Sciencemadness Discussion Board - Lewis acids: SnCl4 and SbCl5

Lewis acids: SnCl4 and SbCl5

blogfast25 - 1-3-2014 at 04:26

I have a relatively easy (and chlorine free) way of preparing both perfectly anhydrous SnCl4 and SbCl5, both of which are Lewis acids.

Would these be suitable for Friedel Crafts (FC) reactions? I always see AlCl3 and FeCl3 mentioned as catalysts/Lewis acids in FC, never any others. Is that due to availability or because they're better suited for FC?

Forgive my ignorance: last FC I carries out was a long forgotten uni practicum, 30 odd years ago...

[Edited on 1-3-2014 by blogfast25]

plante1999 - 1-3-2014 at 04:36

The antimony pentachloride mixed with some fluoride in a reaction become a so strong lewis acid that it could be destructive to your substrate, as for the pure pentachloride, I have seen it used in industrial polymerisation where strong lewis acid is mandatory.

I don't have the knowledge for the frield craft, however I would guess Tin tetrachloride would work as it is a reasonably strong lewis acid.

I would think it is due to the fact the aluminium trichloride is easy and cheap to make anhydrous in industry, and that it is a solid most importantly.

Boffis - 1-3-2014 at 07:31

@ Blogfast, might we ask about your "chlorine free" preparation of anhydrous SnCl4 etc. I'm intrigued.

I have not come across the use of SnCl4 in the FC reaction only in some formylation reactions. SbCl5 is an aggressive chlorinating reagent.

Nicodem - 1-3-2014 at 11:00

SnCl₄ is a commonly used acid catalyst for Friedel-Crafts acylations (if you check the literature, you can easily find many examples). I don't remember any example of its use for the Friedel-Crafts alkylations, but it should work at least for the tert-alkyations.
However, not every acid is good enough for just any Friedel-Crafts reaction and you say nothing of what the reaction is. You don't even say the most important things, like whether you talk about an acylation, alkylation, and what kind of a substrate you have in mind.

SbCl₅ can cause electrophilic chlorinations, so it might not be suitable for your reaction.

blogfast25 - 1-3-2014 at 11:02

Quote: Originally posted by Boffis

@ Blogfast, might we ask about your "chlorine free" preparation of anhydrous SnCl4 etc. I'm intrigued.

The method for both is very similar. Both these metals can be dissolved in an excess HCl + a stoichiometric amount of HNO3 to oxidise them to the highest oxidation state, very quickly. For Sb, HCl + H2O2 on ice bath also dissolves the metal effortlessly to what is essentially solution of HSbCl6.

In the case of tin, adding the requisite amount of KCl and reducing the solution down to about half, followed by cooling and chilling, causes the crystalline K2SnCl6 to form. This needs to be washed and dried thoroughly.

In the case of antimony, KCl is also added and the solution reduced but KSbCl6 does not crystallise out due to very high solubility. I have found that ethanol is a suitable anti-solvent though. Add 3 - 4 times the volume of EtOH to the volume of solution and the KSbCl6 precipitates out. Filter and wash with a bit of EtOH, then acetone. Dry carefully.

On pyrolysis (500 - 700 C) both these dry salts lose the volatile SnCl4/SbCl5 which is distilled off and collected in a chilled recipient.

[Edited on 1-3-2014 by blogfast25]

blogfast25 - 1-3-2014 at 11:06

Quote: Originally posted by Nicodem

[...] you say nothing of what the reaction is. You don't even say the most important things, like whether you talk about an acylation, alkylation, and what kind of a substrate you have in mind.

That's because I'm looking for an example of a reaction where this could work, without knowing much (if anything, other than textbook knowledge) about FC reactions. I.o.w. I'm not looking for a specific reaction, rather for one that could be catalysed by either of these acids.

[Edited on 1-3-2014 by blogfast25]

blogfast25 - 2-3-2014 at 13:39

Quote: Originally posted by Nicodem

I don't remember any example of its use for the Friedel-Crafts alkylations, but it should work at least for the tert-alkyations.

Forgive my ignorance but by "tert-alkyations" do you mean alkylations using a tertiary alkyl chloride, like t-butyl chloride (2-chloro-2-methylpropane)? I happen to have some of that. Thought I might try alkylating benzene with it, with SnCl4 as catalyst?

[Edited on 2-3-2014 by blogfast25]

Nicodem - 4-3-2014 at 09:45

Quote: Originally posted by blogfast25

I might give you some references, if you kindly list the aromatics and acylchlorides or alkyl chlorides that you have access to. There are hundreds of examples of SnCl₄ catalyzed Friedel-Crafts acylations and dozens of examples of Friedel-Crafts alkylations in the literature. Surely you don't expect anyone would review all of them. So, what else do you have available besides t-butyl chloride?

blogfast25 - 4-3-2014 at 14:07

Quote: Originally posted by Nicodem

Surely you don't expect anyone would review all of them. So, what else do you have available besides t-butyl chloride?

