[Altered State]

Tomorrow I'm gonna have to obtain an halohydrin from an alkene (E-stilbene) and the procedure I've came up with is this one:

A 500-ml., round-bottomed flask equipped with  a magnetic stirring bar  and a thermometer is
charged with 18.0 g. (0.100 mole) of (E)-stilbene (Note 1), 5.0 ml. (0.28 mole) of water, and 300 ml.
(4.23 moles) of dimethyl sulfoxide (Note 2). The resulting suspension is stirred for 5 minutes at room
temperature (20–25°) (Note 3). Stirring is continued as 35.6 g. (0.200 mole) of N-bromosuccinimide
(Note 4) is added in small portions over ca. 10 minutes. A yellow color appears when the first portion of
N-bromosuccinimide is added, and by the time the addition is complete, the solution is bright orange.
During the addition the temperature of the mixture rises to 50–55°, and all the (E)-stilbene dissolves.
The contents of the flask are stirred for another 15 minutes and poured into 1 l. of ice water; the product
separates immediately as a white solid (Note 5). The aqueous slurry is transferred to a separatory funnel
with the aid of 50-ml. portions of water and diethyl ether, and extracted with four 200-ml. portions of
ether. The combined ethereal extracts are washed with 250 ml. of water and 250 ml. of sodium chloride
solution, dried over anhydrous magnesium sulfate, and evaporated with a rotary evaporator at a water
bath temperature of ca. 30°. The pale yellow, crystalline residue is dissolved, to the extent possible, in
600 ml. of hot hexane, and the resulting suspension is filtered while hot, removing a small amount of an
insoluble impurity. Cooling the filtrate provides colorless fibers of analytically pure erythro-2-bromo-
1,2-diphenylethanol, m.p. 83–84° (Note 6); a second crop of crystals is obtained by concentrating the
mother liquor to 200 ml. (Note 7). The combined yield is 22.0–24.9 g. (80–90%) (Note .

And the only thing I do not really understeand is "The combined ethereal extracts are washed with 250 ml. of water and 250 ml. of sodium chloride".

It's obvious that proccess is done to clean the product, but I do not get what exactly do we eliminate cleaning with those 2 extractions, specially the NaCl one.

Any thoughts on this?

[Dan]

It's obvious that proccess is done to clean the product, but I do not get what exactly do we eliminate cleaning with those 2 extractions, specially the NaCl one.

1. What is likely to be in the reaction mixture at this point?
2. Which components of this mixture might be more soluble in water than in ether?

NB: The NaCl wash means a "brine wash" <- food for google

[Altered State]

It's obvious that proccess is done to clean the product, but I do not get what exactly do we eliminate cleaning with those 2 extractions, specially the NaCl one.

1. What is likely to be in the reaction mixture at this point?
2. Which components of this mixture might be more soluble in water than in ether?

NB: The NaCl wash means a "brine wash" <- food for google

Ok, NaCl wash is for more effinciently eliminating water from the mixture.

But Im not sure what can you clean using water.

Obviously the water I add in the begining will be removed.

The other components in the mixture are DMSO and NBS excess. An maybe Br2 formed (which gives orange colour to de mixture, is this right???)
But I do not know if they are more likely solved in water than in ether. It makes sense that all of those compounds are removed in this step, because then, when you evaporate ether, you get just the final product. But on the other hand it doesnt make too much sense that organic compounds like NBS or DMSO would rather solve in water than in ether.

What is it?

[Dan]

NBS is not very soluble in water, but DMSO is and can be removed by aqueous washing.

That is not all. What is the chemical equation for the reaction you are doing?

[Altered State]

NBS is not very soluble in water, but DMSO is and can be removed by aqueous washing.

That is not all. What is the chemical equation for the reaction you are doing?

E-stilbene (1 eq) + NBS (2 eq) ----(H2O, DMSO)---> eritro-2-bromo-1,2-diphenylethanol

Later we do a recrystalization solving the product in hexane (50°C)
Could this be the process in which we remove NBS?

[Dan]

E-stilbene (1 eq) + NBS (2 eq) ----(H2O, DMSO)---> eritro-2-bromo-1,2-diphenylethanol

Not balanced. For example, you have N on the left (in NBS), but no N on the right.

[Altered State]

You are right. But I'm still lost.

I've found this in bibliography: "4. N-Bromosuccinimide purchased from Arapahoe Chemical Company was used without purification. If
the purity of the N-bromosuccinimide is in doubt, it should be titrated before use by the standard iodide–
thiosulfate method and purified, if necessary, by recrystallization from 10 times its weight of water.2
Solutions of N-bromosuccinimide in dimethyl sulfoxide cannot be stored, since the solvent is oxidized
by the brominating reagent."


I guess that last paragraph is the key for why do we use 2,00 equivalents of NBS and it turns to... ¿?
And oxidates dimethyl sulfure to...


This is my first year studying organic chemistry, and this is my first lab. experiment in this area, I'm quite lost with this, can't see as fast as I want the compounds I have in my mixture every moment (I'm sorry), and I really want to understeand it at all so I can achive the best possible mark.



This is the bibliography source they gave us:
http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0184



An here we go again:

"7. The submitters found that the residue (2.8 g.) obtained upon evaporation of the mother liquor was
largely erythro-2-bromo-1,2-diphenylethanol contaminated with a small amount of succinimide."

We got succinimide, and it is not removed from the product...

So, we simply don't eliminate it in this procedure?
Orange colour comes from dimethyl sulfure oxidated by the excess of NBS? What is formed so?

[Dan]

So what is the chemical equation of the reaction (forget about the DMSO for now)?

Hint: The authors say there was contamination of a small amount of succinimide - so logically there must be succinimide as a product in the chemical equation, right?

[Altered State]

E-stilbene (1 eq) + NBS (2 eq) ----(H2O, DMSO)---> eritro-2-bromo-1,2-diphenylethanol (1 eq), succinimide (1 eq), NBS (1 eq)

or


E-stilbene (1 eq) + NBS (2 eq) ----(H2O, DMSO)---> eritro-2-bromo-1,2-diphenylethanol (1 eq), succinimide (2 eq), Br2 0,5 eq? HBr 1 eq?

[Dan]

First equation looks good, though water is a reagent in this reaction:

alkene + NBS + water halohydrin + succinimide

You can also consider:

NBS + water succinimide + HOBr

What is water soluble here and what isn't?

[Altered State]

First equation looks good, though water is a reagent in this reaction:

alkene + NBS + water halohydrin + succinimide

You can also consider:

NBS + water succinimide + HOBr

What is water soluble here and what isn't?

I've found about succinimide that "It is very soluble in water, slightly soluble in alcohol, and insoluble in ether and chloroform.", so, we've got water, succinimide and BrOH are water soluble and NBS not soluble.
So if that equilibrium shifts to the right, seems that we can remove all the by-products using water, isn't it?

And then we use NBS excess with because we are sure that we are going to be able to remove it using water.

Does HOBr gives orange colour so?


PS: I didn't even know that Br could form BrOH...

[Altered State]

Still need *delete me*!

In about 15 hours I'm gonna have to explain this !