Houngan
Harmless
Posts: 16
Registered: 19-1-2012
Location: Space
Member Is Offline
Mood: Regioregular
|
|
Basifying Agents
Hi,
I am wondering about choice of basifying agents / solutions.
I have seen sodium hydroxide and ammonium hydroxide used for basifying in liquid-liquid extractions.
I was just wondering when is best to use each of them.
To me it seems like an arguement of weak vs strong bases but is there anything else to the story?
Cheers, Houngan
The spice must flow.
|
|
|
woelen
Super Administrator
Posts: 7977
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
NaOH only adds base and Na(+) ions to your solution, ammonia also adds NH3. The latter can react with many compounds, can dissolve in other solvents
than water (e.g. when extracting things) and it can form coordination complexes with many metals. So, yes, there is more to it than just the strength
of the base. Other suitable weak bases may be sodium acetate, sodium carbonate. It just depends on what you really want.
|
|
|
Houngan
Harmless
Posts: 16
Registered: 19-1-2012
Location: Space
Member Is Offline
Mood: Regioregular
|
|
Thanks woelen!
So is the NH3 required for extraction of alkaloids?
Cheers, Houngan
The spice must flow.
|
|
|
Houngan
Harmless
Posts: 16
Registered: 19-1-2012
Location: Space
Member Is Offline
Mood: Regioregular
|
|
What would be a good system to practice one?
Cheers, Houngan
The spice must flow.
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
There are a whole variety of solutions available for work up. Generally theres strong/weak acid/base solutions, and some other more specialist
solutions.
Saturated sodium bicarbonate has a pH of 8 and will wash acidic compounds into the aqueous layer. It provides a means to remove unreacted acid from an
esterification reaction (for example) facilitating isolation of relatively pure material. It is also used for neutralizing an acidic solution, as the
pH will only ever rise to pH 8.
Sodium hydroxide (various concentrations) is used to basify in liquid-liquid extraction. It will bring the pH to 12-14 quite readily, allowing the
extraction of free amines from the aqueous phase. It is often used occasionally for working up zinc-based reductions, as the soluble zinc (II) first
precipitates (do it before transfer to the sep funnel!) and then dissolves as an excess of NaOH is added (as sodium zincate). Its also used in the
famous Feiser workup for LAH reductions.
Aqueous ammonia is often used as a 5 or 10%w/w solution. It has use for washing organic extracts from reactions utilising copper salts, amongst other
metals. The aqueous layer turns blue (in the case of copper) and when subsequent washes remain colourless you can be fairly confident theres no copper
left. I used this a load when I was doing Stille couplings with CuI as a co-catalyst.
I could elaborate on other solutions but it depends if theres a demand for such info. Much can be gleaned from the not-voodoo site (uni rochester).
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
