α−PROTON CHEMISTRY
TAUTOMERISM
          Keto |
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  Enol |
Tutorial 11.3
How do we know that tautomerism actually exist, since we can never isolate the enol product? Answer
The rate of enol formation is greatly favoured by the presence of an acid. We can carry out an electrophilic addition onto the π−bond and regenerate the ketone.
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H + ⇄ |
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− H + ⇄ |
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When a base is used, it extract a proton from the α−carbon to form an enolate ion. The ability to form a enolate is due to the resonance stability provided by the oxygen atom.
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OH - ⇄ |
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− OH - ⇄ |
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−  � THF |
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HALOGENATION
When halogens - Cl2, Br2 - are added to the α−protonated aldehydes and ketones in the presence of water, the halogens execute an electrophilic addition to the π−bond of the enol.
fast
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Since the tautomerism is in favour of the keto compound the amount of enol present at the initial stage is very low. So the reaction is very slow. However the HBr produced catalyses the formation of the enol compound.
fast
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Very soon the reaction becomes rapid, since the rate determining step is the formation of the enol. In chemistry the time taken for a reaction to take off is known as an induction period.
When the halogens are added to the ketone in the presence of a base the halogen will react with the enolate ion formed.
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ALDOL ADDITION
When aldehydes with an α−proton is stirred with an aqueous sodium hydroxide solution it reacts with itself with the release of a water molecule. Such reaction is traditionally referred to as a condensation reaction - two molecules bonding to each other with the release of a water molecule. This particular condensation reaction is known as the Aldol (aldehyde-alcohol) Addition reaction.
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 Heat |
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If there is a second α−proton to the aldehyde, the mixture can be heated to produce an α,β−unsaturated aldehyde. Yield of about 80% can be expected.
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Heat |
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The Aldol Addition reaction can also be carried out with potassium hydroxide, or lithium diisopropylamide in tetrahydrofuran.
Ketones with α−proton can also show Aldol Addition but the equilibrium is very much in favour of the reagent. We can only get significant amount (up till 80%) of the alcohol or the α,β−unsaturated carbonyl compound, if we remove the water formed during the reaction.
Tutorial 11.4
What compounds would you expect to obtain by heating the following compounds in an aqueous sodium hydroxide solution?
- Butyraldehyde;
- 5-oxo-2-octanol; and
- Cyclohexane and benzaldehyde Answer
SYN ADDITION
GRIGNARD REAGENT
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- the configurational stability of the aldehyde, and
- the steric obstruction of each configuration making it difficult for the carbanion to reach the carbon.
The R−O−MgBr complex can be hydrolysed to give an alcohol.
Experiment with formaldehyde
Experiment with acetone
Tutorial 11.5
When 2-cyclohexylpropanal is treated with methylmagnesium bromide and then hydrolysed in an acidic medium, a mixture of (2R,3R)-3-cyclo-2-butanol and (2S,3R)-3-cyclo-2-butanol is produced in the ratio of 65:35. Explain why the products are not obtained in equal proportion. Answer
| For aldehydes and ketones with a α−proton to the carbonyl, the acidity of this proton will also entice the carbanion to show its basicity property. |
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| Conversely, if the carbanion has a α−proton, the positively charged carbon of the carbonyl may form a bond with it. |
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The carbonyl will also react with alkyllithium in a concerted manner.
| RBr + 2 Li |
� Ether |
RLi + LiBr |
R'2C=O � − 78�C |
R'2RC−OLi | H+ � |
(t-Bu)3C−OH + Li+ ca 80% |
Tutorial 11.6
When 2-methylcyclopentanone is treated with methyllithium and then the product hydrolysed with an aqueous acid, a mixture of (1R,2S)-1,2-dimethylcyclopentanol and (1S,2S)-1,2-dimethylcyclopentanol is produced in the ratio of 9:1. Explain why the products are not obtained in equal proportion. Answer
PHOSPHORANE
Georg Wittig discovered that aldehydes and ketones can react with a phosphorane by a concerted reaction.
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ca − 80�C |
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ca 0�C |
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| oxaphosphetine | ||||||
At about − 80�C, the oxaphosphetane is stable in solution, but when warm to about 0�C it decomposes to give an alkene.
The preparation of alkenes with the help of phosphorane is now referred to as the Wittig Reaction.
Tutorial 11.7
How would you prepare methylenecyclohexane from an alkyl bromide and cyclohexanone using the Wittig Reaction? Answer
DIEL-ALDER REACTION
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     1,4-orientation |
1,3-orientation |
OXIDATION and REDUCTION
Aldehydes are easily oxidised, even by the oxygen in the air, to the corresponding acid.
R−CHO � R−COOH
Common reagents used to oxidise aldehydes are: silver oxide, potassium permangnate, chromate, the peroxides and the hydropeoxides. With oxygen the first product formed is a peroxide, R−C(O)OOH, via a free radical reaction.
So if you have a bottle of aldehyde left standing in the laboratory for quite a while you will have to purify it before use.
It is more difficult to oxidise the ketones. Vigorous condition has to be used resulting in a mixture of products. The only important process is the oxidation of cyclohexanone by nitric acid catalysed by vanadium pentoxide. The product is adipic acid, the raw material for the preparation of nylon 66.
Aldehyes an ketones can also be reduced by hydrogen in the presence of a metallic catalyst to an alcohol.
However in the laboratory the reduction is executed wth metal hydrides such as lithium aluminum hydride or sodium boronhydride. The aldehyde will give a primary alcohol while the ketone will give a secondary alcohol.
COMMERCIAL ALDEHYDES
  Trioxane |
 Paraldehyde |
Formaldehye is a gas and generally sold as formalin, a 37% aqueous solution. However it is also available as a solid in the form of trioxane (M.Pt. = 64�C).
The trimer for acetaldehye is known as paraldehyde; a liquid with a boiling point of 128�C. It will generate acetaldehyde when heated with a little acid.
