Ax165Xj
Harmless
Posts: 25
Registered: 31-5-2013
Member Is Offline
Mood: No Mood
|
|
Jones Reagent from K2Cr2O7
I am attempting to prepare Jones reagent but the only source of hexavalent chromium I have is potassium dichromate. I had read a tek somewhere that
suggested dissolving CrO3 in 23mL conc. sulfuric and diluting to 100mL with water. I can't remember the suggested weight of CrO3 but when I did the
math I ended up needing about 39 grams of K2Cr2O7. I was able to dissolve that weight upon addition of heat. As expected, once the solution cooled, a
dark precipitate formed, possibly reddish.
Does anyone have any suggestions as to where I should proceed?
Should I just dilute the solution because the K2Cr2O7 doesn't have too great of a solubility in water? I am ok with that but I just wanted to make
sure it wouldn't interfere with its use as an oxidant. Also, is that precipitate CrO3?
I suppose this all really boils down to: does anyone have a good recipe for Jones reagent from potassium dichromate?
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
This may help:
http://www.sciencemadness.org/talk/viewthread.php?tid=6116#p...
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
That is a very interesting preparation, Magpie, definitely one for the files.
But the Jones oxidation proceeds vigorously (depending on conditions, of course) also when using acidified solutions of K2Cr2O7, very quick and
exothermically when concentrated solutions of dichromate are used.
It does of course create a bit of a mess from which the desired reaction product may be difficult to extricate, depending on its volatility or water
solubility mainly.
[Edited on 22-6-2013 by blogfast25]
|
|
|
Ax165Xj
Harmless
Posts: 25
Registered: 31-5-2013
Member Is Offline
Mood: No Mood
|
|
I had seen that topic before and that is definitely going to be my plan B if I can't get the job done with the potassium dichromate. In all Jones
reagent recipes they always say "you can use potassium dichromate too" but I can never find a good tek that specifically uses it. Synthesizing CrO3
sounds fun but f I don't need to go through those steps if I don't need to.
|
|
|
Ax165Xj
Harmless
Posts: 25
Registered: 31-5-2013
Member Is Offline
Mood: No Mood
|
|
I want to know the exact concentration of the Cr so I can add the correct amount of reagent to avoid over-oxidation. I am trying to think of good ways
of indicating the reaction is done. I have a solution of Cr and H2SO4, that is for sure but I have no idea what the concentration is. Could I use
KI-starch paper to indicate there are still oxidizing Cr6+ ions in solution? I feel like the darkness and low pH of the solution may interfere with
the starch blue.
I suppose I could just analyze my mystery solution for Cr (I run an ICP-OES). The solution will be less concentrated that it is supposed to be but as
long as a little excess water won't hurt the reaction, it doesn't matter to me.
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
For what it's worth, I can refer to some of my own experiments with KMnO4 and K2Cr2O7 for the oxidation of ethanol, in particular a crude
determination of the exotherm of these reactions:
http://www.sciencemadness.org/talk/viewthread.php?tid=12832&...
Quote: Originally posted by Ax165Xj  | | I want to know the exact concentration of the Cr so I can add the correct amount of reagent to avoid over-oxidation. I am trying to think of good ways
of indicating the reaction is done. |
I wouldn't worry too much about that: these reactions proceed near-quantitatively; the high exotherm indicates a very high equilibrium constant.
Iodine is likely to be oxidised by K2Cr2O7 but I don't think it's useful here.
What is it that you're looking to oxidise?
[Edited on 22-6-2013 by blogfast25]
|
|
|
Ax165Xj
Harmless
Posts: 25
Registered: 31-5-2013
Member Is Offline
Mood: No Mood
|
|
I was planning on oxidizing some random alcohols I have laying around. I figure methanol, ethanol, IPA etc. I don't really have a particular goal in
mind, just sounds fun to make something I don't have.
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
Jones did not write a recipe or tek using dichromate. Jones et al. didn't even mention [JCS 39 (1946), 2548 (1953)] the procedure as a particularly
general method beyond the obscure cpds. there. Jones did not do primary alcohols either. One can do oxidations of primary alcohols with
dichromate/H2SO4, but they aren't Jones'.
BTW quotes of interest IMHO from those articles:
The ready availability of the secondary acetylenyl-carbinols and glycols suggested an investigation of their oxidation, especially of the
aliphatic ethynylcarbinols, with chromic acid and good yields of ketones have been obtained in practically every case.
The nature of the solvent employed, the concentrations of the reagents and the reaction temperature all have a marked influence on the reaction. The
most satisfactory procedure consists in the addition of a solution of chromic anhydride in dilute sulphuric acid to an acetone solution of the
carbinol; in some cases a sharp endpoint is observed after the addition of the theoretical amount of the oxidising agent. Acetone is preferred as a
solvent since the oxidations proceed at a far greater rate in this medium than in acetic acid. The most suitable conditions for the oxidation of any
given carbinol are best determined by preliminary titration of small quantities (ca. 1 g.) with a standard chromic acid solution.
