Organic Fulton Flashcards
Oxidation can be considered as a…. of H2
Reduction can be considered as a … of H2
loss of H2
Gain of H2
Alcohols are oxidised by transition metal oxidising agent such as..
KMnO4 (potassium permanganate)
CrO3 (chromium trioxide)
Na2Cr2O7 (sodium dichromate)
The metals are in high oxidation statesAs the oxidation proceeds, the metals are reduced to lower oxidation states
What is the Jones reagent and what does it do?
Oxidation with Na2Cr2O7(sometimes CrO3) and H2SO4in acetone is called the Jones oxidation
Works best in the oxidation of secondary alcohols
The Limitations of Jones Oxidation
. Over oxidation is a major issue.
. Furthermore, the use of strong acid means this reagent cannot be used on acid-sensitive substrates.
. Chromium has toxicity issues.
. Expensive to get rid of
To oxidise in the absence of water use..
PCC or PDC
The problems with Chromium Reagents
- It is very difficult to separate the chromium residues from the product.
- Chromium is highly toxic so these oxidations are not suitable for use in pharmaceuticals.
- Large amounts of chromium containing waste are produced.
- Cost can be a problem especially if using PCC or PDC on a large (multi Kg) scale.
What is TPAP ?
Therefore what is the driving force of the reaction using this?
a mild oxidising agent which uses catalytic amounts of metal
Driving force for this oxidation is the reduction of Ru (VII) to Ru (V)
Propose mechanism for TPAP oxidation of primary alcohol?
Manganese dioxide (MnO2) is a very mild oxidant (Mn (IV) so it can only oxidise..
Activated Alcohols such as allylic alcohols or Benzylic alcohols.
What is swern oxidation?
what does it use as the oxidant and the activator?
a mild method to convert alcohols to aldehydes.
Uses DMSO as the oxidant and oxalyl chloride as an activator
Draw the sulfur ylide and explain why it is stable?
Because the ylid is resonance stablized it can be formed relatively easy by deprotonation, Sulfur atom has a low energy d-orbital which can accept electrons from carbon.
Advantages od swern oxidation (3)
Disadvantages od Swern oxidation (3)
- Uses only inexpensive reagents.
- Avoids use of metals, so no toxic waste.
- Easy purification as CO, CO2 and Me2S are all gases
and Et3NHCl is water soluble. - Me2S smells (but it can easily be trapped and reoxidised to odourless Me2SO with H2O2).
- The reaction needs low temperatures (-78 oC).
- Not all alcohols / aldehydes are compatible with the
basic reaction conditions.
Why can Dess-Martin Periodinane (DMP) Oxidation oxidise very sensative alcohols?
The Dess-Martin reagent is active under very mild and completely neutral conditions. The iodine wants to be in a lower oxidation state
What is TEMPO, draw it.
how is it stabilised ?
TEMPO is a stable radical
Reagents used to reduce carbonyls (aldehyde, ketone)? and why?
sodium borohydride(NaBH4) —safe and easy to handle, mild. used in a protic solvent, EtOH, MeOH and acid workup
lithium aluminium hydride (LiAlH4) —powerful, potentially hazardous.
in anhydrous orgainc solvent, Et2O, THF. reacts with H2o producing H2 causing fire
more reactive so can also reduce esters and amides
Draw the Mechanism of Borohydride Reduction
Draw Mechanism of LiAlH4Reduction of Esters
Reductive Amination way to make amines is by the reduction of imines.
draw mechanism
the best way to make secondary amines.
Why is NaBH4 a soft nucleophile?
what kimd of additions will it perform?
NaBH4is a source of nucleophilic hydride ions (H-). However the nucleophilic electrons in the B-H bond are diffuse (allow NaBH4to be nucleophilic as rather than basic like NaH) therefore it will perform initial 1,4-additions, then 1,2-additions leading to reduction of the alkene and carbonyl groups.
If we wish to just do 1,2-addition of the hydride (to just reduce the carbonyl) we can add to the reaction CeCl3. This is a Lewis acid which coordinates to the carbonyl carbon activating it to nucleophilic attack. Now the carbonyl group is MOREreactive, the carbonyl now undergoes 1,2-addition with NaBH4.
What is BH3 a good reagent for?
What does it reduce?
BH3is a good reagent for the reduction of unreactive carbonyls
BH3will reduce a carboxylate to an alcohol
BH3will reduce an amide to an amine
BH3 exists as a gaseous dimer B2H6.
How doe we make this easier to use?
Make it a liquid by its complexation with Et2O, THF or dimethyl sulfide (DMS).