is why the ratio benzene/t-butyl chloride is kept high.
| Quote: Originally posted by Cheddite Cheese | | In my organic chemistry class, we alkylated toluene (much safer than benzene!) with t-butyl chloride using iron chloride. We added everything all at
once in an ice bath, let it react for 30 minutes, and then warmed it up to room temperature. |
Interesting, just one addition…
| Quote: Originally posted by Magpie | Does your AlCl3 fume in moist air and make a hissing noise when added to water?
You could test your AlCl3 by making acetophenone by an FC reaction if you happen to have some acetic anhydride. I have done this and it worked well
IIRC. |
Yes and yes. No to acetic anhydride, I’m afraid.
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Chemosynthesis
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I have seen slow additions of HCl (obviously produced in this reaction) to reduce polymerization and heat. Perhaps too rapid an addition causes a
localized XS halide and increased temperature which would lead to side reactions such as the aforementioned polyalkylization despite the overall
stoichiometry favoring the monoalkyl product.
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Metacelsus
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I think that adding the t-butyl chloride slowly would probably have given better yield and purity, but we only had a limited time to do the
experiment. Polyalkylated product was still only on the order of a few percent (but remember, it was toluene, not benzene).
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Pyro
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I made this compound a while ago, when I was adding the t-BuCl to the benzene slowly as recommended by Vogel's 4th, I needed to add it quite a bit
faster to stop the benzene freezing.
Sadly the only evidence i have that the reaction worked was the smell. the product smelt nothing like benzene or t-BuCl at all.
see, after I added the t-BuCl to the benzene I put it in a stoppered flask in a fridge (lab fridge obviously). to make things short, that flask and a
cupboard were all that was left after the lab was packed up (had to go, the building was sold). I made the mistake of asking my dad to carry that home
while I took the rest, after a short chat on how much he thought it smelt like benzene, we went off. What does he do? while crossing the bridge he
DUMPS THE FUCKING CONTENTS IN THE RIVER, he thought it was waste 
I had 200ml of benzene invested in that thing,
lesson learned:don't stop half way through a reaction
all above information is intellectual property of Pyro.  |
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CuReUS
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| Quote: Originally posted by Chemosynthesis | | I have seen slow additions of HCl (obviously produced in this reaction) to reduce polymerization and heat. Perhaps too rapid an addition causes a
localized XS halide and increased temperature which would lead to side reactions such as the aforementioned polyalkylization despite the overall
stoichiometry favoring the monoalkyl product. |
wow ,this is a good way to prevent polyalkylation.
by adding HCl ,you shift the equlibrium left
but isnt the yield of friedel craft very bad
i saw somewhere that it was 1.5%
the only way to get good yiels is to use large quantity of benzene
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CuReUS
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blogfast ,actually FeCl3 is used instead of AlCl3 if you dont want your alkyl chain to rearrange as FeCl3 is a
weaker lewis acid than AlCl3
for ex-if you use n-propyl choride for alkylation in presence of AlCl3
,you get tert-butyl benzene
but if you use FeCl3 you get n-propyl benzene
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Metacelsus
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Don't you mean you get iso-propyl benzene?
Some degree of rearrangement is unavoidable, even with iron chloride.
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CuReUS
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yes ,i am sorry
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