What's The Point in Ethanolic Solution? Watch

ProfessorFitBoy
Badges: 2
Rep:
?
#1
Report Thread starter 12 years ago
#1
Hi

I was wondering if anyone knows why some organic synthesis reactions such as the synthesis of a Nitrile from a Halogenoalkane under reflux and the production of an Alkene from a Halogenalkane (elimination) need to be conducted in Ethanolic solution.

It just seems like one of those little things that nobody thinks of and they might ask you in an exam.

Any help appreciated
0
reply
gyrase
Badges: 0
Rep:
?
#2
Report 12 years ago
#2
Well as you know ethanol is an alcohol, and as such has a specific pH. I would suggest that it is used to provide favourable pH conditions for the reaction to occur under.
0
reply
cpchem
Badges: 12
Rep:
?
#3
Report 12 years ago
#3
I would imagine that the reaction's mechanism involves ethanol - that seems like the obvious answer.
As far as pH-type effects - it has no buffering capability, so it doesn't seem likely to me, it's practically pH 7, after all. According to that hypothesis, you'd be just as well doing the synth in distilled water.

Anyway, that's just my view on it. I could be completely wrong!
0
reply
charco
  • Forum Helper
Badges: 17
Rep:
?
#4
Report 12 years ago
#4
likely to be a solubility effect - ethanol dissolves organics that water wouldn't
0
reply
sea tea
Badges: 2
Rep:
?
#5
Report 12 years ago
#5
ethanol will dissolve both organic reagents and inorganic ones. it also favours elimination as it allows -:OH to act as a base rather than a nucleophile.

alternatively, it could be viewed from the point of view that KOH in ethanol allows the formation of potassium ethoxide, a strong base and (due to it's size) fairly rubbish nucleophile. it's the C2H5O- which actually takes part in the elimination.
0
reply
Nandoz
Badges: 0
#6
Report 12 years ago
#6
ethanol is used as a solvent
0
reply
sea tea
Badges: 2
Rep:
?
#7
Report 12 years ago
#7
the mechanisms both feature ethoxide, which would presumably form if you dissolved KOH in ethanol.

E2 mechanism:
http://en.wikipedia.org/wiki/Image2_reaction.png

E1 mechanism:
http://en.wikipedia.org/wiki/Image1_reaction.png
0
reply
Robusty
Badges: 0
Rep:
?
#8
Report 12 years ago
#8
(Original post by ProfessorFitBoy)
Hi

I was wondering if anyone knows why some organic synthesis reactions such as the synthesis of a Nitrile from a Halogenoalkane under reflux and the production of an Alkene from a Halogenalkane (elimination) need to be conducted in Ethanolic solution.

It just seems like one of those little things that nobody thinks of and they might ask you in an exam.

Any help appreciated
No, I don't think its pH nor mechanisms. This is simply because:

Halogenoalkane reacts with water to form an alcohol (only a little portion) as water is polar/nucleophile and attacks the positive centre of the halogenoalkane. (but to make an alcohol from a halogenoalkane, you would use aqueous NaOH solution since OH- is a better nucleophile than water)That is why ethanol is used as the solvent otherwise an alcohol will be formed as an impurity.

Hope this helps
0
reply
Robusty
Badges: 0
Rep:
?
#9
Report 12 years ago
#9
and yeh, i think this would be a crafty question to ask...
0
reply
sea tea
Badges: 2
Rep:
?
#10
Report 12 years ago
#10
(Original post by Robusty)
No, I don't think its pH nor mechanisms. This is simply because:

Halogenoalkane reacts with water to form an alcohol (only a little portion) as water is polar/nucleophile and attacks the positive centre of the halogenoalkane. (but to make an alcohol from a halogenoalkane, you would use aqueous NaOH solution since OH- is a better nucleophile than water)That is why ethanol is used as the solvent otherwise an alcohol will be formed as an impurity.

Hope this helps
-OH is still present in ethanolic solution, and is a cracking nucleophile even if there isn't any water present. the only thing which favours elimination over substitution is high basicity or stearic hindrance of the nucleophile, forcing it to act as a base (as in the case of organic bases eg alcoxides).

alcohols are always formed when you carry out an E1 or E2 elimination, more so with an E1 as the mechanism shares an intermediate carbocation with SN1. all you can do is favour elimination over substitution, not do one or the other.

how would using ethanol as a solvent stop the alcohol impurity in the way you described? the only case would be where you are reacting haloethane, as the equilibrium would be driven towards the haloalkane by the large excess of ethanol.

it is mechanism thing.
0
reply
silent ninja
Badges: 19
Rep:
?
#11
Report 12 years ago
#11
Like people have said, just solubility. Alcohols are excellent solvents.
Water cant dissolve for example halogenoalkanes-- you should also learn why water cant do this despite ethanol and water both being polar and so too halogenoalkanes.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Cranfield University
    Cranfield Forensic MSc Programme Open Day Postgraduate
    Thu, 25 Apr '19
  • University of the Arts London
    Open day: MA Footwear and MA Fashion Artefact Postgraduate
    Thu, 25 Apr '19
  • Cardiff Metropolitan University
    Undergraduate Open Day - Llandaff Campus Undergraduate
    Sat, 27 Apr '19

Have you registered to vote?

Yes! (356)
37.63%
No - but I will (74)
7.82%
No - I don't want to (66)
6.98%
No - I can't vote (<18, not in UK, etc) (450)
47.57%

Watched Threads

View All
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise