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17 Reactions of Aromatic Compounds -- Part 8
Under acidic conditions, acetals are stable to strong bases and nucleophiles.
It is easy to make acetals from the corresponding aldehydes and ketones.
We used silyl ethers to protect alcohols.
The following proposed synthesis is an example.
The aldehyde carbonyl group would react with its own group.
The Grignard reagent can be used to react with cyclohexanone.
The alcohol and hydrolyzing the acetal give the deprotected aldehyde.
Because aldehydes form acetals more quickly than ketones, we can protect them in the presence of a ketone.
ketone can be modified under neutral or basic conditions without disturbing the more reactive aldehyde group.
A variety of carbanion-like reagents have been added to carbonyl groups.
Wittig discovered a way to add a carbanion to a ketone.
The product is not alcohol because the intermediate undergoes elimination.
Wittig received the prize for his discovery in 1979.
In a two-step process, tri-phenylphosphine and alkyl halides are prepared.
The first step is an attack by triphenylphosphine on an alkyl halide.
The product is a salt.
The salt is treated with a strong base to abstract a protons from the carbon atom.
There are two forms of the phosphorus ylide, one with a double bond between carbon and phosphorus, and the other with charges on carbon and phosphorus.
The charged structure is the major contributor to the weak pi bond between carbon and phosphorus.
The carbon atom has a partial negative charge and a positive charge.
Trimethylphosphine is a stronger nucleophile than triphenylphosphine.
The ylide carbon atom has a strong carbanion character.
There is a negatively charged oxygen and a positively charged phosphorus on adjacent carbon atoms.
The four-membered ring collapses to give the alkene and triphenylphos phine oxide.
The driving force for the Wittig reaction is provided by the conversion of triphenylphosphine to triphenylphosphine oxide.
The carbonyl is attacked by the ylide.
O bond was formed.
O bond was formed.
The Wittig reaction can be used to form carbon-carbon double bonds.
When geometric isomerism is possible, cis and trans isomers often result.
Triphenylphosphine attacks and opens epoxides.
The initial product collapses to an alkene and triphenylphosphine oxide after being cyclized.
-2,3-epoxybutane is used to give but-2-ene.
The Wittig reaction can be used to convert a carbonyl group to a carbon-carbon double bond.
The Wittig reaction can produce a wide variety of alkenes.
To determine the necessary reagents, mentally divide the target molecule at the double bond and decide which of the two components should come from the carbonyl compound or the ylide.
The ylide should come from an alkyl halide.
The best way to react Triphenylphosphine is with unhindered primary and methyl halides.
Sometimes it reacts with secondary halides, but these reactions are slow and give poor yields.
The following example shows the planning of Wittigeses.
There are two double bonds in this molecule.
There are two ways in which the central double bond could be formed.
The mixture of cis and trans isomers will be produced by both of these synthetics.
The solution should be drawn out by you.
Plan a Wittig synthesis so that the alkyl halides are included.
Wittig reactions could be used to synthesise the following compounds.
aldehydes are easily converted to carboxylic acids by common oxidants such as bleach and permanganate.
Slow oxidation by atmospheric oxygen can be avoided by keeping the air out of the containers.
Mild reagents such as Ag2O can oxidize aldehydes in the presence of other functional groups.
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