Surely not, sorry if I gave that impression.

Not a great deal besides t-butyl chloride. Definitely 2-chloropropane, n-hexanol (to prepare 1-chlorohexane with?). Ethyl bromide but I'd have to prepare some.

Aromatics: benzene, toluene, a few more simple ones that are easy to acquire.

I'd be happy with a proposed synthesis with broad outline of method, just as a starting point.

Thanks for your help.

[Edited on 4-3-2014 by blogfast25]

Nicodem - 5-3-2014 at 10:24

I found no specific example of SnCl₄ catalyzed alkylation of benzene or toluene with ^tBuCl or ⁱPrCl, but there are some relevant examples with acids of comparable strenght (ZrCl₄, TiCl₄, ZnCl₂...).

Of the SnCl₄ catalyzed Friedel-Crafts alkylations there are examples of alkylation of benzene with 2-methyloxetane [1], ^tBuNO₂ and related nitroalkanes [2], mandelic acid [3], and benzyl chloride [4].
... of toluene with styrene oxide [5].
... of chlorobenzene with mandelic acid [6].

t-Butylations of benzene, toluene or related aromatics are otherwise best catalyzed by sulfuric acid when you use t-butanol, isobutene or methyl t-butyl ether as the alkylating reagents (hundred of literature examples, too many to cite). ZrCl₄ and related acids can also be used with these reagents.[7]
For a sulfuric acid catalyzed Friedel-Crafts benzylation of benzene with benzylalcohol see kmno4's report.

BnCl and ^tBuCl are of comparable reactivity, so if you want to try the t-butylation of benzene, I suggest you to adapt the procedure from ref. 4 (DOI: 10.1063/1.1555631):

Quote:

Benzyl chloride (3.80 g, 30.0 mmol) and benzene-d (15mL) in a three-necked flask equipped with a calcium chloride guard tube, a magnetic stirrer and a water-cooled condenser were cooled with an ice bath under stirring and tin(IV) chloride (0.40 g, 1.53 mmol) was added slowly in five equal amounts. After stirring for 15 min D₂O was added (5×10 ml) to decompose the catalyst. The mixture was washed with water (2×10 ml) and 1 M HCl (2×10 ml), the organic layer was separated and dried over magnesium sulphate. Volatile impurities were removed at 100 °C/3 Torr, leaving behind the desired diphenylmethane-d₅.

Leave it overnight, if you can't monitor the reaction (it is hard to follow such volatile compounds with TLC).
You will have to fractionally distill the mixture to isolate t-butylbenzene, which means you will need to do this on a larger scale or multiple times.

1. Bulletin of the Chemical Society of Japan, 1982, 55, 167-170.
2. Journal of the Chemical Society, Chemical Communications, 1986, 1285-1287.
3. Collection of Czechoslovak Chemical Communications, 1949, 14, 551-554.
4. J. Chem. Phys., 2003, 118, 6417.
5. Journal of Organic Chemistry, 1983, 48, 592-596.
6. Zeitschrift fuer Naturforschung, B: A Journal of Chemical Sciences, 2011, 66, 95-97.
7. Chemistry & Industry, 1985, 762-763. (see also US6172262 for an example with TiCl₄ or WO2011047315 for FeCl₃)

blogfast25 - 6-3-2014 at 06:36

Thanks, Nicodem. I might see if I can procure some benzyl chloride, if not it will have to be t-butyl chloride.

I also looked around in orgsynth.org but although some published syths do prescribe SnCl4, all of them required reagents well outside of my reach.

Organikum - 6-3-2014 at 15:50

I did an FC on benzene (reactand and solvent) and mono-chloroacetone with SnCl4 and it worked well. Nothing special regarding conditions IIRC, 50°C was reaction temperature.

Was from some article where they claimed SnCl4 being superior to AlCl3 in this reaction and this made me curious so I tried it and found it was true.

regards
/ORG

blogfast25 - 7-3-2014 at 10:38

Thanks Org. Mono-chloroacetone is probably out of my reach though.

Metacelsus - 7-3-2014 at 17:55

Organikum, just why were you making phenyl-2-propanone? (Yes, it does have legitimate uses.)

Making chloroacetone is easy if you have a chlorine source and acetone. You can even electrolyze a mixture of acetone and hydrochloric acid, although this gives low yields and is quite messy (tar alert).

blogfast25 - 8-3-2014 at 06:36

Quote: Originally posted by Cheddite Cheese

Organikum, just why were you making phenyl-2-propanone? (Yes, it does have legitimate uses.)

Making chloroacetone is easy if you have a chlorine source and acetone. You can even electrolyze a mixture of acetone and hydrochloric acid, although this gives low yields and is quite messy (tar alert).

I don't much care for the tone of that first question. It's bad enough having parts of Law enforcement looking suspiciously at some hobbyists' experimentation, so lets not start an internal Inquisition too. If there really are any kooks on this forum they're hardly going to tell us.

As regards direct chlorination of acetone, do you have any experience/references with that?