The reagent concentrations can usually be adjusted so that an upper layer, consisting mainly of an acetone solution of the ketonic product, separates
during the course of the oxidation from the lower green (usually saturated) solution of chromium salts. The segregation of the product in this way
appears to be of considerable importance in protecting it from further oxidation; the colour of this upper layer also gives a useful indication of the
presence of any excess of chromic acid. When the ketone produced is itself readily oxidised, it is advisable to introduce the chromic acid solution
well below the surface of the liquid, so as to avoid undue contact with the upper layer.
With readily oxidisable carbinols, satisfactory results can usually be obtained by using the theoretical quantity of oxidising agent but, in those
cases where oxidation is slow, the use of an excess of chromic acid is desirable. The particular temperature employed, in the range 0-20°, depends
upon the ease of oxidation of the carbinol; the temperature and the rate of addition of the chromic acid solution are mutually adjusted so that the
reaction proceeds at a reasonable rate without any appreciable excess of oxidising agent ever being present in the reaction medium.
================
General Method of Oxidation with Chromic Acid.-A cold solution of chromic acid (267 g.) in concentrated sulphuric acid (230 c.c.) and water (400 c.c.)
was made up to 1 L. This solution is 8N with respect to oxygen. The compound to be oxidised was dissolved in pure acetone (distilled over potassium
permanganate) at 20°, and the reagent added dropwise from a microburette until a persistent orange-brown coloration indicated that oxidation was
complete.
[Edited on 22-6-2013 by S.C. Wack]
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
| Quote: Originally posted by S.C. Wack | Jones did not write a recipe or tek using dichromate. Jones et al. didn't even mention [JCS 39 (1946), 2548 (1953)] the procedure as a particularly
general method beyond the obscure cpds. there. Jones did not do primary alcohols either. One can do oxidations of primary alcohols with
dichromate/H2SO4, but they aren't Jones'.
|
I don't think it matter much what we call it. Why it's often referred to as 'Jones' is quite obvious. Wiki mentions the dichromate/acid oxidiser in
the entry on 'Jones oxidation'. Granted, that doesn't mean much but language is in constant flux.
So what were the 'obscure compounds there'?
[Edited on 23-6-2013 by blogfast25]
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
Jones reagent is a specific thing, not necessarily the same as K2Cr2O7/H2SO4, so if someone does use K2Cr2O7 with or without acetone, that should be
stated clearly IMHO. The reaction
is often represented (e.g. March's) in general and even with K2Cr2O7/H2SO4 named instead of CrO3 (March's) and there are reviews along those lines,
easily the best (to 1982) being Chromium Oxidations in Organic Chemistry (seechemport), but there are also those more strict in definition (Hudlicky).
The properties of (m)ethanol may cause problems when they're plugged in there, while known methods for oxidizing those might use chromic acid
chromate/sulfuric or whatever but are definitely not in any case a Jones oxidation, so I think it's important to keep all this straight. Sometimes a
certain (di)chromate works better for some reason than CrO3 with acetone or not, whether for the reaction or isolation or both. Then there's all the
other things that can be done with hexavalent chrome, probably more interesting, again not Jones.
Some weird (fungus-related) items related to polyporenic acid in the latter, some (edited) selected from many similar from the former, no (m)ethanol
in sight:
A solution of chromium trioxide (10.3 g.) in water (30 c.c.) and concentrated sulphuric acid (8.7 c.c.) was added during two hours to a stirred
solution of oct-3-yn-2-ol (15 g.) in acetone (30 c.c.) at 5-10". After stirring for a further 30 minutes, the mixture was diluted with water to 250
C.C. and the product isolated with ether; distillation then gave oct-3-yn-2-one (11.5 g.), b. p. 70.5-71.5"/14 mm
To a stirred solution of phenylethynylcarbinol (342 g.) in acetone (500 c.c.) a solution of chromium trioxide (175 g.) in water (500 c.c.) and
concentrated sulphuric acid (148 c.c.) was slowly added; the operation, carried out at 5" in nitrogen, lasted 3-4 hours. After stirring for a further
30 minutes, the mixture was diluted with water and the product extracted with ether. Evaporation of the ethereal solution gave a yellow solid which
was recrystallised from aqueous methanol to give phenyl ethynyl ketone (258 g.), as pale yellow needles, m. p. 50-51"
Deca-2 : 8-dien-5-yn-4 : 7-diol (3 g.) was oxidised in the usual manner in acetone (20 c.c.) with a solution of chromium trioxide (2.45 g.) in water
(10 c.c.) and concentrated sulphuric acid (2.3 c.c.). Isolation by means of ether gave deca-2 : 8-dien-5-yn-4 : 7-dione (1.2 g.) as an oil, b. p.
80-81"/0.005 mm.
A solution in acetone (15 c.c.) of a mixture (m.p. 129-130') of the stereoisomers of 1 : 4-diphenylbut-2-yn-1 : 4-diol (2 g.) was cooled to 0-5" and
treated over a period of 15 minutes with a solution of chromium trioxide (1.5 g.) in concentrated sulphuric acid (1.36 c.c.) and water (4 c.c.). After
shaking for a further 5 minutes, the mixture was diluted to 100 C.C. with water and the pale yellow solid separated and washed well with water to
remove all traces of chromium salts. Recrystallisation from either ligroin (b. p. 80-100°) or alcohol gave dibenzoylacetylene (1.8 g.) as very pale
yellow leaflets, m. p. 111-112"
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
Fair points.
|
|
|
Ax165Xj
Harmless
Posts: 25
Registered: 31-5-2013
Member Is Offline
Mood: No Mood
|
|
It also seems like there are large variations in the amount of H2SO4 required in different preparations. Does anyone know of a general trend as to how
much sulfuric acid should be added for a specific substrate? Of course less acid would be better for acid sensitive compounds but I haven't found much
info on how much to use.
I feel like I saw somewhere that the amount needed is based on the metathesis between sulfuric acid and potassium dichromate and that the end goal is
K2SO4 and H2Cr2O7 which decomposes to the chromic acid/anhydride mixture. In this case, you would need an excess of sulfuric acid (seeing as the
second H doesn't like to dissociate as much) to produce the desired effect. Also, all of this could be conjecture, I can't find the source.
And for now on, I won't call this a Jones Oxidation, thank you for the correction.
[Edited on 1-7-2013 by Ax165Xj]
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
@Ax165Xj:
A good starting point is stoichiometry. For instance, oxidation of a primary alcohol to carboxylic acid:
R-CH2-OH + H2O === > R - COOH + 4 H+ + 4 e- ... Eq.1
Reduction of the Cr2O7(2-):
Cr2O7(2-) + 14 H+ + 6 e- ==== > 2 Cr3+ + 7 H2O ... Eq.2
Then 6 x Eq.1 + 4 x Eq.2, eliminate any redundant H+ and H2O on either side of the equation, and add SO4(2-) to the left to convert H+ to H2SO4 and K+
to obtain K2Cr2O7.
This gives you the theoretical minimal amount of potassium dichromate and sulphuric acid per mole of primary alcohol. A stoichiometric amount of
K2Cr2O7 probably works but you'll need some excess acid.
If you're dealing with a water soluble substrate you should have no problems but two phase reactions could be a lot trickier.
[Edited on 1-7-2013 by blogfast25]
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
In Laboratory Experiments in Organic Chemistry by Adams et al, 5th ed, isoborneol is oxidized to camphor using a Jones' reagent. It is
prepared by dissolving 27g of CrO3 in 23 mL of con sulfuric acid, followed by cautious dilution with water to 100 mL.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Ax165Xj
Harmless
Posts: 25
Registered: 31-5-2013
Member Is Offline
Mood: No Mood
|
|
| Quote: Originally posted by blogfast25 | @Ax165Xj:
A good starting point is stoichiometry. For instance, oxidation of a primary alcohol to carboxylic acid:
R-CH2-OH + H2O === > R - COOH + 4 H+ + 4 e- ... Eq.1
Reduction of the Cr2O7(2-):
Cr2O7(2-) + 14 H+ + 6 e- ==== > 2 Cr3+ + 7 H2O ... Eq.2
Then 6 x Eq.1 + 4 x Eq.2, eliminate any redundant H+ and H2O on either side of the equation, and add SO4(2-) to the left to convert H+ to H2SO4 and K+
to obtain K2Cr2O7.
This gives you the theoretical minimal amount of potassium dichromate and sulphuric acid per mole of primary alcohol. A stoichiometric amount of
K2Cr2O7 probably works but you'll need some excess acid.
If you're dealing with a water soluble substrate you should have no problems but two phase reactions could be a lot trickier.
[Edited on 1-7-2013 by blogfast25] |
Thank you very much this is exactly what I needed. It has been a while since I have had to balance redox reactions and to be honest I kind of forgot
about that approach. To make sure I am doing things correctly, I attempted to balance the oxidation of a secondary alcohol.
I came up with (minus spectator ions):
3(RCOHR)+ Cr2O7(2-)+ 11H(+) ===> 3(RCOR)+ 2Cr(3+) + 7H2O
Because the pKa2 of H2SO4 is fairly low and the solution will be diluted, I treated H2SO4 as a monoprotic acid. Therefore 11HSO4(-) and 2K(+) are the
spectator ions.
So does this mean a minimum of about 11moles of H2SO4 for every 1mole K2Cr2O7 is required for secondary alcohol oxidation by this method?
[Edited on 2-7-2013 by Ax165Xj]
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
Perhaps you should just do whatever you're interested in, exactly as some article or lab manual says. That could be the most important part.
Acetic acid from Cumming:
To 80 g (excess) of finely powdered potassium or sodium dichromate, and 100 g of 50% sulphuric acid placed in a reflux apparatus, 70 g of 25%
alcohol are slowly added. The mixture is heated for 30 minutes and distilled until only very little acid passes over. The distillate is neutralised
with caustic potash, and evaporated to dryness on a water bath. The residue is powdered and distilled with conc. sulphuric acid to 139°, and the
distillate fractionated between 115° and 120°.
Yield.--80% theoretical (18 gms.).
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
Work out the moles and this would probably work for oxidation of a secondary alcohol too, possibly with minor adjustments...
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
